APPENDIX P4.10 Background Information for Section 4.10, Geology and Soils

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1 Central Link Light Rail Transit Project Final Supplemental Environmental Impact Statement North Link APPENDIX P4.10 Background Information for Section 4.10, Geology and Soils

3 Appendix P4.10 Background information for Section 4.10, Geology and Soils Geologic units Various geologic units are encountered along the corridor alternatives. Very few geologic units have precise boundaries or contacts. The geology of an area can change drastically both horizontally and vertically within a few feet or, in some instances, can remain fairly consistent for hundreds of feet. The geologic units younger than Vashon-age glacial till have not been glacially overridden. The Vashon-age glacial till and the older units have been glacially consolidated and are typically very dense or hard. Modified Land (Mc and Mf) The term modified land is used to describe surficial geologic conditions that have been modified by human activities such as, but not limited to: cutting (Mc), filling (Mf), grading, leveling, sluicing, and shoreline protection. Fill material is usually composed of glacial soils or alluvium from different locations and may consist of clay, silt, sand and/or gravel. Dumped rock, construction debris, and boulders may also be present. Locally, some effort at compaction may have been made during placement of these fills, and their relative density varies widely. The engineering properties of fill can be very different from location to location. Alluvium (Ha) Alluvial soils were transported and deposited by water in streams, rivers, and creeks. They are typically comprised of silt and fine to medium sand, but the size of the particles in a particular deposit depends on the velocity of the water at the time of deposition. High velocity alluvium may include coarser materials, such as medium to coarse-grained sand, gravel, cobbles, and boulders. Fine-grained soils such as silt and fine sand are low-velocity alluvium. Recessional Outwash (Vr) Recessional outwash was deposited by meltwater streams emanating from retreating glaciers during the last episode of glaciation. This unit is typically found directly overlying glacial till. It has not been overridden by glacial ice and is usually medium dense. In composition, it ranges from silty fine sand to clean coarse gravel with occasional cobbles and boulders. Due to the fluvial nature of deposition of recessional outwash, these materials are generally stratified. Glacial Till (Vt) Glacial till typically consists of a heterogeneous mix of gravelly sand with scattered cobbles and boulders in a clay/silt matrix. It is very dense and is locally referred to as hardpan. The predominant glacial till encountered in the area is Vashon-age glacial till. However, glacial till from previous glaciations may be encountered at depth along some portions of the corridor. Glacial till typically exhibits high shear strength, low compressibility, and low permeability characteristics. It is generally considered the most competent bearing soil in the area, aside from bedrock. Temporary excavations in glacial till will generally stand near vertical. Excavation can be difficult due to its compact nature. Advance Outwash (Ve) Glacial advance outwash soils were deposited by meltwater streams emanating from advancing glaciers. Advance outwash is similar in composition to recessional outwash, except it has been glacially over-ridden. Advance outwash soils typically range from silty fine sand to coarse gravel with cobbles and occasional boulders. Internal stratification of these deposits is the result of the fluvial environment of deposition. This unit is regionally important as an aquifer and is locally referred to as Esperance Sand. Because the advance outwash has been glacially overridden, it is generally dense to very dense. March 2006 P North Link Final SEIS Appendix P4.10: Geology and Soils

