Study On Mechanical Properties And Engineering Stability Of Neogene Loose Sediments In Eastern Zhejiang

Loose sedimentary interlayers are generally distributed in the Neogene basalt strata in Eastern Zhejiang.The engineering properties of the loose sediments are relatively poor,under the influence of precipitation or engineering disturbances,the slopes have the risk of instability and destruction at the loose sediments.Therefore,it's of great theoretical and engineering significance to take systematic research on loose sedimentary layers.In this paper,loose sedimentary interlayer of Neogene Shengxian Formation in Eastern Zhejiang is studied.Comprehensive research methods such as geological survey,laboratory test and numerical simulation are carried out to study its distribution characteristics,material composition and engineering characteristics.By combining with typical engineering project,PFC^2D software is used to simulate the excavation of slopes with loose sedimentary,and the stability of surrounding rock is evaluated.The main research contents and results of this paper are as follows:?1?Geological data of the strata in Shengxian Formation is collected,and the formation causes,distribution and sedimentary environment of loose sedimentary interlayer are studied.Through site investigation,thickness distribution and material composition characteristics of loose sediments are determined.?2?Unconfined uniaxial compression tests are carried out on loose sediment remodeling samples at different loading rates to study uniaxial compressive strength rate sensitivity characteristics and deformation failure characteristics of remolded samples.Test results show that compressive strength of the sample increases with the increase of loading rate.In addition,during the loading process,the sample undergoes radial expansion and cracking.After crack develops,the sample loses its bearing capacity and the cracked surface layer is peeled off.?3?Based on the similar grading method,3 sets of different maximum particle size d_max and different particle grading remodeling samples are designed.Particle size distribution of granular mass fraction D_F is used to quantify the size distribution of loose sediments.Take large-scale direct shear test to study the effect of particle size composition on the shear strength of loose sediments,obtaining the relationship between cohesion c and internal friction angle?and d_maxand D_F is studied.?4?Screening fine fractions with a particle size of less than 5 mm in loose sediments,and large-scale direct shear tests are carried out on coarse particle samples of 3 particle size ranges and 9 sets of particle grades.Coarse particles have obvious particle breakage during the direct shearing process,and the degree of fracture is controlled by the normal load and the sample particle gradation.The shear strength of the sample caused by particle breakage is no longer applicable to the Mohr-Coulomb strength theory,but more in line with the power function strength model,?=f?_n^m+c.It is found that the shear strength of coarse-grained materials has a particle size effect,and friction coefficient f has a unique relationship with the characteristic particle size of the sample,which can be calculated by the given formula.?5?Based on a typical project,an excavation model of the slope with loose sedimentary layer is established based on PFC^2D software.Stress field,displacement field and fracture development process of the surrounding rock of the face after excavation are monitored,and the instability and failure process of the excavation face is analyzed.And by adjusting the distribution position of loose sediments,the influence law of loose sediments on the deformation and failure characteristics of surrounding rock is explored.