On the evaluation of settlement measurements using borehole extensometers

Borehole extensometers are commonly utilized in field monitoring programs to measure settlement. In order to secure the targets of the extensometer, the borehole is typically backfilled with cement-bentonite grout. In the absence of standards for grout mix designs and due to non-universality of the properties of the backfilling grout, the accuracy of settlement furnished by borehole extensometers may be questionable. A preliminary parametric study was conducted to investigate the influence of grout stiffness on settlement measurements using borehole extensometers. The finite element program ABAQUS was used to model a borehole extensometer installed in an isotropic homogeneous elastic soil layer of a finite depth subjected to a uniform surface surcharge. Effect of the grout-soil elastic moduli ratio, soil/grout interface parameters (friction angle and shear strength), and extensometer embedment depth were investigated. Soil/grout interface behavior was assumed to comply with the elastic-perfectly plastic Coulomb friction model. The mechanism of measuring settlement using borehole extensometers was studied. The numerical study revealed that minimum errors in settlement can be obtained if the grout-soil elastic moduli ratio is made equal to about 1.0. Depending on the grout-soil moduli ratio, larger grout/soil interface friction angle and interface shear strength are generally associated with smaller settlement errors. Compared to fully embedded extensometers, partial embedment yields smaller settlement errors for grout-soil moduli ratio significantly above unity. In view of the findings of this preliminary parametric study, it is suggested to adopt the grout-soil moduli ratio in conjunction with grout compressive strength as the controlling criterion rather than the grout compressive strength only (current standard of practice). Additionally, it is evident that cement-bentonite grout mix should be carefully designed as part of the settlement monitoring programs to match the expected deformability (stiffness) of the soil mass under consideration. Future laboratory, field and numerical studies are needed to investigate more complex soil stratifications and to evaluate the behavior of various grout mixes.