4 Glacial Lacustrine (Vl) Underlying the advance outwash, glacial lacustrine deposits (also known locally as Lawton Clay) are typically encountered. Most of these fine grained soils were deposited in glacial meltwater lakes and were subsequently covered with coarser outwash before being overridden and densely consolidated by the ice. The glacial clays and silts typically encountered in the project area range from massive to laminated and are frequently blocky or fractured. Locally they may be distorted or sheared, thereby having a lower mass strength than the surrounding soil. Glacial Marine Drift (Vd) These sediments result from the deposition of suspended, sand, and fine-grained material in proglacial lakes or marine water with variable amounts of clastic debris from melting icebergs, floating ice, and gravity currents. These deposits generally consist of poorly graded granular material within a clayey matrix (a clayey diamict). Composition varies from very dense, gravelly, silty sand with a trace of clay to silty, clay-like sand and hard, silty clay with varying percentages of sand and gravel. This unit can contain shell fragments, and occasional cobbles and boulders. Pre-Vashon Deposits (Pv) Material from previous glaciations, such as older gravel, sand, clay, and gravelly clay were encountered underlying the Vashon deposits. These deposits have been glacially consolidated and have properties similar to those of the Vashon-age deposits. Steep Slope and Landslides Hazard Areas Steep slope areas are generally defined as those that rise at an inclination of 40 percent or more with a vertical change in elevation of at least 10 feet. There are many areas of steep slopes along the I-5 corridor. Figures P and P show landslide hazard areas and other geologic critical areas. Generally, Landslide Hazard areas can be defined as areas that include: Any area with a combination of: Slopes greater than 15 percent Impermeable soils (typically silt and clay) frequently interbedded with granular soils (predominately sand and gravel); and Springs or groundwater seepage. Any area which has shown movement during the Holocene epoch (from 10,000 years ago to present) or is underlain by mass wastage debris of that epoch. Any area subject to instability as a result of rapid stream erosion, stream bank erosion, or undercutting by wave action. Any area that shows evidence of, or is at risk from, snow avalanches. Any area located on an alluvial fan that is presently subject to, or potentially subject to, inundation by debris flows, or deposition of stream transported sediments. Areas of known landslides are included in the mapped landslide hazard areas. Some of these areas have a history of repeated landsliding while others do not. Frequently, these areas of repeat landsliding are located within areas mapped as steep slope hazard areas. Landslide deposits and landslide scars are indicators of historical or past landslides. North Link Final SEIS P March 2006 Appendix P4.10: Geology and Soils

5 EI_T SEA / Sound Transit / NLink2005Final SEISNewPrefAlt / Fig P4.10-1_SegA_GeologicCriticalAreas / / lw LEGEND - A1.1 12th Avenue NE Tunnel A2.1b 8th Avenue NE (West Portal) A2.1c 8th Avenue NE (East Portal) - Revised Landfill LIquefaction Zone Steep Slope/ Landslide Hazard Coal Mine Hazard 99 WALLINGFORD AVE NE TPSS N 85TH ST N 80TH ST 5 1ST AVE NE A1.1, A2.1b & A2.1c 2ND AVE NE Northgate (Preferred Alternative and Options E1, C2, P C3 & C4) NE 100TH ST 5TH AVE NE NE 85th Street TPSS NE NORTHGATE WAY ROOSEVELT WAY NE Maple Leaf Reservoir NE 103RD ST NE 92ND ST 15TH AVE NE LAKE CITY WAY NE The holder of this map has a limited, non-exclusive license to reproduce the map, solely for purposes which are: a) internal or personal; b) non-commercial. All other rights reserved. 99 Green Lake NE 50TH ST NE 47TH ST NE 45TH ST Roosevelt (Option C) A2.1b 5 A2.1b & A2.1c 8TH AVE NE A2.1c East Portal Vent 12TH AVE NE Green Lake Reservoir Roosevelt (Preferred Alternative and Option A) A1.1 15TH AVE NE 20TH AVE NE NE 75TH ST NE 65TH ST N 0 1,000 2,000 FEET Tunnel Elevated At Grade Retained Cut-and-Fill and Cut-and-Cover Tunnel P Remote Vent/TPSS Park-and-Ride Figure P Segment A Geologic Critical Areas

6 B3.D & B3.G E1_T SEA / Sound Transit / NLinkFinal SEISNewPrefAlt / Fig P4.10-2_ Seg B_GeologicCriticalAreas / / lw The holder of this map has a limited, non-exclusive license to reproduce the map, solely for purposes which are: a) internal or personal; b) non-commercial. All other rights reserved. PREFERRED ALTERNATIVE Capitol Hill - University of Washington B1.A First Hill - 15th Avenue NE B1.D LEGENDFirst Hill - Montlake B1.G First Hill - West Tunnel B3.D Eastlake - Montlake B3.G Eastlake - West Tunnel B4.D Capitol Hill - Montlake B4.G Capitol Hill - West Tunnel Elliott Bay ROY ST MERCER ST DENNY WAY WESTERN AVE Landfill LIquefaction Zone Steep Slope/ Landslide Hazard 99 N 45TH ST N 35TH ST WESTLAKE AVE N Westlake University Street West Tunnel Lake Union Convention Place (Optional) PINE ST PIKE ST N 40TH ST Brooklyn Southwest Campus Option A (one station) Option B (two stations) EASTLAKE AVE E SR 520 Vent Shaft/ TPSS EASTLAKE AVE E Harrison E OLIVEWAY Broadway Option B4.D & B4.G BROADWAY E 5 B3.G B1.A, B1.G, B3.G & B4.G 12TH AVE NE BROOKLYN NE (Options A & C) (Preferred and Options B & D) 10TH AVE E First Hill B1.A, B1.D & B1.G B3.D B1.D, B4.D Portage Bay 15TH AVE E E JOHN ST Nagle Option E PINE ST E PIKE ST 15TH AVE NE Montlake/ Rainier Vista B1.D 520 Capitol Hill (Preferred ) Pacific E LYNN ST E MADISON ST NE 50TH ST NE 47TH ST NE 45th (Options A & B) 19TH AVE E E ALOHA ST B3.D B4.D E UNION ST MONTLAKE AVE NE B1.D, B3.D & B4.D BOYER AVE E 23RD AVE E University of Washington (Preferred) 520 Remote Vent/TPSS: Preferred and Option C Option A Option B N 0 1,000 2,000 FEET Tunnel Elevated At Grade Retained Cut-and-Fill Optional Route Remote Vent/TPSS University of Washington Central Link Initial Segment Figure P Segment B Geologic Critical Areas

