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

Everett Housing Authority April 22, 2020 Page 4 testing period, the water was turned off and the water level was recorded for an additional hour or until the test pit was drained. By periodically refilling the pit to the starting level, then allowing the water level drop between two pre -determined levels, the apparent infiltration rate for each cycle was determined accurately using measurements from the pressure transducer. The rate of decline in the falling water levels recorded after each filling cycle was used to calculate the apparent infiltration rate for each stage of the test. The overall testing process for each PIT took about 7 hours with water levels measured continuously (every 5 seconds). PIT-2 was completed in a similar fashion, however the water level only drained approximately 11/2 inches over the entirety of the 7-hour PIT (pre-soaking, testing phase and drain down). After each test pit was drained, they were excavated an additional 21/2 to 3 feet below the infiltration testing depths. The test pits were overexcavated to observe and sample the soils below the level of the PITs. Hydraulically resistive layers consisting of glacial till were observed in both test pits below the elevation where the PITs were performed. We also observed slow groundwater seepage from 6 to 7 feet below the ground surface during overexcavation of PIT-1 following the infiltration test and this seepage was likely residual water used during testing, suggesting that the water was mounding on top of the dense glacial till. Measured Infiltration Rate Plots of the apparent short-term infiltration rates calculated during each stage of the PITs are presented in Figures 4 and 6. Field measured infiltration rates were higher during the initial stages of the test as additional water was required to fill the pore space of the initially unsaturated soils until it approaches full saturation toward the end of the test. In PIT-1, 11 stages were analyzed with a repeated head drop of 0.2 feet (Figure 4). The geometric mean of the testing phase head -change stages was calculated to determine a measured infiltration rate of 3.89 inches per hour in PIT-1. This is a conservative analysis method. In PIT-2, there was very little infiltration duringthe 7-hour testing period. The infiltration rate was calculated during one stage over the entire test duration (61/2 hours). The field measured infiltration rate during the testing stage was 0.23 inches per hour in PIT-2. Design Infiltration Rates Measured infiltration rates used for the design infiltration rates for Site 1 from the PITs are summarized in Table 1. TABLE 1. MEASURED AND RECOMMENDED DESIGN INFILTRATION RATE Location PIT-1 (Facility 1) PIT-2 (Facility 2) Measured Infiltration Rate (In/hr) 3.89 0.23 Design Infiltration Rate (In/hr) 1.2 0.1 GWENGINEERS� �7 n 1 F: e No 212SS 002-00