Case Study: Ground Improvement of Yangtze River Floodplain Soils with Combined Vacuum and Surcharge Preloading Method

A coal storage wharf located in Zhenjiang, Jiangsu province, China, has a planned annual capacity of 15 million tons. The thick Yangtze River floodplain is rich in muddy-silty clay with high compressibility, low bearing capacity, and poor stability, which needs to be improved to meet engineering requirements. In this paper, a case study on soft clay treatment with the vacuum preloading method (VPM) and combined vacuum and surcharge preloading method (CV-SPM) is introduced and discussed. Negative pore-water pressure was induced by vacuum pressure in the VPM, which led to the seepage of water and consolidation of soils. However, the vacuum pressure could never exceed 101,325 Pa (1 atm), so the additional filling surcharge was usually adopted to raise the preloading load in the CV-SPM when the bearing capacity requirement was higher. To perform ground improvement in the field, first in situ soil samples were taken from the construction site before the treatment, and the properties of soils were investigated through laboratory tests, which were used as the reference to analyze the effect of ground improvement. Second, during the construction process vacuum pressure, ground settlement, and pore-water pressure as well as lateral displacement of soils were monitored and analyzed. Finally, vane shear tests and shallow plate bearing tests were carried out to investigate and evaluate the effect of ground improvement. Test results show that the undrained shear strength increased remarkably, and the bearing capacity reached the design requirements after around 100 days of preloading, indicating that VPM and CV-SPM are appropriate and effective for ground improvement of the thick muddy-silty clay laying in the Yangtze River floodplain.