Study On The Constitutive Model Of Marsh Turfy Soil In The East Of Jilin Province Considering Stress Path

Turfy soil is a special kind of soil which is widely distributed in the Changbai Mountains, Great Khingan Mountains, Lesser Khingan Mountains, Sanjiang Plain, most mountain areas in Xinjiang, Western Sichuan Zoige Plateau, Qinghai-Tibet Plateau, Yunnan plateau, and in the Middle and Lower Reaches of the Yangtze River in China. Due to the unique climate, topography and hydrological conditions, the plant remains (including roots, stems, leaves, fruits, etc.) in the surface swamp environment turn into humus soil under the effects of oxygen and microorganisms (aerobic bacteria). The material composition, physical properties, chemical properties, water physical properties and mechanical properties is very complex. It forms a class with unique properties different from other soil properties, namely high organic matter, high-compression, high moisture, high permeability, high void ratio, low degree of decomposition because of the special external environment, and it also has the general characteristics of soft soil, namely:nonlinear, press hard, expansion and contraction, plasticity, anisotropy and rheology. With the development of economic construction, carrying out large-scale construction projects in these turfy soil distribution areas has become a development trend. In this way, it will bring many problems in geotechnical engineering design at the region of turfy soil.The equilibrium conditions, physical conditions and deformation conditions should be considered in numerical computation of soil, of which only the physical condition (constitutive relation) depend on the nature of the intrinsic material, which is the key to numerical calculation. With the continuous development of computer technology and the increasing maturity of numerical calculation method, solving the numerical equations is not a big problem for many scholars. Therefore, the key to solve the problem is to establish the constitutive model that can reasonably describe the soil strength and deformation characteristics. Experimental researches have shown that the stress history, stress path and physical state of soil composition, structure, etc. have a significant impact on its constitutive model. In many types of soil, turfy soil contains a lot of plant residues and organic matter, and the influence of stress path on the constitutive model will be more significant due to the complexity of its composition and structure. At present, the engineering design in turfy soil area mostly adopts the general soil constitutive model, and it is difficult to match the actual engineering conditions, which is bound to bring a large deviation between the theoretical calculation and the actual situation. Solving this kind of problem is in urgent need of turfy soil constitutive model considering stress path, which can provide theoretical basis on the solution of the engineering problems and geotechnical engineering design in turfy soil area.Based on the above reasons and the case studies, this paper studied further on the constitutive model of marsh turfy soil in the east of Jilin province considering stress path under the support of the National Natural Science Foundation of China’Study on the structural property and constitutive relations for marsh peat of seasonally frozen region’and multiple related transportation construction projects in Jilin Province. The main contents are as follows:Firstly the correlation between the degree of decomposition and the organic matter content and physical and mechanical indicators of turfy soil was analyzed. Then, the stress path test plan is designed with orthogonal experimental design theory, and four groups of undisturbed turfy soil were conducted consolidated drained triaxial compression tests under three kinds of stress paths: CUAI stress path (Confining Pressure is Unchanged and Axial Pressure Increases); CIAI stress path (Confining Pressure Increases and Axial Pressure Increases); CDAI stress path (Confining Pressure Decreases and Axial Pressure Increases), researching on the deviatoric stress - axial strain or deviatoric strain characteristics, volumetric stress-strain characteristics, strength characteristics and volumetric contraction chatacteristics after unloading of the undisturbed turfy soil under the different stress path. Combined with the degree of decomposition, the organic matter content and the mesoscopic structures, the mechanism of their formation were studied. The constitutive model of the undisturbed turfy soil in the east of Jilin province considering stress path has been established using the non-linear incremental theory, elastic-plastic theory and the equivalence principle of stress path and the model was validated. The main conclusions are as follows:1. The correlation of the turfy soil material composition and the physical and mechanical basic indicators was discussed, which found that:(1) With the increase of organic matter content (not including plant residues and plant fibers), the natural density and the soil particle density show a decreasing trend, but the moisture content shows an increasing trend. The organic matter within turfy soil is mostly humus, whose specific gravity and density are smaller than the soil particles. Meantime, the organic granule has strong adsorption capacity, which makes the particle surface attach more water. The smaller the degree of decomposition is. The greater the share of plant residues are, whose proportion and density is less than the soil particles. At the same time, its tubular structure can hold a lot of water, which make the moisture content great.