Destabilizing Effects of Chemical Weathering and Root Jacking on Sub-Tropical Volcanic Rock Slopes

James Kwong1, Ph.D., P.E., M. ASCE, Kealohi Sandefur2, P.E., M. ASCE and Shentang Wang3, M. ASCE, Ph.D., P.E.

1Princiapl, Yogi Kwong Engineers, LLC, 677 Ala Moan Bvld., # 710, Honolulu, HI 2Associate, Yogi Kwong Engineers, LLC, 677 Ala Moan Bvld., # 710, Honolulu, HI 3 Senior Geotechnical Engineer, formerly Yogi Kwong Engineers, LLC, 677 Ala Moan Bvld., # 710, Honolulu, HI

ABSTRACT: Forensic investigations of rock falls and rock fall mitigation studies in Hawaii and American Samoa revealed that unique combinations of volcanic lava flows lithologies, rock mass characteristics, chemical weathering and/or the presence of aggressive tree roots on sub-tropical volcanic hillsides resulted in unstable rock slope situations that can be independent of rock discontinuities attitudes and engineering geology. The resultant engineering geologic situations expose the natural hill slope to disturbing forces such as gravity induced stress cracking of weaker bearing rocks under large rocks, or severe seismic loading. These natural rock failure triggering mechanisms often resulted in periodic rockfalls to massive rock slides. Results of back analysis of rock fall paths and probable seismic loading effects on steep canyon walls are presented and discussed. Mitigation measures for some of the cases discussed are summarized.

INTRODUCTION

On Pacific Islands such as the Hawaiian Islands and American Samoa, basaltic lavas originated from oceanic “hot spots” generated dense to vesicular, to clinkery flows with tuff and cinders interbeds. Upon chemical weathering, the more advance degree of decomposition and weakening of the more porous lavas, such as clinkers and highly vesicular basalts, in contrast to the dense basalt flows, resulted in differential weathering on steep hill slopes or canyon walls, on the subsiding islands. Combined with potential significant seismic shaking, aggressive trees roots growth on steep tropical hill slopes, the contributing factors to potential rockfall hazards, often involve more than rock discontinuities attitudes, rock joint conditions, slope angles and shear strength of the rock discontinuities.

Page 1 PACIFIC ISLANDS GEOLOGIC SETTING

Hawaiian Islands The 15 volcanoes that comprise the eight principal Hawaiian Islands are the youngest in a linear chain of about 125 volcanoes that stretches for about 3,600 miles across the north Pacific Ocean. The assembly line that forms the volcanoes is driven by a "hot spot," or plume of hot material, deep within the Earth that partially melts to produce magma as it rises beneath the Pacific Plate. As the plate moves west- northwest, each volcano moves with it from its place of origin above the hot spot. Today the Pacific Plate migrates northwest at a rate of about 10 cm/year (Moore, 1987). When a new volcano forms, eruption rates gradually increase over a period of several hundred thousand years, attain their peak for perhaps 500,000 years, and then decline rapidly (Hawaii Volcano Observatory, 1995 to 2000). The end of the post- shield stage was followed by a period of erosion and subsidence, during which deep canyons may form along the flanks of the volcano. As the islands subside, fringing coral reefs grow. Typical Hawaii volcanoes grow into ponderous large island masses, too heavy for the underlying lithosphere to support without bending under the weight. As the Pacific Plate flexes down the volcanic island masses subsides, resulting in an island wide rise in sea level upon its shore.

American Samoa American Samoa and the Tutuila Island are part of a volcanic archipelago formed by occurrence of a “hot spot” under the westward moving Pacific Plate under the Pacific Ocean (Stearns, 1941 and National Park of Samoa, 2002). Tutuila Island erupted above the ocean about 1.5 million years ago. Since then the volcanic craters forming the island have weathered and eroded. Combined with continuing submergence of mid oceanic “hot spot” volcanoes, the island has been subsiding since its formation. The Pacific Plate dives under the Australian Plate at the 6-mile-deep Tongan trench, located between American Samoa and Fiji and Tonga, only about 100 miles south of Tutuila. The continual collision of these two plates and “hot spot” means American Samoa will continue to be subjected to earthquakes, tsunami, and volcanic eruptions. In Western Samoa, the last eruption was in 1905 and in the Manu’a islands (east of Tu’tuila) the last surface volcanic eruptions occurred in 1866 (National Park of Samoa, 2009). The most recent earthquake at the Tongan trench subduction zone that directly affected American Samoa occurred on September 29, 2009. The earthquake measured a magnitude 8.1 on the Richter scale. The earthquake was centered at Latitude 15.509 degrees South and Longitude 172.034 degrees west, approximately 120 miles south-west of Pago Pago, at a depth of 11.2 miles. The volcanic rocks described at the case study sites are principally of the alkali olivine basalt suite within tuff and cinder beds (McDougall, 1987).

