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 Geologic Unit (Map Symbol) Description General Constructability Modified land (hatching) or (m) a Fill and/or graded natural that obscure or alter the original deposit. Variable Very soft to stiff or very loose to dense Density/ Hardness Strength Permeability Liquefaction Potential b Potentially low Variable Potentially high Wetland (Qw) Mass wastage (Qmw) Landslide (Qls) Tideflat (Qtf) Lake (Ql) Vashon recessional outwash () Vashon recessional lacustrine (l) Organic-rich sediment, peat, and fine-grained alluvium, poorly drained and intermittently wet. Not all have been mapped. Colluvium, soil, landslide debris, and organic matter. Common below springs where peaty are also present. Mapped on steep slopes. Deposits, both mapped and unmapped, include abundant landslides up to 500 feet long. Blocks of surficial transported downslope en masse by gravity. Numerous unmapped areas of both landslide and related mass-wastage occur along slopes. Landslide terrain often includes benches that slope back into the hillside and have wetlands and peat. Silt, sand, organic sediment, and detritus, with some shells, historically exposed in broad coastal benches at low tide and now covered with fill. Silt and clay with local sand layers, peat, and other organic sediments deposited into Lake Washington. Most mapped areas are lake bottom sediments exposed when Lake Washington was lowered in 1916. At many locations, the lake are thin and overlie a dense substrate. Commonly capped by fill to improve building sites. Layered sand and gravel. Deposited in outwash channels of melting glaciers during the ice retreat. Also includes that accumulated in or adjacent to recessional lakes. Deposits less than about 3 feet thick not shown on map. Thinly layered silt and clay with local sand layers, peat, and other organic sediments, deposited in slow-flowing water and temporary lakes. Low Very soft to medium stiff or very loose to medium dense Low Loose to dense and soft to stiff; variable degree of consolidation depends on material in colluvium and its coherency Low Very loose to very dense or soft to hard; variable degree of consolidation depends on material coherency Low Very loose to dense or very soft to stiff Low Very soft to medium stiff or very loose to medium dense Potentially low Potentially low Potentially low Potentially low Potentially low Medium Loose to dense Low t o medium Variable Potentially high Variable Potentially high Variable Potentially high Variable Potentially high Low Potentially high Low Potentially high Low to medium Very soft to stiff Medium Low Medium Draft EIS F4.11-1 December 2008

Appendix F4.11 Geologic Unit Summaries and Hazard Areas TABLE F4.11-1 Summary of Geologic Units and their Engineering Properties Geologic Unit (Map Symbol) Description General Constructability Vashon till () Vashon advance outwash () Lawton Clay of Mullineaux and others (1965) () Deposits of pre-fraser glaciation, undifferentia ted () Deposits of pre-fraser glaciation age, nonglacial (n) Deposits of age, finegrained () Deposits of age, coarse grained () Deposits of age, glacial (Qpog) Compact mixture of silt, sand, and gravel, glacially transported and deposited under ice. Contains layers of sand and gravel and cobbles are common. Upper 3 feet of till generally weathered and only medium dense to dense. Sand and gravel deposited by streams from advancing ice sheet. Silt lenses locally present in upper part and are common in lower part. Thinly layered to a mixture of silt and clay with scattered pebbles deposited in lowland proglacial lakes. Layers of sand, gravel, and silt, with and mixtures. Locally divided into: Coarse grained (c) Sand and gravel, clean to silty, with some silt layers. Fine grained (f) Silt and clay, may have sand layers. High Dense to very dense; sand is commonly less dense High Dense to very dense High Very stiff to hard High Very dense and hard Sand, gravel, silt, clay, and organic. High Very dense and hard Density/ Hardness Strength Permeability Liquefaction Potential b High Low Low High c High Low High Low Low High Variable Low High Variable Low Silt and clay, may have sand layers. High Hard High Low Low Sand and gravel, clean to silty, with some silt layers. High Very dense High c High Low Silt, sand, and gravel. High Very dense and hard High Variable Low F4.11-2 Draft EIS December 2008

