Mechanical behavior of intact completely decomposed granite soils along multi-stage loading–unloading path

Completely decomposed granite (CDG) soils in Shenzhen, China are usually subjected to loading-unloading due to the demolition/reconstruction of buildings and excavation/backfilling of foundation pits during the rapid development of the city. To investigate the mechanical behavior including the failure mode of the CDG soils under loading-unloading, consolidated drained triaxial tests along multi-stage loading-unloading paths were carried out in this study. Four types of loading sequences with one, two, three, and four loading-unloading cycles, respectively, were applied to investigate the mechanical properties of the CDG soil. Test results show that, as the number of the loading-unloading cycles increased from one to four, the effective cohesion (c') increased from 45.4 to 63.6 kPa; however, the effective friction angle (φ') first increased from 22.0° to 27.1° and then decreased to 19.0°. These results are mainly related to the strain-hardening property of the CDG soil after compressed by a relatively low deviator stress in the first two cycles and the structural damage of the specimens caused by a higher deviator stress in the last two cycles. In addition, the secant modulus also increased in the first two cycles, but decreased in the last two cycles due to the aggravation of soil structural damage. However, the unloading modulus (E_ur) decreased all the way from 162.9 to 22.7 MPa is attributed to the number of loading-unloading cycles and the different confining stresses applied in the tests. The unloading modulus was not only affected by the confining pressure and, more importantly, also affected by the accumulated structural damage of soils.