Improvement of Soft Foundations Under a Rapid-Transit Tram Rail System

A case study is presented on the soft soil substrate under a rapid-transit tram system in a coastal area of southeast China. Attempts were made to improve the stability of this soil by the combined use of deep cement mixing (DCM) piles and prefabricated vertical drains (PVDs) method. Three test sections were improved separately by the DCM-PVD method, the replacement method, and vacuum preloading with PVDs (the VPL-PVD method). The DCM-PVD method yielded the largest settlement at the foundation surface during the construction period, namely 20.7% and 14.2% larger than those of the replacement method and VPL-PVD methods, respectively. In addition, the DCM-PVD method required the shortest consolidation time among the three methods and gave the highest degree of settlement (81.5%). After the rapid-transit tram system had been open to traffic for 6 months, the DCM-PVD method gave the smallest amount of post-construction settlement. As a result, the DCM-PVD method is found to be an effective method to control the settlement of soft foundations under a rapid-transit tram system.