Appendix F4.11 Geologic Unit Summaries and Hazard Areas TABLE F4.11-1 Summary of Geologic Units and their Engineering Properties Geologic Unit (Map Symbol) Description General Constructability Deposits of age, finegrained glacial Density/ Hardness Strength Permeability Liquefaction Potential b Silt and clay, may have sand layers. High Hard High Low Low (Qpogf) Deposits of Till-like material, but finer grained and with fewer gravel. High Very dense High Low Low and hard age, glacial diamict (Qpogd) Deposits of Sand, gravel, silt, clay, and organic. High Very dense High Low Low and hard age nonglacial () Deposits of Silt and clay, may have sand layers and peat. High Hard High Low Low age finegrained nonglacial (f) a Areas mapped as artificial fill are locations where fill placement is relatively extensive and is likely to be thick enough to be of geotechnical significance (greater than about 6.5 feet thick). Liquefaction depends in part on density of the material and the groundwater table elevation. These ratings assume a shallow groundwater condition. High strength unless cut vertically below the water table, then potentially low to medium strength. b c Notes: The descriptions and density/hardness are based on the following sources: Troost et al. (2005), Troost and Wisher (2006). The terms low, medium, and high were determined based on professional opinion from experience with the soil types. Hazard susceptibility was determined based on criteria in the cities of Seattle, Bellevue, and Redmond Code; King County Code; and professional opinion. Draft EIS F4.11-3 December 2008

1ST AVE S Qb 2ND AVE S 5TH AVE S International District/ Chinatown Station Qpog 12TH AVE S Qvi S JACKSON ST Qi Qal GOLF DR S A1 ALASKAN WAY S Qob Qpogf Qp " 90 S E A T T L E Qtf 6TH AVE S S HOLGATE ST Qpdf l M L KING JR WAY S Qpogd Lake Washington 5 Qvi Qpogd Qal S LANDER ST S FOREST ST Qpogd S MC CLELLAN ST Qi Qpog Source: Data digitized from GeomapNW at the University of Washington (2007); Sound Transit (2007) and King County (2006). 15TH AVE S l c 23RD AVE S Qp To Sea-Tac Airport Route at Grade Elevated Route Route in Retained Cut Route in Tunnel " Traction Power Substation Proposed Station Central Link Alignment and Station 0 0.25 Miles Exhibit F4.11-1 Surficial Geology Segment A - Seattle F4.11-4

90 Qpovt Ql Ql " Lake Washington Qob Ql 72ND AVE SE n 77TH AVE SE SE 27TH ST l Qpogf Qpogd Ql n Qp 80TH AVE SE Qw A1 Mercer Island Presbyterian Church SE 40TH ST Qp l M E R C E R I S L A N D Qw Qal l Qw Ql Qal Qpogf Qal Mercer Is United Qp Qal Methodist Church Qpogf Qal Mercer Island Qw f l l Qal Source: Data digitized from GeomapNW at the University of Washington (2007); King County (2006). Qw Qpogf Qal Ql Qvi n f SE 36TH ST l l l Segment A Ql " Route at Grade Elevated Route Route in Retained Cut Route in Tunnel " Traction Power Substation Proposed Station City Limits 0 0.25 Miles Exhibit F4.11-2 Surficial Geology Segment A - Mercer Island F4.11-5

Proposed ÂΡ l Grace Lutheran Church 405 Segment Lake Bellevue l Segment C D Kelsey Creek m Sturtevant Creek B E L L E V U E Qyal Segment C Mercer Slough East Mercer Slough West Sturtevant Creek Wilburton ÂΡ Proposed Kelsey Creek Lake Washington B E A U X A R T S South Bellevue ÂΡ Qw Mercer Slough East Segment Segment A 90 B 90 405 Source: Data from City of Bellevue (2005) and King County (2006). Route at Grade Elevated Route Route in Retained Cut Route in Tunnel Proposed Station City Limits 0 0.25 Miles Exhibit F4.11-3 Surficial Geology Segments B and C F4.11-6

