Full scale consolidation test on ultra-soft soil improved by prefabricated vertical drains in MAE MOH mine, Thailand

The reclamation of slurry pond with ultra-soft soil deposit using the prefabricated vertical drains (PVDs) with preloading technique in the Mae Moh mine, Lampang, Thailand is a challenging work and is illustrated in this paper. Geotextile reinforcement was used to strengthen bearing capacity of the soil foundation prior to the installation of sand platform. The delay of excess pore water pressure dissipation at the early loading stage occurred despite the occurrence of large settlements was a distinct behavior of the ultra-soft soil. Within the delayed time, the calculated average degree of consolidation based on measured settlements, U-s increased while the calculated average degree of consolidation based on measured excess pore water pressures, U-e and the undrained shear strength, S-u remained unchanged. Beyond the delayed time, both U-e and S-u increased significantly with time and when U-s > 90%, the difference between U-e and U-s was observed to be small. It was suggested to use U-s for approximation of Su when U-s > 90% based on the SHANSEP's method. The successful installation and application of PVD to improve ultra-soft soil in this research is applicable for the ground improvement of similar problematic ultra-soft soils in international land reclamation projects.