4805 BELVEDERE AVE 2016-01-01 MF Import

SubsurJace Erplorarion. Geologic Nazard, and Bel��edcre Lors East Preliminaq� Genrechnicaf Engineering Rrpott E�crrrr LSashin�mn Preliminarv Design Rrcommendnrions indicare that a severe seismic event (100-year design earthquake of 0.2g horizontal ground acccleration) could produce landsliding with substantial downslope movement within 20 feet o( the top of the steep slope. To mitigate the risk of placing the buiiding foundations within this area we recommend the use of pile foundations designed to address the following conditions: . Transitory lateral earthquake soil loads. • Retaining slope soils beneath the house foundations, which are susceptible to undermining by poter'.ial landslide scarps. • Settlement of the loose surface soil due to both foundation loading and landsliding. The pile foundations should consist of a slope-side row of large-diameter drilled piles, and a system of interior, small-diameter driven pipc piles. The slope side piles should be designed to carry vertical and lateral foundation loads, and act as a soil arch retaining wall, preventing potential landslide soil from undermining the residences. The soil arch wall is intended to retain slope soils helow the residences, but a�ill experience some movement should landsliding occur downslope of the wall. Therefore, the interior, small-diameter driven pipe piles should be designed to provide vertical foundation support to mitigate downset and creep associated with movement of the soil arch wall. I3ased on our anatysis, it is our opinion that long-term mitigation of landslide activiry below residences constructed within 20 feet of the top of thc slope can be achieved by installing the drilled pile soil arch wall along the side of the structures facing the slope crest. The piles would be drilled through the upper loose soils and potential landslide soil, and anchored into the dcnse, r.atural sand soils underlying the site. The piles are not intended to stabilize the slopc do��nhill of the residences. The piles ��ould be closely spaced to rely on soil arching between piles to retain the slidc soils beneath the homes, and eliminate the need to place lagging bea+�cen the piles. �1'e anticipate that the piles will be 18 to 24 inches in diameter, and placed �+�ith a maximum separation of 4 feet, edge to edge. The piles will extend to depths ranging het��een 35 to 50 feer beloti� U�e existing grades. Strucmral analysis and design of the pile wall and interior pipe piles should be completed by the project structural engineer, based on the information provided below. Thc piles shuuld be designed to resis� the lateral acti��e earth and seismic soil pressures devcluped hy the retained soils. The soil design loads presented below assume a horizontal hackslope condition. Any inclined backslope or non-pile-supported foundation loads should be included as surcharge loading on the pile ��all. The site's upper sand soils are •.vell drained. 7'hus, no surcharge loading due [o hydrostatic pressures should be required in the pile soil arch ��•.i11 dcsign, provided the strucwres are provided �4�ith foo[ing drains and dow'nspout tightlines, ��hich comey drainagc hack to the street. We recommend that no fill soils or non-pile- supported strucwres be placed �cithin 20 feet of the top of the slope to prevent overloading the suil arch w'all. Alnrch S, _'lXi4 ASSOCIATED EARTN SClENCES, 1NC. uri�� e�- c�� r,.�,.,:�,,,�.�uv�:e ��r u:n Page 15