Field Study And Calculation Analysis Of The Influence On Surrounding Soil Due To Vacuum Preloading

Field studies and finite element analyses of the impact of vacuum preloading on surrounding soils were carried out for a soft ground improvement project of Nansha hydraulic filling area of Guangzhou harbor. The emphasis of the field studies were placed on the observation of settlement and subsurface lateral displacement during vacuum preloading of three representative pilot test sites. A finite element program was coded based on the Biot consolidation theory, and the rationality of the program was verified by comparing the calculated results with the field test results. With the program, the influence of soil constitutive model parameters on the settlement and the lateral displacement of the soils surrounding the ground improvement area was studied. The protection effect of grid-shape cement-mixing piles and spiral spray piles were also analyzed. It has been shown that the extent and the depth of the influencing area of the vacuum preloading are related to the soil conditions outside the preloading area. The better the soil conditions, the lesser the influence extent and depth will be, and vice versa. Under the soil conditions of the pilot test sites, the maximum influence distance is over 50 m, the influence depth is over 15 m. For the ground-improved test site outside the improving site, the influence distance is approximately 15 m due to its relatively well soil conditions. The influence of vacuum preloading on the surrounding soils weakens gradually with the duration of vacuum pumping, and the significant influence occurred in the first thirty days. During this period, over 80% of total settlement and lateral displacement was completed. With increasing soil parameters K, e0,k, the settlement and lateral displacement increase, but reduce gradually with increasing distance away from the preloading boundaries. With increasing soil strength parameters c and φ the extent of tension fracture area reduces obviously. However, when the shear strength of upper soil layer is lower, the tension fracture may appear within a distance of 60 m away from the preloading boundaries.Jet grouting pile wall and grid-shape soil-cement deep mixing pile wall play an important role in adjusting the settlement and lateral displacement of surrounding soils. With these piles, soil deformations concentrated mostly inside the walls. The soil deformations outside the wall and the tensile stress on buildings induced by the vacuum pumping were reduced greatly. Comparatively, the influencing effect of the grid-shape cement-mixing piles is better than that of the spiral spray piles. Therefore, relative wide grid-shape cement-mixing pile wall should be adopted at the first priority to isolate the influence of vacuum preloading on the surrounding soils.