Responses of excess pore water pressure in soft marine clay around a soil-cement column

The soil ground treated by deep cement mixing (DCM) in the field normally consists of cement-soil mixed columns and untreated soils. Although many attempts have been made, research on the consolidation behavior of the treated soil ground has been limited. To better understand the consolidation process of the DCM treated ground, in this study, an axisymmetric physical model test with full instrumentation was carried out. The physical model ground consisted of a central cement-soil column and surrounding soft soil. Excess pore water pressures in the soil and vertical pressures carried by the DCM column and the untreated soil were recorded throughout the test. Responses of excess pore pressure under loading and unloading stages are highlighted. Based on the data analysis, it is revealed that the improved ground consolidates faster than the pure soil ground. The major reason is considered to be that the DCM column reduces the vertical stress increment in the soil and results in a lower value of excess pore pressure. The decrease of excess pore water pressure in the middle of soil seems to be controlled by the reduction of total stress in the soil. Besides, a delayed pore water pressure increase was observed in the early period of the loading stage. During the unloading stages, the stress on the DCM column was found to be reduced by approximately the same magnitude as the decrease in the vertical pressure on the model ground. In addition, a small residual pore pressure in the soil was also found at the end of the unloading stages.