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Environmental and Engineering Geoscience

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Field-Scale Measurement of Groundwater Profiles in a Drained Slope

¹ Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401
² Blackhawk Geologic Hazard Abatement District, 4125 Blackhawk Plaza Circle, Danville, CA 94506

In 2003, Crenshaw and Santi developed a method for calculating average piezometric heads between horizontal drains in a slope. The method relies on drain flow rates, slope geometry, and soil hydraulic conductivity. The corrugated shape of groundwater profiles between drains and the departure of the groundwater surface from the drain near its uphill end were verified using laboratory-scale physical and computer modeling. In 2007, a study was conducted seeking to confirm these findings using field-scale modeling. The test site consists of a 2H:1V (2 horizontal : 1 vertical), 30 x 12 ft (9.1 x 3.7 m) concrete slope representing low-permeability bedrock. Five perforated pipes embedded in the concrete are used to simulate base-flow recharge. The concrete was covered with a lean clay, and two wick drains were installed at a spacing of 8 ft (2.4 m). Fifty standpipe piezometers were installed to measure groundwater profiles between and along drains. Measurements were taken during recharge and drawdown events. The test was repeated with a clayey sand. Test results generally confirm the findings of Crenshaw and Santi, with some localized variations in groundwater profiles. The variations are most likely due to factors such as heterogeneous soil properties, the development of preferential pathways, boundary effects, etc. Field-testing also yielded additional information about the behavior of drained slopes that may be useful for future slope-stability projects. As a result of this research, recommendations for using Crenshaw and Santi's method to estimate piezometric heads between short, high-angle sub-horizontal drains in shallow landslides for use in slope-stability analyses are provided.

Key Words: Groundwater • Slope-Stability • Horizontal • Drains • Field-Measurement