FINAL GEOTECHNICAL ASSESSMENT REPORT SINKHOLE DEVELOPMENT AT THE TROY MINE AND IMPLICATIONS FOR THE PROPOSED ROCK CREEK MINE LINCOLN AND SANDERS COUNTIES, MONTANA

Prepared For

FOREST SUPERVISOR KOOTENAI NATIONAL FOREST

U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE REGION 1

June 15, 2006

Prepared By

TETRA TECH, INC. Power Block Building, Suite 612 7 West Sixth Avenue Helena, Montana 59601

and

R Squared Incorporated 1332 East 22nd Avenue Denver, CO 80205

TABLE OF CONTENTS

GLOSSARY ...... iii

ACRONYMS AND ABBREVIATIONS...... iii

EXECUTIVE SUMMARY ...... ES-1

1.0 INTRODUCTION ...... 1

2.0 BACKGROUND ...... 3 2.1 GEOLOGY...... 3 2.1.1 Physiography and Regional Geology...... 3 2.1.2 Regional Stratigraphy ...... 3 2.1.3 Troy Mine (Spar Lake Deposit) – Geology ...... 7 2.1.4 Rock Creek Project -Deposit Geology...... 13 2.2 HISTORY AND DESCRIPTION OF SINKHOLES...... 19 2.2.1 Introduction...... 19 2.2.2 Sinkhole #1 ...... 25 2.2.3 Sinkhole #2 ...... 27

3.0 SITE-SPECIFIC OBSERVATIONS ...... 29 3.1 AERIAL PHOTOGRAPH REVIEW ...... 29 3.2 DRILL HOLE LOGS REVIEW...... 30 3.3 EAST FAULT, SINKHOLES, AND THE TROY MINE WORKINGS RELATIONSHIP ...... 31 3.4 OPEN EAST-WEST FRACTURES ...... 32 3.5 EAST FAULT ROCK MECHANICS PROPERTIES ...... 33 3.6 ANALYSIS OF SINKHOLE FORMATION ...... 34 3.6.1 Sinkhole #1 Formation...... 34 3.6.2 Sinkhole #2 ...... 37

4.0 POTENTIAL FOR SUBSIDENCE AT THE ROCK CREEK PROJECT...... 39 4.1 SUFFICIENCY AND ADEQUACY FOR IMPLEMENTATION OF THE STIPULATIONS IN THE ROCK CREEK FEIS AND ROD TO MINIMIZE THE POTENTIAL FOR GROUND SUBSIDENCE...... 40

5.0 REFERENCES ...... 42

Final Geotechnical Assessment Report, i Troy Mine, Montana

FIGURES

1 Location Map of Project Area...... 4 2 Regional Stratigraphic Section ...... 5 3 Geology of Troy Mine Area ...... 8 4 Cross Section Across the East Fault in the Vicinity of the Cross Fault...... 9 5 Tension Fractures in the Revett Formation...... 12 6 Geology of the Rock Creek Mine Area ...... 14 7 North-South Cross Section of the Rock Creek Deposit...... 15 8 Map Showing Rock Creek Project Area Lakes, Faults, and Proposed Buffer Zones...... 20 9 Mine Level Plan Map Showing Sinkholes and Geometry of Caved Materials ...... 21 10 Northeast-Southwest Drill Cross Section, Troy Mine Subsidence Area ...... 22 11 Photographs of Sinkholes #1 and #2...... 23 12 Schematic Cross Section of Sinkhole # 1 and #2 ...... 24

TABLES

1 Open Fractures Mapped Underground...... 32 2 Summary of Rock Mechanics Properties Surmised for East Fault...... 33

ATTACHMENTS

1 Selected Drill Hole Logs 2 Summary of Opinions, Kuipers’ Review of Final EIS and ROD, Proposed Rock Creek Mine, Libby, Montana, Kootenai National Forest

Final Geotechnical Assessment Report, ii Troy Mine, Montana

GLOSSARY

Sinkhole: A sinkhole is a surface depression resulting from a collapse of near-surface materials along sharply defined, steep-sided perimeter walls, into an open space at depth beneath the sinkhole. Sinkholes, other than those produced by the solution of limestone, are usually the result of a chimney type caving mechanism that extends upward from the underground opening.

Slump Features: Slump features are near-surface disturbances that result from rotational failure of usually unconsolidated surficial materials. The failures typically occur in seasonally saturated materials and are almost invariably developed on steep or undercut slopes.

Sag or Trough Subsidence: An area where surface materials have sunk or settled over a generally wide-area with no obvious sharp, vertically-displaced, perimeter features.

Swell Factor: The percentage increase in the volume of rock material as it is broken during mining to form spoil. The increase in volume results from the creation of voids between the broken rock fragments that were not present in the original unbroken rock.

ACRONYMS AND ABBREVIATIONS

CNI Call and Nicholas, Inc. CY Cubic yard

FEIS Final Environmental Impact Statement

KNF Kootenai National Forest

MDEQ Montana Department of Environmental Quality

mya Million years ago

Q Quality index value

R Squared R Squared, Inc.

RMR Rock mass rating

ROD Record of Decision

RQD Rock quality description

Tetra Tech Tetra Tech, Inc

USFS United States Forest Service

Final Geotechnical Assessment Report, iii Troy Mine, Montana

EXECUTIVE SUMMARY

PURPOSE

At the request of the United States Department of Agriculture, United States Forest Service, Kootenai National Forest, Tetra Tech, Inc. (Tetra Tech) and R Squared, Inc. (R Squared) have prepared this geotechnical assessment report to (1) describe two sinkholes which developed at the Troy Mine in 2005 and 2006, and (2) develop conclusions regarding the potential for sinkholes, such as those that occurred at the Troy Mine, to occur at the proposed Rock Creek Project. In addition, the geotechnical comments prepared by Mr. Jim Kuipers regarding the potential for mine subsidence at the Rock Creek Project near Noxon, Montana were evaluated.

SUMMARY AND CONCLUSIONS

• The ultimate cause of the sinkholes at the Troy mine was mining activity that did not leave buffer zones of solid rock between the workings and East Fault zone. Failure at the level of the mine workings propagated upward as chimney failures through the intensely fractured and deeply weathered rock of the East Fault and resulted in the two sinkholes.

• Using buffer zones to prevent mining up against the East Fault and properly sizing and securing drifts (mine roads) that pass through the fault would have prevented caving and surface subsidence. It is our understanding that the Troy mine operating permit does not specifically provide for buffer zones or preclude surface subsidence; consequently, the mine operators were not obligated to take measures to mitigate subsidence.

• The potential for chimney-type subsidence to occur at the Rock Creek project is minimal to nonexistent. The sinkholes at Troy do not raise new questions or provide information that would lead to major changes in the mitigation for the Rock Creek project. The mitigation measures the agencies have required at Rock Creek will prevent contacting fault zones during mine operations. One hundred foot wide buffer zones are required around the Copper Lake Fault and Moran Fault, and a 450 foot vertical buffer zone would be maintained between mine workings and the surface. The available drill hole data from the Rock Creek project suggest that the fault zones are more competent than the East Fault zone at the Troy mine. In addition, no secondary recovery of pillars will be allowed.

• At hard rock room and pillar mines, such as the proposed Rock Creek project, surface subsidence is not an inevitable consequence of mining. Provided that the mine is properly designed to prevent subsidence, the potential for subsidence to occur is minimal. This view is shared by many experts in the field of subsidence, including Peng (1992), Whitaker and Reddish (1989), Agapito (1991), Golder Associates (1989), and Cullen and others (2002). Room and pillar mining is well established as a means to significantly reduce or prevent surface subsidence.

Final Geotechnical Assessment Report, ES-1 Troy Mine, Montana

• The Final Environmental Impact Statement (FEIS) and Record of Decision (ROD) properly considered and informed the public of subsidence issues at Rock Creek. The FEIS and ROD minimize the potential for subsidence to occur by requiring buffer zones as well as the collection and analysis of data to complete detailed mine designs prior to entering areas where mining could result in impacts to the surface.

• Mine planning and subsidence prevention are an iterative process at operating mines. Even mines that have collected considerable rock mechanics information fully expect to modify and optimize the plans as more information is obtained. No mine in the development stages could possibly be expected to produce a single definitive plan. The approach taken by the FEIS — that mine planning and subsidence prevention are dynamic processes that should be continually under review and modified as information is collected — is reasonable and proper.

BACKGROUND

TROY MINE SINKHOLES

Two sinkholes have developed over the southeast end of the Troy Mine since April 2005. Sinkhole #1 was first noted on October 31, 1997 and was described as being 8 feet deep and 20 feet across. On April 29, 2005, the sinkhole enlarged to approximately 50 feet wide and 20 feet deep (Call and Nicholas, Inc. [CNI] 2005). By May 5, 2005, the sinkhole had deepened to about 50 feet while remaining approximately 50 feet in diameter. The volume of the sinkhole at the time of backfilling was approximately 2,550 cubic yards (CNI 2005).

Sinkhole #2 is approximately 150 feet north-northwest of the first sinkhole. It is oblong and elongated in an east-west direction. It is about 135 feet long in an east-west direction and about 100 feet wide in a north-south direction. It is about 20 to 30 feet deep at the north end and about 20 feet deep at the south end. The volume of the second sinkhole has been estimated at 8,800 cubic yards (Kirk 2006). The two sinkholes have developed at the surface about 270 and 320 feet, respectively, above the top of the mine workings.

Based on geologic mine mapping by ASARCO in 1990 and 1991 (Genesis, Inc. 2006), the projection of the East Fault lies directly beneath sinkhole #1 and the surface projection of the fault runs directly through sinkhole #2.

TROY MINE EAST FAULT GEOLOGY

The East Fault zone is a heavily fractured, weak zone of Proterozoic bedrock composed of broken rock and gouge of negligible width to as much as 10 feet wide (CNI 2005). As headings were driven up to or through the East Fault zone, it was not uncommon for a run of material to occur from the East Fault zone into the open workings (CNI 2005). The East Fault is covered by unconsolidated glacial material along the surface projection of its fault trace.

Final Geotechnical Assessment Report, ES-2 Troy Mine, Montana

CAUSES OF TROY MINE SINKHOLE FORMATION

It is our opinion that sinkhole #1 and associated underground failures occurred as the result of a “chimney” type cave event. These are progressive caves that start at the underground opening and work their way towards or to the surface. Fault zones, highly fractured zones, and other geological weak zones tend to enhance propagation of chimney caves (Crane 1929; Rice 1934). Caving of the East Fault where it is pierced by the 20 foot wide top headings at the Troy mine is not unexpected because of poor rock quality. Even if local ground support was installed, the span design approach developed by Lang (1994) suggests that spans greater than 19 feet would not be stable in a zone of poor rock mass quality such as the zone along the East Fault.

In the case of sinkhole # 2, propagation of the collapse from the underground workings to the surface was relatively quick, from essentially instantaneous to as little as four days (Erickson 2006). The location of sinkhole #2 within the weak, fractured material of the East Fault zone, along with the timing of the second set of underground failures and the good volume to swell factor match, leave little doubt that the formation of sinkhole #2 is directly related to the underground failure. The magnitude of this failure leads us to suspect that either the pillar along the East Fault between the E76 and E77 drives, or a portion of the exposed back within the workings, may have failed.

MINIMAL POTENTIAL FOR SUBSIDENCE AT ROCK CREEK

At the Troy Mine, top cut workings were mined into the East Fault in perhaps as many as 70 drive headings (Erickson 2006). As discussed above, caving of the East Fault where it is pierced by the 20 foot wide top headings at the Troy mine is not unexpected. Failure at the level of the mine workings propagated upward as chimney failures through the intensely fractured and deeply weathered rock of the East Fault and resulted in the two sinkholes. The ultimate cause of the sinkholes at the Troy mine was the result of mining activity that did not leave buffer zones of solid rock between the workings and the East Fault zone. Both sinkholes at the Troy Mine site occurred in overburden that was less than the minimum over-burden thickness proposed for the Rock Creek Project. The Troy mine did not require buffer zones, which are proposed for the Rock Creek Project. The Rock Creek deposit lies approximately 16 miles southeast of the Troy Mine. Rock Creek stratigraphy and mineralization is similar in style and grade to that of the Troy Mine; however, available drill hole data from the Rock Creek Project suggest that the fault zones are more competent than the East Fault zone at the Troy mine. Proposed mitigations at Rock Creek will prevent contacting fault zones during mine operations. For example, one hundred foot wide buffer zones are required around the Copper Lake Fault, Moran Fault, and other faults designated by the operator, and a 1,000 foot buffer zone is required around Cliff Lake in order to provide hydrologic integrity of the groundwater levels in faults that may recharge wilderness lakes. In addition, a 450 foot vertical buffer zone would be maintained between mine workings and the surface (again for hydrologic purposes, but it also minimizes the potential for subsidence ) and no secondary recovery of pillars will be allowed, which would greatly reduce the potential for subsidence (USFS 2003, Mitigation 28). As such, we consider

Final Geotechnical Assessment Report, ES-3 Troy Mine, Montana

the potential for chimney type subsidence to occur at the Rock Creek Mine to be minimal to nonexistent.

COMMENTS ON THE FINAL ENVIRONMENTAL IMPACT STATEMENT AND RECORD OF DECISION

An evaluation of the geotechnical comments prepared by Mr. Jim Kuipers regarding the potential for mine subsidence at the Rock Creek Project near Noxon, Montana was made as part of this geotechnical assessment report. The central issues stated in Mr. Kuipers’ reports in connection with surface subsidence are the following:

• Subsidence is an inevitable consequence of underground mining. • Subsidence is independent of depth. • Subsidence can not be prevented by such measures as limited room and pillar extraction. • Faults, folds, and other inconsistencies in the overlying strata may increase the subsidence potential. • Buffer zones to prevent subsidence are unproven in hard rock mines.

Our responses to Mr. Kuipers statements have been presented in this geotechnical assessment report and are summarized below.

At hard rock room and pillar mines, such as the proposed Rock Creek Project, surface subsidence is not an inevitable consequence of mining. Provided that the mine is properly designed to prevent subsidence, the potential for subsidence to occur is minimal. This view is shared by many experts in the field of subsidence, including Peng (1992), Whitaker and Reddish (1989), Agapito (1991), Golder Associates (1989), and Cullen and others (2002). Room and pillar mining is well established as a means to prevent damaging surface subsidence. Peng (1992) states that “if chain pillars are properly designed to support the overburden, surface subsidence will not occur.” In addition, room and pillar extraction is the accepted method in Pennsylvania to prevent or minimize unplanned subsidence (Pennsylvania Department of Environmental Protection 1997).

There is no known research or case studies on the effects of depth on the extent of surface subsidence caused by pillar failures. However, such studies do exist for full extraction mining (Peng 1992; Kratzsch 1983; National Coal Board 1975). These studies indicate that the amount of surface subsidence decreases with increased depth of mining. Provided that the mine is properly designed to prevent pillar failure, the potential for the pillars to fail is minimal. Because, in a broad sense, pillar failures over a large area will create a condition similar to full extraction mining, it follows that subsidence caused by pillar failures likewise decrease with increased depth of mining. If pillars are properly designed with an appropriate factor of safety,

Final Geotechnical Assessment Report, ES-4 Troy Mine, Montana

there should be no massive pillar failures in the underground mine. The Troy ore zone averages sixty feet thick, whereas the Rock Creek ore zone averages twenty-seven feet thick which also substantially reduces the chance of subsidence if pillar failure were to occur.

The influence of geologic structure on subsurface ground movements and ultimately on surface subsidence has been recognized by many researchers (Crane 1929, 1931; Heslop 1974; Boyum 1961; Kantner 1934; Fletcher 1960; Kotz 1986; Mahtab 1976; North and Callaghan 1980; Hoek 1974; Peng 1992; Nelson and Fahrni 1950; Holla and Buizen 1990; Lee 1966; Shadbolt 1987; Hellewell 1988; Whittaker and Reddish 1989; Cullen and Pakalnis 1995). Many observations of the influence of geologic structure have been made but only a modest amount of research work has been carried out. Hellewell (1988) reports that understanding the effects of geologic structure is complicated by the fact that the results of scientific investigations are in some instances contradictory. Therefore, limited data exists to verify that faults, folds, and other inconsistencies in the overlying strata may increase the subsidence potential. The final Environmental Impact Statement (FEIS) took a very conservative approach and assumed these geologic structures might affect the potential for subsidence. As a result, the Record of Decision (ROD) required mitigation and monitoring plans to minimize the potential for subsidence.

The FEIS and ROD contain stipulations (including mitigations and monitoring plans) that require collection and analysis of data necessary to complete detailed mine designs prior to entering areas where mining could result in impacts to the surface. These designs are to be approved by the U.S. Forest Service prior to their implementation. The U.S. Forest Service believes that if properly carried out, this would reduce the potential for subsidence to minimal. We concur with the U.S. Forest Service. If the following strategies are employed at the proposed Rock Creek Mine, as outlined in the final EIS and ROD, we consider that the potential for subsidence will be minimal:

• Properly design the pillars to support the overlying rocks with an appropriate factor of safety. Consideration must be given to the factors that will affect the long-term pillar strength. Artificial support such as rockbolts should not be relied upon for long-term stability. • Properly design the size of the openings such that they cannot readily cave. Artificial support such as rockbolts should not be relied upon for long-term stability. • Properly design the height of the rooms such that if caving does occur it does not reach surface or areas of potential hydraulic connectivity with the surface. • Establish buffer zones around faults and dikes that extend to the surface as well as minimum overburden thickness to reduce the potential of surface subsidence.

Final Geotechnical Assessment Report, ES-5 Troy Mine, Montana

Using buffer zones (to prevent mining up against the fault) and properly sizing and securing drifts (underground mine roads) that pass through the fault would have prevented caving and surface subsidence. It is our understanding that the Troy Mine operating permit does not specifically preclude surface subsidence; consequently, the mine operators were not obligated to take these measures to mitigate subsidence. Conversely, the proposed Rock Creek Mine is required to employ buffer zones and other measures to mitigate subsidence such that the potential for chimney type caves to progress up faults is considered minimal.

While it is possible to minimize the potential for subsidence, it is not possible to completely eliminate its potential. The nature of geotechnical engineering is such that it is never possible to completely characterize all rock mass conditions. It is conceivable that unexpected conditions and rock mass response may occur. Carrying out an ongoing testing and monitoring program will reduce the potential of encountering unexpected conditions.

It is well documented that subsidence and caving creates fractures, which may affect ground and surface waters. However, if there is no subsidence or caving, a fractured zone will not develop and there will be no subsidence induced hydrologic changes.

It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements to ensure that the Rock Creek Mine is designed and operated with minimal potential for mining-induced ground subsidence.

We concur with the Forest Service that the ROD and FEIS inform the public of the potential impacts from subsidence and puts forward an alternative with stipulations that will reduce the potential for subsidence to minimal.

Mine planning and subsidence prevention are an iterative process at operating mines. Even mines that have collected considerable rock mechanics information fully expect to modify and optimize the plans as more information is obtained. No mine in the development stages could possibly be expected to produce a single definitive plan. The approach taken by the FEIS — that mine planning and subsidence prevention are dynamic processes that should be continually under review and modified as information is collected — is reasonable and proper. As discussed in the U.S. Forest Service document, the ROD requires that rock mechanics data be collected during the evaluation adit and mine operation stages, and that this information be used to prepare mine plans and subsidence mitigation measures that are subject to review and approval by the Agencies.

