Analysis On Engineering Properties Andconstitutive Model Of Coarse-grained Soil

Among the many types of earth-rock dam,rock-fill dam is an important type of dam.Compared with other types of earth-rock dams,rock-fill dam has significant advantages under different foundation conditions,such as low cost,short construction period,good safety,strong adaptability and so on.Thus it has been applied in many practical projects of earth-rock dam.As the main material for rock-fill dam,coarse-grained soil is a kind of granular particles,which is filled by particles of different sizes.It has good engineering properties.Its high strength can meet the requirements of shear strength of rock-fill dam,small deformation can meet the needs of stability of rock-fill dam,and strong water permeability can make the major rockfill areas reach the design standard of free drainage.At present,it has been widely used in the construction of rock-fill dam.For the actual projects,it is very important to study the engineering properties of coarse-grained soil.Although many scholars have done a lot of research in this field,due to the complexity of its mechanical properties,how to carry out tests reasonably and how to establish an appropriate constitutive model are the key contents of coarse-grained soil research.During the construction inspection in the major rockfill area of Hanshan Reservoir,there is a phenomenon that the coarse material content cannot meet the design requirements,and whether the shear strength and the permeability coefficient of this area can meet the design standards needs to be verified.In order to deeply study the engineering characteristics and constitutive model of coarse-grained soil,and to provide relevant theoretical and experimental basis for actual projects,according to the self-test data,this paper selects the upper and lower envelope of the design gradation curves and the other three groups of gradation curves with unqualified coarse-grained content,a total of five groups are selected,and the five groups of coarse-grained soil with different coarse-grained content are selected as the research object.Focus on seepage,strength,deformation and constitutive model.The main research contents include:(1)Through in-situ permeability test to study the permeability characteristics of coarse-grained soil,the results show that the permeability of five groups of graded samples can meet the requirements of free drainage in the major rockfill area;the influence of coarse-grained content on the permeability is mainly caused by the change of relative content between coarse-grained soil and fine-grained soil;and the mud content of 10%is the characteristic point,which affects the permeability of coarse-grained soil.In the actual project,the mud content should be controlled.(2)Through in-situ direct shear test to study the direct shear strength of coarse-grained soil,the results show that the direct shear strength of five groups of samples can meet the strength requirements;the internal friction angles of five groups of graded rockfill materials are around 44 degrees,and the internal friction angle is less affected by gradation;in practical engineering,the friction strength is the main strength source of coarse-grained soil,and occlusion has little effect on the strength of coarse-grained soil.(3)Through a series of large-scale triaxial consolidation drainage shear tests on coarse-grained soil.The results show that when the confining pressure is constant,with the decrease of coarse content,the peak deviation stress increases first and then decreases,the more obvious the strain softening and dilatancy effect are.When the content of coarse material is constant,with the increase of confining pressure,the peak deviation stress gradually increases,and the coarse-grained soil gradually changes from strain softening type to strain hardening type,in terms of volume deformation,the more obvious the shear shrinkage deformation is.The shear strength of the samples with higher coarse material content than the design standard cannot meet the design requirements,in practical engineering,the coarse-grained soil in this area should be reduced;the shear strength of the samples with lower coarse material content meets the requirements.(4)Combined with the triaxial test data,the applicability of the constitutive model of coarse-grained soil is analyzed,The application of Duncan-Chang model will cause large errors when describing the tangential Poisson ratio and tangential volume modulus of the coarse-grained soil,and the model needs to be improved.When use the rowe dilatancy model,the dilatancy parametershas have good normalization at the larger axial strain(?_a>3%).The Duncan-Chang model is improved and verified.The results show that the improved model has advantages in describing the strain softening and dilatancy characteristics,the tangent Poisson's ratio calculated by the improved formula tends to be consistent with that obtained by the test.