Research On The Microstructure Effect Of Unsaturated Shear Strength Of Remolded Net-like Red Soil

The net-like red soil with special structure is widely found in Xuancheng.Previous researches on the strength of net-like red soil were mainly focused on saturated remolded soil,but few on unsaturated strength and microstructure.The net-like red soil in Xuancheng is often in unsaturated state,thus the strength researches on unsaturated net-like red soil is of important influence in order to ensure the safety of the construction of net-like red soil.A series of experiments performed on remolded soil using an uniaxial unsaturated soil direct shear apparatus were carried out to discuss the matrix suction and net normal stress effects on the shear strength,also according to Fredlund and Bishop equations,the corresponding parameters were specified.In addition,the quantitative and qualitative analysis of the microstructural changes on the shear surface of the specimen were also performed by a scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS)and to try to obtain the connection of microstructural characteristic and macro-strength and deformation.The major conclusions were as follows:(1)The results of shear tests indicated that the failure patterns were brittle and plastic patterns.And the brittle failure pattern appeared in specimens performed at a higher matrix suction and lower net normal stress.(2)According to the analyses using by Fredlund,the relationsh between interval friction angle,cohesive force corresponding and matrix suction didn't display an linear variation,when the suction was below 100 kPa,the cohesive force increased with the increment of matrix suction,the interval friction angle gradually decreased with the increase of matrix suction.However,when the suction was over 200 k Pa,the interval friction angle corresponding to cohesive force increased with the increment of matrix suction,while corresponding to net normal stress increased with the matrix suction.(3)Based on the strength equation proposed by Bishop,it can be seen that the effective stress parameter did not locate in the interval [0,1],the parameter did not change with net normal stress when matrix suction is over 200 k Pa,it decreases with the increment of net normal stress when the net normal stress is200 k Pa and 300 k Pa,while it increases with the increment of net normal stresswhen the net normal stress is 100 k Pa and 410 k Pa.(4)Based on the measured data,a curved surface relationship among unsaturated strength,matrix and net normal stress was set up by means of Matlab program.It is distinct discrepancy with the plane relationship expressed by Fredlund equation,especially in the condition of net normal stress with100 k Pa and 200 k Pa,matrix suction of 100 k Pa and 200 k Pa,This may suggest that the limitation occurs in the Fredlund equation.(5)The SEM and EDS indicated that the pores on shear failure surface of remolded net-like red soil were mainly consisted of micro-pores with less than0.1?m2,small pores with 0.1~0.2?m2,medium pores with 0.2~0.3?m2,large pores with more than 0.3?m2.This may be due to the specimen sieving by the ground processing.The chemical elements in specimen were mainly consisted of oxygen,aluminum,silicon,iron and a small amount of potassium,magnesium and calcium.(6)An interesting ridge-like microstructure was observed on the shear failure plane.It can be noticed that the special microstructures only appeared before the formation of friction scratches.By comparing and analyzing the SEM photos,this kind of microstructure only appeared on the specimens performed with matrix suction of 200 k Pa and 350 k Pa.