Final Geotechnical Assessment Report, ES-6 Troy Mine, Montana

1.0 INTRODUCTION

At the request of the United States Department of Agriculture, United States Forest Service (USFS) Region 1, Kootenai National Forest (KNF), Tetra Tech, Inc. (Tetra Tech) and R Squared, Inc. (R Squared) have prepared this geotechnical assessment report (1) to describe a sinkhole that developed at the Troy Mine in 2005, and (2) to evaluate the geotechnical comments prepared by Mr. Jim Kuipers regarding the potential for mine subsidence at the Rock Creek Project near Noxon, Montana. In addition, a second sinkhole developed in the vicinity of the first sinkhole sometime between February 6 and 10, 2006. The relationship between the two sinkholes is discussed.

The Troy Mine sinkholes are located above the Troy Mine historic underground workings, near Troy, in northwestern Montana. The purpose of the geotechnical assessment effort is to (1) understand the conditions and circumstances that caused the Troy Mine Sinkhole, and (2) compare and contrast the conditions at both the Troy Mine and Rock Creek Mines to assess the potential for sinkholes to occur at the proposed Rock Creek Mine. For the purposes of this report, a sinkhole is defined as an areal collapse of near-surface materials along sharply defined, steep-sided perimeter walls, into an open space at depth beneath the sinkhole. Sinkholes are distinguished from subsidence features that result from the sinking of near surface materials over a generally wide-area with no obvious sharp, vertically displaced, perimeter features. In general, neither of these features exhibits a large amount of horizontal displacement.

In preparing this report, Tetra Tech reviewed documents identified in the reference section of this report; reviewed historic aerial photographs from the Troy Mine area; and discussed the sinkhole with Mr. John McKay of the KNF, Mr. Ray Tesoro of the USFS Region 1, Mr. Larry Erickson and Mr. Dirk Nelson of Genesis, Inc, operators of the Troy Mine, and Mr. Paul Cicchini of Call and Nichols, Inc. (CNI). Tetra Tech and R Squared personnel visited the site on November 1, 2005 and Tetra Tech visited the site again on February 16, 2006.

This geotechnical assessment report describes our independent assessment of the Troy Mine Sinkhole events and presents the results of evaluations of the natural conditions and nearby human activities (e.g., mining activities, road construction, and logging) to assess to what extent, if any, they may have contributed to the event. This included an evaluation of both (1) the geologic characteristics and conditions of the site and (2) the engineering aspects (rock mechanics, nearby mining, etc.). Further, Tetra Tech and R Squared provided their analysis and opinion on the sufficiency of available information regarding subsidence potential at Rock Creek with respect to (1) the agencies’ findings in the Final Environmental Impact Statement (FEIS); and (2) the 2003 Record of Decision (ROD) from the Rock Creek project. Tetra Tech and R Squared also included conclusions and recommendations regarding the effectiveness of permit conditions and mitigation requirements in the ROD to manage the risk of subsidence at Rock Creek.

Final Geotechnical Assessment Report, 1 Troy Mine, Montana

In addition, Tetra Tech and R Squared prepared an evaluation report to address the following: (1) our review of James Kuipers’ report regarding the potential for subsidence at Rock Creek and (2) the response to the Kuipers report completed by the USFS. The evaluation report is included as Attachment 1.

This geotechnical assessment report is organized into the following sections:

• Section 1.0 contains an introduction to the report. • Section 2.0 provides background information on the geology of the Spar Lake (Troy Mine) and Rock Creek deposits and the development of sinkholes #1 and #2. • Section 3.0 contains site specific observations and an interpretation of the formation of the underground collapses and sinkhole formation. • Section 4.0 describes the potential for a similar type of subsidence to occur at the proposed Rock Creek Mine. • Section 5.0 contains references.

The report also contains two attachments:

• Attachment 1 contains copies of selected drill hole logs examined as part of this review. • Attachment 2 contains the Summary of Opinions on Mr. Kuipers’ review of the FEIS and ROD for the proposed Rock Creek Mine.

Final Geotechnical Assessment Report, 2 Troy Mine, Montana

2.0 BACKGROUND

2.1 GEOLOGY

The geology of the Troy Mine and Rock Creek deposit is described in the following sections.

2.1.1 Physiography and Regional Geology

The Troy Mine (Spar Lake deposit), Rock Creek deposit, and Rock Lake deposit (Montanore Project) are located in the Cabinet Mountains of northwestern Montana (see Figure 1). The Cabinet Mountains occur as a northwest-trending range with as much as 5,500 feet of relief. Rocks in the Cabinet Mountains are Precambrian metasediments that are strong, dense, hard and not easily eroded. Prominent valleys in the area are generally the result of erosion along major faults.

The Spar Lake, Rock Creek, and Rock Lake deposits (see Figure 1) are stratabound and stratiform sediment-hosted copper/silver deposits. The regional geology consists of a thick Proterozoic sequence of sedimentary mud, silts, sands, and carbonates. Copper and silver mineralization generally took place in unlithified quartz-rich sandy deposits of the Revett Formation along oxidation reduction (roll) fronts probably during sediment compaction and dewatering (Lange and Sherry 1986). Sediments, including the Revett Formation, were subsequently lithified, silica-cemented, and altered under low-grade metamorphism to argillites, siltites, and quartzites of the Proterozoic Belt Supergroup. Deposits have been transported laterally by thrust faulting, folded into broad open anticlines and synclines, and subsequently broken up into blocks by high angle faults.

2.1.2 Regional Stratigraphy

The Belt Supergroup sediments consist of as much as 46,000 feet of fine-grained clastic sedimentary rock that were deposited into the Belt Basin during the Proterozoic 1,450 to 850 million years ago (mya) (Harrison and Cressman 1993; Harrison and others 1974). The Belt Supergroup is comprised of four conformable groups. From oldest to youngest these groups are: Lower Belt, Ravalli, Middle Belt Carbonate, and Missoula Groups (see Figure 2) (Harrison and Cressman 1993).

The principal unit of interest with respect to the Spar Lake (Troy Mine) and the Rock Creek deposits is the Ravalli Group. The Ravalli Group is underlain by the Prichard Formation and overlain by the Middle Belt Carbonate Group (see Figure 2). The Prichard is comprised of about 15,000 feet of argillites (clay-rich well-indurated [hard] sediments) interbedded with minor amounts of siltite (silty units) and gray sandy quartzite units. The overlying Middle Belt Carbonate is represented by the Wallace and Helena Formations that interfinger with one another laterally from west to east, respectively. Both are shelf–type carbonate units with the western derived sediments of the Wallace Formation having a higher clastic component.

Final Geotechnical Assessment Report, 3 Troy Mine, Montana Troy Libby

Troy Mine Site * Montanore Mine Site *Rock* Creek Mine Site

N SOURCE OF BASE: National Geographic Map Set 1:24,000 Montana Scale 1” = 10 Miles (approx.)

Insert Map

M O N T A N A TROY MINE SINKHOLE EVALUATION USDA Forest Service, Region 1, Kootenai National Forest

FIGURE 1 Location Map of Project Area STRATIGRAPHIC COLUMN

TROY MINE SINKHOLE EVALUATION USDA Forest Service, Region 1, Kootenai National Forest

FIGURE 2 Regional Stratigraphic Column

Modified from Harrison and Cressman 1993

The Ravalli Group consists of three Formations: the Burke, Revett, and St. Regis Formations. The Burke Formation has a transitional contact with the underlying Prichard Formation and is composed of about 3,000 feet of predominantly green, gray, and purple siltites. In the Cabinet Mountains area, the overlying Revett Formation occurs as a northward thinning wedge of clastic sediment, typically consisting of a lower (300 to 1,300 feet), middle (300 feet) and upper unit (250 feet). The Revett can, however, locally be as much a 2,400 feet thick. The upper and lower units are predominantly quartzites with interbedded siltite and argillite, while the middle unit is predominantly siltite with interbedded quartzite and argillite. The overlying St. Regis Formation is locally very thin (30 to 40 feet) but can be as much as about 700 feet thick, and comprises siltite and argillite units. In the mine area, it is approximately 250 feet. In the Cabinet Mountains area, the St. Regis Formation interfingers with the Empire Formation, a thinly laminated dark green dolomitic argillite, as much as 2,000 feet thick.

The Revett Formation is host to the Spar Lake (Troy Mine), Rock Creek deposit (proposed Rock Creek Mine project), and the Rock Lake deposit (proposed Montanore Mine project). The Troy deposit occurs in the middle quartzite beds of the Upper Revett and the Rock Creek and Montanore deposits occur in the upper part of the Lower Revett (Balla 2000). The Revett and St. Regis Formations contain many other small, sub-economic copper-silver deposits and occurrences. The other Belt Supergroup units mentioned do not contain known mineralization or deposits in the Cabinet Mountains area.

The area in the vicinity of the Troy Mine differs significantly from the proposed Rock Creek and Montanore Projects area in its glacial history. The area around the Troy Mine lies within an area that has undergone continental glaciation, where a large, thick, aerially extensive ice sheet covered and buried the regional topography of the area. During the melting of this ice sheet extensive and continuous sheets of unconsolidated sediment were laid down beneath melting glaciers. These deposits are either fluvial outwash or ground moraine deposits. Ground moraine sediments are deposited at the base of the glacier and are produced by glacial erosion; they are typically comprised of poorly sorted clay to gravel sized material. In the Troy Mine area these sediments can range from 6 to as much as 70 feet thick, with thin, more recent organic soils developed on top of them. Rather than continental glaciation, the Rock Creek and Montanore Projects area has been subjected only to valley or mountain-type glaciation. Glacial ice in these areas was restricted to the flanks of the higher mountain (glacial cirques) and glacial ice accumulated in and moved down the drainages carving U-shaped the valley systems. As a result, the mountain tops are sharper (horns) with many “knife edge” ridges (arêtes) connecting the higher peaks. In addition, there are no deposits of ground moraine in the higher mountain areas, and glacial deposits are restricted to valley bottoms. Therefore, there is little unconsolidated sediment overlying large portions of the Rock Creek and Montanore Projects area. Rock outcrops are extensive and the bedrock geology is well exposed and more easily mapped over these two deposits than in the Troy Mine area.

Final Geotechnical Assessment Report, 6 Troy Mine, Montana

2.1.3 Troy Mine (Spar Lake Deposit) – Geology

The Troy Mine was brought into production by ASARCO in 1981 and they operated the mine through 1993, mining and processing more than 34 million tons of ore. The mine was closed from 1993 through 2004 due to low metal prices. Genesis, Inc. (a subsidiary of Revett Minerals, Inc) reopened the mine in 2004 and brought it back into production. The Spar Lake deposit (Troy Mine) is a stratabound, stratiform copper–silver deposit hosted in Proterozoic rocks of the Belt Supergroup (Balla 1982). It has formed as a roll front (oxidation / reduction front) copper- silver deposit during early burial and diagenesis of the Revett Formation. The preservation of the deposit is also related to diagenetic changes associated with lithification and quartz overgrowth cementation of the sandy, quartz-rich sediments to a quartzite (Hayes 1983; Hayes and Einaudi 1986). Low-grade metamorphism has also served to further lithify the sediments hosting the deposit. The result is a strongly lithified, dense, hard impermeable quartzite with almost no open space porosity that is host to the stratabound copper-silver deposit. Principal reference works on the Troy deposit include Hayes 1983; Hayes and Einaudi 1986; Hayes and Balla 1996; Balla 2000.

The Spar Lake Deposit is located in the middle quartzite unit of the upper Revett Formation. The deposit is flat-lying (horizontal), approximately 7,500 feet long in a northwest-southeast direction, 1,800 feet wide, and averages about 60 feet thick (see Figures 3 and 4). Soil/colluvial cover of the Troy deposit averages about 40 feet thick on the north side of Mt. Vernon and is variable, commonly about 6 to 20 feet, but can be as much as 70 feet thick on the south side.

2.1.3.1 Troy Mine Area - Stratigraphy

The stratigraphy in the vicinity of the Troy Mine is depicted on the stratigraphic section (see Figure 2), on the geologic map (see Figure 3), and cross section (see Figure 4). The units exposed in this area consist of the Upper, Middle and Lower Revett (Revett Formation is locally as much as 2,335 feet thick), the St Regis (locally 670 feet thick) and the Wallace Formations (more than 600 feet thick) (Balla 2000). Mineralized outcrops, major faults, a plan view of the Spar Lake deposit, and the location of northeast-southwest oriented cross sections are shown on Figure 3. A northeast-southwest geologic cross section in the vicinity of the junction of the East Fault and Cross Fault is shown as Figure 4.

Near the East Fault along the southeast end of the Spar Lake (Troy) deposit the mineable unit within the Revett Formation was about 60 feet thick (the open space void left from mining is 60 feet high). The overlying bedrock ranges in thickness from about 220 to 300 feet and is in turn overlain by about 20 to 50 feet of unconsolidated overburden or surficial material. In this area, there is a combined total of about 270 to 320 feet of material overlying the mined out portions of the mine.

Final Geotechnical Assessment Report, 7 Troy Mine, Montana N

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pCre pCsr

pCsr pCre pCre

pCsr pCw pCsr

pCw pCw pCsr

pCw pCsr pCw

B’ B

pCsr pCsr

Modified from Balla 2000; Hayes 1986 Scale 1” = 2000 feet LEGEND

Fault, Showing Relative pCw Wallace Formation Horizontal Movement TROY MINE SINKHOLE EVALUATION pCsr St. Regis Formation USDA Forest Service, Region 1, Kootenai National Forest Fault, U on Up Thrown Side; U D on Down Thrown Side D pCre Revett Formation - Upper FIGURE 3 pCre Revett Formation - Middle Geology of Troy Mine Area Syncline, Showing Traces of Revett Formation - Lower Axial Plane and Plunge of Axis pCre

2.1.3.2 Troy Mine Area -Structure

The sediments hosting the Spar Lake deposit are gently folded across the axis of a very broad, open southward-plunging (7 degrees) syncline called the Mt. Vernon Syncline, shown on Figure 3. The syncline likely formed during an extended period of regional thrust faulting between 200 and 60 mya (Harrison and Cressman 1993). The syncline is offset by the east-west trending Cross Fault.

Faults

Two major faults and a number of smaller faults have offset the Spar Lake deposit (Troy Mine). The two major faults are the East Fault and the Cross Fault (see Figures 3 and 4). The East Fault is important to the evaluation of the sinkholes and is described below.

The East Fault can be traced over a distance of 6,500-7,000 feet adjacent to the Troy Mine workings (see Figure 3). The fault is a normal fault with a trend of north-northwest at 330 degrees with a dip that averages about 65 degrees to the east (see Figures 3 and 4). The dip of the fault at the mine level on the southeastern end of the Troy Mine ranges from 52 degrees to 67 degrees. The East Fault forms the western flank of a north-south trending grabben structure. The fault is downthrown to the east approximately 210 to 250 feet (Balla 2000). At the mine level, near the southeast end of the deposit, the Revett Formation quartzites are in fault contact with silty argillites of the Saint Regis Formation (see Figure 4). As such the mineralization in the Revett appears to be cut-off to the east along the East Fault; however, work by Hayes (1983) determined that the East Fault was a post mineral fault, and detailed mine mapping by ASARCO indicates that the deposition of the Spar Lake deposit stopped in the host quartzite units prior to reaching the present location of the East Fault. The East Fault is Proterozoic in age (1,450 to 543 mya) and has been reactivated on numerous occasions through time, including the Mesozoic (248 to 65 mya) and into the Cenozoic periods (65 mya to present) (Hayes 1983). The fault plane is described as being tight, with broken rock and gouge occurring over negligible to as much as 10 feet wide (CNI 2005). As headings were driven up to or through the East Fault zone, it was not uncommon for a run of material to occur from the East Fault zone into the open workings (CNI 2005). In addition, the hanging wall St. Regis material is described as being typically highly fractured and broken (personal communication, Larry Erickson, Genesis, Inc. 2006). This highly fractured hanging wall rock may significantly increase the effective width of sheared and broken rock along the East Fault zone. The reactivation of the East Fault through time may also contribute to the highly sheared and broken character of rock along the fault. In addition, near-surface weathering may have expanded the effective width of the fault zone and weakened the rocks to greater depths as well. The East Fault is covered by unconsolidated glacial material along the surface projection of its fault trace.

Final Geotechnical Assessment Report, 10 Troy Mine, Montana

Joints and Fractures

Because the copper-silver mineralization was deposited in unlithified sediments, joints and fractures in the Troy Mine are post-mineral. Many of the fractures are coated with remobilized secondary copper minerals. Most of the joints and fractures have either developed along bedding planes or approximately perpendicular to bedding.

Jointing or fractures developed in the near-surface, parallel to gently dipping bedding or along bedding planes proper are often called release fractures. These fractures have been formed and accentuated by a phenomenon known as glacial unloading, whereby the melting of many thousand feet of glacial ice since the last ice age (approximately 10,000 years ago) has relieved stresses induced by the weight of the ice such that the near horizontal fractures and bedding planes expand or open-up with the unloading. These fractures are most common within several hundred feet of the surface, where compressive loading and unloading is most effective. At greater depths the lithostatic load of overlying rock begins to approach or exceed the weight of the ice that once was present.

Tension fractures and a north-south trending fault at the northeast end of the Spar Lake deposit have developed parallel to the north-northeast-trending Mt. Vernon Synclinal fold axis (see Figure 3) (Balla 2000). These tension fractures are typically open and result from various tensional and compressional stresses generated by folding of the brittle quartzite units of the Revett (see Figure 5). Another set of east-northeast-trending joints and fractures are prominent in the subsurface and were mapped during historic mining operations by ASARCO (see Figure 9). These fractures have also been called tension fractures and are thought by some (CNI 2005; Genesis, Inc. 2006) to be related to the development of subsidence features at the surface at the southeast end of the Spar Lake deposit, as discussed below.

2.1.3.3 Troy Mine Area –Surficial Geology

The characteristic surface topography in the vicinity of the southeast end of Troy Mine is an undulating surface developed in unconsolidated glacial sediments (poorly-sorted ground moraine) deposited on steep topographic slopes. The undulating topography is defined by numerous surface swales and apparently recent near surface slumps. Slump features are near- surface disturbances that result from rotational failure of usually unconsolidated surficial materials. The slumps typically occur in seasonally saturated materials and are almost invariably developed on steep or undercut slopes. The local distribution of these slump features are discussed in the aerial photography section below (see Section 3.1) and there is some indication that these features were more prominent on these steep south facing slopes prior to the most recent episode of logging in 1997.

Final Geotechnical Assessment Report, 11 Troy Mine, Montana TROY MINE SINKHOLE EVALUATION USDA Forest Service, Region 1, Kootenai National Forest

Modified from Balla 2000 FIGURE 5 Tension Fractures in the Revett Formation

Surficial material in the vicinity of the Troy Mine is a mix of colluvial, glacial and residual soil material consisting of unconsolidated sandy, locally clayey to gravelly material, near the surface that grades downward into a weathered in place and rubbilized bedrock representing a “C” soil horizon. Organic-rich “A” horizon soils are thin (a few inches thick), which is typical of mountainous, high altitude soils developed beneath predominantly pine vegetation. Surficial material overlying the southeast end of the Troy Mine ranges in thickness from as little as 6 to 15 feet to locally more than 70 feet.

2.1.4 Rock Creek Project -Deposit Geology

The Rock Creek deposit lies approximately 16 miles southeast of the Troy Mine (see Figure 1). The geology of the area has been described by Wells and others (1981) and by Harrison and Cressman (1993). The geology of the Rock Creek deposit has been described in detail by Balla (1993). A geologic map of the Rock Creek deposit area is shown as Figure 6. Figure 7 is a north-south cross section (D-D’) through the Rock Creek deposit. Mineralization is essentially flat-lying or exhibits low angle dips to the west off the flank of a large broad and open anticline. Outcrops of the Rock Creek deposit occur at elevations around 5,700 feet.

