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Geotechnical Report Gravina Access Project

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Geotechnical Report Gravina Access Project Phase I Geotechnical Report Gravina Access Project Agreement No: 36893013 DOT&PF Project No: 67698 Federal Project No: ACHP-0922(5) Prepared for: State of Alaska Department of Transportation and Public Facilities 6860 Glacier Highway Juneau, Alaska 99801 Prepared by: Shannon & Wilson Under contract to: HDR Alaska, Inc. 712 West 12th St. Juneau, AK 99801 May 2000 Phase I Geotechnical Report, Gravina Access Project EXECUTIVE SUMMARY This preliminary geotechnical report presents the results of our review of existing literature and limited geotechnical engineering analysis for the Gravina Access Project, Ketchikan, Alaska. The purpose of the literature review was to identify the likely subsurface conditions at various potential water crossing locations. Preliminary geotechnical engineering studies were then conducted to develop conceptual foundation design recommendations and assist the design team in narrowing the crossing alternatives to the more promising alignments. Because most existing data is near the shoreline and there is essentially no drilling data in the middle or deep parts of the Tongass Narrows, the recommendations we have provided should be recognized as being conceptual for use in planning and feasibility studies for the project. For this preliminary study, four subsurface cross-sections were developed to represent our rough estimate of the anticipated depths and possible subsurface conditions that could be encountered in the various water crossing corridors. The sections are situated to the North, directly across from downtown Ketchikan and to the south crossing Pennock Island where two water crossing are required to complete the hard link. These profiles are conceptual at this stage and will have to be refined with follow-on exploratory studies. The profiles generally show one of three conditions: 1) a thin veneer of sediments over bedrock, 2) a thick package of glacial drift material overlying bedrock, or 3) loose sands overlying glacial drift and bedrock. The loose sands that occur near Carlanna Creek near downtown Ketchikan would present the most challenge in designing and constructing foundations for the bridge from a geotechnical view. A fault may exist within the Tongass Narrows and in the channels on either side of Pennock Island. While others are considering tunnel or other crossing methods, the focus of our preliminary analyses is on a bridge crossing. Preliminary design capacities for bridge pier foundations followed by a brief discussion of construction considerations are therefore presented to aid the designers in establishing rough estimates of pier diameters, lengths, and numbers at the various crossings. Conceptual design recommendations are also presented for the abutments, for the approach fills, and retaining structures. Seismic considerations are discussed at the end of our recommendations. In the offshore areas of the narrows, foundations to support the bridge columns would penetrate up to 200 feet (60 m) of water, and various thicknesses of loose and dense soils, and derive foundation support in bedrock or the dense glacial soils. Generally bridge foundations are tentatively envisioned to be drilled piers with diameters in the order of 4 to 12 feet (1.2 to 3.6 m). Because much of the construction work must be carried out on floating or jacked-up platforms and in deep water, construction of the piers will likely be carried out in the wet or without i May 2000 Phase I Geotechnical Report, Gravina Access Project dewatering using “wet” construction methods. As the bridge extends out from the shorelines, foundations for abutments can vary depending upon local topography and subsurface conditions. If competent soils or rock are present, either the large diameter piers or embankments and spread footings can be used. Once the more feasible three or four corridors are identified from this Phase 1 study, we recommend that exploratory borings be conducted in each area to refine our interpretations of the preliminary subsurface conditions we have described herein. At each of the three or four crossings, exploratory drilling work should consist of a minimum of two offshore borings on either side of the inferred fault and two onshore borings (one on each side) in areas where bedrock is not exposed on the surface. After a preferred alignment is chosen, for final design, we recommend additional explorations at each anticipated bridge pier location and in areas where special structures are planned. The preliminary engineering recommendations contained in this report are based on site conditions as they were extrapolated and interpreted from limited existing literature and are assumed to be typical of the subsurface conditions throughout the site. It is possible (and likely) that some of our interpretations of offshore sediment thickness and material properties may have to be adjusted to accommodate different conditions. These changes may also lead to some changes in our preliminary recommendations. ii May 2000 Phase I Geotechnical Report, Gravina Access Project TABLE OF CONTENTS 1.0 INTRODUCTION ...................................................................................... 1-1 1.1 Purpose & Scope..................................................................................................1-1 1.2 Authorization .......................................................................................................1-1 2.0 SITE AND PROJECT DESCRIPTION................................................... 2-1 2.1 Site Conditions.....................................................................................................2-1 2.2 Geography ............................................................................................................2-1 2.3 Project Description..............................................................................................2-2 3.0 PRIOR EXPLORATIONS AND TESTING............................................ 3-1 4.0 GEOLOGICAL CONDITIONS................................................................ 4-1 4.1 Regional Geology.................................................................................................4-1 4.2 Regional Tectonics...............................................................................................4-1 4.3 Local Bedrock Geology .......................................................................................4-2 4.4 Local Unconsolidated Deposits...........................................................................4-3 5.0 SUBSURFACE CONDITIONS................................................................. 5-1 5.1 Soils .......................................................................................................................5-1 5.2 Bedrock.................................................................................................................5-2 5.3 Local Faulting......................................................................................................5-3 6.0 FOUNDATION CONSIDERATIONS ..................................................... 6-1 6.1 Foundation Types ................................................................................................6-1 6.2 Drilled Piers..........................................................................................................6-1 6.2.1 Pier Bearing Capacities.........................................................................6-2 6.2.2 Pier Skin Friction ..................................................................................6-2 6.2.3 Pier Lateral Resistance..........................................................................6-2 6.2.4 Pier Settlements .....................................................................................6-3 6.2.5 Pier Construction...................................................................................6-3 6.3 Abutment Support...............................................................................................6-4 6.3.1 Footing Bearing Capacities...................................................................6-4 6.3.2 Abutment Earth Pressures and Lateral Resistance..............................6-5 6.4 Fill Considerations...............................................................................................6-6 6.4.1 Material Types........................................................................................6-6 6.4.2 Fill Placement........................................................................................6-6 6.4.3 Embankment Slopes..............................................................................6-6 6.4.4 Slope Protection.....................................................................................6-7 6.5 Retaining Structures............................................................................................6-7 6.6 Seismic Design Criteria .......................................................................................6-8 6.6.1 Seismically-induced Geologic Hazards.................................................6-8 6.7 Landslides.............................................................................................................6-9 7.0 ADDITIONAL EXPLORATIONS AND STUDIES ............................... 7-1 8.0 LIMITATIONS........................................................................................... 8-1 9.0 REFERENCES............................................................................................ 9-1 iii May 2000 Phase I Geotechnical Report, Gravina Access
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