2710 14TH ST BLDG A Geotech Report 2023-12-29

IEverett Housing Authority April 22. 2020 Page 3 GROUNDWATER Groundwater seepage was not encountered in any of our original test pit excavations as summarized in our geotechnical report. Groundwater seepage was also not observed during initial excavation of PIT-1 and PIT-2; however, slow groundwater seepage was observed directly above dense glacial soils in PIT-1 during overexcavation of the test pit following the infiltration test. The seepage in PIT-1 was likely residual water from the infiltration testing and not natural groundwater seepage. Perched groundwater should be expected to vary as a function of season, precipitation, and other factors. Seepage zones should be expected to develop in the fill material, perched above the dense glacial till deposits. ' PILOT INFILTRATION TESTING ' PIT-1 and PIT-2 were conducted as small-scale PIT tests. The following is a summary of test methods, field measured infiltration rates, and design infiltration rates for the small-scale PITs. ' Test Method Test pits PIT-1 and PIT-2 were initially excavated with a trackhoe to depths of approximately 5'h and 5 feet, or approximately Elevation 791h and 91 feet, respectively. The test pits were excavated to the proposed subgrade elevations of the future infiltration facilities, as requested by KPFF and EHA at the locations shown on Figure 1. The bottom of each test pit was approximately 4 feet wide by 4 feet long. The soil in PIT-1 generally consisted of fine to medium sand (fill) at the depth tested. The soil in PIT-2 generally consisted of silty fine to medium sand with occasional gravel (glacial till) at the depth tested. ' Groundwater was not observed during the initial excavation of the test pits; however, slow groundwater seepage was observed between depths of approximately 6 and 7 feet below the ground surface in PIT-1 following the infiltration test, ,which was interpreted as being residual water from the infiltration testing. ' The groundwater seepage was observed during overexcavation of the test pit (as required by 2014 SWMMWW). Water -level records recorded for PIT-1 and PIT-2 are plotted on Figures 3 and 5, respectively. Plots of the measured infiltration rates calculated during each stage of the small-scale PITs are shown on ' Figures 4 and 6. Low infiltration rates were observed in both PITs. During the PITs, a graduated yard stick was driven into the floor of the test pit as a visual reference for ' monitoring water levels during testing. A piezoelectric pressure transducer was secured to the bottom of the yard stick to provide accurate water level records in 10-second intervals throughout the duration of the tests. Water used for the infiltration tests was provided by hose bibs from adjacent housing units. ' Initial filling and maintaining of the water level in PIT-1 was performed by monitoring the declining water level in a series of stages. The test pits were filled to a predetermined depth (approximately 14 inches) for ' the 6-hour pre-soaking period. During the pre-soaking, the water level was allowed to drain approximately 2 inches before the pit was refilled to the initial level again to maintain a constant minimum depth of 1 foot. This draining and refilling process was repeated a number of times (stages) in succession. At the end of the 6-hour pre-soaking period, the successive filling and draining was repeated at the same 12 and 14 inches of water depth for an additional hour, which is considered the testing period. Following this I FF',e Nc, 212SS-002-00 GEOENGINEERS