The stratigraphy in the vicinity of the deposit is similar to that of the Troy mine with deposits hosted in the Revett Formation that are overlain by the St. Regis and Wallace Formations and underlain by the Burke Formation (see Figures 3 and 6).

The Rock Creek deposit occurs over a distance of about 16,000 feet in a north-south direction (see Figure 7) and about 7,200 feet in an east-west direction. Mineralization is generally restricted to one quartzite unit within the Revett Formation, but there may be stacked deposit zones (see Figure 7), (locally as many as four). Copper/silver mineralization averages about 27 feet thick; however, it ranges from as little as 6 to as much as 235 feet thick, where it occurs in multiple stacked units of quartzite near the Copper Lake Fault.

Rock Creek mineralization is similar in style and grade to that of the Troy Mine; however, the Rock Creek deposit occurs lower in the geologic section in the middle and upper part of the lower quartzite members of the Revett Formation (see Figures 3, 6, and 7). In addition, these disseminated copper-silver deposits are interpreted to have resulted from oxidation/reduction front deposition associated with basin brines migrating down the hydrologic gradient in unlithified sediments. Important ore controls include the porosity and permeability of unconsolidated sediments during mineralization, the diagenesis and cementation of the quartz- rich sandstones to quartzite, and in the case of the Rock Creek deposit, indications that at least some of the primary mineralization is associated with high-angle faults along which ore-grade metals may have mobilized and deposited (Lange and Sherry 1986) (also see discussion of Structure, Section 2.1.4.1 below). Subsequent Cretaceous faulting has broken up the ore deposits into distinct, discontinuous and sometimes isolated blocks.

Final Geotechnical Assessment Report, 13 Troy Mine, Montana Modified from Balla 2000 TROY MINE SINKHOLE EVALUATION USDA Forest Service, Region 1, Kootenai National Forest

FIGURE 6 Geology of Rock Creek Mine Area D D’

TROY MINE SINKHOLE EVALUATION USDA Forest Service, Region 1, Kootenai National Forest

FIGURE 7 North-South Cross Section of the Modified from Balla 2000 Rock Creek Deposit

The Rock Creek deposit outcrops along the headwall of a glacial cirque on the south sides of St. Paul and Chicago Peaks (see Figure 6). The deposit also outcrops along the valley walls and headwaters area of the North Basin drainage and along the northeast side of St. Paul Peak (see Figure 6). The ore deposit depth ranges from zero in outcrops of the North and South Basin to as much as 2,000 feet beneath St. Paul Peak. The west side of the deposit lies approximately 1,000 feet beneath the surface and the deposit lies at depths of about 900 and 1,100 feet respectively below Copper Lake and Cliff Lake. Both of these lakes are small and occur along major fault traces. Copper Lake covers an area of approximately 2 acres and reaches a mean depth of about 1.3 feet. It can dry up during some years. Cliff Lake has water year around and covers an area of about 2 acres and has a mean depth of about 5 feet. Mineralization between the Moran Basin and the southern flank of Chicago Peak also lies approximately 1,000 feet below the surface.

2.1.4.1 Rock Creek Project - Structure

Faults

In mineralized areas, the host-quartzite units are very competent. Based on logs from core drilling in the Rock Creek deposit (Revett Minerals, Inc. written communication, March 2006) and on information presented in a hydrologic report (Montana Department of Environmental Quality [MDEQ] 2001), the rock can be moderately to densely fractured in the vicinity of some fault zones. According to Lange and Sherry (1986) major faults in the Rock Creek area were formed during active deposition of the Revett Formation. Such faults are commonly called growth faults and they usually form in areas of very rapid deposition in saturated sediment. Movement along these faults occurs in unlithified sediments and once the movement is initially established, movement can continue over time under the weight of ongoing sediment accumulation. As stated above, copper silver mineralization is thought to have occurred in unlithified sediments of the Revett Formation. As further evidence of this, Lange and Shelly state that there is some evidence in the Rock Creek deposit that sulfide mineralization has migrated up these syndepositional northwest-trending fault zones and moved laterally into adjacent permeable horizons (Lange and Shelly 1986, p. 270). In addition, there is evidence in drill hole data and from geologic mapping in the Rock Creek deposit area that that the major faults have not seen a great deal of reactivation through geologic time (Burnside and Thompson, 1985). Evidence that the faults have been annealed by silicification and perhaps by the migration of mineralizing solutions up the fault zones is observed in drill hole data from the Rock Creek project drill holes RC 57A and RC 57B. This is further evidenced by the fact that these faults do not weather as negative topographic features (as they do in the Troy area) but are resistant to weathering and cross major topographic features with little evidence of erosion along the faults. Both the Copper Lake and Moran Faults are considered inactive with no significant movement over the last 10,000 years (Knight Piesold, LLC 1997).

Final Geotechnical Assessment Report, 16 Troy Mine, Montana

Drill hole logs from holes RC 57A & RC 57B that penetrate the Copper Lake Fault at the north end of the deposit were reviewed. Both of these holes crossed the fault at steep oblique angles and both holes encountered a minor amount (when corrected for true thickness) of fracturing and gouge and in general reported good to excellent core recovery, suggesting that the rock was coherent where drilled.

The Rock Creek deposit is broken by two major fault zones, the Copper Lake Fault and the Moran Fault (see Figure 6 and 7), into three blocks, the Chicago, St. Paul, and North Basin blocks. The Chicago block occurs as a broad open anticline that plunges gently to the north (Balla 1993). The Copper Lake Fault truncates the anticline to the north (see Figure 6). To the north of the Copper Lake Fault, the beds of the St. Paul block are highly contorted. The Moran and Copper Lake Faults form the margins of a northwest-trending 3,200 foot-wide grabben (see Figures 6 and 7) within which the St. Paul block has been down dropped some 250 to 500 feet with respect to the Chicago block. Within this block, the beds of the Revett Formation are folded into a northwest-trending anticline. Both the Copper Lake and Moran Faults are considered inactive with no significant movement over the last 10,000 years (Knight Piesold, LLC 1997).

Both the Copper Lake and the Moran Faults trend northwesterly, with the Copper Lake Fault occurring over a greater strike length distance (see Figure 6). The Copper Lake Fault exhibits a scissors type of displacement with offsets being on the order of 100 feet at the north end of the Rock Creek Project area and as much as 500 feet on the southern end. The Moran Fault exhibits a similar sort of displacement with offsets being about 100 feet at the north end of the Rock Creek Project area and about 400 feet at the south end.

The hydrology report on wilderness lakes (MDEQ 2001) concludes that the overall hydraulic conductivity of the rock mass in the Rock Creek area is very low (10-6 centimeters per second), while the conductivity of the major fault zones may be significantly higher (10-3 centimeters per second) but still low, especially where major faults are associated with parallel fractures in rock that result in secondary porosity and permeability. The lack of glacial cover and the excellent outcrop and exposure has probably allowed for the identification and mapping of all of the major faults within the Rock Creek project area. There may, however, be other smaller faults present in the project area that that were too subtle to be mapped at the surface or detected by exploration drilling. In general, drilling of pilot holes in advance of drifting into new areas of the mine can identify faults and other water bearings structures in advance of mining into them. This allows for rock quality data and water sources and quantities to be evaluated and proper precautions to be taken with respect to ground stability and water control prior to mining or providing access through the zone. Having this knowledge in advance of mining could eliminate or minimize the danger of mining into structurally unstable ground conditions such as those identified along the East Fault at the Troy Mine.

Final Geotechnical Assessment Report, 17 Troy Mine, Montana

Joints and Fractures

As with rocks in the Troy Mine area, most fractures observed in the outcrop and in drilling are either parallel to, or perpendicular to, the bedding. Both horizontal release fractures as described above and vertical tension fractures as described in the Troy deposit likely also exist to some extent in the Rock Creek deposit (Balla 2000). However, as discussed previously, the Rock Creek deposit was not overridden by thick continental ice sheets as occurred at the Troy deposit. Rock Creek experienced smaller valley glaciers instead. Thus, there should be less potential at Rock Creek for glacial unloading and crustal rebound due to the melting of thousands of feet of ice to form or accentuate fractures as at Troy. Regardless of the type of glaciation, the thicker rock formations overlying the Rock Creek deposit would have likely borne the lithostatic load from glacial ice much better than at Troy and thus reduced the potential for glaciation-induced fractures. As mentioned, these fractures are most common within several hundred feet of the surface, where compressive loading and unloading is most effective. At greater depths the lithostatic load of overlying rock likely would begin to approach or exceed the weight of the ice that once was present.

Because any fractures that may exist at Rock Creek are associated with brittle rock and an overall anticlinal structure, it is reasoned that these fractures would open upward (V-shaped) as shown in Figure 5 as opposed to those of the Troy Mine that are of an inverted V-shape. Due to the "wedging" that would occur, fractures that open upward would be less likely to contribute to subsidence than those that open downward. Principal joint sets trend both north-northwest (associated with major faulting) and east-northeast and both sets are predominantly vertical (Chen-Northern 1988).

2.1.4.2 Rock Creek Project – Surficial Geology

As described above, there is little unconsolidated glacial cover in the Rock Creek deposit area, because the area did not undergo continental-type glaciation. As a result soils, except those along the valley floors are largely thin and formed from the accumulation of materials weathered in place and by the accumulation of decayed organic materials. This lack of glacial ground moraine material provides excellent outcrop throughout most of the Rock Creek area.

2.1.4.3 Rock Creek Project – Buffer Zones

Figure 8 shows the major faults, lakes and some of the proposed buffer zones established by mitigations in the ROD (USFS 2003) and the FEIS for the Rock Creek Project (MDEQ and U.S. Forest Service 2001). Relevant portions of the mitigation section are reproduced at the end of this report in Enclosure 2. Buffer zones were designed to protect the hydrologic integrity of the potentiometric surface near Cliff Lake (1,000-foot buffer zone), and to prevent hydro-fracturing of bedrock in near surface (shallower than 450 feet) and near outcrop areas (nearer than 1,000 feet laterally) in order to prevent the formation of new surface seeps and springs during flooding of the mine workings at closure. Buffer zones under lakes and near faults and outcrop zones are

Final Geotechnical Assessment Report, 18 Troy Mine, Montana

expected to be the most effective mitigations for reducing impacts to surface water bodies (MDEQ 2001).

2.2 HISTORY AND DESCRIPTION OF SINKHOLES

2.2.1 Introduction

Two sinkholes have developed over the southeast end of the Troy Mine since April 2005. This section of the report describes the sinkholes and the history of their formation.

A number of figures have been prepared to aid in the discussion of the sinkholes. Figure 9 is a plan map of the sinkhole locations in relationship to underground workings, faults, and fractures. The geology on Figure 9 is depicted at the level (elevation) of the mine workings. The information was obtained from original mine maps prepared by ASARCO in 1990-1991 (written communication, Genesis, Inc. 2006). The East Fault traces and the sinkhole locations (see Figure 9) are shown as projections from the surface onto the mine level plan map. Figure 10 is a northeast-southwest cross section drawn across the East Fault in the vicinity of the sinkholes (cross section B-B’, on Figure 3). This cross section is based in part on historical drill data provided by Genesis, Inc. and this drill data is projected into the plane of the cross section (written communication, Genesis, Inc 2006). Figure 11 provides photographs of the sinkholes, shortly after their formation. Figure 12 provides schematic or diagrammatic northeast-southwest cross sections through the East Fault depicting conceptual relations between the sinkhole locations and the described characteristics and geometry of the East Fault.

Important geologic features shown on Figure 9 include the East Fault and a prominent set of east-northeast-trending fractures. The East Fault can be traced over a distance of 6,500 to 7,000 feet adjacent to the Troy Mine workings (see Figures 3 and 9). The East Fault is a normal fault with a trend of north-northwest at 330 degree with a dip that averages about 65 degrees to the east (see Figures 3 and 9). The actual dip of the fault at the level of the mine workings at the southeastern end of the Troy Mine ranges from 52 degrees to 67 degrees based on geologic mapping by ASARCO in 1990 and 1991. The east side of the fault is downthrown approximately 210 to 250 feet (Balla 2000). At the mine level, near the southeast end of the deposit, the Revett Formation quartzites are in fault contact with silty argillites of the St. Regis Formation (see Figure 4). The East Fault is covered by colluvial material along the surface projection of its fault trace.

Final Geotechnical Assessment Report, 19 Troy Mine, Montana TROY MINE SINKHOLE EVALUATION USDA Forest Service, Region 1, Kootenai National Forest FIGURE 8 Rock Creek Project Area Lakes, Faults, and Proposed Buffer Zones

Modified from Balla 2000

Photograph of Sinkhole No. 1 (May 2005)

Photograph of Sinkhole No. 2 (February 2006) (Note: Red line on above photograph is hazard warning tape)

TROY MINE SINKHOLE EVALUATION USDA Forest Service, Region 1, Kootenai National Forest

FIGURE 11 Photographs of Sinkholes Numbers 1 and 2

The prominent east-northeast trending sets of fractures (see Figure 9) are mostly vertical and do not show significant displacement along the fracture sets. Where seen in underground workings at the south end of the mine, these fractures are frequently associated with open voids (see Section 3.3). These fractures and the open space voids have been enlarged and have stoped naturally upwards by rock fall into the open workings of the mine. These fractures are reported to have had open space voids along their trends when encountered during initial mining (personal communication, Larry Erickson, Genesis, Inc. 2006). These east-northeast-trending fractures are probably not the tension fractures described above in Section 2.1.3.2 above, and described by Balla (2000). If they were, they should trend north-northeast, parallel to the synclinal axis.

The two sinkholes (#1 and #2) have developed at the surface about 270 and 320 feet respectively above the top of the mine workings (see Figure 9). (It should be noted that the thickness of the overburden material at the Troy mine is much less than the 450 foot vertical and 1,000 foot lateral buffer zone requirement for the Rock Creek Project).

2.2.2 Sinkhole #1

Sinkhole #1 (see Figure 9) was first noted on October 31, 1997 by Cindy Betlach of the KNF. Ms. Betlach documented the appearance of the sinkhole in an email to Mr. McKay on May 27, 2005 (Betlach 2005), in which she described the sinkhole in 1997 as being about 8 feet deep and 20 feet across. In September 2004, Larry Erickson of Genesis, Inc. reported the size of the sinkhole to be essentially unchanged from 1997 (CNI 2005, p. 4). On April 29, 2005, the sinkhole enlarged to approximately 50 feet wide and 20 ft deep (CNI 2005). By May 5, 2005, the sinkhole had deepened to about 50 feet while remaining approximately 50 feet wide (see Figure 11). The sinkhole is located about 80 feet west of the East Fault at the level of the mine workings and lies just to the east of the likely surface trace projections of the East Fault (see Figures 9 and 10). Sinkhole #1 developed in unconsolidated sediment at the surface that is approximately 50 feet thick. The unconsolidated sediment is underlain by about 220 feet of rock before encountering the mined out drive heading in the mine at depth (see Figure 12). The sinkhole was backfilled with waste rock (obtained from near the south adit) a couple of weeks later. Additional subsidence in sinkhole #1 has not been reported since backfilling.

The volume of the sinkhole at the time of backfilling was approximately 2,550 cubic yards. This volume was determined by CNI (2005) using the following methods:

1. A visual estimation of the sinkhole dimensions and volume calculations. 2. A count of the number of 10-cubic yard truck loads required to fill and grade the sinkhole to original topography. 3. A before and after survey of the fill borrow pile (CNI 2005, page 5, first paragraph).

Final Geotechnical Assessment Report, 25 Troy Mine, Montana

The sinkhole was described by CNI in their 2005 report. Excerpts from this report follow:

p4, par4 The backplane and long axis of the sinkhole appears to be bounded by an east- west trending fault structure with an azimuth of approximately 280 degrees that steeply dips to the north…(CNI 2005, page 4, last paragraph)…. No trace of the north-south trending East Fault could be found in the backplane of the sinkhole. The material at the bottom of the sinkhole was comprised of fine colluvium that had slumped into the void. The lowest portion of the sinkhole occurs at the southeast end. Groundwater inflow of approximately 5 to 10 gpm was noted along the colluvial contact on the northwest end of the sinkhole. The direction of flow was northwest to southeast, away from the East Fault and toward the deeper end of the sinkhole (CNI 2005, page 4, last paragraph, and page 5, first paragraph).

This structure along the backplane of sinkhole #1 and described above by CNI is oriented close to the direction of the prominent set of fractures mapped in Figure 9.

2.2.2.1 History and Description of East Fault Caving in E79 Drive

The Troy Mine was closed due to low metal prices and high-production costs and placed in care and maintenance status from 1993 until 2004. Sometime during this period a cave occurred within the mine along the East Fault where it had been pierced by the E79 Drive top heading (see Figure 9). The cave resulted in the formation of muck piles in the E79 and E80 drives. The location of these muck piles is shown in Figure 9. The cave and muck piles were first noted in 2004 when the mine reopened. According to CNI, the muck piles have not changed since this time (CNI 2005).

Caving features spatially related to the sinkholes at the level of the mine workings are shown on Figure 9. These include a dashed magenta colored line depicting the “brow” associated with the line along which the underground failure of the back is believed to have occurred thus allowing material to accumulate in the open mine workings. Another black dashed line depicts the toe of the angle of repose slope of caved material originating at the brow and extending south and westward into the mine.

Genesis Inc. surveyed the muck piles in June 2005. The volume of material in the muck piles was estimated to be 4,657 cubic yards. CNI reports that this volume may include oversize rock left in the E79 Drive at the time of shutdown in 1993. The CNI report indicates that Genesis, Inc. estimated the volume of oversize material to be as much as 1,000 cubic yards. This being the case, then the volume of caved material in the muck pile could be as little as 3,657 cubic yards. Any reduction in volume of caved material results in a more significant mismatch between the volume of the sinkhole and the volume of the muck pile. This is discussed in the following sections as well as in the CNI report.

We believe that if oversize rock is present in the muck pile it is unlikely to be more than about 300 cubic yards based on the following assessment:

Final Geotechnical Assessment Report, 26 Troy Mine, Montana

1. Oversize rock piles at mines are typically placed with little or no stacking; thus, the maximum height of an oversize pile is unlikely to be more than 3 yards. 2. Oversize rock piles at mines are typically placed loose; a void ratio of 50 percent is typical. 3. Oversize rock piles at mines typically have voids of sufficient size to be partially filled by the caved material. 4. No oversize material is visible in the E79 Drive muck pile. As such, the maximum area that could “hide” oversize rock is approximately 3 yards high by 12 yards wide by 15 yards long (i.e, 3 yards × 12 yards × 5 yards = 540 cubic yards × 50 percent swell factor = 270 cubic yards < 300 cubic yards).

2.2.3 Sinkhole #2

A second ground failure and cave occurred in the underground workings of the Troy Mine along the west side of the East Fault near the ends of the E76 and E77 Drives on February 6 2006 (see Figure 9). The failure occurred between 5:30 AM and 7:30 AM shifts. Some material sloughed over a top cut between pillars located between the E77 and E78 drive and deposited a small amount of caved material in the E78 Drive (see Figure 9). Four days later on February 10, 2006, sinkhole #2 was noted on the surface (see Figure 11). No evidence of subsidence was noted in the vicinity of sinkhole #2 based on a helicopter survey in the late fall of 2005 (George Furniss, personal communication and photographs, March 2006).

