Effects of freezing-thawing and cyclic loading on pore size distribution of silty clay by mercury intrusion porosimetry

With the development of urban rail transposition, the long-term deformation of soft clay under the subway vibration loading has drawn wide attention by researchers and engineers recently. In addition to the situ tests and laboratory triaxial tests, the microscope tests also provide an effective way to clarify the dynamic characteristics of soft clay. Moreover, the variations of the pore size distribution (PSD) of the silty clay before and after freezing-thawing have less been investigated. In this paper, the mercury intrusion porosimetry (MIP) tests were conducted to study the microscope pore structures of the silty clay before and after freezing-thawing which had experienced the cyclic loading by the cyclic triaxial tests. A bottleneck phenomenon with the pore radii of 0.1-0.6 μm exists in the mercury intrusion process. The log-differential pore volume curves of the silty clay show a unimodal mode, where the mode with pore radii range from 0.05 to 0.6 μm represents intra-aggregate pores. After freezing-thawing, the final mercury intrusion volume and the most probable pore size of the samples without cyclic loadings increase about 6.0% and 30.64%, respectively. Under the same cyclic loadings, the mercury intrusion volume increases with the freezing temperature decreasing. The lower the frequency and the larger the cyclic stress ratio (CSR) of cyclic loadings are, the larger the deformations of the samples are and the smaller the most probable pore size becomes. The pore volume of samples after freezing-thawing (F) with frequency of 2.5 Hz is 8.74% larger than that with 0.5 Hz, while it is 5.03% of undisturbed samples (U). The most probably pore size of the samples after freezing-thawing with CSR of 0.125 is about 81.2% larger than that with CSR of 0.375. The process of mercury intrusion in MIP tests is similar to the air injection in a saturated soil as described along the drying water retention curve (WRC). The mercury intrusion pressure and the normalized volume are equivalent to the soil suction and the air volume within the soil, respectively. The MIP derived WRC is suitable for the prediction of the soil-water characteristic curves with the van Genuchten model. In addition, the microscope pore structures of the silty clay exhibit fractal characteristics with the thermal fractal dimension model in MIP tests.