Using coefficient of consolidation to assess response time and reading accuracy of piezometers in grouted boreholes in fat clays
Brewster, Alexander C.
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Reading accuracy of a piezometer in a fully grouted borehole has typically been assessed by comparing the hydraulic conductivity of the grout (K[subscript grout]) to that of the soil (K[subscript soil]). The field conditions of interest to this study represent the case of a relatively permeable cement – bentonite grout (the fill material of choice for borehole applications) being installed in a relatively impermeable fat clay. The soil and grout are assumed to be incompressible and fully saturated with an incompressible fluid. Using a steady state approach, error is defined for a given K[subscript grout]/K[subscript soil] ratio. Each of the three studies that investigate K[subscript grout]/K[subscript soil] propose a significantly different failure criterion. An experimental program was developed, with the critical area of focus being on the highly inconsistent K[subscript grout]/K[subscript soil] criteria used to predict grouted piezometer reading error. This study investigated the merits of coefficient of consolidation (c[subscript v]) theory in predicted such errors. It was discovered that pressure pulses (like those experienced within a grouted borehole) migrate through a specimen under a different gradient than those experienced in a typical consolidation environment. This discovery extends beyond existing literature, which makes no distinction between consolidation and pressure propagation. As such, a new term has been coined: the coefficient of pore pressure propagation (C[subscript p]). C[subscript p] was found to be characterized as a single-valued function of C[subscript v], making it a particularly useful term (i.e. calculating C[subscript v] allows for C[subscript p] to be accurately predicted). The testing program consisted of CRS and triaxial approaches that measured the K, C[subscript p] (attained from pulse tests), and C[subscript v] (attained from conventional consolidation tests) as a function of consolidation pressure and volumetric water content. These parameters were successfully measured for sands, sand-bentonite mixtures (SBM), and fire clay specimens. It was discovered that a C[subscript p] framework is better suited to assess reading error than the prevailing K theory. Experimental data demonstrates that pressure equalization rates within the grout are highly dependent on stiffness and degree of saturation, neither of which is adequately captured in the K data. Furthermore, relying on K theory led to widely varying predictions of response time that were experimentally determined to be inaccurate. A numerical model was developed using the C[subscript p] theory. Even in a reasonably conservative state, the model predicts that no currently-available cement-bentonite grouts are suitable for use in a fat clay installation. If future studies fail to identify a viable grout for such installations, the fully grouted method may need to be abandoned altogether for installations in fat clays.