5900 36TH AVE W Geotech Report 2025-07-29

Mr.Richard Burrell February 9, 2006 The two dispersion systems in the northern portion of the site are located on the lower portion of a proposed north-facing structural fill embankment having a maximum slope inclination of 1.5:1 (Horizontal:Vertical). The maximum height of the fill embankments at the northwestern and northeastern outfall locations are about 36 and 14 feet, respectively. The dispersion pipes are shown at an elevation approximately four to eight feet above the toe of the fill embankment. SOIL CONDITIONS Our observations of on-site exposures indicate that the native soils in the project area consist primarily of glacial till. This is supported by the results of a previous geotechnical study for the site prepared by Associated Earth Sciences, Inc., dated March 31, 2000, including the observation of dense to very dense glacial till in 3 test pits excavated in the location of the proposed northeastern outfall. We anticipate that the structural fill embankment will be constructed using material derived primarily from similar on-site glacial till or glacial till-like soils. Subsurface conditions at the site, including the northern outfall locations will be verified by additional site ' explorations performed as part of our forthcoming geotechnical report. ' DISCUSSION Northern Outfall Locations Based on our review, it is our opinion that the soil conditions in the northern outfall locations are suitable for construction of the proposed stormwater outfall systems provided the recommendations discussed below are ' followed. In our opinion, adequate slope face protection and flow energy dissipation can be provided by • dispersing onto a reinforced rock mat (Maccaferri Reno mattress or equivalent) that is at least 12 inches thick. The rock mat should extend at least 5 feet upgradient and 15 feet downgradient from the dispersion pipe, and at least 5 feet laterally from the ends of the dispersion pipe. A geotextile separation layer, such as Mirafi 140N or equivalent, should be placed between the soil subgrade and the Reno mattress. The dispersion pipe should be oriented with holes pointing down into the Reno mattress. If other types of pipe perforations are considered, we ' should review the proposed pipe to determine if additional or alternative measures are needed for adequate flow energy dissipation at the pipe. We also recommend constructing a gabion barrier adjacent the lower margin of the Reno mattress at the toe of the structural fill embankment. The gabion bather will provide a secondary measure of flow energy dissipation and ' will buttress the Reno mattress. The need for additional anchoring of the Reno mattress will be evaluated prior to construction. Schematic details showing the above recommended dispersion structures both in section and plan view are attached. ' Sub Fill Drainage S' The structural fill embankment in the northern portion of the site will require construction of a toe drain in the excavation for the keyway cut at the toe of the fill slope. In addition, a sub-fill drain will be required where the structural fill embankment is constructed over the natural drainage corridors where the two northern stormwater outfalls discharge. Project No.T-5738 Page No.2