3620 36TH PL NE 2018-01-02 MF Import

For walls that are free to yield at the top at least one -thousandth of the height of the wall, soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing. Assuming 0-at the walls are backfilled and drainage is provided is outlined in the following paragraphs, we recommend that yielding walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf (triangular distribution), while nonyielding walls supporting horizontal backfill be designed using an equivalent fluid density of 55 pcf (triangular distribution). We recomment? that the allowable passive resistance be computed using an equivaent fluid density of 300 pounds per cubic foot (pcf) if these elements are surrounded by structural fill. The structural fill should extend rut from the face of the foundation element for a distance at least equal to three times the height of the element and be compacted to at least 95 percent of the NMI). An additional rectangular distribution of 6H psf (rectangular distribution) should be applied for the seismic lateral earth pressure, where H is :he exposed height of the retaining structure. The above soil pressures assume that the walls will be (1) constructed on a minimum 2-foot thick structural fill pled as described for building foundations in Section 4.4, (2) backfilled with properly compacted imparted structural fill, and (3) that wall drains will be installed to prevent the buildup of hydrostatic pressure behind ttue walls, as discussed below. Positive drainage should be provided behind any permanent conventional cast -in -place subsurface walls by pl•,cing a minimum 2-foot wide zone of pea gravel or gravel backfill for walls immediately adjacent to the walls. Gravel backfill for walls should conform to Section 9-03.12(2) of the 2002 I WSDOT Standard Specifications. The gravel backfill for walls zone should extend from the base of the wall to within I foot of the finished ground surface behind the wall. The top 1-foot of fill should consist of relatively impermeable soil to prevent infiltration of surface water into the wall drainage zone. Weep holes at about 8-foot centers at the base of the wall should he installed to drain water from exterior walls. Alternatively, a perforated drainpipe may be embedded at the base of the wall in the t gravel backfill for walls to remove water that collects in this zone. 4.7 EARTHQUAKE ENGINEERING 4.7.1 Liquefaction Potential Liquefaction refers to the condition where vibration or shaking of the ground, usually from earthquake forces, results in the development of excess pore pressures in saturated soils with subsequent loss of strength in the deposit of soil so affected. In general, soils that are susceptible to liquefaction include very loose to medium dense, clean to silty sands that are below the water table. The results of our analyses indicate that several zones in the loose to medium dense sandy soils encountered in the explorations below the organic silt have a moderate to high potential for liquefaction during a design earthquake event, and a moderate potential during an event with a lower level of ground shaking. The organic silt and silt deposits located above the sand have a low potential for liquefaction during a design earthquake (a magnitude 6.5 earthquake with a PGA of 0 27g). Our analyses indicate that settlements caused by liquefaction of the saturated loose to medium dense sand layers at this site during a design earthquake could be on the order of 3 to 6 inches. Foundations for the planned structures will be constructed over the liquefiable soils and will therefore be prrne to L� G c 0 E n g i n e e r s 19 File No. 10625 001-02.)I23003