Theoretical Calculation Method Of Swelling Capacity Of Expansive Soil And Its Application In Slope Engineering

In engineering practice,swelling pressure is an important parameter for expansive soil foundation pit,its right selection is very important to the success or failure of foundation pit support.Swelling stress is caused by volume expansion of expansive soil containing hydrophilic minerals after water absorption.At present,the parameters of swelling pressure and swelling ratio are obtained on the basis of a large number of test data.For the same type of expansive soil,different test results can be obtained by different test methods and conditions.How to reasonably choose the parameters is lack of a theoretical basis.In this paper,a theoretical model of transformation from micro-particle expansion to soil mass expansion and then to macro-expansion stress based on the expansion mechanism and chemical thermodynamics principle is attempted to be established.The theoretical model can be used to calculate the swelling pressure acting on the supporting structure.From the perspective of weakly bound water film thickening and taking the principle of chemical thermodynamics and the existing expansion mechanism as the research foundation,a theoretical model of volume expansion of soil particle caused by the thickening of water film of soil particle is established based on the change of surface free energy of soil particle in the process of water absorption.The theoretical formulas for calculating the water film thickness of soil particle are deduced,and the calculation method for the swelling ratio of soil particle during water absorption is established.The calculating method provides a basis for deducing the swelling deformation of expansive soil mass.In order to reduce the influence of structure effect caused by particle size difference on free swelling ratio,an improved free swelling test method is proposed.Based on the theory of particle accumulation,the geometric structure model of expansive soil mass composed of mineral particles with different expansibility and with various particle accumulation forms is established.And combined with the theoretical formula of the swelling ratio of single soil particle,the theoretical formulas of the swelling ratios of soil mass under three test conditions of triaxial expansion,lateral expansion and improved free expansion are derived.Swelling deformation and swelling stress are related to water content of soil samples.Therefore,expansive modulus is a variable related to water content and it can be defined as a function of water content.Through improving the test device and method,the relationship between expansive modulus and water content satisfying engineering accuracy is obtained,then the swelling stress can be got combing the theoretical formula of the swelling ratio.The external cause of swelling is the increase of soil water content,so it is necessary to study the changes of humidity field in expansive soil.Since the existence of cracks improves the permeability and aggravates the change of humidity field in expansive soil mass,it is necessary to study the development degree of cracks.A method is established to describe the crack network of expansive soil by the following five parameters: crack length,the average crack width,depth of crack node,angle between cracks and number of zero edges of hexagon.Through the combination of surface crack photography,image processing,depth measurement by wire insertion and modified crack depth by CT scanning,the geometric parameters of expansive fracture are obtained,and a practical method based on monte carlo method is established to simulate and calculate the ratio of cracks,which can provide parameters for the calculation of humidity field of expansive soil mass with cracks.The expansive soil pressure acting on the supporting structure is the additional load formed by the coordinated deformation of the slope soil and the supporting structure after the expansion potential is generated due to the change of humidity field in expansive soil slope.By incorporating the principle of micro-layer method into the theoretical calculation of swelling stress,the formulas for calculating active earth pressure and passive earth pressure of expansive soil slope are derived.Based on the above achievements,a set of theoretical calculation methods from the theoretical expansion of expansive soil to the expansive soil pressure of slope is established.A related indoor test of undisturbed soil foundation pit slope with cracks is conducted.It is preliminary proved that the theoretical expansion of expansive soil derived by thermodynamics theory and its application on slope engineering is feasible and practical.