Land Subsidence Caused By High-rise Building Group Based On The Varying Groundwater Table

More than 60 countries are suffering from the costly land subsidence. The economic loss induced by land subsidence in Shanghai has reached over 46 billion dollars, which means that each millimeter subsidence costs Shanghai nearly 25 million dollars. This paper focuses on the land subsidence in Shanghai. The methods of theoretical analysis, numerical simulation and physical model test are applied to study land subsidence caused by the high-rise building group and the pumping and recharging of groundwater. The results provide references for the policy-makers to control the land subsidence in soft soil area induced by the high-rise building group and the pumping and recharging of groundwater.(1)The model of land subsidence caused by pumping of single well in multi-layered soil region is developed. Take the 2-layered soil as an example, when the consolidation of aquitard is not considered, compared with the maximum vertical displacement, the maximum horizontal displacement is 1 order magnitude less. The scope of horizontal and vertical displacement is approximately the same. When the differential settlement of the aquifer is significant, the settlement of the surface of the overlying aquitard is less than that of the aquifer. When the differential settlement of the aquifer is not indistinctive, the settlement of the surface of the overlying aquitard is close to that of the aquifer. The 1D theory of consolidation is applied to consider the consolidation process of the overlying aquitard. The constitutive model of elastic and Merchant visco-elastic is adopted. At the beginning of consolidation, the consolidation degree of the two situations is close. With the consolidation developing, the difference of the consolidation degree of the 2 situations increases. The maximum difference value is 29.6% when the calculation with the parameters in this thesis is conducted.(2)The pile length, the distance between the buildings, the drawdown and the pumping aquifers are changed to study the land subsidence caused by the high-rise buildings and the groundwater pumping with the numerical modeling. The subsidence value decreases with the pile length increases, yet the distribution of land subsidence is nearly the same. When change distance between the buildings, the subsidence in the building position is the largest. The subsidence value of the building region center decreases sharply with the distance between the buildings increasing. Land subsidence of pumping the deeper aquifer is much less than that of the shallow one. The influence scope of the building is about 0.5 time the width of the building base when pumping. When pump the first aquifer, the existence of the building reduces the subsidence. The subsidence of the building position is about 50% of the same position that has no building. The axial force of the pile in the corner increases about 1% and the pile in the center about 1.5% with the drawdown increase 1m. The amplitude of variation of the axial force is distinct and the existence of the building influences the soil deformation greatly. When the second aquifer is pumped, the existence of the building influences the soil deformation to a small degree. The change of the axial force of the pile is the main reason why soil in the building position and the nearby place changes greatly. According to the vertical influence scope of building and pumping, when the pumping policy and the layered soil compression in Shanghai is analyzed, it can be concluded that the main reason of subsidence in urban city of Shanghai is transferring from pumping to engineering construction. The subsidence caused by engineering construction is about 40% of the total subsidence.(3) Model test of land subsidence was conducted. The Particle Image Velocimetry(PIV) is used to study the layered subsidence caused by building load and pumping. The prior constructed building makes the soil in the posterior building position consolidate and the subsidence of the posterior building is less than that of the prior one. The posterior building increases the subsidence rate of the prior building. The superimposed effect of land subsidence caused by building load is found. The subsidence in the interior zone increases because of the superimposed effect. The differential settlement is greater in the exterior zone. When the exploitation volume of groundwater is less, building influences the soil compression at the pumping period barely. The surface subsides evenly. When the exploitation volume of groundwater increases, the negative friction effect decreases the subsidence of the building and the surrounding soils. The differential settlement increases. Compared with the subsidence caused by building load, subsidence caused by pumping develops slowly but keeps much more time. The compression of the soft soil layer counts for the main subsidence of the total in both loading and pumping period. The recharge of ground can help suppress the subsidence development. However, the rebound value is little. Land subsidence is irreversible.(4) Compare the physical model test with numerical modeling was conducted. At the loading period, the subsidence in the interior zone increases but the differential settlement decreases because of the superimposed effect. Under the influence of the existing building, the subsidence decreases but the differential settlement increases because of the pile-soil interaction at the pumping period. The pile axis force increases at the same time. The soil influenced by the superimposed effect at the loading period greatly is also influenced greatly by the pile at the pumping period. The combined action of building load and exploitation of groundwater decreases the differential settlement and the subsidence tends to develop evenly. However, if the exploitation volume is too much, the differential settlement may increases reversely.