2400 33RD ST GEOTECHNICAL INVESTIGATION 2018-01-02 MF Import

sc 130 Milbor-Pita Woanrra arc impermeable due to the variety nf grain sizes. Typically, it does have discontinuous seams of sandier material that may contain water. 4.2.2.5 Silty Sand — Advance Outwash Below the till in the deeper borings is a silty to clean medium sand unit. It is interlayered with silt and clay seams, probably formed as the glacier made minor advances and retreats. 4.2.3 Groundwater There are several :..)nes of groundwater below the site, however they appear to be discontinuous. There is a zone within the upper five feet of material, along the top of the stiff silt unit. This appears to be seasoi:al, indicating that is it fed by precipitation rather than a regional groundwater zone. The silt/clay itself does have some sandier lenses that appear to be slightly more permeable and contain small amounts of water. There is another zone on the top of the till at a depth of approximately 18 to 20 feet, which is sometimes present as a softer zone within the overlying silt. These water -bearing layers are referred to as "perched" water zones. The true regional groundwater level is at approximately 35 to 39 feet below the site. This zone was encountered in MP-5 at a depth of 38 feet, and the water level rose to 17.5 feet within minutes, indicating that it is confined below the till and silt. A true water level was not obtainable due to water flowing into the well from an upper perched water zone. The drilling became difficult below 38 feet due to the sand "heaving" into the boring due to the upward pressure of the confined water. 5.0 CONCLUSIONS AND RECOMMENDATIONS The results of our exploration program indicate that compressible silt/clay deposits overlying dense, glacially consolidated till deposits are bedded relatively uniformly beneath the site. The fill and silt/clay will not provide adequate support for the proposed structure due to settlement. If the silt/clay is over -excavated and replaced with structural fill placed from the underlying dense soil to the footing elevation, the resulting bearing capacity on the structural fill would be 5,000 pounds per square foot. Deep foundation alternatives include drilled shafts (piers), auger -cast piles, and driven piles. These are discussed in Section 5.3.2 below. The following sections contain recommendations on seismic design criteria, building foundation design, pavement design, deep foundation design (including axial compression, pull-out and lateral analysis), lateral earth pressures, general earthwork considerations, temporary excavations, and site drainage and erosion considerations. 5.1 Site Earthwork 5.1.1 Pre-Subgrade Preparation Prior to earthwork operations, the site's demolition debris should be removed and the ground cleared of all deleterious materials, including buried pipelines, building foundations and floor slabs, vegetation and buried organic rich soil and/or wood debris, (n-otcchnical, tuuicl mid Environmental Ingmeas 3080 125" Ave NE Phone (425) 869-5778 ncllme, WA 98005 Pages Fax (425)861-0677