Strength Characterization Of Damaged Expansive Soil And A Generalized Soil Water Characteristic Curve Considering Two Stress State Variables

Expansive soil is widely distributed in over 60 countries and regions in the world.Because of its remarkable swell-shrink and crack characteristics,economy lose caused is more than $15 billion every year.One of the important characteristics of expansive soil is its structural damage,which has crucial influences on the deformation,strength and stability of expansive soil.It is the current research task to study on the characterization and the evolution theory of damage,while the correctness of a damage constitutive model is determined essentially by the rationality of damage variables.The key reason causes damage is induced by the change of external moisture state.Luckily,the state of water in the soil can be characterized by soil-water characteristic curve(SWCC)expediently.Based on SWCC equation,it is extremely easy to predict the strength,permeability and stability of soil aggregate.But the acquisition of experimental data is extremely time-consuming and labour-requiring,and is the key to limit the engineering application of SWCC equation.This article firstly carried out the strength characterization and assessment damage variables of damaged expansive soil,then based on van Genuchten soil-water characterization equation and Fredlund double stress variable theory,a generalized SWCC of unsaturated soil was proposed,which takes soil deformation and multimodal probability pore-size density function(M-PDF)into account.It has the feature of fitting SWCC by at least three experiment data with high characterization precision.Multi-dimension verifications were carried out.The main research work is as follows:(1)To evaluate the applicability of several existing methods for characterizing the damage variables,a series of CT-triaxial tests was conducted on the expansive soil sampled from the site at the middle part of the South-to-North Water Transfer Project in Taocha,China.Damage variable characterization methods based on the loaded area,mean ME and variance SD of CT numbers are quantitatively examined by means of CT-triaxial shear tests under 100 kPa net confining stress and 50 kPa suction respectively.Based on the Kachanov's continuous damage variable principle and the Fredlund's effective stress theory for unsaturated soil,a strength characterization method for structure-damaged expansive soil is proposed.15 reconstituted specimens are divided into three groups with different cylindrical-pore damages.Whereafter,triaxial compression tests are conducted by using a modified multifunction unsaturated soil triaxial apparatus under controlled net confining pressures of 50,100 and 150 kPa respectively.The influences of the initial pore-damage primitives and fissure evolution under triaxial loading on the mechanical behaviors of expansive soil are analyzed quantitatively.(2)The current classic SWCC equations were compared,the concept of generalized SWCC was defined,and relative volumetric water ratio was proposed too.Based on Fredlund two stress variables theory and van Genuchten soil-water characteristic equation,a generalized SWCC equation was derived,which has the capacities of inclusiveness for complex factors,and could deal with soil deformation and solid-liquid-gas hydro-mechanical coupling effects and M-PDF conditions.(3)The influences of geostatic stress of expansive clay(depth of 0m~10m)on the soil structural evolution were investigated with two Geo-Expert high one-dimensional stress controllable volumetric pressure plate extractor systems based on axis translation technique.With the obtained data and literature data,the generalized SWCC which considering two stress state variables and M-PDF conditions we proposed previously was validated against a wide range of experiments under the following different conditions:(1)K0 consolidation tests and desorption-SWCCs for slightly double-series porosity Nanyang expansive under different geostatic stress.(2)Double-series porosity diatomaceous earth mixtures SWCCs with zero net normal stress state.(3)One porosity Korean residual soil SWCCs at different net confining stress.(4)Desorption-adsorption path of Bapaume loess from France under different stress path,and Edosaki sandy soils from Japan under different initial dry density.Performances of van Genuchten equation,Fredlund&Xing equation and the equation proposed were compared.The main conclusions obtained are as follows:(1)The damage variable defined with the damage area and the SD value is appropriate.However,the ME value should not be used to define the damage variable of expansive soil,though it is agreeable in the characterization of the damages of rock.The mechanical characterization of unsaturated soil associated with structure and damage can describe the mechanical behaviors of soil commendably.The strengthening or weakening effect of pore damage is closely related to the initial water content.At 24 % water content,the strength of no damage sample is lower than 3 mm center hole sample,6 mm center hole sample,6 mm eccentric hole sample and four 3 mm peripheral holes sample.With inital water content of 7%,pore damage has weakening effects on the strength,and the evolution form of fissure plays a leading role in the structure and strength of the soil.Particularly,its mechanical behaviors are characterized by two stages of fissure development and residual strength,and four damage types of linear-hardening,elastic-plastic,brittle-plastic and linear-softening failure modes.(2)The proposed generalized SWCC equation takes M-PDF situation and soil deformation into consideration,modified van Genuchten SWCC from stress history to a general stress state situation.Compared with the two-parameter Brooks&Corey equation,three-parameter van Genuchten equation as well as the four-parameter Fredlund&Xing equation,the proposed SWCC has only three parameters,two of which could be obtained by least-square linear fitting method in the plane of double logarithmic coordinate system of ?relative volumetric water ratio versus suction?,only one left parameter could be achieved by nonlinear least-square fitting method.(3)The unimodal SWCCs with zero net normal stress,bimodal SWCCs with two stress state variables,unimodal SWCCs under isotropic stress state,different desorption-adsorption paths and initial dry density SWCCs fitting as well as predicting curves show that,the new generalized SWCC methodology we proposed involves two stress state variables as well as mult-pore-size density distribution function has the ability of inclusion for complex influence factors,it's flexible and applicable.It not only can be applied to fit experiments data as the traditional way,but also to fit the SWCC with at least three experiment data with a high-precision performance.An entire unimodal SWCC could be achieved with at least three soil-water measured data at different stress condition by the ?three point fitting method?,which greatly shortens the acquisition time of SWCC.The model provides with a new way to predict the strength,permeability and soil-water retention of the soil quantitatively in an easier and timesaving process.