Vibrating wire piezometer nests in Champlain clay: Bias in hydraulic head measurements in installations with excessively-permeable backfill

ABSTRACT: An extensive network of multilevel vibrating-wire piezometers (VWP) exists within the St. Lawrence Lowlands to monitor the spatial and temporal variation of pore pressures (and hydraulic heads) in clay slopes. Well construction logs indicate that some of the VWP installations use well-sorted crushed stone as well backfill instead of bentonite pellets or cement-bentonite grout. The higher hydraulic conductivity (K) of crushed stone relative to standard backfill materials can result in biased measurements of hydraulic head due to preferential flow within the backfill (i.e., a hydraulic short circuit). While crushed stone is not recommended for use as well backfill in new piezometer installations, there is a practical interest in understanding the extent of the bias that may be present in installations previously constructed in this manner. This study uses 2-D radial coordinate models to quantify the possible extent of this bias. The analysis focused on the relative importance of the following components: K the of crushed stone, length of the backfill intervals, length of the bentonite plugs, magnitude of the vertical gradient, and the degree of vertical and horizontal anisotropy within the clay. Simulation results show that the use of crushed stone as backfill results in measurements of hydraulic head that substantially differ from undisturbed conditions, regardless of the values assigned to the parameters of interest. Results further show that the error observed in the simulated piezometers is the result of a series of hydraulic short circuits caused by water preferentially flowing through the crushed stone intervals. Larger vertical gradients and vertical anisotropy increased the magnitude of the bias, which ranged from -0.25 cm to +210 cm across all simulations. These biases were observable up to 2 meters from a given piezometer installation, while the largest biases were consistently found in the deepest piezometers.