Sinkhole #2 is approximately 150 feet north-northwest of the first sinkhole1 (see Figure 9). Sinkhole #2 is oblong and elongated in an east west direction. It is about 135 feet long in an east-west direction and about 100 feet wide in a north-south direction. It is about 20 to 30 feet deep at the north end and about 20 feet deep at the south end. The volume of the second sinkhole has been estimated at 8,800 CY (Kirk 2006). Sinkhole #2 is developed in about 20 feet of glacial ground moraine material at the surface that is in turn underlain by about 320 feet of bedrock before encountering the mined out drive headings at depth in the mine (see Figure 12).

There is approximately 12 to 15 feet of deeply weathered residual soil or sandy colluvial material exposed in the caved subsidence walls. This material grades downward into a weathered in place bedrock that is highly fractured (“C” soil horizon) and extends down to the bottom along the walls of the sinkhole. The floor of the sinkhole appears to be comprised mostly of unconsolidated surficial material. The ground surface was covered with about three feet of snow around the sinkhole. There were large irregular, jagged cracks in the snow leading away from the edge of the collapsed depression. Slump blocks of potentially frozen colluvial material had slid down the walls in places. The snow along the southwest portion of the depression was covered with orange-tan colored dust. This apron of dust was probably generated during the collapse of sinkhole #2 suggesting rapid subsidence or failure of the materials at the surface.

1 Genesis, Inc. surveyed in the location and dimensions of the sinkhole.

Final Geotechnical Assessment Report, 27 Troy Mine, Montana

2.2.3.1 Description of East Fault Caving in E76 and E77 Drives

The volume of muck associated with the second underground failure has been estimated by calculating the volume of workings filled with collapsed or caved material at about 22,000 cubic yards (Kirk 2006). This volume of material is almost 10 times more than that believed to be associated with sinkhole #1.

Figure 9 shows the area of these drives which are filled to the brow with caved material (workings are filled sill to back, dashed magenta line); and a second area that shows the toe of the angle of repose slope of caved material from the brow, westward down the sill of the drift (dashed black line). Most of the material is Revett Formation; however, there may be some St. Regis Formation (the lithologies are interbedded near the contact). Debris fragments are broken along fracture surfaces and fractures are iron-oxide coated. Size of material is generally much coarser than that associated with sinkhole #1; a few blocks at the toe of the debris pile are 2 feet × 2.5 feet × 5 feet that likely resulted from failure of more competent portions of the back at or near the brow area.

As with the underground caving associated with sinkhole #1, the area of actual failure is not visible as the workings are completely filled with debris fall material, but it is likely that the failure originated in the East Fault (which is exposed in the back of the top-cuts in the E76 and E77 Drives) and expanded along the back, westward down the Drive headings.

Final Geotechnical Assessment Report, 28 Troy Mine, Montana

3.0 SITE-SPECIFIC OBSERVATIONS

3.1 AERIAL PHOTOGRAPH REVIEW

On November 2, 2005, Tetra Tech reviewed four sets of available aerial photographs of the Troy Mine site. The photographs were taken in 1975, 1982, 1992, and 1997 and were reviewed at the KNF office in Libby, Montana. The photographs reviewed and summary of the review results are provided below.

1975 Infrared Aerial Photo (August)

The south adit was visible on the aerial photograph. Switch backs were present on the road above the south adit but the slope had not yet been logged. A slump feature appears to be present in the photo above the south adit. No sinkhole was observed in the photograph.

1982 Color Aerial Photograph

The south adit is visible, but the slump feature is difficult to see because the area of interest is on the edge of photograph. However, the slump observed in the 1975 aerial photo is still visible in the photo and appears to be vegetated by small brush. The slope had not been logged. No sinkhole was observed in the photograph.

1992 Color Aerial Photograph (June)

The slump feature appears to fan out from a narrow linear depression, possibly a small drainage, which extends up towards the upper switch-back and may extend above the road. The area had not been logged and evidence of the slump feature is easy to identify on the aerial photograph. Field examination of this slump activity would be necessary to determine its origin. No sinkhole was observed in the photograph.

1997 Color Aerial Photograph (August)

The area above the south adit had been logged and the slump feature is not readily visible in the aerial photograph. No sinkhole was observed in the photograph.

The development of surface slump features appears to be related to the rotational failure of probably seasonally saturated, unconsolidated near surface material deposited on steep slopes. There is no direct evidence that would tie these slump features to either the East Fault or the development of sink holes.

Final Geotechnical Assessment Report, 29 Troy Mine, Montana

3.2 DRILL HOLE LOGS REVIEW

Drill hole logs from the vicinity of the East Fault and sinkholes #1 and #2, completed during exploratory drilling, were reviewed for evidence of subsurface voids noted in the drill holes on a cross section prepared by Genesis, Inc. The following drill hole logs were reviewed:

• DDH 20 • DDH 31 • DDH 50 • DDH 53 • DDH 55 • DDH 75 • DDH 83 • DDH 94 • DDH 95

The approximate location of several of the drill holes is shown on Figures 8 and 9. Copies of the drill hole logs are included in this report as Attachment 1.

On the basis of our reviews, the following observations are noted:

• No voids are reported in the drill logs; however, several sections of “missing core” 2. are noted. This means that core was not recovered as it should have been during the drilling of the hole. • The log for DDH-53 reports “highly faulted and broken, 50 ft core missing” between 70 ft to 120 foot depth (see Figure 10). This section of missing core corresponds well with the projection of the East Fault (see Figure 9).

2 There are several possible explanations for missing core: (1) a mislatch, a mechanical failure of equipment that does not allow the core drilled to enter the core sampling device mislatches rarely occur over multiple or sequential coring intervals in a drill hole (usually 10 feet); (2) voids or large amounts of open space exists in the rock, the presence of open space is usually indicated by other pieces of evidence observed during drilling, lost circulation of core drilling fluids, drill rods physically fall into or offer no resistance to penetration into the open space (daily drillers log or site geologist logs are not available for these holes which were drilled in 1958 by Kennecott’s Bear Creek Exploration group, L. Erickson, Genesis, Inc. personal communication March 28, 2006); (3) the presence of very broken rock with some open space in the rock, extremely fractured or broken rock can fall out of the sampling device when it is picked up off the bottom of the hole, if the rock falls into an open space it may not be recovered in subsequent core runs and it would indicate that there was at least some open space in the hole; if the rock just falls down the hole and rests confined in the drill hole at the bottom of the hole, pieces of the core are usually picked up in the next core run as milled fragments of broken rock. In the case of 50 feet of missing core in drill hole # DDH-53, this would seem to be a very thick zone of core not to be recovered unless there were voids or significant open space present for broken and unrecovered core to accumulate.

Final Geotechnical Assessment Report, 30 Troy Mine, Montana

• The log for DDH-50 reports “fault + gouge 114 to 137 feet, 20 feet core missing.” This section of fault and missing core corresponds well with the projection of the East Fault. • The only other missing core is reported in DH-20, located to the northwest of the East Fault. The missing core does not correspond to faults or highly fractured zones. Drilling equipment or recovery problems are suspected, although driller’s daily reports were not reviewed.

3.3 EAST FAULT, SINKHOLES, AND THE TROY MINE WORKINGS RELATIONSHIP

The sinkholes overlie areas of the Troy mine that were mined out during the period 1979 to 1993 (probably 1990 to 1991 based on period of active geologic mapping by ASARCO). Figure 9 shows the location of the sinkholes in relation to these mined out areas. The East Fault runs in a north-south direction and in general forms the eastern boundary of the ore body. The location of the East Fault in the E76, E77, E78, E79, and E90 Drives is shown on Figure 9; and Figure 10 shows the mined out projection of the E79 drive in cross section. These mapped locations of the East Fault were obtained from original mine mapping by ASARCO in 1990 and 1991 (Genesis, Inc. 2006) during a site visit in February 2006. Some earlier versions of mine maps reviewed show the location of the East Fault as being approximately 50 feet further to the west.

The East Fault is described in the CNI 2005 report as follows:

p3, par1 (The East Fault) is characterized as tight with a thickness of broken rock and gouge varying from negligible to as much as 10 feet, with three feet being the typical thickness. The azimuth of the fault is 330 degrees with a dip of 65 degrees. Dips as flat as 55 degrees and as steep as 70 degrees have been recorded.

Mr. Erickson from Genesis, Inc. reports that the St. Regis Formation in the hanging wall of the East Fault is typically highly fractured and broken, and that the East Fault contains splays (for example see, the East Fault in E71 and E72 drives) and that the fault zone is often more than 10 feet wide.

Based on geologic mine mapping completed by ASARCO in 1990 and 1991 (Genesis, Inc. 2006), the East Fault in the vicinity of the sinkhole dips at between 52 and 67 degrees. The most likely surface projections of the East Fault using these dips are shown on Figures 9 and 10. Both the 52 and 67 degree projection lines are shown. Note that these projections do not account for surface topography; consequently, they are only accurate in the plane of the cross section.

Based on these projections, the East Fault is located between 20 and 120 feet west-southwest of the sinkhole #1 (see Figure 9). Thus, the East Fault lies directly beneath sinkhole #1 (see Figures 10 and 12). In addition, the 67 degree surface trace projection of the East Fault runs directly through sinkhole #2 (see Figure 9).

Final Geotechnical Assessment Report, 31 Troy Mine, Montana

3.4 OPEN EAST-WEST FRACTURES

Several open east-west fractures were mapped by Dr. Michael Cullen on November 1, 2005 and Allan Kirk on February 16, 2006. The mapping results are summarized in Table 1. The mapped fractures were located to the west of the East Fault. Numerous other east-northeast-trending fractures are shown on Figure 9 and are based on geologic mapping by ASARCO in the early 1900s (written communication, L. Erickson, Genesis Inc., February 2006).

Table 1. Open Fractures Mapped Underground

Location Dip Strike Condition Comments 197100N 90 70° Strong parallel fracture sets create a Larry Erickson reports that the fracture 202750E 90 70° fracture zone that is greater than 10 was open 6 inches when first 85 70° feet wide. Orthogonal joints result encountered. Caving caused by in failure of rectangular blocks undermining has resulted in a void that approximately 1 cubic feet in size. is now ~10 feet wide by 30 feet high by 50 feet long. unknown 85 70° 6 to 12 inch wide fracture filled with This fracture is now void for distances site 2 a brown clayey-silt of firm in excess of 20 feet. The fill material consistency. is progressively raveling such that the size of the void is increasing. It is not known if a fracture was completely filled or if a void was present when the fracture was first encountered. 197250N 90 70° Parallel fracture set 10 feet wide in Steel plates rock bolted in place across 203200E back , two zones: one 6 inches wide and longitudinally along zone to and another 18 inches wide both minimize damage from potential rock crossing 45 foot wide drift show fall, fractures in west rib show no open open space voids that have likely space other than minor openings stoped up 10 or more feet (tenths of and inch) along fractures. 197200N 90 75° 6 foot wide zone of strong parallel 202700E fractures in west rib with no open space, as zone crosses the back a 6-to-12-inch-wide open zone that has stoped up several feet, by blocky rock fall 196400N 90 70° 125 foot long fracture zone (extends Stoping up height estimate by 204350N to over 2 drive headings and a pillar) L. Erickson. 240475N open space along this fracture was 18 to 24 inches wide during initial mining (Erickson, personal E81 Drive communication Feb 16, 2006) and has now opened up in one drive heading to 6 to 10 feet wide, over a 35 foot length, and has stoped up vertically 30+ feet

Final Geotechnical Assessment Report, 32 Troy Mine, Montana

3.5 EAST FAULT ROCK MECHANICS PROPERTIES

Available information indicates that the East Fault zone is heavily fractured and weak. However, no rock mechanics properties are available for the East Fault. Based on a review of drill logs and geologic mapping provided from the mine coupled with engineering judgment, we have surmised that the uncorrected rock mass rating (RMR) value is approximately 20 (Bieniawski 1976, 1989) and the tunneling quality index Q value is approximately 0.1 (Barton and others 1974). The rock mass properties used to arrive at these classification values are summarized in Table 2. Both the RMR and Q classification systems qualitatively classify the East Fault rock within a category designated as very poor.

Table 2. Summary of Rock Mechanics Properties Surmised for East Fault

Value Used in Classification Parameter Explanation for Value Q RMR Rock Quality Designation (RQD) In a section of the drill log for DDH-50 that <25 3 corresponds to the location of the East Fault, the log reports 150 fractures in 10 feet. This corresponds to a mean joint spacing of 0.8 inches. Based on the correlation developed between RQD and mean fracture spacing developed by Priest and Hudson (1976) the RQD would be 10. A review of the size of rock in the cave muck pile suggests a somewhat larger RQD is appropriate.

Joint Set Number (Jn) Rectangular blocks observed in the caved rock 12 muck pile suggest 3 joints sets plus random (fault) Joint Roughness Number (Jr) In a section of the drill log for DDH-50 that 1.0 0 Discontinuity condition corresponds to the location of the East Fault the log reports "fault + gouge. Gouge was also noted in the cave muck pile. This was assumed to indicate the presence of >5-mm-thick gouge zones. Joint Alteration Number (Ja) Discussions with Larry Erickson of Genesis Inc 8.0 indicate that the East Fault zone consists of rock and gouge. This is also indicated in drill holes. Joint Water Reduction (Jw) As is standard practice in underground mines, the 1.0 10 Ground water condition cave is assumed to be wet but free draining with no water pressure. Stress Reduction Factor The East Fault was considered to be a single weak 2.5 zone in competent rock that intersected the excavation at depth greater than 160 feet. Rock Strength Estimated at less than 25 MPa 2 for fault zone material. Spacing of discontinuities A section of DDH-50 that corresponds to the 5 location of the East Fault the log reports 150 fractures in 10 feet. This corresponds to a mean joint spacing of 0.8 inches.

Final Geotechnical Assessment Report, 33 Troy Mine, Montana

3.6 ANALYSIS OF SINKHOLE FORMATION

3.6.1 Sinkhole #1 Formation

CNI completed an independent review of the sinkhole #1 event for the mine operator (CNI 2005, page 2, conclusions section). Their findings and conclusion are summarized as follows:

p2, par2 A cursory examination of the situation at Troy would lead to the quick conclusion that the sinkhole development was directly related to mining. However, upon closer examination, none of the pieces of the puzzle fit. The key problems are summarized below: 1. The mass balance does not work out without stretching the limits of reasonable volumes, cave diameters, and percent swell. 2. The backplane of the sinkhole is bounded by an east-west trending fault plane, not a north-south trending fault plane, such as the East Fault. 3. The steepest surface projection of the East Fault does not intersect the sinkhole but is actually off to the side that is opposite from the lowest portion of the sinkhole. 4. Groundwater inflow into the sinkhole is flowing to the southeast, away from the direction of the East Fault. 5. Surface water entering the sinkhole is not reporting to the underground muck pile. 6. The greatest amount of surface subsidence was not accompanied by an influx of muck into the underground. Although it is not possible to completely rule out a connection between underground extraction and the surface sinkhole, this conclusion is very difficult to support based on available data…. It appears much more likely that the surface sinkhole was the result of the collapse and erosion of surface colluvium into a void along a dilated segment of an east-west trending structure. Open voids are common along this structural system, particularly in the dip slope direction.

The main arguments put forward by CNI is that the volume of the caved material in sinkhole #1 only matches up with the volume of the underground muck pile if the swell factor for the material is less than about 32 percent and the diameter of the pipe connecting sinkhole #1 to the mine is less than about 22 feet. Because they consider that these values for swell factor and pipe diameter to be extreme, they conclude that the sinkhole is probably unrelated to mining.

CNI (2005) proposed that the 2,550 cubic yards of material from the sinkhole entered a pre- existing void along an east-west trending structure. Assuming a 20 percent swell factor, the material that must be accounted for is 3,060 cubic yards. If the proposed existing void is 3 feet wide (a value that is consistent with "several feet" reported by CNI) and the angle of repose for the displaced material is 34 degrees, the void required to accommodate the material must be at

Final Geotechnical Assessment Report, 34 Troy Mine, Montana

least 135 feet high and 405 feet long. If the void was less than 100 feet long, it would need to be more than 225 feet deep in which case it would intersect the mine workings. Based on our underground and surface observations, review of drill hole logs, and discussions with Troy Mine personal, there is no record of natural voids with sufficient volume to accommodated 3,060 cubic yards of material in the vicinity of the sinkhole. In addition, although the intersection of these east-northeast oriented tension fractures with the East Fault may produce zones of weakness in the plane of the East Fault, no tension fracture structures were mapped by ASARCO in the immediate areas of underground failures (see Figure 9). The nearest ENE-trending “tension” fractures mapped at the 1:40 scale of underground mine mapping, lie about 150 feet north of sinkhole #2 and 125 feet south of sinkhole #1 (see Figure 9). None are mapped at the mine level between the two sinkholes.

It is our opinion that sinkhole #1 occurred as the result of a “chimney” type cave event (see Figure 12). These are progressive caves that start at the underground opening and work their way up toward or to the surface. Once initiated, the failure will progress until one of the following conditions occurs:

• The cave is choked off by bulking (swell of the broken material exceeds the open space available) of the caved material. • A competent rock unit capable of spanning the caved zone is encountered. • Stress redistribution and/or change in chimney shape no longer favor failure. • The ground surface is reached.

In the case of sinkhole #1, the progressive cave could have been initiated with caving at the mine level anytime between 1993 and 2004. Initial subsidence at the surface was first noted in 1997 (Betlach 2005), with larger scale subsidence occurring in April and May of 2005 (CNI 2005). In this case, the propagation of the collapse from the underground working to the surface (about 270 feet) in an upward stoping chimney-type collapse (see Figure 12) was apparently relatively slow, occurring over a period of 1 to 12 years.

Fault zones, highly fractured zones, and other geological weak zones tend to enhance propagation of chimney caves. Crane (1929) and Rice (1934) report good examples in Michigan Copper and Iron Mine. In one case a 14 feet by 28 feet chimney extended to surface through almost 1,000 feet of cover along a fault zone. This unusual example occurred in a mine where the material within the collapsing chimney of rock was constantly removed for use as underground fill material. Therefore, with the caved material being continuously drawn out or removed, choking off and blockage of the collapsing chimney could not occur. Despite the fact that this is an unusual example, it indicates the intensity of the fracturing and degree of instability that may be developed in rock materials along fault zones. (It should be noted that structures with such intense fracturing are not known to be present at Rock Creek; buffer zones will be used to mitigate mining adjacent to faults).

Final Geotechnical Assessment Report, 35 Troy Mine, Montana

We concur with the CNI analysis with respect to swell factor and pipe (chimney) size: in order for there to be a direct connection between sinkhole #1 and underground workings, the swell factor of the caved material must be less than about 32 percent and the “chimney” connecting sinkhole #1 to the mine must have an equivalent diameter less than about 22 feet. However, for the reasons discussed below, we consider that these values for swell factor and chimney size are not unreasonable, and that sinkhole #1 and the East Fault cave in Drives E79 and E80 are directly related.

