Investigation For Uniaxial Compression Mechanical Properties Of Fissured Loess Disturbed By Seismic Load

The loess region in the northwest of China was affected by multiple seismic zones,the earthquake is active and serious earthquakes occur frequently.Moreover,due to the initial defects in the loess,such as joint fissures,micro-cracks and pores,it has high vibration vulnerability,and geological disasters such as landslides and collapses can occur under the action of moderate and strong earthquakes.Therefore,studying the strength and deformation characteristics of fissured loess after seismic load disturbance is of great significance for seismic design and stability analysis of slope projects in loess area.In this paper,the fissured loess is taken as the research object.The horizontal sine wave output from the shaking table is used to simulate the disturbance of the seismic load on the soil in the horizontal direction.Uniaxial compressive strength(UCS)experiment and numerical simulations are carried out to researched the fracture characteristics of fissured loess after disturbance of different vibration parameters,determined the fractured loess stress-strain characteristics,deformation and strength parameters change laws,revealed the failure evolution mechanism of fractured loess after disturbed by seismic load.The results show that:(1)Undisturbed fissured loess has the following damage characteristics:when the fracture angle is 0°,it is mainly split failure;when the fracture angle is 15°and 30°,the fracturing failure occurs on the whole samples;when the fracture angle is 45°,the failure form of the sample is slip-shear compression failure;when the inclination angle of the fissure is 60°,the sliding shear failure occurs;when the fracture angle is 90°,the failure mode is compression-shear failure.(2)With the increase of vibration amplitude,the more secondary fractures occurred on both sides of the fracture surface when the soil sample reached the failure limit state.With the increase of vibration frequency,micro-cracks are generated inside the loess.With the increase of vibration time,the input energy continues to increase,the damping energy consumption of the loess structure increases,and the cumulative damage effect of the soil is obvious.(3)The stress-strain curve of fissured loess has obvious characteristics of strain softening.The variation law of the stress-strain curve of loess samples with fracture angles of 0°、15°、30°are the same.It goes through four stages:compaction,elastic deformation,plastic yielding and residual deformation.When the fracture angle is 45°,the curve has multiple stress peaks;when the fracture angle is 60°,the stress-strain curve decreases rapidly after reaching the peak strength,and the peak stress is minimum;when the fracture angle is 90°,the stress-strain curve has obvious peak point,and the peak stress is the largest.(4)As the vibration amplitude increases,the slope of the stress-strain curve gradually decreases during the elastic deformation phase.With the increase of vibration frequency,the peak strength of the curve decreases slightly,and the influence of vibration frequency on the fissured loess is limited.The peak strength of fractured loess samples with vibration time of 300s and 600s is obviously greater than that of samples under the vibration time of 900s,and the damage effect of the soil is accumulated.(5)The UCS of fissured loess presents a"U"shape that decreases first and then increases with increasing of the fracture angle.With the rise of vibration amplitude,the UCS of fissured loess samples decrease linearly.The UCS decreases linearly with the increase of vibration frequency.The UCS of fractured loess sample decreases linearly with the increase of vibration time.(6)According to the Mohr-Coulomb criterion,the failure conditions of fractured loess along the fracture surface are judged.When the fracture angle of the soil satisfies ψ_f<α<90°,the specimen suffered sliding shear failure along the fracture surface.If not satisfied,the specimen will be fractured with sliding shear composite damage.The finite element software was used to simulate the uniaxial compression process of the fractured soil.The axial deformation of the soil samples is concentrated in the weak layer of the fracture surface and the soil on both sides.The axial displacement field at the tips of the upper and lower structural soil blocks is symmetrically distributed in the center.The stress concentrated at the fracture surface of the soil sample,and subjected to shear and tensile stress.The two sides of the fissure are mainly the distribution of tensile stress.During the failure process,the fractured soil is greatly affected by the fissure surface and has the risk of sliding shear failure.