Development of a Field Torsional Direct Shear Test Apparatus

The current study proposes a novel field Torsional Direct Shear Test (TDST) that can measure the field shear strength of soils with particle sizes up to 12.5 mm. TDST apparatus comprises an open cylinder 300 mm in diameter and 90 mm in height. The cylinder has two vanes fixed at 90° to one another at the center. It also has a 25 mm thick circular cover plate, quarter plates covered by the vanes, and a vertical loading column. Here circular cover plate transfers the normal load applied to the loading column, and the torsional load is applied to the loading column using the lever arm. Quarter plates allow the soil to dilate while shearing, enabling the measurement of shear strength under a known, constant applied normal stress. To validate the performance of the TDST apparatus, set up modified, and used in the laboratory large box direct shear test setup. The results of the laboratory TDST setup were compared with those obtained using the laboratory DST. It was found that the angle of internal friction (ϕ) measured using the proposed TDST was nearly identical to that measured using the laboratory DST. In contrast, the cohesion (c) measured using TDST was about 1.2 times greater than that measured using DST. The proposed in situ TDST method does not require a large horizontal force. Instead, as the length of the lever arm increases, the required force to induce torsional stress in soil reduces, making the method much easier to perform than the conventional field DST.