(2) The smaller the degree of decomposition is, the higher share of plant residues is and the more loose structure is. Meanwhile the plant roots inside soil occur lateral bending deformation under vertical loads while it will produce the lateral tensile stress that will make greater rebound deformation. So the compression coefficient, the compression index and the rebound index become greater. However, the greater the organic matter content is, the stronger the hydrophilic of turfy soil is, the larger the void ratio is, and the stronger the compression is. Meantime, the organic matter particle size is small that will produce an easier shift after unloading. So the compression coefficient, the compression index and the rebound index become greater.2. Derived from CUAI stress path tests of turfy soil:(1) Under the same consolidation state, the strain of the turfy soil that has the larger degree of decomposition and lower organic matter content develops slowly. It is because this kind of soil contains more clayey soil particles, which has stronger resistance to deformation ability due to the interlocking and the friction effect between particles. However, the strain of the turfy soil that has the larger degree of decomposition and the higher organic matter content develops rapidly. This is mainly because it contains a lot of organic matter content. The strong absorption capacity can make void ratio increase, which has the weak ability to resist deformation.(2) The volumetric strain rate of the turfy soil that has the larger degree of decomposition and the higher organic matter content is the biggest under the smaller consolidation stress, which can’t make the internal micro-pores be fully tamped. The organic particles may therefore have the relative displacement under the action of shear loads. However, the volumetric strain rate of the turfy soil that has the larger degree of decomposition and the lower organic matter content is the biggest under the larger consolidation stress.’Skeleton’ structure formed by clayey soil particles is destroyed by the shear loads while the original micro pores are filling by the clayey soil particles that were cut into pieces and organic matter particles.(3) The turfy soil is a kind of highly compressible material, which will likely have a greater deformation under load. The nonlinear stress-strain relationship of the turfy soil therefore can be described through a piecewise linear approximation. Elastic parameters of turfy soil in each period are constant. But there will be differences in the various time periods, depending on the stress state of the current period.3. Derived from CIAI stress path tests of turfy soil:(1) When the consolidation stress level is high, the undisturbed turfy soil will appear different degrees of volumetric contraction phenomenon after unloading. On the one hand, it is relative to the position energy transformation of turfy soil and the extension tectonics of plant residues and plant fibers; on the other hand, it is also relative to the extrusion damage of hydraulic path while turfy soil is forming.(2) Under the same consolidation state, the yield deviatoric stress and the yield ball stress of undisturbed turfy soil are both affected by the degree of decomposition and the organic matter content. When the degree of decomposition is low, a large number of non-decomposed plant residues and plant fiber within the turfy soil can form more "skeleton" root-like structure, which can provide drainage path and prevent turfy soil particle relative sliding when sheared, showing the larger yield stress. The smaller the organic matter content is, the larger occlusal force of big diameter clay should be, showing the larger yield stress.(3) The turfy soil has obvious structural property. When the suffered ball stress is less than the yield ball stress, its primary structure has not been destroyed, volume deformation is smaller. As the ball stress increases, the primary structure is continuously destroyed, and the volume deformation is developing rapidly. So in the semilog coordinate system, its ball stress-volumetric strain curve and is combined with power function curve and straight line. The mathematical relationship of volumetric strain and deviatoric strain increment can be obtained by the nonlinear incremental theory.4. Derived from CDAI stress path tests of turfy soil(1) The phenomenon of volumetric contraction phenomenon after unloading under CDAI stress path is more obvious than that under CIAI stress path. This is mainly because the turfy soil’s microstructure state under CDAI stress path is more easily convert from point-surface contact to plane-plane contact. Meanwhile, the transverse hydraulic path formed by plant roots in turfy soil is more difficult to destroy, thus resulting in a larger volumetric contraction content, which presents a tendency of decrease with the initial consolidated stress increasing.(2) Under the same consolidation stress, if organic matter content of the undisturbed turfy soil is similar, the less the degree of decomposition is, the stronger the original structural property is and the more obvious the turning point of the deviatoric stress- volumetric strain curve is. Meanwhile the hydraulic path formed by plant roots in turfy soil will occur phased destruction when subjected to shear stress, make the deviatoric stress show an obvious evidence of fluctuation along with the development of the volumetric strain.(3) The deviatoric stress and deviatoric strain shows hyperbola relationship under CDAI stress path, the mathematical relationship of deviatoric strain increment can be obtained by the nonlinear incremental theory. However, the volumetric strain increment is all produced by the deviatoric stress, but it is also relate to the initial consolidated stress, combining with the dilatancy equation of modified Cambridge model and the elastoplastic incremental theory, the mathematical relationship of plastic volumetric strain increment of turfy soil can be obtained.5. Using the principle of equivalent stress path and unloading criterion established can reflect in eastern Jilin province original state of peat soil nonlinearity, hard pressure, discharge carrier shrinkage and expansion shrinkage stress path constitutive model, combined with the path of conventional triaxial test of the model is verified, the results show that the calculation of stress and strain regularity result was consistent with the experiment, and can better reflect the original state of deformation and strength characteristics of peat soil.