4003 NASSAU PL 2016-01-01 MF Import

� 1 Geptechnical Engineering Report Kvemmo Single-Famil}� Residence 4003 Nassau Place, Everett September 17, 2002 NGA File No. 356102 Page 8 Rock Butress We have evaluated the stnbility of the final planned site configuration by performing a numerical global stabiliry analysis of the planned geometry at Cross-Section A-A'. The purpose of this analysis is to evaluate the extent of stabili7ation methods that would be needed to maintsin a stable wndition. We analyzed global site stabiliry using a computer program titled XSTABL, version 5.2. XSTABL is a two-dimensional, limittquilibrium, slope stability program. XSTABL generetes random potential failure surfaces and detertnines their corresponding factors of safety with rcspect to failure. •The factor of safety is defined as the ratio of the intemal soil strength divided by the gravity driving forces ihat cause failure. By generating a large number of random surfaces, the factor of safery can be obtained as the lowest number calculated. We did not perform laboratory shesr testing on the on-site materials; however, we established design strengths from visual evaluation end classification of the soils, based on our experience with similar soils. The geometry of Cross-Section A-A' was used to perform a static stability analysis and estimate the siu of the rock butress needed to maintain stability. The factor of safety against slope failurcs was found to be at least 1.5 using a rock butress at the tce of the fill. Our stebility analysis indicates that a rock spell with a minimum thickness of 4 feet and a minimum width of 10 fat is required near the tce of the fill. 7'he rock spalls should extend at least 2 feet above and below the bottom of the fill material. The rock spalls should be placed on a level subgrade excavated into the stiff or better native silt soils inferred to � underlie the fill on the site. We estimate that an excavation approximately 70 fat long and up to 12 feet deep will be necessary to expose the native soils for butress placement. The eastem edge of the rock � butress should be Iceated as close to the utility easement as conditions permit, without undertnining the existing sewer line. A rock buttress detail is shown on Figure 8. ' The remainder of the rock butress excavation should be backfilled with granular material placed and compacted as swctural fill, as outlined in the Structural Fill subsection of this rcport. 7'his fill should be compacted to at least 90% of the modified Proctor maximum dry density. Cut-Off Drain We recammend that a cut-off drain be installed on the uphill side of the planned residence to intercept NELSON GEOTECHNICAL ASSOC/ATES, INC. ��