Qtu Qoal Qyal R E D M O N D Qf 405 ¾À 908 Redmond Qls Qls Bear Creek Qyal Proposed ÂΡ Â Ρ Qoal ¾À 202 Qob ¾À 520 ÂΡ Segment E Segment D Overlake Transit Center Lake Sammamish Qoal Segment D Segment C l Proposed ÂΡ B E L L E V U E Source: Data from City of Bellevue (2005) and King County (2006). m Overlake Bellevue Christian Reformed Church Qob Qyal Qob Lake Sammamish Qob Route At Grade Elevated Route Route in Retained Cut Route in Tunnel Proposed Station Maintenance Facility City Limits 0 0.5 Miles Exhibit F4.11-4 Surficial Geology Segments D and E F4.11-7

5 To University of Washington B E L L E V U E S E A T T L E L a ke Wa sh in g t o n Segment C Segment A 90 Segment A M E R C E R I S L A N D 5 To Sea-Tac Airport Liquefaction Hazard Type (City of Bellevue including Mercer Island) Low to Moderate Peat Moderate to High Bedrock Transition Zone Liquefaction Hazard Area (City of Seattle) Landslide Hazard Area (City of Seattle) Steep Slope Greater than 40% (City of Seattle & King County) Route at Grade Elevated Route Route in Retained Cut Route in Tunnel F4.11-8 Proposed Station Central Link Alignment and Station City Limits 0 1 Miles Source: Data from City of Seattle (2003), City of Bellevue (2003), King County (2006), and Sound Transit (2007). Exhibit F4.11-5 Geologic Hazard Areas Segment A

908 À ¾ R E D M O N D 405 202 ¾ À Segment E Segment D Segment C Segment D 520 À ¾ Lake Sammamish C Segment B E L L E V U E 405 ke La n to ng hi as W Liquefaction Hazard Type Low to Moderate Moderate to High Bedrock Peat Transition Zone Draft EIS December 2008 90 0 0.5 Mile Source: Data from City of Bellevue (2003), City of Redmond (2007), and King County (2003 and 2006). Landslide Hazard Area (City of Redmond) Landslide Hazard Area (King County) Steep Slope Greater than 40% (City of Bellevue & King County) F4.11-9 Route at Grade Elevated Route Route in Retained Cut Route in Tunnel Proposed Station City Limits Exhibit F4.11-6 Geologic Hazard Areas Segments B,C,D, and E

NE 6TH 5 University of Washington 405 ¾À 202 ( ¾À 908 ( R E D M O N D ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ¾À 202 ¾À 520 ( ¾À 520 Segment E Segment D ( (( ( ( L a k e U n i o n Segment D Segment C ( ( ((( ((((( ( (( (( ((( ( (( (( ( ( ((((((( ( ( (( (( ( ( ( (( ( ( ( Elliot Bay ( (( ( ( (( ( ( ( ( ( Segment A 5 S E A T T L E ( ( ( ( (( (( ( (( ( ( To Sea-Tac Airport 90 L a k e W a s h i n g t o n ( M E R C E R I S L A N D Source: Data from GeoMapNW at the University of Washington (2007), Sound Transit (2007), and King County (2006). Segment C ( ( ( Segment ( A ( ((( ( ( ( ( (( ( (( (( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( B E L L E V U E 90 405 L a k e S a m m a m i s h ( Existing Soil Boring Within the Alternative and Maintenance Facility Right-of-Way East Link Proposed Alternative Central Link Alignment and Station 0 1 Mile Exhibit F4.11-7 Existing Soil Borings Segments A,B,C,D, and E F4.11-10