EFFECTS OF WEATHERING AND STRESS INDUCED CRACKING

Investigation of several rock falls occurred in the last 10 years on the island of Oahu revealed a common set of hazardous engineering geological situations that contributed to rockfall from slopes that had not experienced rockfall onto residential

Page 2 areas for at least 20 to 40 years. In the cases investigated, large blocks of less weathered dense basaltic rocks were supported by extremely to highly weathered volcanic clinkers (Figure 1) or vesicular highly to moderately weathered basalt lavas on hill slopes (Figure 2).

FIG. 1. Case A: observed differential weathering and erosion of highly weathered clinker layers and dense slightly weathered basalt units. Note abundance of trees roots at this rockfall source area.

Volcanic clinkers generally form by de-gassing of lavas and the highly vesicular lava fragments may be weakly fused at contacts between the lava fragments. With chemical weathering particularly of the predominant feldspar minerals, the skeletal frame work of primary bonds between honey combed and partially weathered minerals weaken with time (Kwong, 2007). Gases in lavas contributed to the formation of highly vesicular basalt and the presence of blisters or voids in the lavas (Figures 2 and 3).

FIG. 2. Case B: large (6 m x 2 m x 3 m) overhanging dense slightly weathered basalt (center photograph) causing stress cracking in still intact highly weathered, highly vesicular basalt with voids (left and right photographs) at the base.

A similar situation was observed during a rockfall forensic study, where stress cracking and breakage of more weathered basalt under dense, slightly weathered basalt probably resulted in fallen rocks crashed into a house (Figures 3 and 12). The

Page 3 presence of tree roots near the sloping base of the dislodged rock was also observed.

FIG. 3. Case C: dense slightly to moderately weathered basalt over highly to moderately weathered basalt and probable lava blister (void), left photograph. Note close up of tree roots (top right insert) and fractured fragments of moderately to highly weathered basalt near lava blister upon rock dislodgement.

In high rainfall areas, these rock fall potential situations are exacerbated by combinations of root jacking, probable more intense chemical decomposition and development of widespread hair line cracks in the highly weathered volcanic rocks with time, and erosion of the friable highly to extremely weathered zones in the rock mass during rainstorms (Kwong 2007). Further, rock falls from such natural slopes were common in significant earthquakes.

SUSCEPTIBILITY TO SEISMIC DISTURBACNES

Evaluation of massive rockslides inside several canyons in the northeast portion of the Island of Hawaii (Figure 4) and widespread rock falls along the coastal cliff slopes of eastern Maui (Figure 8) after the October 16, 2006 earthquake that originated under the northwest coast of the Big Island (M=6.7) revealed the vulnerability of bedded and differentially weathered basaltic lavas to cracking, spall, and overtopping during a significant earthquake. It is also suspected that repeated significant seismic shaking over time contributed to the opening of pre-existing rock discontinuities and probably contributed to the formation of new cracks in highly weathered rocks. In this earthquake, no significant damage was observed inside the mostly unlined small (average 2.5 m diameter, part of a 21 miles system) irrigation tunnels in weathered to unweathered basalts, however, the rock slides resulted in debris blocking the tunnels through numerous entries from stream intakes and over flow tunnels (Figure 4).

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FIG. 4. Case D: Photograph of massive rock slides on 600 m high canyon walls in Waima Valley (top left), Koiawe Valley (top right) and Alakahi Valley (bottom) after the October 2006 earthquake.