1. Based on the East Fault projections described above, the East Fault either pierces the bottom of the sinkhole #1 or passes within approximately 50 feet of the bottom of sinkhole #1 (see Figures 10 and 12). 2. The East Fault zone consists of fractured rock with gouge and highly fractured St. Regis Formation siltite in the hanging wall (personal communication, Larry Erickson, Genesis, Inc. 2006); such material has a relatively low swell factor. 3. The material exposed in the sinkholes #1 and #2 was silty sand with some gravel with cobble; such material typically has a low swell factor. The lower portion of the walls of sinkhole #2 passed into broken-up and weathered in-place bedrock typical of “C” horizon soils. 4. The broken material within the chimney would be subject to a compressive force due to the overlying rock. At the bottom of the chimney, this pressure could be as much as 1.6 MPa (33,000 pounds per square feet). This pressure would result in a further reduction in swell factor for the caved material. 5. The East Fault zone is heavily fractured and weak. Although no rock mechanics properties are available for the East Fault, both the RMR and Q classification systems qualitatively classify the East Fault rock as very poor. The rock mass properties used to arrive at these classification values are summarized in Table 2 (see Section 3.5 above). 6. Using the above rock mechanics information, the maximum unsupported stable excavation span is predicted to be less than 6 feet based on work by Barton (1977) and Bieniawski (1976). As such, caving of the East Fault where it is pierced by the 20 foot wide top headings at the Troy mine is not unexpected. Even if local ground support was installed, the span design approach developed by Lang (1994) suggests that spans greater than 19 feet would not be stable in a zone of “very poor” rock mass quality such as that at least locally developed along the East Fault. 7. Using the above rock mechanics information we determined that the Mathews Method “N” (stability number after Potvin 1988) value is approximately 0.16. Based on the modified stability graph (Nickson 1992), instability in an unsupported excavation may occur once the hydraulic radius exceeds 2 meters (equivalent to a 26 foot diameter circle).

Final Geotechnical Assessment Report, 36 Troy Mine, Montana

In summary, although we are of the opinion that the first sinkhole is directly related to a chimney-type collapse associated with mining into the East Fault zone, a definitive answer would require a more detailed field investigation that would include the following:

1. Evaluate the composition of material within the underground muck pile (size, material or rock type, oversize composition). 2. Evaluate the swell factor of the material in the muck pile. 3. Perform a surface drill program. Ideally this should be done with a rig equipped for tricone, coring, and sampling. 4. Conduct a geophysical survey. Since we are looking for small near vertical features the survey should use down hole methods. Surface geophysical surveys, such as resistivity or side-scanning radar, may provide useful information.

3.6.2 Sinkhole #2

In the case of sinkhole # 2, the progressive cave was initiated at the mine level early on the morning of February 6, 2006 and sinkhole #2 (see Figures 11 and 12) identified on the surface on February 10, 2006 (although it may have been present before this date). In this case, propagation of the collapse from the underground workings to the surface (as much as 320 feet) (see Figure 12) was relatively quick, from essentially instantaneous to as little as four days.

The timing of the second set of failures (underground and surface sinkhole development), along with a better volume to swell factor match in volumes leave little doubt that the formation of sinkhole #2 is directly related to the underground failure (see Figure 12). The magnitude of this failure leads us to suspect that either the pillar along the East Fault between E76 and E77 Drive or a large portion of the exposed back within the workings may have failed (as suggested schematically on Figure 12). Mine maps indicate that this pillar was approximately 40 feet by 40 feet, which is slightly smaller than the design size of 45 feet by 45 feet. ASARCO 1994 reports that pillars in deeper sections of the Troy Mine have collapsed as a result of stresses exceeding strength. Although the location of this cave is in a shallow section of the mine, it is conceivable that mining into the East Fault produces a local stress concentration that exceeded the strength of the somewhat undersized pillar. A more detailed evaluation is beyond the scope of this report.

It is our opinion that there must have been a considerable amount of ground preparation (specific geologic events such as faulting, fracturing, weathering, etc that might prepare the ground to fail) within or adjacent to the East Fault zone in order to allow this type of failure to occur (essentially instantaneously at depth) and then be translated upwards through about 250-270 feet of rock and 20 to 50 feet of colluvial material to produce a collapse feature at the surface (see Figure 12). These ground preparation events must have taken place over geologic scales of time (thousands to millions of years) and have resulted in highly fractured, faulted, sheared and broken-up zones perhaps with open-space preserved within the near-surface weathering environment. This

Final Geotechnical Assessment Report, 37 Troy Mine, Montana

ground preparation was necessary in order to allow the failure of rock from an initiation point at depth, to be translated up through the column or chimney of broken rock to the surface without a bulking or bridge blockage stopping the upward propagation of the failure.

It is also clear that at least in the case of the second underground failure and sinkhole development, the failure at the mine level did not cave up gradually through the overlying rock column over time. The last mining in this area of the mine was in 1993 or earlier, and although there was locally a minor amount of debris on the top cut sills, most of the very large volume of material that moved into the drift in the second subsidence incident did so essentially instantaneously. The process of failure must have been instantaneous at the mine level and propagated up through the column of rock very quickly (instantaneously to a few days maximum), almost as if a plug of broken rock dropped down into the mine and then the overlying column (chimney) of rock dropped in a “fluidized” fashion to fill the void left at depth and producing the second sinkhole at the surface. It is unclear if current mining activity and recent blasting in the East Ore Body (see Figure 12) may have triggered the initial instability and collapse at the mine level on the west side of the fault. Current mining activity in the East Ore Body is taking place almost 500 feet east and 250 feet lower in elevation than the recent collapses on the west side of the East Fault (see Figure 12).

Finally, the fact that the second sinkhole is so convincingly related to the second underground failure, adds credibility, at least circumstantially, to our opinion that that the first sinkhole may also be related to the first ground subsidence event.

Final Geotechnical Assessment Report, 38 Troy Mine, Montana

4.0 POTENTIAL FOR SUBSIDENCE AT THE ROCK CREEK PROJECT

At the Troy Mine, workings mined into the East Fault, which defines the edge of the ore with top cuts in perhaps as many as 70 Drive headings (Erickson 2006). Therefore, previous mining activity did not leave buffer zones of solid rock between the workings and the sheared and broken fault zone (MDEQ 2006). At the proposed Rock Creek project, one hundred foot wide buffer zones are required around the Copper Lake and Moran Faults in order to provide hydrologic integrity of the potentiometric surface (groundwater levels) in faults that may recharge wilderness lakes. In addition, a 450 foot vertical buffer zone would be maintained between mine workings and the surface (again for hydrologic purposes) (see Figure 8).

At hard rock room and pillar mines, such as the proposed Rock Creek Project, surface subsidence is not an inevitable consequence of mining. Provided that the mine is properly designed to prevent subsidence, the potential for subsidence to occur is minimal. Proper design includes the following:

• Design the pillars to support the overlying rocks with an appropriate factor of safety. Consideration must be given to the factors that will affect the long-term pillar strength. Artificial support such as rock bolts should not be relied upon for long-term stability. • Design the size of the openings such that they do not readily cave. Artificial support such as rock bolts should not be relied upon for long-term stability. • Design the height of the rooms such that if caving does occur it does not reach surface or areas of potential hydraulic connectivity to surface. • Use buffer zones around faults and dikes that extend to surface and have the potential to induce surface subsidence.

At the Rock Creek Mine the company must provide updated mine design plans prior to evaluation adit and mine adit construction (USFS 2003, Mitigation 26) and prior to entering areas where mining could result in impacts to the surface (USFS 2003, Mitigation 27). Finally, no secondary recovery of pillars will be allowed, which would reduce the potential for subsidence (USFS 2003, Mitigation 28). The proposed Rock Creek Mine must avoid surface disturbance related to subsidence. The sinkholes that occurred over the Troy Mine are the result of a progressive chimney cave up at a weak fault zone with a relatively shallow depth of cover. Limited geologic exploration data from the Rock Creek Project area and mapping, and description of faults suggests that the two major faults the Copper Lake and Moran Faults, are in fact, not similar to the East Fault with respect to the degree of shearing, fracturing and broken character of the rock within the fault zones. In addition, the Moran and Copper Lake Faults are much better silicified and resistant to weathering than the East Fault at the Troy Mine location.

Final Geotechnical Assessment Report, 39 Troy Mine, Montana

It is conceivable that similar subsidence could occur at the proposed Rock Creek Mine if the faults are mined into at depths less than about 300 feet with mining heights greater than 30 feet. It is understood that mining up to the Moran and Copper Lake Faults and other faults, as proposed by the applicant, will not be allowed at the proposed Rock Creek Mine and that hydrogeologic buffer zones will be required around these faults. It is also understood that the depth of mining at Rock Creek will be greater than 450 feet and that the height of mining will typically be 25 feet. As such, we consider the potential for chimney type subsidence to occur at the Rock Creek Mine to be minimal to nonexistent.

4.1 SUFFICIENCY AND ADEQUACY FOR IMPLEMENTATION OF THE STIPULATIONS IN THE ROCK CREEK FEIS AND ROD TO MINIMIZE THE POTENTIAL FOR GROUND SUBSIDENCE

It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements to ensure that the proposed Rock Creek Mine is designed and operated with minimal potential for mining induced ground subsidence. The stipulations within the ROD that are related to ground subsidence and hydrology are contained in Attachment 2 of this report.

James Kuipers, in his report “Technical Comments on the Proposed Rock Creek Project FEIS and ROD”, dated February 19, 2002, asserts that subsidence is an inevitable consequence of mining and that the ROD and FEIS is flawed. As discussed in Attachment 2 of the report, we disagree with Mr. Kuipers regarding the potential for subsidence at Rock Creek and the sufficiency and adequacy of the ROD and FEIS.

Stipulation #29 in the ROD requires that for hydrologic purposes a 100-foot buffer be maintained around the Copper Lake and Moran Faults and other faults as proposed by the applicant, and a minimum depth of cover of 450 feet be maintained throughout the mine (see Figure 8). In addition, the applicant is required to submit updated mine plans prior to entering areas that could result in impacts to the surface (Stipulation 28). The stated purpose of Stipulation 29 is to guard against impacts to the potentiometric surface based on hydraulic connectivity with these fault zones; however, it will also prevent chimney subsidence along the faults. The Forest Service indicated that the purpose of Stipulation 76 and the Rock Mechanics Monitoring Program (Appendix K in the ROD) is to require Revett Minerals to provide scientific analysis of the adequacy of this 100-foot value once additional geotechnical and rock mechanic data is available and before development will be allowed to start (MDEQ and U.S. Forest Service 2003). We are of the opinion that the buffer zone may ultimately consider direction and angle of dip of fault planes and require a minimum thickness of competent rock between the mine opening and the fault plane such that blockage or bulking will occur before a collapse reaches either the fault or the surface.

Final Geotechnical Assessment Report, 40 Troy Mine, Montana

The proposed Rock Creek Mine will be allowed to drive access drifts through all or some of the faults. Driving access drifts through fault zones is distinguishable from active mining of ore within fault zones. The former involves penetration of the fault with a single small drift for access to mine rock on the other side of the fault, whereas active mining would involve multiple penetrations and mining in stopes or along drive headings within the fault zone. This later type of mining will not be allowed in fault zones in the Rock Creek deposit. Hydrologic isolation of the mine access workings from the regional groundwater system in the vicinity of faults is a common practice and is accomplished by the placement of grout rings or curtains around the areas to be mined prior driving the access drift through them. Proper engineering design assures stability of the access drift through the fault. Access to the St. Paul and North Basin Blocks would require driving mine development access through these faults. Experience at the Troy Mine indicates that under some conditions, caving may occur in drift size openings that cross through some significantly weakened zones along faults. This matter of access drifts crossing faults should be clarified as it potentially has both hydraulic and subsidence implications. Where access drifts are allowed to cross the faults, Stipulation 76 and the Rock Mechanics Monitoring Program (Appendix K in the ROD) require Revett Minerals to provide scientific analysis and rational (that will be reviewed by the USFS) for the size and support requirements such that the potential for mining induced subsidence is minimal and the potential loss of groundwater to the workings from the fault zone are eliminated or minimized. .

As is characteristic of most new mining operations, all of the information that is needed to develop a final detailed mining plan that can anticipate all possible variations prior to beginning operations is not available and in most cases, obtaining this information at the pre-development stage is not feasible. Rather, the needed rock mechanic data must necessarily be gathered sequentially and continuously as new areas are opened up to development drifting or actual mining operations. This is the reason the project is proposed in two phases, the Evaluation Adit Phase and the Construction Phase. In this way, the mining plan evolves over time with mine designs taking advantage of newly developed rock mechanic data for new areas in the mine or for changing geologic settings and characteristics.

Final Geotechnical Assessment Report, 41 Troy Mine, Montana

5.0 REFERENCES

ASARCO. 1994. Rock Creek Project Rock Mechanics Analysis. Prepared by Dave Young.

Balla, J. C. 1982. Geology of the Troy Deposit, Northwestern Montana. Proceedings of the Denver Regional Exploration Geologists Society Symposium, The Genesis of Rocky Mountain Ore Deposits: Changes with Time and Tectonics, Denver, Colorado, p.29.

Balla, J.C. 1993. Geology of the Rock Creek Deposit, Sanders County, Montana. Program and Abstracts, Belt III, August 14-21, 1993, 2p.

Balla, J.C. 2000. Geologic Comparison of the Troy Mine/Spar lake Deposit and the Rock Creek Mine/Rock Creek Project Northwest Montana. Company report prepared for Sterling Mining Company. 97p.

Barton, N., Lien, R., and Lunde, J. 1974. “Engineering Classification of Rock Masses for the Design of Tunnel Support.” Rock Mechanics. Vol. 6, No. 4, 1974, pp 189 – 236.

Barton, N., Lien, R., and Lunde, J. 1977. Estimation of Support Requirements for Underground Excavations. Proceedings 16th US Rock Mechanics Symposium.

Betlach, C. 2005. Email message to John McKay, KNF, regarding the Troy sinkhole. May 27.

Bieniawski, Z.T. 1976. Geomechanics Classification of Rock Masses and its Application in Tunnelling. Proceedings, 3rd International Congress on Rock Mechanics, ISRM, Vol. 11A.

Bieniawski, Z.T. 1989. Engineering Rock Mass Classification—a Complete Manual for Engineers and Geologists in Mining Civil and Petroleum Engineering. Wiley and Sons, New York.

Burnside, M and Thompson R, 1985, Rock Creek Mineral Report; Cur and Lynn lode claims, Kootenai National Forest, Supervisor’s Office, Libby, Montana, Minerals Administrative Records.

Call and Nicholas Inc. (CNI). 2005. Memorandum on Troy Sinkhole Evaluation. July 28.

Chen-Northern, Inc. 1988. Montana Mining Venture, Hydrology Investigation Report. Prepared for Noranda Minerals Corp. November 21.

Crane, W.R. 1929. Subsidence and Ground Movement in the Copper and Iron Mines of the Upper Peninsula, Michigan. U.S. Bureau of Mines Bulletin 285.

Erickson, L. 2006. Personal communication between Allan Kirk, Tetra Tech, and Larry Erickson, Genesis, Inc. February.

Furness, G. 2006. Montana Department of Environmental Quality, personal communication, March.

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Genesis, Inc. 2006. Written communication including copies of three 1:40 scale geologic maps of workings in the southeast portion of the Troy Mine. 3p.

Harrison, J.E., and Cressman, E.R. 1993. Geology of the Libby Thrust Belt of Northwestern Montana and Its Implications to Regional Tectonics. U.S. Geological Survey Professional Paper 1524, 42p.

Harrison J.E., Griggs, A.B., and Wells, J.D. 1974. Tectonic Features of the Precambrian Belt Basin and Their Influence on Post-Belt Structures. U.S. Geological Survey Professional Paper 866, 15p.

Hayes, T.S. 1983. Geologic Studies on the Genesis of the Spar Lake Stratabound Copper-Silver Deposit, Lincoln County, Montana. Dissertation for Ph.D submitted to the Department of Applied Sciences, Stanford University, Stanford. November 1983. Typescript.

Hayes, T.S., and Balla, J.C. 1996. “Troy Mine.” In: Geological Association of Canada, Mineralogical Association of Canada, Canadian Geophysical Union Joint Annual Meeting, 1986, Ottawa, Ontario, 10 pgs.

Hayes, T.S., and M.T. Einaudi. 1986. Genesis of the Spar Lake Stratabound Copper-Cilver Deposit, Montana: Part I Controls Inherited from Sedimentation and Preore Diagenesis. Economic Geology. Volume 81:1899-1931.

Horlocker, N. 1967. Annual Report, Rock Creek Project, Sanders County, Montana. Unpublished report, Bear Creek Mining Company. 7 pages, plus appendices.

Kirk, A.R. 2006. Notes including rough calculations of volume of material caved into the E76, E77 and E78 Drive headings and of sinkhole #2. February 20, 2006. 1p

Knight Piesold, LLC. 1997. Conceptual Design Report on Alternative Methods for Paste Deposition. ASSARCO Inc. Proposed Tailings Storage Facility, Rock Creek Project, Near Noxon, Montana. Company report prepared for ASARCO. May 1997.

Lange, I., and R. Sherry. 1986. “Nonmassive Sulfide Deposits in the late Precambrian Belt Supergroup of Western Montana.” In: Montana Bureau of Mines and Geology 1986 Belt Supergroup: A Guide to Proterozoic Rocks of Western Montana and Adjacent Areas. pp. 269-278.

Lang B. 1994. Span Design for Entry Type Excavations. M.A.Sc. Thesis University of British Columbia.

Montana Department of Environmental Quality (MDEQ). 2001. “Technical Report. Hydrology and Chemistry of Wilderness Lakes and Evaluation of Impacts from Proposed Underground Mining, Cabinet Mountains Wilderness, Montana.” March.

MDEQ. 2005. Hard Rock Program, Operating Permit – Field Inspection Report of Troy Mine. May 12.

Final Geotechnical Assessment Report, 43 Troy Mine, Montana

MDEQ. 2006. Email from George Furniss, MDEQ to Pat Plantenberg, MDEQ, regarding inspection of Troy Mine. February 15.

MDEQ and U.S. Forest Service. 2001. “Final Environmental Impact Statement Rock Creek Project.” September 2001. Volumes 1 and 2.

MDEQ and U.S. Forest Service. 2003. “Record of Decision Rock Creek Project.” June. 89p. and attachments.

Nickson S. 1992. Cable Support Guidelines for Underground Hard Rock Mine Operations. M.A.Sc. Thesis University of British Columbia.

Potvin Y. 1988. Empirical Open Stope Design in Canada. Ph.D. Thesis, University of British Columbia.

Priest, S., and J. Hudson. 1976. “Discontinuity Spacing in Rock.” International Journal Rock Mechanics. Min Sci. Volume 13.

Rice, G.S. 1934. “Ground Movement From Mining in Brier Hill Mine, Michigan.” Transactions American Institute of Mining Engineers. Volume. 109, pp 118-152.

Wells, J.D., Lindsey, D.A., and Van Loenen, R.E. 1981. Geology of the Cabinet Mountains Wilderness, Lincoln and Sanders Counties, Montana. U.S. Geological Survey Bulletin 1501-A.

Final Geotechnical Assessment Report, 44 Troy Mine, Montana

ATTACHMENT 1 DRILL HOLE LOGS

ATTACHMENT 2 SUMMARY OF OPINIONS, KUIPERS’ REVIEW OF FINAL EIS AND ROD PROPOSED ROCK CREEK MINE, LIBBY, MONTANA, KOOTENAI NATIONAL FOREST

ATTACHMENT 2: INTRODUCTION

R2 Incorporated, with assistance from Tetra Tech, Inc., prepared this report to present our assessment of the opinions of James M. Kuipers, P.E. concerning the sufficiency and adequacy of the information in the Final Environmental Impact Study (FEIS) and Record of Decision (ROD) relating to subsidence at the proposed Rock Creek Project near Libby, Montana. Our assessment consisted of meeting with representatives of Tetra Tech and the U.S. Department of Agriculture, Forest Service (Forest Service) in Libby, Montana and reviewing the following documents relating to mining induced surface subsidence at the proposed Rock Creek Project.