7 Generally, steep slope and landslide hazards are of more concern for the at-grade and elevated portions of the corridor. The tunnel alternatives are typically only affected by steep slopes and landslide hazards near the tunnel portals, vent shafts, stations, and construction of cut and cover sections. Erosion Hazards Erosion hazards are defined as those areas containing soils that may experience severe to very severe erosion from construction activity. The susceptibility to erosion is generally a function of soil type, topography, occurrence of groundwater seepage or surface runoff, and the built environment. In general, the erosion hazard will be more serious for the at-grade portions of the route in relatively non-developed steep areas as compared to tunnel sections or elevated sections in urban areas. According to the King County Area Soil Survey (Snyder et al. 1973), a majority of the segments are classified as urban development, which does not receive a rating. Along the remainder of the segments in the area covered by the soil survey, the soil types and topography are such that the erosion potential of the soils along the bulk of the corridor is ranked as slight. Seismic Hazards General Seismic hazard areas are generally defined as those areas subject to severe risk of earthquake damage as a result of seismically induced settlement, ground rupture, landsliding or soil liquefaction. The Uniform Building Code (ICBO 1997) defines the Puget Sound region as Seismic Zone 3, which represents an area susceptible to moderately high seismic activity. Consequently, moderate to high levels of shaking should be anticipated during the design life of the proposed project. For comparison, Alaska and California are within Seismic Zone 4, which are susceptible to higher degrees of seismic activity. Since the 1850 s, over 25 earthquakes of magnitude 5.0 (Richter Scale) and greater have occurred in the Puget Sound and north-central Cascades region. Four events may have exceeded magnitude 6.0. In addition to the recorded historic earthquakes, paleo-geologic evidence has been discovered over the last 10 years which suggests that a large (greater than magnitude 7) earthquake occurred about 1,100 years ago on a fault (Seattle Fault) that roughly parallels Interstate 90 and passes below downtown Seattle near the Pioneer Square area (Johnson et al. 1994). Paleo-geologic evidence also suggests that large subduction zone earthquakes (magnitude 8 to 9) can occur along the Washington and Oregon coast. The paleo-geologic record suggests five or six subduction zone events may have occurred over the last 3,500 years; the most recent being about 300 years ago (Yamaguchi et al., 1997). Liquefaction When shaken by an earthquake, certain soils lose strength and temporarily behave like a liquid. This phenomenon is known as liquefaction. The seismically induced loss of strength can result in failure of the ground surface that is most typically expressed as lateral spreads, surface cracks, settlement and sand boils. A structure can sustain substantial damage during a large seismic event if it is supported in, or on, a soil susceptible to liquefaction. Seismically induced liquefaction typically occurs in loose, saturated, sandy material commonly associated with recent river, lake, and beach sedimentation. In addition, seismically induced liquefaction can be associated with areas of loose, saturated fill. March 2006 P North Link Final SEIS Appendix P4.10: Geology and Soils

Appendix F4.11 Geologic Unit Summaries, Hazard Areas, and Boring Locations

Appendix F4.11 Geologic Unit Summaries, Hazard Areas, and Boring Locations Appendix F4.11 Geologic Unit Summaries and Hazard Areas TABLE F4.11-1 Summary of Geologic Units and their Engineering Properties