1815 HOLBROOK AVE Geotech Report 2025-03-06

June 9,2022 Page to of 15 Geotechnical Evaluation Wall Design Criteria "At-rest"Conditions(Lateral Earth Pressure—EFD') 55 pcf(Equivalent Fluid Density) "Active"Conditions(Lateral Earth Pressure—EFD') 35 pcf(Equivalent Fluid Density) Seismic Increase for "At-rest" Conditions 21H* (Uniform Distribution) 1 in 2,50o year (Lateral Earth Pressure) event Seismic Increase for "At-rest" Conditions 14H*(Uniform Distribution)1 in 50o year event (Lateral Earth Pressure) Seismic Increase for "Active" Conditions 7H*(Uniform Distribution) (Lateral Earth Pressure) Passive Earth Pressure on Low Side of Wall Neglect upper 2 feet,then 25o pcf EFD' (Allowable,includes F.S.=1.5) Soil-Footing Coefficient of Sliding Friction(Allowable; 0.40 includes F.S.=1.5) 'H is the height of the wall;Increase based on one in 50o year seismic event (io percent probability of being exceeded in 5o years), -,EFD—Equivalent Fluid Density The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by water accumulation behind the retaining walls. Uniform horizontal lateral active and at-rest pressures on the retaining walls from vertical surcharges behind the wall may be calculated using active and at-rest lateral earth pressure coefficients of o.3 and o.5,respectively. A soil unit weight of 125 pcf may be used to calculate vertical earth surcharges. To reduce the potential for the buildup of water pressure against the walls, continuous footing drains (with cleanouts) should be provided at the bases of the walls. The footing drains should consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed down and enveloped by a minimum 6 inches of pea gravel in all directions. The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should consist of free-draining granular material. All free draining backfill should contain less than 3 percent fines (passing the U.S. Standard No. 200 Sieve)based upon the fraction passing the U.S. Standard No. 4 Sieve with at least 3o percent of the material being retained on the U.S. Standard No. 4 Sieve. The primary purpose of the free-draining material is the reduction of hydrostatic pressure. Some potential for the moisture to contact the back face of the wall may exist,even with treatment, which may require that more extensive waterproofing be specified for walls, which require interior moisture sensitive finishes. We recommend that the backfill be compacted to at least 90 percent of the maximum dry density based on ASTM Test Method D1557. In place density tests should be performed to verify adequate compaction. Soil compactors place transient surcharges on :he backfill. Consequently, only light hand operated equipment is recommended within 3 feet of AN so that excessive stress is not imposed on the walls. www.cobaltgeo.com (206)331-1097