7505 SEAWAY BLVD Geotech Report 2022-10-25

November 11, 2016 HWA Project No. 2014-117-21 also recommend that the face of the signal pole closest to the slope be located at least 1 shaft diameter away from the top of the slope. 4.7 STORMWATER MANAGEMENT The increased stormwater associated with the proposed project will necessitate upgraded stormwater management facilities. The entire alignment is predominantly underlain by glacial till soil,present at shallow depths. Glacial till soils possess high fines contents and are generally not conducive to onsite infiltration. Consequently, the use on onsite infiltration as a means of stormwater management is not feasible for this project. We understand that an underground stormwater detention vault is proposed as means of stormwater management for this project from which the stormwater will be conveyed off site. We understand the underground detention vault will consist of a cast-in-place concrete system. Inlet pipes will be used to convey stormwater into stormwater conveyance systems and into the detention vault. Outlet pipes will be used to convey the stored stormwater from the detention vault to appropriate discharge locations Based on the available soil information, we expect that the detention vault will be founded on very dense glacial till soil. An allowable bearing pressure of 5,000 psf should be assumed for design. We assume that the vault will be backfilled with compacted gravel borrow backfill possessing a unit weight of approximately 135 pcf. We assume that the vault will not be free to deform under static loading conditions (at rest earth pressures assumed for static loading)but will be free to deflect sufficiently to develop active loading conditions during the design earthquake. Based on these assumptions, the lateral earth pressures provided in Figure 5 should be used for design of the vault structure. Figure 5 provides the anticipated pressures associated with traffic loading(250 psf at ground surface), soil over the vault, earth pressure and hydrostatic pressure. No ground water seepage was encountered during our explorations in the vicinity of the vault (BH-2 and BH-3),which were drilled during a dry summer period. However,we expect that the vault excavation backfill will fill with water over time due to the very low permeability of the glacial till soils (bath tub affect). Therefore, the vault should be designed to resist buoyancy forces. For our earth pressure diagrams, we assumed the worst case scenario, where groundwater accumulates up to the proposed ground surface. However, it may be that water will drain out of the fill before this extreme case occurs. For buoyancy calculations, we recommend the groundwater level be assumed at the highest level it can raise before flowing out through the surface or through more permeable trench backfill extending from the vault excavation. 2014-117 Final Geotechnical Report 12 HWA GEOSCIENCES INC.