In Situ Measurement of Nonlinear Shear Modulus of Silty Soil

A new field test method to evaluate in situ nonlinear shear modulus of soils was developed. The method utilizes a drilled shaft as a cylindrical, axisymmetric source for shear loading of soil at depth. The applicability of the test method was studied by conducting small-scale, prototype experiments at a “calibration” field site in Austin, Texas. Numerous conventional in situ and laboratory measurements were performed to characterize the soil at the field site. The “small-scale” nature of the tests involved using a $381\mathrm{mm}$ $\left(15\mathrm{in.}\right)$ diameter, $3.7\mathrm{m}$ $\left(12\mathrm{ft}\right)$ long drilled shaft. Experimental results from this field study provided an opportunity to compare laboratory and field measurements of the $G-\log \gamma$ and $G∕G_{\max }-\log \gamma$ 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 $1.8\text{to}2.1\mathrm{m}$ $\left(6\text{to}7\mathrm{ft}\right)$ in a silt (ML). The field $G∕G_{\max }-\log \gamma$ curve for this soil at low confining pressures are in general agreement with the laboratory curve from an intact specimen as well as empirical curves.