2621 ROCKEFELLER AVE Geotech Report 2022-10-25

11 The contractor performing thewo rk g w rk has the primary responsibility for protection of workmen and adjacent improvements. In our opinion, the contractor will be in the best position to observe subsurface conditions continuously throughout the construction process and to respond to variable soil and groundwater conditions. Therefore, the contractor should have the primary responsibility for deciding whether or not to use open cut slopes for much of the excavations rather than some form of temporary excavation support, and for establishing the safe inclination of the cut slope. Acceptable slope inclinations for utilities and ancillary excavations should be determined during construction. Because of the diversity of construction techniques and available shoring systems, the design of temporary shoring is most appropriately left up to the contractor proposing to complete the installation. Temporary cut slopes and shoring must comply with the provisions of Title 296 WAC,Part N,"Excavation,Trenching and Shoring." The excavations for the parking level of the building will be completed primarily in dense to very dense glacial till deposits. The following sections summarize the general excavation recommendations. If conditions allow, the entire excavation for the proposed building may be accomplished using temporary 11 cut slopes. However, depending on the final building layout and site constraints, temporary shoring may also be needed. Temporary shoring appropriate for the soil conditions at the site may include soldier pile and timber lagging walls or soil nails. Temporary retaining walls must account for groundwater seepage in their design and during construction. In the following sections we provide a brief description of soldier pile and timber lagging walls, which in our opinion will be more cost effective than a soil nail wall for temporary shoring. Temporary Cut Slopes 1 For planning purposes, temporary unsupported cut slopes more than 4 feet high for the parking level may be inclined at 3/4H:1V maximum steepness within the dense to very dense glacial till. If significant seepage is present on the cut face of the dense to very dense till then the cut slopes may have to be flattened. If dry conditions exist and the dense native till and outwash soil are evaluated during construction and deemed competent by the geotechnical engineer, then the temporary cuts may be steepened to 0.5H:1V. However, temporary cuts should be discussed with the geotechnical engineer during final design development to evaluate suitable cut slope inclinations for the various portions of the 11 excavation. For open cuts at the site we recommend that: • No traffic, construction equipment, stockpiles or building supplies be allowed at the top of the 1 cut slopes within a distance of at least 5 feet from the top of the cut. • Exposed soil along the slope is protected from surface erosion using waterproof tarps or plastic sheeting. • Construction activities are scheduled so that the length of time the temporary cut is left open is reduced to the extent practical. • Erosion control measures are implemented as appropriate such that runoff from the site is reduced to the extent practical. • Surface water is diverted away from the excavation. • The general condition of the slopes is observed periodically by a geotechnical engineer to confirm adequate stability. If temporary cut slopes experience excessive sloughing or raveling during construction, it may become necessary to modify the cut slopes to maintain safe working conditions. Slopes experiencing problems I File No.12547-001-00 Page 9 GEOENGiNEERS/ February 3,2006