Seismic Mechanism And Dynamic Response Characteristics Of Complex Stress Condition In Situ Loess

The Loess Plateau is located in the north-south seismic zone,and earthquakes have caused a large number of landslides,seismic traps and other geological disasters.The study of the dynamic response characteristics,seismic trap mechanism and damage characteristics of loess is a difficult and focal point of geotechnical engineering.In this paper,the dynamic response acceleration,shear deformation,seismic deformation,fracture development and dynamic shear damage of in-situ loess,as well as the damage deformation law of shallow loess in centrifugal vibration model,seismic deformation characteristics and potential fracture slip damage mode are analysed by using dynamic torsional shear test and centrifugal machine shaking table in-situ loess model test under complex stress conditions.The research results achieved the following understanding.(1)Through the dynamic torsional shear tests of homogeneously consolidated and biased consolidated in-situ loess under complex stress conditions,the changes in dynamic modulus,damping ratio and dynamic strength curves under different mid-major stress ratios were tested and analysed.Expressions for the relationship between maximum dynamic shear modulus,constitutivity and consolidation envelope pressure were established.It was revealed that two sets of intersecting shear damage surfaces were generated in the loess specimens under torsional shear under complex stress conditions,and the damage state was closely related to the original loess fracture cracks,and the soil units between the damage surfaces maintained their original structure.(2)The relationship between loess seismic subsidence and dynamic stress,vibration frequency,consolidation envelope pressure and constitution was established,and the three-dimensional strain characteristics of loess under three-way principal stress consolidation were analysed to obtain the change law of dynamic characteristics of in-situ loess under complex stress conditions.The variation patterns of dynamic shear modulus and damping ratio on dynamic shear strain of loess under different medium principal stress ratios are similar,and converge under the damage criteria.The relationship between the decay variation of dynamic shear modulus with dynamic shear strain and the relationship between dynamic elastic shear yield strength and consolidation mean spherical stress is revealed(3)The dynamic torsional shear properties of in-situ loess under complex stress conditions are systematically studied to reveal the variation of dynamic shear yield and damage strength of loess under different mid-major stress ratios,and the relationship between dynamic shear damage strength and consolidation mean spherical stress of loess under different mid-major stress ratio consolidation conditions is analysed.The yield and damage states of the loess shear deformation under dynamic shear exist in the stress space and the dynamic shear damage surface is located within the static shear damage surface.(4)The in-situ loess slope model tests reveal the deformation development of loess seismic trap and shear.Different moisture contents and slope ratios have different effects on the dynamic deformation and dynamic amplification coefficients of different soil layers of the loess slope under the same excitation effect.The continuous development of tension fissures and cracks creates water seepage channels and potential slip surfaces.It is shown that the development of fractures under the action of strong earthquakes in history provided sliding surface conditions for natural loess slope landslides to arise.(5)The ground vibration response pattern and seismic trapping characteristics of the in-situ loess foundation are revealed.The lower the moisture content of the foundation under the same excitation,the more obvious the amplification effect is;the cumulative development of seismic trap deformation is produced by the foundation model under different excitation.The seismic intensity,stratigraphic thickness and water content are the main causes of seismic deformation in loess foundations.The interaction between shear deformation and seismic settlement under seismic action causes the development of dynamic response of soil cracks,serious shear damage in the shallow layer of the foundation,collapse of the soil structure from the inside to the outside,and the development of internal penetrating crack extension.