• “Technical Comments on the Proposed Rock Creek Project FEIS and ROD” by James R. Kuipers, P.E., dated February 19, 2002. • “Underground Hard-Rock Mining: Subsidence and Hydrologic Environmental Impacts” by Blodgett, S., Kuipers, J., dated February 2002 • “Kootenai National Forest, USDA Forest Service Technical Review of Technical Comments on the Proposed Rock Creek Project FEIS and ROD” by James R. Kuipers” prepared for Forest Supervisor Kootenai National Forest, dated April 10, 2003. • “Final Environmental Impact Study, Rock Creek Project” by Montana Department of Environmental Quality and Forest Service, dated September 2001. • “Record of Decision, Rock Creek Project” Forest Service, dated June 2003.

Other documents reviewed as part of our assessment are referenced in Enclosure 1. Stipulations and orders contained in the ROD and FEIS are presented in Enclosure 2. Subsidence data from other hard rock mines are summarized in Enclosure 3.

BACKGROUND

The Rock Creek project is a proposed underground copper and silver mine in northwestern Montana. The mine, mill, and other facilities would be located in Sanders County, Montana, near Noxon, Montana (FEIS 2001). The mining company holds mineral rights under the Cabinet Mountain Wilderness Area. The proposed mine would extract ore from beneath the wilderness through an underground tunnel.

CONSIDERATIONS

The assessment completed as part of this review considered only the technical aspects of mine subsidence. Non-subsidence issues, hydrological issues, legal issues, and the secondary effects of mine subsidence on the environment, are beyond the scope of this work.

Final Geotechnical Assessment Report 2-1 Troy Mine, Montana

The central issues stated in Mr. Kuipers’ reports in connection with surface subsidence are the following:

• Subsidence is an inevitable consequence of underground mining. • Subsidence is independent of depth. • Subsidence can not be prevented by such measures as limited room and pillar extraction. • Faults, folds, and other inconsistencies in the overlying strata may increase the subsidence potential. • Buffer zones to prevent subsidence are unproven in hard rock mines.

The proposed Rock Creek Mine deposit is hosted in the upper members of the lower Revett Formation (Balla 2000). This formation consists of layered quartzites and siltites. Both these rock types are hard crystalline rocks produced by the metamorphism of quartz-rich sedimentary rocks. Rock strength values of 34,500 pounds per square inch are reported for quartzite from the nearby Troy Mine (ASARCO 1994). It is our understanding that there are no rock mechanic test results specific to the Rock Creek deposit at this time.

Mine subsidence is defined as the vertical and horizontal movements of the ground surface that occur due to the collapse of the overlying strata into mine voids. Most of our present knowledge of subsidence comes from experience at full extraction mining (that is, longwall or pillar retreat) in soft rock where no rock pillars were left to hold the overlying rock formations. Mine subsidence in soft rocks (sedimentary) has been found to differ significantly from that of hard rocks (crystalline, such as the quartzite rocks found at the Rock Creek Project). Although studies are limited, it is generally accepted that the amount of subsidence decreases as the hardness and strength of the rocks increase. Partial extraction mining methods such as room and pillar have been shown to significantly reduce or prevent mine subsidence. The proposed Rock Creek Mine Project will utilize partial room and pillar mining methods in strong, hard rock.

The following sections present an overview of mine subsidence with specific reference to Mr. Kuipers’ concerns at the proposed Rock Creek Mine site.

POTENTIAL FOR MINE SUBSIDENCE

At hard rock room and pillar mines, such as the proposed Rock Creek Project, surface subsidence is not an inevitable consequence of mining. Provided that the mine is properly designed to prevent subsidence, the potential for subsidence to occur is minimal. This view is shared by many experts in the field of subsidence including Peng (1992), Whitaker and Reddish (1989), Agapito (1991), Golder Associates (1989), and Cullen and others (2002). Room and pillar mining is well established as a means to prevent damaging surface subsidence. Peng (1992) states that “if chain pillars are properly designed to support the overburden, surface

Final Geotechnical Assessment Report 2-2 Troy Mine, Montana subsidence will not occur.” Room and pillar extraction is the accepted method in Pennsylvania to prevent or minimize unplanned subsidence (Pennsylvania Department of Environmental Protection 1997).

There are hundreds of mines that have operated without any specific subsidence prevention designs that have not experienced any detectable surface subsidence. There are also numerous examples of hard rock mines that have been designed to prevent subsidence and then successfully mined without any damaging surface subsidence being observed. Several examples of these mines that have operated in the last 20 years include the following:

• Cannon Mine, Washington State. One of the permitting requirements for the mine was to prevent significant surface subsidence. The mine design successfully achieved this requirement. • Overlook Mine, Washington State. One of the permitting requirements for the mine was to prevent significant surface subsidence. The mine design successfully achieved this requirement using room and pillar mining followed by backfill then pillar extraction. • Eskay Creek Mine, British Columbia. Mined to within 15 meters of the surface. Subsidence would have a significant impact on mine operations. Specific designs to prevent significant subsidence have been successfully implemented. • Eagle Point Mine, Saskatchewan. Subsidence would have a significant impact on mine operations. Specific designs to prevent subsidence have been successfully implemented. • Detour Lake Mine, Ontario. Subsidence would have a significant impact on mine operations. Specific designs to prevent subsidence have been successfully implemented. • Myra Falls Mine, British Columbia. Tailings were to be placed over shallow mine workings. Subsidence or crown pillar failure would result in a potentially catastrophic inrush of tailings. Assessments indicated that the crown pillar was stable and tailings placement proceeded. • Bafokeng Mines, South Africa. Experienced significant subsidence in past. Subsidence was terminated with implementation of room and pillar mining in one mine, and backfilling in another mine.

Hard rock mines have experienced unexpected surface subsidence. Where this has occurred at room and pillar mines it has usually been attributed to failure of mine pillars, collapse of the mine roof into the openings, or movement along faults. A list of hard rock mines that have experienced expected and unexpected subsidence is included in Enclosure 3. The list was compiled from recent literature searches but should not be considered as exhaustive. Of the mines on the list, only 8 are known to have experienced unexpected subsidence. This low

Final Geotechnical Assessment Report 2-3 Troy Mine, Montana

number is an indication that the problem of unexpected surface subsidence in hard rock mines is not widespread. It should also be noted that most of these mines operated more than 20 years ago and did not have specific measures in place to mitigate subsidence.

Surface subsidence above room and pillar mines, such as the proposed Rock Creek project, can be caused by one of the following:

• Failure of the pillars. • Failure of the roof rock over mine openings. • Deformation of the pillars in response to mining. (This subsidence is minimal to nonexistent, and can be considered “non-damaging”). • Fault interaction.

The following sections provide a brief review of these possible causes of subsidence.

SUBSIDENCE RESULTING FROM PILLAR FAILURE

Pillar failure occurs when the load on the pillar exceeds the pillar strength. Once a pillar fails its load is transferred to adjacent pillars that may in turn fail. As such, it is possible (although not common) for numerous pillars in an area to fail. After the pillars fail, the overlying strata may cave or simply sag. The cave and/or sag will migrate upwards and may eventually reach surface. Subsidence associated with pillar failures typically takes the form of trough subsidence.

There is no known research or case studies on the effects of depth on the extent of surface subsidence caused by pillar failures. However, such studies do exist for full extraction mining (Peng 1992, Kratzsch 1983, National Coal Board 1975). These studies indicate that the amount of surface subsidence decreases with increased depth of mining. Because, in a broad sense, pillar failures over a large area will create a condition similar to full extraction mining, it follows that subsidence caused by pillar failures likewise decrease with increased depth of mining. If pillars are properly designed with an appropriate factor of safety, there should be no massive pillar failures in the underground mine; hence, no significant subsidence.

For the reasons discussed above, we disagree with Mr. Kuipers’ assertion that “subsidence is independent of depth”. This assertion appears to be extracted from the Society of Mining Engineers’ “Mining Engineering Handbook” (1996). In the handbook, the reference for this comment is to a 1964 paper on backfilling longwall coal mines. We do not believe this reference to total extraction mining in soft sedimentary rocks is relevant to the Rock Creek project since the project does not propose longwall mining or backfilling, nor does it involve soft sedimentary rocks.

Final Geotechnical Assessment Report 2-4 Troy Mine, Montana

SUBSIDENCE RESULTING FROM ROOF ROCK FAILURE

When the roof rocks over mine openings fail, a cave will progress upwards. This is often termed a chimney cave. Caving is terminated when one of the following occurs:

• The cave is choked off by bulking (expansion) of the caved material. • The ground surface is reached. • A competent rock unit capable of spanning the caved zone is encountered. • Stress redistribution or change in shape no longer favors failure.

Little data on the height of chimney caves in hard rock room and pillar mines is available. In soft rock mines, the height of chimney caves is typically 3 to 6 times the height of the excavation; in unusual circumstances, caving may extend upwards of 10 times the height of the excavation (Garrard and Taylor 1988, Whittaker and Reddish 1989, Piggot and Eynon 1978, Peng 1992). A conservative approach commonly used at hard rock mines is to assume that the maximum possible height of a chimney cave is 10 times the height of the excavation. The Rock Creek project has an average excavation of 27 feet with a minimum overburden thickness of 450 feet (MDEQ and U.S. Forest Service, 2001).

SUBSIDENCE RESULTING FROM PILLAR DEFORMATION

As mining proceeds, pillars take up the load that was previously carried by the unmined rock. This load will cause a very small reduction in the height of the pillars. This will in turn result in sagging of the overlying rocks. In theory, this sag will propagate up to the ground surface. In simplistic terms the change in pillar height can be calculated by elastic theory as follows:

ε = σ/E

where E = modulus of elasticity σ = stress ε = strain

For example, if we consider 25-foot high pillars, at a mining depth of 500 feet, with 70 percent extraction, and E = 8.7x106 pounds per square inch (psi), the strain induced in the pillars will be ε = 0.02 percent, and the reduction in pillar height (and corresponding roof sag) will be 0.005 foot. If such movements did propagate through the overburden to surface, it would result in less than 0.005 foot of surface subsidence which could only be detected by extremely sensitive instruments. Since miniscule ground movement such as this are not detectable by commonly employed instrumentation, it is typically not considered in assessments of subsidence at hard rock room and pillar mines.

Final Geotechnical Assessment Report 2-5 Troy Mine, Montana

SUBSIDENCE RELATED TO FAULTING

The influence of geologic structure on subsurface ground movements and ultimately on surface subsidence has been recognized by many researchers (Crane 1929, 1931, Heslop 1974, Boyum 1961, Kantner 1934, Fletcher 1960, Kotz 1986, Mahtab 1976, North and Callaghan 1980, Hoek 1974, Peng 1992, Nelson and Fahrni 1950, Holla and Buizen 1990, Lee 1966, Shadbolt 1987, Hellewell 1988, Whittaker and Reddish 1989, Cullen and Pakalnis1995). Many observations of the influence of geologic structure have been made; however, only a modest amount of research work has been carried out. Hellewell (1988) reports that understanding the effects of geologic structure is complicated by the fact that the results of scientific investigations are in some instances contradictory. The following conclusions are drawn from a literature review of subsidence at hard rock mines (Enclosure 3):

• Mining may reactivate faults and induce surface subsidence. • Most of the anomalous subsidence from deep hard rock mines is attributed to geologic structure such as faults. • Faulting can result in surface subsidence from considerable mining depth.

POTENTIAL FOR SUBSIDENCE AT THE PROPOSED ROCK CREEK MINE

The FEIS and ROD contain stipulations (including mitigations and monitoring requirements) that require collection and analysis of data necessary to complete detailed mine designs prior to entering areas where mining could result in impacts to the surface. These designs are to be approved by the MDEQ and U.S. Forest Service prior to their implementation. The U.S. Forest Service believes that if properly carried out, this would reduce the potential for subsidence to minimal. We concur with the U.S. Forest Service. If the following strategies are employed at the proposed Rock Creek Mine, as outlined in the final EIS and ROD, we consider that the potential for subsidence will be minimal:

• Properly design the pillars to support the overlying rocks with an appropriate factor of safety. Consideration must be given to the factors that will affect the long-term pillar strength. Artificial support such as rockbolts should not be relied upon for long-term stability. • Properly design the size of the openings such that they do not readily cave. Artificial support such as rockbolts should not be relied upon for long-term stability. • Properly design the height of the rooms such that if caving does occur it does not reach surface or areas of potential hydraulic connectivity with the surface. • Use buffer zones around faults and dykes that extend to surface and have the potential to reduce surface subsidence.

Final Geotechnical Assessment Report 2-6 Troy Mine, Montana

While it is possible to minimize the potential for subsidence, it is not possible to completely eliminate the potential for subsidence. The nature of geotechnical engineering is such that it is never possible to completely characterize all rock mass conditions. It is conceivable that unexpected conditions and rock mass response may occur. Carrying out an ongoing testing, monitoring, and probing program is a reasonable and prudent way to reduce the potential of encountering unexpected conditions, but can never completely eliminate the potential for their occurrence.

Revett plans to obtain design-level information on rock properties at the proposed Rock Creek mine from data gathered during construction of the evaluation adit. Rock properties information is available from the adjacent Montanore Project (Redpath 1991, Agapito 1991). It is understood that the USFS utilized the Montanore data in their assessment of the Rock Creek project. Preliminary mine design work for the proposed Rock Creek mine completed by ASARCO in 1994 utilized data from the Troy Mine. This is reasonable considering the Rock Creek Mine is situated in similar rock formations as those at the Troy Mine. It is understood that the operators are planning to complete a detailed rock mechanics study during the evaluation adit stage, and that the information collected will be used to complete detailed mine design plans.

REVIEW OF “TECHNICAL REVIEW OF TECHNICAL COMMENTS ON THE PROPOSED ROCK CREEK PROJECT FEIS AND ROD BY JAMES R. KUIPERS” PREPARED BY J. MCKAY, P. WERNER AND J. GURRIERI

We have completed a review of the Forest Services’ response to the Technical Comments submitted by Mr. James Kuipers. As stated previously our review is limited to technical issues of mine subsidence. Non-subsidence issues, hydrological issues, legal issues, and the secondary effects of mine subsidence on the environment, are not included.

The Forest Service document responding to Mr. Kuipers' comments is well thought out and does an excellent job covering the technical issues around subsidence. We agree with the technical comments and analysis related to mine subsidence described in this document. Much of the historical mine subsidence reported in the literature is associated with mines using caving methods and/or at mines that operated when there was little concern for subsidence; hence, little or no effort to enact mitigative measures was necessary. The subsidence mitigation measures discussed in the document (i.e., utilizing buffer zones, utilizing minimum depth of cover, ensuring adequate pillar size and room spans) are well accepted practices in the mining industry.

We concur with the Forest Service that the ROD and FEIS inform the public of the potential impacts from subsidence and put forward an alternative with stipulations that will reduce the potential for subsidence to minimal.

Mine planning and subsidence prevention are an iterative process at operating mines. Even mines that have collected considerable rock mechanics information fully expect to modify and optimize the plans as more information is obtained. No mine in the development stages could possibly be

Final Geotechnical Assessment Report 2-7 Troy Mine, Montana

expected to produce a single definitive plan. The approach taken by the FEIS – that mine planning and subsidence prevention are dynamic processes that should be continually under review and modified as information is collected – is reasonable and proper. As discussed in the Forest Service document, the ROD requires that rock mechanics data be collected during the evaluation adit and mine operation stages, and that this information be used to prepare mine plans and subsidence mitigation measures that are subject to review and approval by the Agencies.

The Forest Service document compares the potential for subsidence at the proposed Rock Creek Mine to the Troy Mine. The Troy Mine is operating in similar rock conditions using similar mining methods to those proposed at Rock Creek. Until 2005 there was no known evidence that the Troy Mine had experienced any surface subsidence. As discussed in our report “Geotechnical Assessment Report Troy Mine Sinkhole”, two sinkholes have recently occurred above the Troy Mine. The sinkholes are the result of chimney type caves that extended up a major fault structure. The caving is a result of mining against and into the East Fault. If the fault had not been mined into there would have been no cave and therefore no surface subsidence. To the best of our knowledge these sinkholes are the only subsidence features that have occurred over the Troy Mine.

Using buffer zones (to prevent mining up against the fault) and properly sizing and securing drifts (mine roads) that pass through the fault would have prevented caving and surface subsidence. It is our understanding that the Troy Mine operating permit does not specifically preclude surface subsidence; consequently, the mine operators were not obligated to take measures to mitigate subsidence. Conversely, the proposed Rock Creek Mine is required to employ buffer zones and other measures to mitigate subsidence such that the potential for chimney type caves to progress up faults is considered minimal.

THIRD PARTY OPINION ON THE SUFFICIENCY AND ADEQUACY OF THE INFORMATION ON SURFACE SUBSIDENCE CONTAINED IN THE FEIS AND ROD

Each of Mr. Kuipers’ comments on the sufficiency and adequacy of the information on surface subsidence contained in the FEIS and ROD is re-printed below (in bold italics), followed by our opinion on the matter. Mr. Kuipers’ report often mixes comments on subsidence and hydrological effects of mining that are unrelated to subsidence. This report only addresses mining subsidence aspects.

In many instances, Mr. Kuipers has simply made statements about subsidence without reference to the sufficiency and adequacy of the information contained in the FEIS; for completeness these statements have also been addressed.

Final Geotechnical Assessment Report 2-8 Troy Mine, Montana

Section 3.1 Environmental Impacts Subsidence is an inevitable consequence of underground mining. In the case of the Rock Creek Project because of its scale it will extend over large areas and the room and pillar mining technique may delay subsidence for many years.

Because Alternative V does not address reclamation on any resulting subsidence areas from the proposed underground mining operations, it does not require reclamation of those areas, and cannot ensure that the affected areas will support a post-mining land use with comparable stability of that of the adjacent undisturbed landscape. Subsided surface features such as landslides, sinkholes, and other surface discontinuities would significantly alter the site from that of the adjacent wilderness area… Alternative V does not ensure that the integrity of the natural resource values in the area will not be significantly altered or harmed. By not addressing subsidence areas and hydrologic impacts from the proposed underground mining operations Alternative V does not minimize adverse environmental impacts on surface resources.

It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements to ensure that the Rock Creek Mine is designed and operated with minimal potential for mining induced subsidence. As such, there is no real need to develop a subsidence reclamation plan.

Section 3.3 Impacts to Wetlands The wetlands analysis did not include wetlands area which might be affected by subsidence of the underground mine either during or post-mining. Mining subsidence induces fissuring in overlying and surrounding strata which influences hydrologic systems in ways that cause changes to both water quality and quantity. Subsidence caused fracturing of overlying rocks strata can enhance vertical flow, which could lead to drainage of overlying aquifers. Permeability increases when fractures reach the ground surface, which may lead to increased ground water recharge and surface water depletion. Surface and near-surface soils and unconsolidated materials are an important factor relative to hydrologic impacts because they affect the exchange of surface water and ground water. Ground water drainage gradients may be altered by disturbances of the strata around mine areas. Rocks may become weakened by saturation and erosion patterns could change. Where surface water is present it may migrate more easily to fractures and fissures in the strata and into the mine area and may induce subsidence. Subsidence can cause the formation of open cracks, fissures or pits which, if connected either directly or indirectly to surface water (streams, lakes, ponds), may lead to partial or complete loss of water that is drained to lower strata or mine workings.

As a result subsidence and/or hydrologic impacts from the underground mining operation, Alternative V may result in unintended consequences to the wetlands and associated surface water systems that make up the overlying surface area that is within the Cabinet Mountains Wilderness Area.

It is well documented that subsidence and caving creates fractures, which may affect ground and surface waters. However, if there is no subsidence or caving, a fractured zone will not develop and there will be no subsidence induced hydrologic changes.

