Energy correction of dynamic cone penetration index for reliable evaluation of shear strength in frozen sand-silt mixtures

Abstract

Previously, in situ tests have been conducted in cold regions since infrastructures such as pipelines have been actively built on frozen ground. However, the engineering properties such as shear strength have not been directly evaluated from in situ tests. The objective of this study is to correlate the shear strength of frozen soils determined by the direct shear tests with the dynamic cone penetration index (DCPI) measured by the instrumented dynamic cone penetrometer (IDCP). The IDCP, which incorporates strain gauges and an accelerometer to measure the energy transferred to the cone tip, is used to estimate the energy-corrected dynamic cone penetration index (energy-corrected DCPI). The direct shear apparatus and the calibration chamber for the IDCP application test are placed in the freezer. The sand-silt mixtures are prepared in the shear box and the calibration chamber at the degree of saturation of 10% and relative density of 60%. Vertical confining stresses are applied to the specimens during the freezing and strength evaluating phases, such as direct shearing or penetrating the IDCP, to determine the effect of the vertical confining condition. The experimental results show that the shear strength of the frozen soils increases in a nonlinear parabolic shape with an increase in the vertical confining stress. Furthermore, the vertical confining stress during the strength evaluating phase has more influence on the strength than that during the freezing phase because the degree of saturation of specimens is low. As the energy transferred to the cone tip is affected by the soil conditions under the cone tip, the energy-corrected DCPI, which is inversely proportional to the shear strength, has a better relationship with the shear strength. This study demonstrates that the energy-corrected DCPI can be effectively used for the evaluation of the shear strength of frozen soils.