Example of sustainable rockfall protection measures

During the emergency design-built efforts to design and repair the damaged stream intake at the Alakahi canyon, rock fall statistical analysis and finite element analysis using commercially available software programs were performed, and considering the site geologic engineering and observed stream characteristics, the following understanding of the Alakahi canyon intake repair site were developed:

1. Based on the canyon geometry at the Alakahi Intake, future large rock falls with unusually high kinetic energy will likely occur at mid stream to the west side (left side in Figure 4 bottom photograph), away from the existing buried intake. The 2006 earthquake released an almost 6 meters size rock that crashed through the concrete roof of the stream crossing tunnel landing on the west side of the stream crossing. A parametric study was performed to evaluate if the roof angle of a constructed rock fall canopy could be utilized to reduce rock fall impact energy. An example of the analysis print out is presented below (Figure 5).

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FIG. 5. Rock fall statistical analysis performed to assess risks of future rock fall relative to stream intake location.

2. A plane strain finite element program was used to back-calculate probable stresses and shear force distribution using known seismic loading magnitude from the M=6.7 earthquake, and the field-measured geo-mechanics rock mass conditions (Figure 6). The analysis confirmed the actual size, height and deformation observed of the spall in the canyon wall (37 m high, 61 m across, Figure 4, bottom photograph).

These analyses also provided an indication of the probable mechanics of vertical canyon wall failure and formation of the overhanging rock ledge. It is very likely that similar failure will recur with future significant earthquakes and continuing tropical weathering. Due to the narrow canyon, the height of the canyon walls, the anticipated size of the source rocks, potential use of rockfall set back, rockfall barrier, and rockfall control netting was considered not applicable to impractical.

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FIG. 6. Plot of back-calculated probable stresses and shear force distribution using known seismic loading magnitude from the M=6.7 earthquake.

3. The following design concept was built to restore and protect the stream intake:

a. Use of grouted rip rap to provide additional protection to future rock falls over the stream crossing tunnel roof, the repaired sand box, and the stream intake roof. Large fallen boulders at the site were used as shields and were piled as steep as practical (Figure 7). An excavator was used to place riprap boulders and pour concrete between the rocks to minimize worker exposure to continuing rockfall under the overhang and canyon base.

b. Reinforced concrete slab, 12-inch thick, was selected to replace the damaged tunnel crossing and was added onto the sand box roof (under the overhang). The Contractor elected to pre-cast the slabs in sizes that the excavator could lift. Most of the work would be conducted away from the precarious and dangerous overhang, and from the potential rock fall impact zone.

c. Keeping the Alakahi stream intake at the same location prior to be being buried by rock falls would not require construction of new stream diversion walls. Although the stream intake and sand box were covered with rock fall debris from the earthquake, analyses performed indicated that this location is the least susceptible to future damage caused by future high-velocity rock falls (Figure 7). A riprap protected galvanized steel liner plate tunnel was constructed for future maintenance access to this portion of the irrigation tunnel system.

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FIG. 7. The restored stream intake with protection from rockfalls using fallen rocks (left). Note analyses performed indicate the original location appears least risky (right, superimposed results from two analyses).

Earthquake induced rockfalls, Maui

Similar earthquake induced rockfalls occurred on southeast Maui during the same earthquake, in highly to slightly weathered basalts (Figure 8).

FIG. 8. Case E: Rockfalls during the October 16, 2006 earthquake, southeast Maui (top left, photograph courtesy of Sato & Associates); post earthquake emergency study revealed separation cracks common on bedded, weathered basalt lavas (bottom left), preliminary rockfall statistical analysis (right).

Page 8 During helicopter and repelling reconnaissance on some of the cliffs along the coastal road, opened rock fractures and fissures in highly weathered volcanic rocks were observed, and believed to be a part of past and recent earthquakes impact.

Preliminary rockfall statistical analysis indicated due to the potential very high kinetic energy and rockfall velocities from the assumed rock size and source area, and very limited space at the base of the high slopes, a rock fall set back or barrier would not be a feasible mitigation option (Figure 8). For this site, drape nets were used as an interim measure to control potential rockfalls.

TREE ROOTS JACKING

In relatively higher rainfall and tropical environment, large protruding tree roots, typically 1 inch to 2 feet in diameter or larger were observed on natural slopes (Figures 10 and 11). These aggressive tree roots were seen penetrating into rock masses, weaving in and out of rock outcrops, widening joints and fracturing rocks through root jacking on the steep slopes. Large trees were also observed growing from fractures in massive to widely fractured rock masses. It is believed that root jacking alone can result in toppling of large blocks of rocks.