Final Geotechnical Assessment Report 2-9 Troy Mine, Montana

It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements to ensure that the Rock Creek Mine is designed and operated with minimal potential for mining induced ground subsidence. As such there is only minimal potential of subsidence induced hydrologic changes.

Section 3.6 Impacts to Wetlands The EIS and ROD do not mention or address consequences to public safety from subsidence due to underground mining. Subsided areas may be unstable and it may also make existing steep slopes and cliff areas more unstable and create an unacceptable risk to public safety.

It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements to ensure that the Rock Creek Mine is designed and operated with minimal potential for mining induced ground subsidence. As such, there is no real need to address the affects of subsidence on public safety.

Section 3.7 Aesthetic Impacts Sloping ground like hillsides tends to emphasize the surface manifestation of subsidence while it is less accentuated on flatter ground and in valleys. The area overlying the rock creek Project underground mining operations consists of steep hillsides and mountain tops, with very unique inlying flat areas. As a result, subsidence impacts will be more dramatic and noticeable and possibly significantly affect the geomorphology of the surface land overlying the underground mine.

Subsidence from underground mining will disturb the natural landform of the Cabinet Mountains Wilderness area and surround areas in Kootenai National Forest.

It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements to ensure that the Rock Creek Mine is designed and operated with minimal potential for mining induced ground subsidence. As such, there is no real need to address the affects of subsidence on aesthetic impacts.

Section 3.8 Subsidence and Hydrologic Impact Mitigations

The existence of a mine plan in and of itself, or the use of the most up-to-date information and technology, does not relieve the possibility of impacts to wilderness lakes above the mine workings….

…The effectiveness of mitigation measures greatly depends upon the accuracy of prediction of subsidence and associated parameters.

Mining at any depth can result in subsidence. Greater depths of overburden do not prevent subsidence but may prolong the time period before subsidence effects are observed at the surface…

…thick ore zones are not a pre-requisite for subsidence, which will occur to at least some extent regardless of the thickness of the ore zone.

Final Geotechnical Assessment Report 2-10 Troy Mine, Montana

Subsidence damage may be reduced by alteration in mining techniques, including by limiting the recovery of pillars. However, while potentially limiting the extent of subsidence, this measure will not entirely prevent subsidence. According to the Society of Mining Engineers Mining Engineering Handbook “Recent studies, however, have shown that no mater how well-designed a room and pillar layout might be, the additional weight transferred to the pillars due to the excavations will cause measurable deformations on the pillars, and these movements will eventually be transmitted to the surface…”

The use of “buffer zones” to protect surface structures or hydrologic features is unproven as a preventative technique in hard rock mining. Buffer zones are not true mitigation measures because subsidence will still occur.

The statements made in Section 3.8 that are related to the potential for subsidence have been discussed previously in this report. It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements to ensure that the Rock Creek Mine is designed and operated with minimal potential for mining induced subsidence.

Contrary to the assertion by Mr. Kuipers, buffer zones have long been used to protect surface resources from the effects of subsidence. Shaft pillars are the probably the oldest form of buffer zones successfully employed in both hard and soft rock mines; i.e., the South African Gold Fields and O’Donahues’s rule of 1907 (Morrison 1976). One of the early objectives of the research into the angle of draw at hard rock mines (Crane 1929) was to establish the setback distance of surface facilities to avoid subsidence damage; this is essentially the establishment of a buffer zone.

Stipulation No. 29 (in the ROD and FEIS) requires that a 100-foot buffer be maintained around all faults and a minimum depth of cover of 450 feet be maintained for all mining The stated purpose of Stipulation No. 29 is to guard against hydraulic connectivity; however, it will also prevent subsidence along the fault Our interpretation of the stipulations in the ROD and FEIS require that Revett provide scientific analysis and justification for these values, and that the value will be adjusted as required to ensure the potential for subsidence is minimal.

The SME handbook quote provided in Mr. Kuipers’ report concerning the potential for subsidence above room and pillar mines is incomplete. The complete quote is as follows. The underlined section is the text previously omitted by Mr. Kuipers.

“Recent studies, however, have shown that no matter how well-designed a room and pillar layout might be, the additional weight transmitted to the pillars due to excavations will cause measurable deformation on the pillars, and these movements will eventually be transmitted to the surface. Depending upon the extent of pillar loading and the characteristics of the pillars and the superincumbent material, the surface deflection may vary from considerable to negligible, and sometimes is nearly undetectable. The long-term stability of mine pillars is extremely difficult to determine.”

Final Geotechnical Assessment Report 2-11 Troy Mine, Montana

As discussed in the previous section on Subsidence Resulting from Pillar Deformation the amount of pillar deformation (and potential for detectible surface movement) can be limited by controlling pillar size (which in turn dictates extraction ratio). To minimize the potential for subsidence pillars must be designed to ensure both short and long-term stability. This is typically achieved by designing pillars with either an adequate factor of safety (FOS) or acceptable probability of failure. Both FOS and probability analysis relate the forces resisting pillar failure to the forces driving failure. The factors that must be considered in determining the long-term stability of a pillar include the following:

• geologic structure • material properties • environment (temperature, moisture) • pillar geometry (size, shape, orientation) • pillar/roof, and pillar/floor interaction • in situ stress (typically related to depth) • mining induced stress (related to extraction ratio) • weathering and time dependent changes

The FOS analysis provides a discrete value for pillar stability. In theory a FOS greater than 1 represents a stable condition and a FOS less than 1 represents an unstable condition. In practice, the FOS is increased as the level of confidence in the input parameters is reduced, or as the importance of the need for stability is increased. FOS Values between 1.2 and 2.0 are typical for pillar design in the mining industry (Cullen 2002). For coal mines Bieniawski (1981) proposed that the FOS of between 1.5 and 2.0 for long-term pillar stability. Hoek and Brown (1980) consider the FOS should be a minimum of 1.6 for permanent support in hard rock mines.

Probability analysis provides a likelihood of failure based on a sensitivity analysis. A series of calculations are performed in which each significant factor affecting pillar stability is varied systematically over its maximum credible range is completed, and the effect on FOS assessed. At this time there is no generally accepted “acceptable probability of failure” for mine pillars. One of the concerns of probability analysis for pillar failure is that the stress driven failure of a rock mass is essentially an indeterminate problem that is not easily represented by a simple set of equations (Hoek et al 1995).

In the event monitoring detects wetland impacts due to subsidence or changes in lake levels due to mining operations, unidentified contingency mitigation approaches are proposed by the ROD. However, only limited means of mitigating impacts to wetlands and surface water are available to the mining operation. Grouting has been successfully used in some applications, however it is not always effective... The site-specific geology and characteristics of the wetlands and surface water occurrences in the area overlying

Final Geotechnical Assessment Report 2-12 Troy Mine, Montana

the Rock Creek Project underground mining operations, because it is highly faulted and jointed, is highly favorable for water ingress, but not highly favorable for grouting to be effective as a mitigation measure.

Remediation of subsidence effects on wetlands or lakes would be difficult. It is unlikely that the results of any Wetland Mitigation Plan could be determined ahead of time. However, the need for wetland mitigation is not relevant if the potential for subsidence is expected to be minimal.

Section 4.0 Mine Area Geology The geology, hydrology, and mine plan for the proposed Rock Creek Project exhibits several characteristics that are favorable for subsidence.

1. The orebody and overlying and surrounding rocks are highly fractured with two vertical joint sets that intersect each other, numerous faults, and brecciated veins.

2. The Revett quartzite, the formation to be mined, has interbeds of siltite and argillite, and is highly fractured due to numerous metamorphic episodes in the past 1.5 million years. The argillite and siltite interbeds are lithified clays and silts that have low structural strength and could form slip-planes under the stress of overburden after mining is done.

3. Measurable seismic activity has occurred in the Cabinet Mountains over the past 20 years.

4. The overburden thickness at Rock Creek averages 800 feet, with a maximum of 2000 feet. This overburden is also highly fractured. Many historical mines have experienced subsidence at these and even greater depths.

5. An extraction zone that averages 250 feet will be created underground when ore is mined. In places this zone could be 400 ft thick. (Note: Mr. Kuipers’ report reads 250 feet; however, it is our understanding that the average extraction height will be only 25 feet.

As discussed above, geologic structure and especially faults play a significant role in mine subsidence. Most anomalous subsidence occurring at deep mines (greater than about 1,000 feet), is related to faults (i.e., the Athens Mine referenced in the report by Blodget and Kuipers [2002]). It is our opinion that the information in the FEIS and ROD is both adequate and sufficient in its requirements for buffer zones around faults to result in minimal potential for mine subsidence.

The Revett Formation consists of quartzites, siltites and argillites. Based on data from the nearby Troy Mine and proposed Montanore mine, all these rocks are strong. The FEIS and ROD require that the mechanical properties of the Rock Creek rocks and rockmass (including the influence of fractures) be determined and that this data be used to design the mine such that the potential for subsidence will be minimal.

Final Geotechnical Assessment Report 2-13 Troy Mine, Montana

It is theoretically possible that earthquakes may induce subsidence. However, we are not aware of any case studies that have identified earthquakes as the cause of mine subsidence. The greatest potential for seismic activity to cause subsidence would be fault reactivation. The FEIS and ROD requirements for buffer zones around faults appear to adequately address this concern.

The FEIS and ROD place limits on mining heights by requiring that the Rock Creek Mine be designed and operated such that there is minimal risk for mine subsidence. The ore body averages 27 feet thick and at its maximum is only 235 feet thick.

CONCLUSIONS

An overview of mine subsidence with specific reference to Mr. Kuipers’ concerns at the proposed Rock Creek Project was completed. Specific issues addressing Mr. Kuipers’ concerns for subsidence at the Rock Creek Mine were addressed in detail. It is our opinion that the potential for mine subsidence associated with the Rock Creek project will be minimal given the requirements set forth in the FEIS and ROD.

Final Geotechnical Assessment Report 2-14 Troy Mine, Montana

ENCLOSURE 1 REFERENCES

Agapito, J.F.T., and Associates, Inc. 1991, Evaluation of subsidence potential, Montanore Project, Noranda Minerals Corp. Submitted to IMS Inc.

ASARCO Inc. 1994 “Rock Creek Project Rock Mechanics Analysis” prepared by Dave Young.

Balia R., Manca P.P., Massacci G., 1990, “Progressive Hangingwall Caving and Subsidence Prediction at the San Giovanni Mine, Italy”, Proc. 9th Int. Conf. On Ground Control in Mining, ed. S. Peng., pp 303-311.

Balla J.C. 2000. Geologic Comparison of the Troy Mine/Spar Lake Deposit and the Rock Creek Mine/Rock Creek Deposit, Northwest Montana.

Betlach, C. 2005. “Slump on Mt. Vernon”. Email to John McKay, Kootenai National Forest Geologist from Ms. Cindy Betlach. May 27.

Bieniawski, Z.T., 1981, “Improved Design of Coal Pillars for US Mining Conditions”, Proceedings, International Conference on Ground Control in Mining, ed. S. Peng.

Boyum, B.H., 1961, “Subsidence Case Histories in Michigan Mines”, 4th US Symposium on Rock Mechanics, Penn. State University.

Brown E.T. Ferguson G.A.,1979, “Prediction of Progressive Hangingwall Caving, Gath’s Mine, Rhodesia”, IMM Section A, July, pp A92-A102.

Brumleve C.B., Maier M.M., 1981 “Applied Investigations of Rock Mass Response to Panel Caving, Henderson Mine, Colorado” in Design and Operation of Caving and Sublevel Stoping Mines, ed R.D. Stewart, AIME, New York, 843.

Brumleve C.B. 1987, “Rock Reinforcement of a Caving Block in Variable Ground Conditions, King Mine Zimbabwe” Trans. Inst. Min. Metall., Sect A, April.

Camp Dresser and McKee Inc. 1989 “Final Engineering Assessment for the Asarco Rock Creek Environmental Impact Statement”.

Crane, W.R., 1929, “Subsidence and Ground Movement in the Copper and Iron Mines of the Upper Peninsula, Michigan”, USBM Bulletin 285.

Crane, W.R., 1931, “Essential Factors Influencing Subsidence and Ground Movement”, USBM IC 6501.

Cullen M, Pakalnis R. 1995 “Subsidence Over Shallow Heavily Faulted Coal Mines”. Proc. 48th Canadian Geotechnical Conference.

Cullen, M., 1996, “Assessment of Nature and Potential for Subsidence at the Alaska Juneau Mine”, consulting report submitted to Golder Associates/Echo Bay Mines.

Final Geotechnical Assessment Report 2.1-1 Troy Mine, Montana

Cullen M., Pakalnis R., Galovich K., 2002, “Protecting the Quinsam Coal Mine Access Road from Subsidence Damage” Proceedings Canadian Institute of Mining, Metallurgy, and Petroleum, Annual General Meeting.

Cullen, M., 2002, “Geotechnical Studies of Retreat Pillar Coal Mining at Shallow Depth”. Ph.D. Thesis, University of British Columbia.

Dickhout M.H., 1963 “Ground Control at the Creighton Mine of the International Nickel Company of Canada Limited”, Proc. Rock Mechanics Symposium, McGill University, Mines Branch, Ottawa.

Fletcher, J.B., 1960, “Ground Movement and Subsidence from Block Caving at Miami Mine”, AIME Transactions, Vol. 217.

Garrard, G.F.G., Taylor, R.K., 1988, “Collapse Mechanisms of Shallow Coal Mine Workings From Field Measurements”, Proceedings, Engineering Geology of Underground Excavations, eds: Bell, Cueshaw, Cripps, Lowel.

Goel S.C., Page C.H., 1982, “An Empirical Method for Predicting the Probability of Chimney Cave Occurrence Over a Mining Area”, Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., vol 19, pp 325-337.

Golder Associates, 1989 “Report to Echo Bay Mines on Surface Subsidence as a Result of Mining the Overlook Orebody” , prepared by M. Cullen.

Hellewell, F.G., 1988, “The Influence of Faulting on Ground Movement due to Coal Mining: The U.K. and European Experience”, Mining Engineer, 147, pp 334 – 337.

Heslop T.G. 1984, “The Application of Interactive Draw Theory to Draw Control Practice in Large Chrysotile Asbestos Mines”, Chamber of Mines Journal (S.A.) June 1984, pp 23-41.

Heslop, T.G., 1974, “Failure by Overturning in Ground Adjacent to Cave Mining at Havelock Mine”, 3rd ISRM Congress, Denver.

Hoek, E., 1974, “Progressive Caving Induced by Mining an Inclined Orebody”, Trans. Inst. Min. Metall., No. 83, pp A113 – A119.

Hoek, E, Brown, E.T., 1980, “Underground Excavations in Rock”, IMM, London.

Hoek, E., Kaiser, P.K., Bawden, W.F., 1995, “Support of Underground Excavations in Hard Rock”, A.A.Balkema, Rotterdam.

Holla, L., Buizen, M., 1990, “Strata Movement Due to Shallow Longwall Mining and the Effect on Ground Permeability”, Proceedings, Aus. IMM, No. 1, pp 11 – 18.

Johnsone G.H., Soule J.H., 1963 “Measurements of Surface Subsidence. San Manuel Mine, Pinal County, Arizona”, U.S. Bureau of Mines R.I. 6204.

Final Geotechnical Assessment Report 2.1-2 Troy Mine, Montana

Kantner, W.H., 1934, “Surface Subsidence over Porphyry Caving Blocks, Phelps Dodge Corporation, Copper Queen Branch”, AIME Transactions, Vol. 109.

Kvapil R., Baeza L., Rosenthal J., Flores G., 1989, “Block Caving at El Teniente Mine Chile”, Trans Instn Min. Metall. Sect A, No. 98.

Kotz, T.J., 1986, “The Nature and Magnitude of Surface Subsidence Resulting From Mining at Relatively Shallow Depths on Platinum Mines”, SANGORM Symposium, The Effect of Underground Mining on Surface, South Africa Institute of Civil Engineers, Yeonville, S.A.

Kratzsch, H., 1983, “Mining Subsidence Engineering”, Springer Verlag, Berlin.

Lee, A.J., 1966, “The Effect of Faulting on Mine Subsidence”, Mining Engineer, 125, pp 417 – 427.

Mahtab, M., 1976, “Influence of Rock Fractures on Caving”, Transactions, AIME, Vol. 260, 1976.

MDEQ. 2006. “Inspection of Troy Mine”. Prepared by Mr. George Furniss to Mr. Pat Plantenberg. February 15.

Mills C.E., 1934, “Ground Movement and Subsidence at the United Verde Mine”, AIME Transactions ,V. 109, pp 153-171.

Morrison, R.G.K., 1976, “A Philosophy of Ground Control”, Ontario Department of Mines.

National Coal Board, 1975, “Subsidence Engineers Handbook II”, National Coal Board Mining Department.

Nelson, J. 1981. “Faults and Their Effects on Coal Mining in Illinois” Illinois State Geological Survey Circular Number 523.

North, P.G., Callaghan, R.P., 1980, “Subsidence Associated with Mining at Mt. Lyell”, The Aus. IMM Conference, New Zealand.

Obert L., Long A.E., 1962, “Underground Borate Mining, Kern Country, California”, USBM R.I.

Owen K.C., 1981 “Block Caving at Premier Mine” in Design and Operation of Caving and Sublevel Stoping Mines, ed R.D. Stewart, AIME, New York, 843 p.

Panek L.A. 1981, “Ground Movements Near a Caving Stope”, in Design and Operation of Caving and Sublevel Stoping Mines, ed R.D. Stewart, AIME, New York, 843 p.

Peng, S.S., 1992, “Surface Subsidence Engineering”, Society for Mining, Metallurgy and Exploration Inc., Littleton, Colorado.

Pennsylvania Department Environmental Protection 1997, “Environmental Protection Code”.

Final Geotechnical Assessment Report 2.1-3 Troy Mine, Montana

Piggot, R.J., Eynon, P., 1978, “Ground Movements Arising from the Presence of Shallow Abandoned Mine Workings”, Proceedings, Conference on Large Ground Movements and Structures, Pentech Press, Plymouth.

Redpath Engineering Inc, 1991. “Supplemental Information in Support of the Environmental Impact Statement Montanore Project”.

Rice, G.S., 1923. “Some Problems in Ground Movement and Subsidence”, Trans AIME, V. 28, pp 374-393 and 414-433.

Rice G.S., 1934 “Ground Movement From Mining in Brier Hill Mine, Michigan”, Trans AIME, V. 109, pp 118-152.

Shadbolt, C.H., 1987, “A Study of the Effects of Geology on Mining Subsidence in the East Pennine Coalfield”, Ph.D. Thesis, University of Nottingham.

Singh U.K., Stephansson O.J., Herdocia A., 1993 “Simulation of Progressive Failure in Hanging Wall and Foot wall For Mining With Sub Level Caving” Trans. Inst. Min. Metal., sect. A, No. 102, pp A188-A194.

Society of Mining Engineers, 1996 “Mining Engineers Handbook.”

Trischka C, 1934 “Subsidence Following Extraction of Ore From Limestone Replacement Deposits, Warren Mining District, Bisbee Arizona” Trans AIME, V. 109, pp 173-180.

Vanderwilt J.W. 1946 “Ground Movement Adjacent to a Caving Block in the Climax Molybdenum Mine”. Technical Paper #200, Mining Technology.

Vongpaisal S. 1974 “Prediction of Subsidence Resulting From Mining Operations”. PhD. Thesis, McGill University, Montreal.

