Experimental Study On Volume Change Of Unsaturated Soils And Its Application To Estimation Of Subsidence

Unsaturated soil is a three-phase media and is composed of soil gain, water and air. And, unsaturated soils are widely distributed and display more complex behaviour compared with saturated soils in engineering practice. The theory of volume change is an important topic in unsaturated soils mechanics, state variables of strain and stress are related by constitutive equation of unsaturated soils. Unsaturated soil is composed of solid phase, fluid phase and gas phase, the pore water pressure and pore air pressure of samples is hard to be controlled and measured in tests, so it is serious lack of testing data of unsaturated soils. Besides, the application of unsaturated soils to practice fall behind of its theory development, many engineers are like to utilize conventional method to solve the unsaturated soil problems. How to perfect the theory of unsaturated soil and develope conventional soil mechanics to generalized soil mechanics is a big challenge for researchers. So, using GDS unsaturated triaxial apparatus, the volume change tests of unsaturated soils are conducted under the condition of controlling matric suction and net mean stress respectively in a predetermined stress path. Based on experimental results and the theory of unsaturated soil, the saturated-unsaturated settlement model is established, this model can be used to consider the settlement taken place in zone of saturated soils and in zone of unsaturated soils. Land subsidence due to drawdown of groundwater table is analyzed by using of the new model. The following portions are included in this dissertation.(1) The empirical relationship between matric suction and void ratio on drying path are established from four types of soils including sand, silt, clay and silty clay by utilizing the shrinkage curves and the soil-water characteristic curves which are determined through pressure-plate tests. It is indicated that during the process of drying path, the soil sample continuously shrinks with increase of matric suction. However, increase of suction will display no effect on the shrinkage deformation of the soil sample once the matric suction increases to a given value which is defined as the shrinkage limit suction s_s. The values of s_s are dependent on plasticityindex of soil. The process of shrinkage of soil is divided by yielding suction s₀ and shrinkage limit suction s_s into three stages, i.e., elastic stage, elasto-plastic stage and stage of shrinkage limit. Moreover, in the process of drying-shrinkage of soil sample, shrinkage of the soil sample is almost fulfilled when degree of saturation of soil sample decreases to 90% and void ratio of the soil sample almost keeps unchanged while degree saturation of soil is reduced to 70%.(2) By using the GDS unsaturated triaxial apparatus, this paper study the change of total volume change and the change of water content under controlled matric suction and net mean stress condition in a predetermined stress path. Two groupe of tests with different stress path are conducted in unsaturated triaxial tests.â‘ Isotropic compression tests. Different net mean stress is applied under the condition that matic suction is controlled;â‘¡Triaxial shrinkage tests. Different matric suction is applied under under the condition that net mean stress is controlled. The results show that the parameter related to compression is suction-dependent and rebound coefficient is dependent on matric suction in a certain degree, shrinkage index and the soil-water characteristic curve are also dependent on net mean stress. At a given matric suction, With the increase of net mean stress, the void ratio and water content of soil sample is decrease, but thedegree of saturation is increased. There is about linear relationship between water content of soil and net mean stress for the soil samples under higher suction. At a given net mean stress, the soil sample will shrinkage in accordance with increase of matric suction. In wetting stage, i.e., zero suction is applied to soil samples in saturated state, the circulating character be analysed.(3) People often like to utilize the theory of effective stress of saturated soil to calculate land subsidence. In fact, Drawdown of groundwater table will cause the change of net mean stress and matric suction in unsaturated soils and then induces volume change of the soil mass. Combined with saturated-unsaturated seepage flow and the constitutive model of volume change, this paper present a new settlement model which can consider settlement in both of saturated zone and unsaturated zone There is an essential distinction between settlement occured in saturated zone and unsaturated zone, so this model utilize effective stress theory to calculate settlement of saturated zone; for unsaturated zone, this model can calculate shrinkage of soil because of increase of matric suction and rebound of soil because of decrease of net mean stress.(4) Based on saturated-unsaturated settlement model, simulation analysis is conducted for Kai-Yuan Ke test model. Through the compare of analysis results and test data, this paper certificates the rationality and feasibility of saturated-unsaturated settlement model. The results of model test indicate that the settlement of unsaturated zone can not be omitted in some case. If we use conventional model to calculate the settlement of ground, the results will not coincidence with facts.Based on the assumed that pore air pressure equal to zero consistently and the magnitude of matric suction in unsaturated stratum is equilibrium with water table, using the saturated-unsaturated model, subsidence occurred in unsaturated soil zone and saturated zone are estimated respectively for different drawdown depth. Computed results indicate that in total amount of subsidence, the portion of subsidence of unsaturated soil zone cannot be overlooked. For unsaturated zone, the shrinkage of soil because of increase of suction and rebound of soil because of decrease of net mean stress are calculated. Also, the result show that shrinkage of soil has the leading position in deformation of unsaturated zone and the rebound of soil can be omitted.(5) During cutting of foundation ditch stage, well-point dewatering is often used to drawdown of groundwater table to avoid the seepage damage of foundation ditch. But, drawdown of groundwater table maybe causes the damage of surrounding building and underground pipeline. Based on simulation of two dimensional steady-state flows induced by foundation pit dewatering, this paper calculate the subsiedce occurred in saturated zone and unsaturated zone respectively. Computed results indicate that in total amount of subsidence, the portion of subsidence of unsaturated soil zone cannot be overlooked. The analyse of influencing factors are conducted, including drawdown depth, initial groundwater table, compression index and shrinkage index of soil, anisotropic permeability, initial matric suction in soil layer, coupIing relationship between suction and net mean stress. Base on simulation of two dimensional saturated-unsaturated unsteady flows, this paper analyse the dynamic process of settlement of ground caused by foundation pit dewatering, and the dependency relation of subsidence and time-space is discussed in detail.