Effect Of Stress History On The Mechanical Properties Of Plane Strain State Powdered Clay

In the construction of foundation projects,problems such as foundation instability and uneven settlement are common.One of the reasons for such engineering accidents is that the stress history of the foundation soil is not considered in the design process.Studying the influence of stress history on the mechanical properties of soil not only helps to reveal the essential characteristics of geotechnical soil,but more importantly,provides basic support for decision making in engineering practice.In projects such as tunnels,narrow pits and embankments,the soil is in a two-dimensional plane strain state;however,conventional triaxial tests can only obtain the mechanical parameters of the soil in an axisymmetric state,which often bring large errors in actual projects.Therefore,it is of great theoretical and practical significance to consider the influence of stress history on the mechanical properties of the soil in the plane strain state.Therefore,this paper uses the self-developed plane strain meter to classify soils into normally consolidated soils and overconsolidated soils by setting different initial consolidation stresses,and conducts plane strain loading tests on the two types of soils to investigate the effects of stress history on the stress-strain relationship,strength and strength parameters,undeformed direction stress and undeformed direction stress coefficient of soils in plane strain state.Direct shear tests were also conducted on the two types of soils,and the strength parameters of the soils obtained from the direct shear tests were compared with the strength parameters of the soils in the plane strain condition.Finally,in order to investigate the effect of stress history on soil deformation in actual projects,numerical simulations of soil footings with different overconsolidation ratios were carried out by MIDAS GTS.The research contents and conclusions are as follows:1.The effect of stress history on the mechanical properties of pulverized clay was investigated using a direct shear instrument,the study found that: the shear property curves of normally consolidated soils were mostly of the hardened type,and the shear property of overconsolidated soils exhibit softening characteristics,and the larger OCR is,the more obvious the strain softening is.For both soils,the soil strength is proportional to the initial consolidation stress,and the larger the initial consolidation stress,the larger the friction angle within the soil,and the change of cohesion is not obvious.Overall,the strength as well as strength parameters of overconsolidated soils are higher than those of normally consolidated soils.2.The state of the mechanical properties of pulverized clay as influenced by the stress history in the plane strain state was studied.The results show that: the stress-strain curves of both normal consolidated soil and overconsolidated soil are hardened type.The stress-strain curve of normal consolidated soil can be divided into two stages,and the first stage has a large deformation under the action of small partial stress;in the second stage of shear,the partial stress gradually increases and the slope of the curve becomes larger.The stress-strain curve of overconsolidated soil can also be divided into two stages.In the first stage,there is almost no deformation of the soil,and the larger the enclosing pressure is,the larger the deformation of the soil sample is;in the second stage of shear,the main strain increases with the increase of the deformation stress.The initial consolidation stress and the surrounding pressure promote the strength of the soil,and the strength of the overconsolidated soil as well as the strength parameters are overall higher than that of the normally consolidated soil.The shear strength,internal friction angle and cohesion of the soil obtained from the plane strain test are greater than those obtained from the direct shear test.The stress in the undeformed direction of the overconsolidated soil increases with the increase of the principal stress,and the trend of the stress coefficient in the undeformed direction with the principal strain first decreases and then stabilizes,and finally stabilizes at about 0.25.3.The effect of stress history on the deformation of the foundation soil and its supporting structure was investigated by numerical simulation of soil pits with different superconsolidation ratios by MIDAS GTS.The results show that: The overall displacement trends of the pit and sheet pile under different overconsolidation ratios are similar,and the deformation of both the pit and the pile decreases with the increase of excavation depth.The larger the overconsolidation ratio,the smaller the deformation of the pit and pile,while the lower the degree of consolidation of the pit soil,the larger the settlement of the surface.The study shows that the stress history of the pit soil has a significant influence on the deformation of the pit and the support structure,so in the design and construction of the pit support structure,it is necessary to consider the soil force parameters such as the overconsolidation ratio of the soil.