Whittaker, B.N., Reddish, D.J., 1989, “Subsidence: Occurrence, Prediction and Control”, Elsevier Science Publishers, Amsterdam.

Final Geotechnical Assessment Report 2.1-4 Troy Mine, Montana

ENCLOSURE 2 STIPULATIONS AND ORDERS CONTAINED IN THE ROD AND FEIS RELATED TO THE MITIGATION OF POTENTIAL MINE SUBSIDENCE AND HYDROLOGIC IMPACTS TO WILDERNES LAKES AT THE PROPOSED ROCK CREEK MINE

Table of Approved Stipulations 26 * Sterling will provide an The objective of this mitigation is to Needed to maintain minimal risk of updated mine design plan prior ensure that the most up-to-date subsidence due to wilderness lakes to evaluation adit and mine information and technology is used above the mine workings. This adit construction. in determining the final mine mitigation is necessary to ensure construction plan. The authority for compliance with 82-4-336(10) this mitigation is 36 CFR 228.8(g). and (12), 82-4-351, 75-5-303, and 75-5-605, MCA, and ARM17.24.105(1)(c). 27. Sterling will submit updated The objective of this mitigation is to Needed to maintain minimal risk of mine plans prior to entering ensure the most current information subsidence due to wilderness lakes areas where mining could is available for review to determine above the mine workings. This result in impacts to the surface if any additional mitigation may be mitigation is necessary to ensure (thick ore zones and ore required to protect the resources. compliance with 82-4-336(10) outcrop zones). The authority is 36 CFR 228.8 and (12), 82-4-351, 75-5-303, and 75-5-605, MCA. 28. No secondary ore recovery This mitigation is needed to This mitigation is needed to from pillars will be allowed to maintain minimal risk of subsidence maintain minimal risk of subsidence reduce the risk of subsidence. due to wilderness lakes above the due to wilderness lakes above the mine workings. The authority for mine workings. This mitigation is this mitigation is 36 CFR 228.8(b) necessary to ensure compliance with and (d), and 228.15. 82-4-336(10) and (12), 82-4-351, 75-5-303, and 75-5-605, MCA. 29.* A 1,000-foot buffer zone will The buffers around the faults and The buffers around the faults and be maintained around Cliff Cliff Lake are necessary to reduce Cliff Lake are necessary to reduce Lake and the north and south the risk of modifying the the risk of modifying the ore outcrop interfaces in potentiometric surface of the ground potentiometric surface of the ground addition to the 100-foot buffer water in the faults that recharge water in the faults that recharge on either side of the Moran wilderness lakes that could affect wilderness lakes that could affect fault, the Copper Lake Fault, lake levels and water chemistry. lake levels and water chemistry. and other faults as proposed by The buffers at the ore outcrop zones The buffers at the ore outcrop zones the applicant. A 450-foot are necessary to minimize the are necessary to minimize the vertical buffer will be potential for creating new seeps and potential for creating new seeps and maintained between the mine springs from water stored in springs from water stored in workings and the surface. underground workings. Those underground workings. Those buffers as well as the vertical buffer buffers as well as the vertical buffer are also required to prevent are also required to prevent hydrofracturing of the bedrock and hydrofracturing of the bedrock and creating new springs and seeps from creating new springs and seeps from water stored in the mine workings, water stored in the mine workings, especially after mine closure. The especially after mine closure. This authority for this mitigation is mitigation is necessary to ensure 36 CFR 228.8b and d, and 228.15. compliance with 82-4-336(10) and (12), 82-4-351, 75-5-303, and 75-5-605, MCA.

Final Geotechnical Assessment Report 2.2-1 Troy Mine, Montana

Table of Approved Stipulations (Continued) 61. Sterling will need to comply The objective of this mitigation is to N/A. Sterling has consented to with all stipulations required lessen the over all potential lost of apply this stipulation to the hard by the COE in its approval of wetland habitat. The KNF can rock operating permit as allowed by Sterling’s 404(b)(1) permit for require this mitigation under 75-1-201(5)(b), MCA. the mine. Items identified in 36 CFR 228.8(e) and (h). the FEIS that will need to be incorporated into the Wetland’s mitigation plan include but are not limited to the following items: a. Sterling needs to add The objective of this mitigation is to N/A. Sterling has consented to contingency measures to its lessen the overall potential lost of apply this stipulation to the hard Wetland Mitigation Plan wetland habitat. The KNF can rock operating permit as allowed by for dealing with wetland require this mitigation under 75-1-201(5)(b), MCA. impacts in the wilderness if 36 CFR 228.8(e) and (h). subsidence or mine operations affects water levels in the wilderness lakes. This should be coordinated with the water resources monitoring and aquatics/fisheries mitigation and monitoring plans and approved by the COE. b. An aquatic life mitigation This mitigation plan is needed in the This mitigation plan is needed in the plan for wilderness lakes unlikely event that mining and/or unlikely event that mining and/or will be prepared in subsidence will affect wilderness subsidence will affect wilderness conjunction with the lakes and streams by draining water lakes and streams by draining water wetlands mitigation plan. and thus affecting aquatic life. The and thus affecting aquatic. The authority for this mitigation is 36 authority for this mitigation is CFR 228.8(a), (e), and (h). 82-4-351 and 75-5-303, MCA. 76.* A subsidence control and The objective of this mitigation is to This plan addresses standard mining monitoring plan will be ensure wilderness characteristics are practices needed to ensure adequate developed and will include an preserved and the risk of impacts to rock bolting, etc. for stability underground mine plan review wilderness lakes is minimized. The purposes. Subsidence risk must by the agencies prior to objective of this plan is to protect remain minimal as wilderness lakes entering areas of potential surface and water resources and to could be affected by massive subsidence. ensure the most current information enough subsidence. This mitigation and technology obtained between is necessary to ensure compliance issuance of the ROD and with 82-4-336(10) and (12), implementation of the project is 82-4-351, 75-5-303, and 75-5-605, issued in the plan. The authority for MCA, and ARM 17.24.103(1)(c). this mitigation is 36 CFR 228.8(b) and (d) and 228.15.

Final Geotechnical Assessment Report 2.2-2 Troy Mine, Montana

From Appendix K of ROD: Agencies Revised Conceptual Monitoring Plan

ROCK MECHANICS MONITORING PLAN

The rock mechanics monitoring plan as envisioned, has a dual purpose: (1) to acquire data pertinent to the site and use this data in mine planning, and; (2) to monitor the surrounding physical environment’s response to mining in order to prevent environmental damage to the surface environment, to surface water and to ground water. Revett (formerly Sterling Mining, Inc.) would develop this plan in conjunction with the Agencies, and the plan’s details and implementation would be subject to Agency approval. The rock mechanics monitoring plan would be submitted to the Agencies prior to construction of the evaluation adit.

The goals of the monitoring plan are:

• To collect site specific data on the host environment. • To confirm assumptions made by Revett concerning physical parameters of the host rock. • To assist in mine planning (e.g., room and pillar size and layout, areas of artificial support, location of monitoring devices, size of buffer zones, etc.) • To provide data to Revett and to the Agencies which would be used in the assessment of potential environmental damage due to mining. • To provide data to assist in determining whether to alter the mine plan to prevent environmental damage.

The scope of this monitoring plan would evolve as the complexities related to construction and mining increase. Initially, the monitoring plan would concentrate on data collection during the evaluation adit phase. In time, as mine development proceeds, the focus of the monitoring plan would be on environmental monitoring in response to mining.

Evaluation Adit Phase

During the development of the evaluation adit, data collection to establish baseline conditions and to confirm physical parameters for the surrounding rock would be the principal objectives. Surface monitoring stations would also be established prior to adit development. These would be installed prior to any mining disturbance, and would be monitored using either conventional land based geodetic measuring systems, or global positioning devices (GPS). Surface monuments would be strategically placed near surface features that may be more susceptible to mine related activities. Areas around Cliff Lake and Copper Lake would have monitoring stations, as well as areas where the ore horizon is particularly thick or near to the surface.

Final Geotechnical Assessment Report 2.2-3 Troy Mine, Montana

Laboratory and In-Situ Testing

Laboratory testing on representative samples collected during the evaluation adit phase would confirm physical parameters of the local host rock. Tests and documentation of material properties would include, but are not limited to: specific gravity, Young’s Modulus, Poisson’s ratio, cohesion, angle of internal friction, uniaxial compressive strength, jointing, and other structural features. This data would be used to develop analytical models for the Rock Creek orebody that in turn would assist in mine design and layout. If mining proceeds beyond the evaluation adit phase, Revett would continue to collect and test samples as the mine advances to confirm material properties as new areas are developed. The frequency of sampling may be determined by either changes in lithology or based on a certain number of samples per volume of material extracted.

In situ monitoring devices would also be installed during the evaluation adit development phase. These may include but are not limited to strain gauges, extensometers and micoseismic monitoring devices. These instruments collect data relating to the how the surrounding rock responds to mining and the excavation of cavities underground. As mining progresses, Revett would continue to install and monitor in situ devices as part of their overall environmental monitoring program. The placement of these devices would be determined through consultation with the Agencies and their representatives.

Areas of known or suspected instability, such as near geologic faults, may get a more concentrated array of devices. The frequency of monitoring would also be resolved with Agency counsel once the adit is underway, however it is difficult to predict both placement and frequency prior to development.

Active Mining Phase

During active mining, surface and in situ monitoring would be ongoing. Deviations from baseline conditions may be indicative of adverse ground reactions to mining. If such conditions occur, the Rock Mechanics Monitoring Plan would have as part of its program, steps and mitigation to retard and stop any deleterious effects. Possible mitigation may include the installation of supplemental supports such as rock bolts, grouting, backfilling the affected area, prohibiting mining in the affected area, or changing the room and pillar sizes to provide more underground support.

The evaluation adit phase would provide ample opportunity to refine the mine plan based on real data so that when active mining does commence, adequate sizing and spacing of pillars and rooms would have occurred. Drilling in advance of new development would intersect unfavorable ground conditions such as faults or extensive jointing, both of which could promote underground instability or ground water drainage stresses on overlying lakes, streams, and wetlands. Mining would not occur in areas where adverse ground conditions could lead to surface subsidence or effects on the wilderness lakes or hydrofracture at outcrop zones (MT

Final Geotechnical Assessment Report 2.2-4 Troy Mine, Montana

DEQ 2001a). The monitoring employed during active mining would provide advance warning of deteriorating ground conditions in response to mining.

The operator or a third party would be responsible for monitoring device installation and data collection. Currently, much of the monitoring equipment is so advanced that mining companies often leave the rock mechanics programs to specialty firms, or at least have a third-party consultant oversee the installation and collection of data. Quality assurance and quality control protocols would be reviewed and authorized by the Agencies to maintain strict regulatory compliance and standards of practice. Revett would submit the results of the monitoring to the Agencies as part of the monitoring plan. These reports may be submitted on an annual, semiannual or quarterly basis depending on what phase of development the mine is undergoing.

EVALUATION ADIT DATA EVALUATION PLAN

This plan would be developed to provide the agencies with data that could not be obtained prior to construction of the evaluation adit. Data from the evaluation adit would be used to verify the hydrologic, geochemical, and rock mechanics data used in the analyses described in the FEIS. It would also be used to modify facility designs and the mine plan to keep impacts at or below the level described for Alternative V, or whatever alternative the Agencies permitted if a decision to permit was made. This plan consists of three components. The first is implementation of the evaluation adit portions of the Acid Rock Drainage and Metals Leaching Plan described above. This plan would provide the geological and geochemical data needed to insure that non-acid generating and non-metals leaching material was used for facility construction. The second plan would require the collection of hydrologic data during evaluation adit construction as described in the Water Resources Monitoring Plan above. This data would be used to better define where ground water is coming from, how much is being produced, and what the quality is to ensure the water treatment system operates as predicted and produces a discharge that would comply with MPDES permit limits (see Appendix D). A better understanding of the impacts of withdrawal of ground water on springs, seeps, and streams could be also obtained as well as the possible impacts the underground reservoir in the mine might have on those same springs, seeps, and streams. The Rock Mechanics Monitoring plan described above contains a description of the third component of the Evaluation Adit Data Evaluation Plan. The rock mechanics data from the evaluation adit would be used to modify the initial underground mine plan to prevent the occurrence of subsidence.

All evaluation adit data would be supplemented by data collected during mine construction and operation that would be used to further modify and refine facility designs and operations. If any data were substantially different from that anticipated and used in the analyses in the FEIS, all appropriate facility designs and mine plans would need to be modified and approved by the agencies to ensure that the impacts would be no greater than as disclosed in the FEIS. The modifications would be requested and processed as defined in the Metal Mine Reclamation Act (MMRA) (sections 82-4-337(4 through 7) MCA). If the changes to the permit were considered to be a major amendment, then the amendment would be subject to additional MEPA/NEPA

Final Geotechnical Assessment Report 2.2-5 Troy Mine, Montana

analysis and public participation. The analysis may be disclosed in either an Environmental Assessment or an Environmental Impact Statement depending upon whether or not there was the potential for significant impacts as a result of implementing the change. Either of these documents would tier to the FEIS for the Rock Creek Mine Project. If the significant impacts could not be mitigated to or below the level of the impacts displayed in the FEIS, then an additional EIS would be required. The project could not proceed beyond the evaluation adit construction phase without approval from the Agencies on the facility designs and mine operation plans as modified due to the results and analysis of evaluation adit construction data.

Final Geotechnical Assessment Report 2.2-6 Troy Mine, Montana

ENCLOSURE 3 SUBSIDENCE DATA FROM HARD ROCK MINES

Stope Ore Dimension Angle Angle Mining Dip Depth (m) Cave Draw Depth/ Mine Method Rock Type (deg.) (m) W L H (deg.) (deg.) Height Comments Reference Cambria Room and iron ore Jackson 300 30 20 Chimney cave. Unexpected. Boyum 1961 pillar formation Michigan Athens Sub level jasper with Plug type chimney care. Controlled 579 61 106 76 0 0 7.6 Vanderwilt 1946 Michigan caving 50m soil by faults and dykes. Unexpected. Negaunee Sublevel jasper with Plug type chimney care. Controlled 300 5 Crane 1931 Michigan caving 50m soil by faults and dykes. Unexpected. Room and pillar with Bier Hill pillar slate and 65 243 0 Chimney Cave. Unexpected. Vanderwilt 1946 Michigan recover jasper and backfill Michigan Iron jasper and caving 10 to 36 Controlled by geologic structure. Crane 1929 Mines slate Michigan jasper and caving 24 Controlled by geologic structure Crane 1929 copper mines slate Room and East Vulcan slate 10 Likely Unexpected. Rice 1934 Pillar sedimentary: Unexpected pillar failure over White Pine Room and shale and 792 7 118 1021m wide area. Maximum of Michigan Pillar sandstone 0.4m surface subsidence. 106 76 27 Not fault controlled Vanderwilt 1946 Copper Queen caving porphyry Kantner 1934 300 250 150 36 45 Fault controlled Joint and dyke controlled angle of Bafokeng North Room and pyroxenite, cave. Unexpected. 50 350 450 Kotz 1986 South Africa Pillar anorthosite Subsidence occurred due to pillar run (inadequately sized pillars) Joint and dyke controlled angle of Bafokeng South Room and pyroxenite, cave. Unexpected. 160 900 600 Kotz 1986 South Africa Pillar anorthosite Subsidence occurred due to pillar run (inadequately sized pillars) Isheming quartzite 65 55 Kantner 1934

Final Geotechnical Assessment Report 3.3-1 Troy Mine, Montana ENCLOSURE 3 SUBSIDENCE DATA FROM HARD ROCK MINES (Continued)

Stope Ore Dimension Angle Angle Mining Dip Depth (m) Cave Draw Depth/ Mine Method Rock Type (deg.) (m) W L H (deg.) (deg.) Height Comments Reference Queen Hill Sublevel limestone 100 25 Fault controlled Trischka 1934 Arizona caving Block El Teniente dacite 30 Kvapil 1989 caving Miami Block schist and 266 30 17 to 47 Usually fault controlled Fletcher 1960 Arizona caving conglomerate Johnson and San Manuel Block conglomerate 26 to 0 Usually fault controlled. 600 37 to 0 Soule 1963 Arizona caving monzanite Panek 1981 Angle of draw was defined by the Climax granite Block presence of tension cracks; however Molybdenum schist 274 152 152 5 30 Vanderwilt 1946 caving no vertical subsidence was Colorado gneiss measured beyond 50. Solbec schist Cut and fill 15 45 Likely Unexpected. Vongpaisal 1974 Quebec limestone 600 40 Kiruna Sublevel Toppling around glory hole resulted 60 Singh 1993 Sweden caving in flattened angle. 100 32 130 50 11 Gangesberg Sublevel 180 50 12 Toppling around glory hole resulted Hoek 1974 Sweden caving 240 50 27 in flattened angle. 300 50 31 United Verde Shrinkage porphyry, Controlled by geologic structure 60 40 to 43 Mills 1934 Arizona Cut and fill chlorite schist Limited control by geologic Mt Lyle Sublevel altered 20 structure North 1980 Australia open stope volcanics Likely Unexpected. norite, Creighton Block granite, 48 560 28 Dickhout 1963 Ontario caving gabbro San Giovanni Sublevel weak shale 75 15 Balia 1990 Italy caving Havelock Sublevel serpentinite, Angle of draw defined by tension 250 50 20 48 Heslop 1974 Asbestos caving chert cracks with little or no subsidence

Final Geotechnical Assessment Report 3.3-2 Troy Mine, Montana ENCLOSURE 3 SUBSIDENCE DATA FROM HARD ROCK MINES (Continued)

Stope Ore Dimension Angle Angle Mining Dip Depth (m) Cave Draw Depth/ Mine Method Rock Type (deg.) (m) W L H (deg.) (deg.) Height Comments Reference Asbestos Corp Sublevel Bl 20, peridotite 317 15 49 42 Vongpaisal 1974 caving Quebec Bell Asbestos Block cave peridotite 60 213 61 61 47 Vongpaisal 1974 100 16 Brown 1979 Gaths Mine Open stope Angle of draw defined by tension peridotite 41 158 25 Zimbabwe caving cracks with little or no subsidence 183 34 45 Heslop 1984 Brumleve 1987, King Block cave serpentinite 80 296 150 14 to 39 Heslop 1984 shale Jennifer Block cave arkosic 100 45 83 10 Obert 1962 sediments. Sublevel graphitic mica Angle of draw defined by tension Rajpura Dariba 70 8 32 Singh 1993 caving schist cracks with little or no subsidence Henderson rhyolite Block cave 750 274 -12 to 1 Joint controlled Brumleve 1981 Colorado granite Limited control by geologic 152 91 20 Open stope metta gabbro structure. Alaska Juneau 65 Cullen 1996 caving phyllite Toppling around glory hole resulted 213 122 25 in flattened angle. Nkana Sublevel quartzite Chimney cave. Goel and Page 12 13 Zambia stoping dolomite Controlled by geologic structure. 1982 Franklin limestone 100 0 Rice 1923 Sublevel gabbro Toppling around glory hole resulted Premier 300 10 Owen 1981 stoping kimberlite in flattened angle. Notes: Subsidence angles are is measured from a vertical line connecting the surface to the edge of the underground excavation. Angle of cave defines the area of complete destruction. Angle of draw defines the greatest extent of affected ground. In many instances the angle of draw is defined by tension cracks with little or no vertical displacement beyond the angle of cave. Where no distinction is made between angle of cave and angle of draw, the angle given in the literature is assumed to be angle of cave. deg Degree H Height L Length m Meter W Width

Final Geotechnical Assessment Report 3.3-3 Troy Mine, Montana