Improved Synchronous and Alternate Vacuum Preloading Method for Newly Dredged Fills: Laboratory Model Study

Abstract

Cost effectiveness is a major factor in most vacuum preloading projects, considering the significantly reduced time allotted to achieve a relatively high degree of consolidation. However, the shallow base and deep layer of newly dredged fills are always treated separately because the traditional vacuum preloading method can barely make sufficient improvement in the one-time treatment in these weak and complex grounds. Therefore, an improved vacuum preloading method was developed to consolidate newly dredged fills in the Lin Gang Industrial Zone of Tianjin City, China. This improved method consists of three main systems: loading devices, a vacuum preloading system, and a moisture separator technique. By using two loading devices, two lengths of the prefabricated vertical drains (PVDs), and a novel moisture separator, the shallow base and deep layer of the newly dredged fills can be simultaneously processed. The results of water discharge and pore water pressure were measured using three parallel laboratory model tests. The moisture content test and the mercury intrusion porosimetry (MIP) test were conducted along with the vane shear strength test. The testing data shows that the improved vacuum preloading method can substantially improve the bearing capacity of an ultrasoft ground. The multiple vacuum preloading method and the improved synchronous and alternate vacuum preloading method increased the water discharge amount by 8.4% and 27.9%, respectively, over the traditional method. The final dissipation of pore water pressure by the traditional method was one-fifth of that of the improved method because the motion directions of soil particles were changed in the improved method. In addition, the pore size distribution (PSD) of soil changed from a bimodal distribution to a unimodal distribution, as shown by the MIP test results. With similar values in total intrusion volume, porosity, and median pore diameter, the data show that by using this newly improved synchronous and alternate vacuum preloading method, the treatment of ultrasoft dredged fills can be more even, which successfully avoids the common problem of uneven settlement of the ground.