Study On Strength Characteristics Of Lime-modified Expansive Soil Based On The Unsaturated Soil Theory

Expansive soils is a special kind of catastrophic soft soil,because of its bad engineering characteristics,many engineering problems or engineering disasters have been caused all over the world.Therefore,it is of great theoretical significance and engineering value to study the treatment method of expansive soil and its strength characteristics after treatment,and to discuss the law of its strength change under unsaturated condition.The research aimed at Nanning expansive soils,and relying on the government's projects funding,the strength characteristics of unsaturated expansive soils and unsaturated lime improved expansive soils have been studied in this thesis.All research works are supported by the following government's fund projects.These projects are included the National Natural Science Fund Project("Experimental study on nonlinear rheological characteristics of expansive soils",No.50978097),and the science&technology plan project which is supported by Hunan science and Technology Department("Experimental study on constitutive model of nonlinear rheological properties of expansive soils",No.2010SK3047)etc.By reading a lot of research literatures home and abroad,the relevant experimental research and theoretical analysis are finished.The main research works carried out are included as follows.Based on the theory of unsaturated soil,and combined with soil-water characteristic curves and triaxial test results,the shear strength model of expansive soil and lime treated expansive soil are established,considering the action of matric suction and radial stress.A method for determining the shear strength of unsaturated soil is proposed,under two stress conditions.The influence laws of matric suction and water content on the strength of unsaturated soil are investigated by analyzing the test results,and so the strength changing mechanism of lime improved expansive soil is revealed.The research results can provide a theoretical basis for the strength analysis of unsaturated expansive soil in practical engineering,it also provides a reliable method for quickly determining the shear strength index of unsaturated soils.The main research works,and the relevant research progresses as well as innovation conclusions have been obtained in this thesis are as follows.1.The main research works in this thesis(1)A series of routine indoor tests on Nanning expansive soils and lime improved expansive soils are carried out.These tests are included limiting moisture content tests,compaction tests,swelling tests,and standard moisture absorption tests and so on.The actual effect of lime improved expansive soil is analyzed,so that the optimum proportion of lime in this experimental study is determined.(2)The soil-water characteristic of Nanning expansive soils and lime-improved expansive soils are tested by using pressure plate,and the soil-water characteristic curves of these two soils are obtained respectively,under the conditions of different compaction degree.The experimental data are fitted by using Fredlund-Xing model.The related model parameters of these two kinds of soils are analyzed respectively,under the same conditions.(3)A series of triaxial tests are performed on the modified expansive soils and unsaturated soils,by using the fully automatic triaxial instrument(GDSTAS)which is produced by the British GDS instrument and Equipment Corporation.(4)Combining Vanapalli&Fredlund's shear strength formula of with Fredlund-Xing's soil-water characteristic curves model,shear strengths of these two kinds of unsaturated soils(expansive soils and improved expansive soils)are analyzed respectively.The variation laws of their shear strengths are investigated,under the action of different matric suction and different net normal stress.2.Main research progress and innovation conclusions obtained in this thesis(1)Based on the Fredlund-Xing model,a soil-water characteristic curves model is established,which is suitable for expansive soils and lime-improved expansive soils.Moreover,by fitting the experimental data,the related model parameters are obtained.Through the analysis of model parameters,it is found that with the decreasing of the intake air pressure value of lime-improved expansive soils,the residual water content is increased.In the same variation range of matric suction,the water content of lime-modified expansive soils changed little.It is shown that the water holding capacity of lime-improved expansive soil is higher than that of expansive soil,and its water stability is better than that of expansive soil.(2)It is found by triaxial tests,that the stress-strain curves of expansive soil are different from that of lime-modified expansive soils,under different conditions.Under the condition of saturated state,the stress-strain relations of expansive soils are mainly shown as strain stabile and strain hardening.But the stress-strain relations of lime-improved expansive soils are mainly shown as strain softening.Furthermore,with the matric suction is increasing,the peak value of shear strength of soil is appeared in advance,and the peak strength is increased obviously.The cohesion of improved expansive soil is affected obviously by matric suction,and it is increased with the increasing of matric suction,while the inner friction angle is slightly affected by the matric suction.(3)Based on the unsaturated soil theory,the strength calculation and analysis of expansive soils and lime-improved expansive soils are carried out.Through the analysis,the variety laws of shear strength of unsaturated soils are explored clearly,under different matric suction and different net normal stress,and the strength envelope surface of unsaturated soil in three-dimensional space is obtained.The influence of matric suction and net normal stress on the strength of unsaturated soil can be reflected directly,by this strength envelope surface.(4)According to the results obtained by consolidation drained triaxial shear tests on saturated soils and the theory of unsaturated soil strength,a method for calculating the shear strength indices of unsaturated soil based on matric suction and water content is proposed.On this basis,the shear strength computational theory of unsaturated soil is established.(5)By analyzing the relations between matric suction and shear strength,it is found that the contribution of matric suction to the shear strength of unsaturated soil is nonlinear.When the matric suction is small,the shear strength of unsaturated soil is increased gradually with the increasing of matric suction.But when the matric suction is reached to a certain value,the shear strength of unsaturated soil decreased gradually with the increasing of matric suction.That is,when the matric suction is reached to an optimum value,the shear strength of unsaturated soil is reached to the maximum.In conclusion,based on the experimental and theoretical researches on the strength characteristics of expansive soils and lime-modified expansive soils,the influence laws of matric suction and water content on the strength of unsaturated expansive soils are investigated.Furthermore,the mechanism of strength change of unsaturated lime-improved expansive soil is revealed,and an analytical model and a strength calculation theory which can better reflect the unsaturated characteristics of expansive soils are established in this thesis.The research results provide a reliable theoretical method for strength analysis of unsaturated expansive soils.