FIELD-SCALE MEASUREMENT OF WATER TABLE PROFILES IN A DRAINED SLOPE

In 2003, Crenshaw developed a method for calculating the average water table height between horizontal drains in a slope. The method relies on drain flow rates, slope geometry, and soil hydraulic conductivity. The corrugated shape of water table profiles between drains and the departure of the water table surface from the drain near its uphill end were verified by Crenshaw using laboratory-scale physical modeling and computer modeling. However, field verification of Crenshaw's findings was not conducted at the time. During the summer of 2007, a study program was conducted seeking to confirm Crenshaw's findings using field-scale physical modeling at the Blackhawk Geologic Hazard Abatement District (GHAD) test site in Danville, California. The Blackhawk GHAD test site is composed of a 2H:1V concrete slope 30 feet long and 12 feet wide, simulating impermeable bedrock conditions. The concrete is embedded with 5 perforated pipes which cross the slope laterally. These 5 pipes are connected to a water system and are used to simulate base flow recharge. For this study, the concrete was covered with compacted layers of a lean clay representing local materials known to be susceptible to slope failures. Two wick drains were installed during construction at a spacing of 8 feet. A total of 50 standpipe piezometers were installed in the slope in order to measure water table profiles both between and along the two drains. Measurements were taken during both recharge and drawdown events. The test was repeated with a locally representative clayey gravel/sand soil. Field test results generally confirm the findings of Crenshaw (2003), with some localized variations in the water table profiles. The variations are most likely due to factors such as inhomogeneity of soil properties and/or compaction, the development of preferential pathways within the slope during recharge, clogging of piezometers, or differential water infiltration in the slope due to a loss of pressure along the perforated recharge pipes.