Evolution of pore structure and creep characteristics of Indian black cotton soil under oedometer conditions

Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India

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Abstract

The present study investigated the macro- and micro-structural creep behavior of remolded Indian black cotton soil under various loading-unloading-reloading cycles. The macrostructural creep behavior of soil was evaluated using multi-stage oedometer tests (MSL tests) under two loading conditions with varying durations. The microstructural analysis included mineral identification using X-ray diffraction (XRD) and evaluation of pore morphology and particle rearrangements using scanning electron microscopy. The XRD analysis revealed a higher proportion of illite, which significantly affected inter-aggregate pores, resulting in a more pronounced reduction in pore area. The reduction in pore area was about 1.54-1.89 times at an intermediate stage of the MSL test and 2.48-3.74 times at the end of the MSL test. Although pore count reduction was 22-73% higher in the vertical plane (i.e., parallel to loading), pore area reduction was 26-64% higher in the transverse plane (i.e., perpendicular to loading). The predominance of micropores was greater in the vertical plane, resulting in a greater reduction in pore count. In contrast, the transverse plane has a significant number of small pores, whose reduction contributes more to the overall reduction in pore area. Rose diagrams revealed that the majority of pores were aligned between 150° and 225° at an early stage of consolidation, then reoriented to 150° and 195° at the intermediate stage, and finally uniformly distributed across all quadrants at the end of the test. The particle orientation index data also showed that the majority of pores initially exhibited anisotropic behavior and transitioned to isotropic behavior by the end of the MSL tests.