As an example, Case F, in American Samoa, 7 mature trees were present at the top of an approximately 10 m high rock columns, with roots jacking and separating major rock joints (Figure 10). The base of the dense basalt rock columns appeared to be a bed of much weaker highly weathered, friable, clinkers or volcanic breccias. The engineering geologic situation at this site appears similar to other rockfall source areas observed on Oahu, described above.

FIG. 10. Note major opened fissure with 7 trees at top of rock column (left), and highly weathered clinkers at base of exposed rock column (right).

Some examples of aggressive tree roots jacking in potential rockfall source areas are shown in Figure 11.

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FIG. 11. Aggressive tree roots on weathered basalt hill slopes: on Oahu (top left), other photographs are taken on steep coastal slopes on Tutuila.

COMPARISON OF ROCKFALL PATHS AND ROCKFALL ANALYSIS RESULTS

Forensic evaluation of rock fall paths and back analysis of rock fall source areas, fall paths, and impact areas using commercially available rock fall statistical analysis rock fall program, revealed comparable field and statistical simulation results based on three cases evaluated.

At the Case A location, Kwong (2007) discussed rockfall statistical simulation can be used to evaluate probable rock fall source area locations. Subsequently forensic studies at Cases B and C locations found that back analysis using commercially available rockfall statistical analysis program, surveyed rock fall paths, estimated boulder sizes and unit weights, selected fall path surface coefficients of restitution, boulder source area identified in the field, yielded results that are consistent with the fall path and location of boulders impact observed on existing buildings (Figures 12 and 13).

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FIG. 12. Case B: Rockfall source area near Figure 2 location (right). Rockfall statistical analysis fall path (left) was consistent with observed actual location of boulders impact to existing building.

FIG. 13. Case C: Rock fall source location shown in Figure 3. Hill side surface conditions shown on left insert, right insert: boulder entered existing house.

Page 11 CONCLUSIONS

Due to basaltic rocks lithologies, effects of differential weathering, and the vulnerability of the Hawaiian and American Samoan islands to seismic shaking and tree roots jacking, it is important that evaluation of rockfall potentials include a detailed field reconnaissance of potential source areas for rockfalls. The approach of extrapolating rock discontinuities measurements and mapped rock conditions at selected locations on a slope will not likely be adequate for weathered basaltic hill slopes.

In the cases studied, where practical, provision of a setback, barrier, or rock fall buffer area between steep rocky hill slopes and residential or infrastructure development can be a simple solution to providing rock fall protection, however such provision is not common in planning and design. Forensic studies also indicated depending on the source area boulder sizes and slope geometry and conditions, rock fall set back may not be practical, particularly for very large boulders, and appropriately designed and installed netting, rock fall shelter or relocation of infrastructure may need to be considered.

REFERENCES

Daly, R. A., 1924. The Geology of American Samoa, Carnegie Inst., Pub. No. 340, P.95-145.

Hawaii Volcano Observatory (1995 to 2000). Volcano Watch by the U.S. Geological Survey / Hawaiian Volcano Observatory dated September 8, 1995; September 15, 1995; September 22, 1995; and September 29, 1995. Last Update: 12/16/00.

Kwong, J. (2007). “Time dependent soil and rock movements on sub-tropical volcanic hillsides in Hawaii: geologic engineering considerations and planning implication”. 1st North American Landslide Conference, 2007, Vail, Colorado.

McDougal, Ian, 1987. “Age and Evolution of the Volcanoes of Tutuila, American Samoa”, Pacific Science (1985), vol. 39, no. 4. Pp. 311 – 320.

Moore JG (1987) Subsidence of the Hawaii Ridge. In: Decker RW, Wright TL, Stauffer PH (eds) Volcanism in Hawaii. Washington, US Government Printing, Office US Geological Survey Professional Paper 1350, pp 85-100.

National Park of American Samoa, 2009. Natural History Guide to American Samoa, 3rd Edition, P. Craig Editor. 130 pages.

Stearns, H.T., 1941. Geologic Map of Tutuila Island.

Stearns, H.T., 1944. Geology of The Samoan Islands, Bulletin of the Geological Society of America, Vol. 55, PP. 1279-1332, November, 1944.

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