Model test on saturated soft ground improved by air-booster vacuum preloading without drained sandy cushion on the surface

The air-booster vacuum preloading without drained sandy cushion is a new method to deal with saturated soft cohesive soil foundations. To fully understand mechanical behavior of the saturated soft ground during its consolidation and drainage improved with this measure, a series of laboratory physical simulation tests have been conducted for a typical pressurized vacuum preloading unit with a geometric similarity ratio of about 1∶6 based on soft ground improvement in an expressway embankment project in Sichuan province. The length, width and height of the modeled ground in the tests are uniformly 1 m. Three pressurization modes are involved in the tests including traditional vacuum preloading (no pressurization), interval pressurization, and constant pressurization, and the variation characteristics of mechanical behavior of the improved ground have been obtained. The test results show that the excessive pore pressure, ground settlement and water discharge in the three modes show the same characteristics of rapid varying in the early stage (the first 40% of total duration of drainage consolidation) and slow changing in the remaining stage. The interval pressurization mode has the best effect on the drainage consolidation. For the interval pressurization, the excessive pore pressure fluctuates growingly by about 10% at the pressurizing moment, and dissipates about 3.1%～4.2% after a pressurization cycle. Compared with interval pressurization and no pressurization, in the constant pressurization mode the excessive pore pressure increases by about 25% and decreases by 9%, the ground settlement reduces by 12% and grows by 25%, and the water discharge diminishes by 15% and rises by 9%, respectively. Water content and parameters of shear strength of the improved ground are significantly decreasing and increasing under the three modes, respectively. Based on the experimental results, an improved numerical modeling method of analyzing such problems is provided. The results of ground settlement obtained using the model test, numerical simulation and theoretical analysis are in good agreement, while the computed early consolidation effect of the ground is slightly better.