310 W CASINO RD 2017-08-11 - Laserfiche WebLink

Acme Homes JN 15373 • September 2, 2015 Page 2 Subsurface Conditions During our visit, we monitored the excavation of three test pits at the approximate locations shown on the attached copy of the site plan. These test pits were evenly spaced along the length of the two potential infiltration trenches. Attached to this letter are the logs for the test pits. In all three test pits, we observed topsoil and a 2- to 3-foot layer of gravelly, silty, fine-grained sand. Underlying this loose, heavily-weathered silty soil was dense outwash gravelly sand. In Test Pit 1, this dense outwash extended to the 9-foot depth of the exploration. In Test Pit 2 we observed dense sand that contained lenses of silt underneath the outwash between a depth of 6 and 10 feet. From 5 to 10 feet below the ground surface in Test Pit 3 we observed slightly silty to sifty sand that contained chunks of silt and glacial till. No groundwater seepage or wet soil conditions were observed in any of the test pits. The explorations were conducted following a hot, dry spring and summer, when groundwater levels would be near their lowest. Based on our observations and the lack of substantial groundwater recharge area upslope of the site, it appears that the seasonal high groundwater level is below the dense outwash soil. CONCLUSIONS Based on the test pits conducted on the site, the soil below a depth of 2 to 3 feet is moderately permeable. The outwash sand and gravel has a negligible silt content, but is very compact, as it has been glacially compressed. The undertying sand and slightly silty sand is dense and is intermixed with chunks of silt and glacial till. The density of the outwash, and the presence of silty inclusions in the underlying sand reduces the permeability of these soils. No impervious layer, such as intact glacial till, or signs of shallow groundwater were observed in the test pits. We recommend that the infiltration trenches all extend through the upper silty soils to reach the underlying outwash sand and gravel. The outwash, which is the most permeable soil encountered in the test pits, is deepest in the southern portion of the site. Better long-term perFormance of the infiltration system would be achieved by placing more of the trenches on the southern portion of the property, if that is possible The trenches should extend no more than approximately 6 inches into this soil, to keep the trenches above the underlying siltier soil found in the northern test pits. Considering the low silt content of the outwash, but its compact condition, we recommend a long- term design infiltration rate of no more than 0.25 inches per hour be assumed. This value should be conservative even for the northern portion of the site where there is less of the outwash soil. The performance of any infiltration system will degrade over time as the surrounding soils become clogged with silt and debris. Gutters, pavements and catch basins should be cleaned regularly to extend the effective life of the infiltration systems. As with any buried drainage system, it is possible that infiltration systems will have to be replaced in the future if their performance becomes inadequate. LIMITATIONS The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our exploration and assume that the soil and groundwater conditions encountered in the test pits are representative of subsurface conditions on the site. If the GEOTECH CONSULTANTS, INC. ,,I� .�+ 1