Shear Strength and Size Effects of Completely Weathered Granite Residual Soil Under Laboratory and In Situ Direct Shear Testing

Completely weathered granite residual soil is a weathering-derived, soil-like geomaterial whose shear strength is difficult to characterize using only conventional small-scale laboratory tests. This study evaluated the effects of specimen size and material structure by comparing in situ direct shear tests, conventional laboratory direct shear tests on undisturbed and remolded specimens, and large-scale laboratory direct shear tests on remolded specimens with box sizes of 150, 200, and 250 mm. The results show that undisturbed specimens exhibited higher shear strength than remolded specimens, indicating a clear structural contribution. With increasing specimen size, cohesion decreased from 41.2 to 31.4 kPa, whereas the friction angle increased from 35.3° to 40.6°. Compared with the conventional undisturbed test, the in situ tests yielded lower cohesion but higher friction angles. These results indicate that both size effect and structural disturbance significantly influence the interpretation of shear strength parameters in completely weathered granite residual soil. For engineering design in weathered-granite terrains, strength parameters derived from larger specimens or in situ tests are likely to be more representative than those obtained from conventional small-scale laboratory tests.