11831 AIRPORT RD WHOLE SITE FILE Geotech Report 2022-10-25

• a proposed new retaining wall along the south side of 128th Street SE immediately west of the south- bound 1-5 ramp, referred to as Wall 1, will require removal of an existing rockery.This new wall will be about 3 to 10 feet in height and be situated close to the existing Denny's restaurant; • a retaining wall for a new station on the east side of Airport Road north of the intersection with 100th Street SW will be situated on an existing fill slope retained by a concrete block wall which is about 8 to 10 feet in height.The new retaining wall will be about 4 feet in height. Geology Mapped soils along the project corridor generally consist of recessional outwash, glacial till, advance outwash, and modified land (fill).The recessional outwash is mainly mapped along SR 527 for the portion of the project area south of about 196th Street SE. Advance outwash, with small windows of glacial till, is mainly mapped north of this area to the intersection of 128th Street SE and 3rd Avenue SE. Glacial till is mapped mainly for the remainder of the project west of 3rd Avenue SE and continuing north on Airport Road. Recessional outwash deposits in the region consist primarily of stratified sand and gravel with occasional layers of silty sand to silty clay deposited in proglacial and ice-marginal environments. Glacial till typically consists of a dense to very dense heterogeneous mixture of sand, gravel, cobbles and occasional boulders in a silt and clay matrix that were deposited beneath a glacier.A zone of weathered till typically overlies the glacial till to depths of several feet below the ground surface.This weathered zone is somewhat drained,whereas the unweathered till is a barrier to vertical drainage.Water percolating into the weathered till will usually pond and migrate laterally between the weathered and unweathered layers. Advance outwash consists mostly of sand and gravel deposited by streams and rivers issuing from advancing ice sheets and is typically almost de-void of silt or clay. Advance outwash has been overridden by thousands of feet of ice and is therefore typically dense to very dense. Fill soils associated with roadway construction, site development, and utilities are also anticipated throughout the project corridor. Surface Conditions The ground surface along the existing roadways are typically relatively level with only gradual gradients. At the proposed new stations, ground surface conditions vary from relatively level to existing slopes that are typically 2H:1V(horizontal to vertical) or flatter. Existing concrete sidewalks are present at most of the station areas. Vegetation along the project segment varies from trees, brush and grass to landscaping depending on the adjacent property use. Pavement Conditions A significant variability in the pavement section thickness was encountered in the explorations. As summarized in Table 1, the hot-mix asphalt (HMA) surfacing varied from 1/ to 15 inches thick and the underlying crushed rock base course varied from 0 to 12 inches thick. Several of the borings and hand augers were completed in grass or landscape areas as noted in the table. GEOENGINEERS April 14,2017 Page3 File No.2095-032-00