Field measurements of the linear and nonlinear shear moduli of soils using drilled shafts as dynamic cylindrical sources

The framework for a generalized field method to evaluate the shear modulus of soils in the linear and nonlinear strain ranges was developed. The field set-up utilizes a cylindrical, axisymmetric source that, in this work, was simply a drilled shaft. The test procedure involves applying vertical dynamic loads to the top of the cylindrical source. Shear waves propagating away from the mid-depth of the source are measured using vertically oriented geophones that are embedded in the soil. The vertical dynamic loads range from small impacts with hand-held hammers to large, sinusoidal loads (6 to 35 kip (27 to 156 kN)) from a Vibroseis. Nonlinearity in the soil in the vicinity of the source (drilled shaft) occurs as the dynamic load level increases. Measurements of linear and nonlinear shear wave propagation permits linear and nonlinear shear moduli and associated shearing strain amplitudes to be evaluated. Shear wave propagation velocity is calculated using the time shift between the waveforms recorded at adjacent sensors. Shear modulus is then calculated using the measured shear wave propagation velocity and the mass density of soil. Strain amplitudes are calculated from dynamic displacements measured at the sensor locations assuming a linear variation with distance.; The applicability of test method was studied by conducting small-scale, prototype experiments at a "calibration" field site in Austin, Texas. Numerous in situ and laboratory measurements have been performed on the soil at the field site. The in situ measurements include Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), Spectral Analysis of Surface Waves Tests (SASW) and Crosshole Seismic Tests. The laboratory measurements include Resonant Column Torsional Shear Tests (RCTS), soil classification and soil index tests as well as pressure chamber tests for determining Soil Water Characteristic Curve (SWCC). The "small-scale" nature of the tests involved using two 15-in. (381-mm) diameter drilled shafts, one was 6-ft (1.8-m) long and the other was 12-ft (3.7-m) long. The field site, prototype experiments and well-characterized soil were used to verify the general field approach. Experimental results from this field study provided information that can be used to improve the developing test method which, in the future, may be used on critical projects and to evaluate difficult soil conditions. Also, the experimental results provided an opportunity to compare laboratory and field measurements of the G/Gmax-log gamma curves. This comparison was used to investigate the accuracy of common procedures relating field and laboratory modulus reduction curves. Nonlinear modulus measurements were performed at depths of 3 to 7 ft (0.9 to 2.1 m) in silt (ML) and silty sand (SM) soils. The field G/Gmax-log gamma curves for these soils at low confining pressures are in general agreement with the laboratory curves as well as the empirical curves.