Test Study On The Unsaturated Shear Strength Of Remoulded Granite Residual Soil

Granite residual soil is widely abundant in Southeast China.It is a kind of typical special soil,which has the softening property.Granite residual soil has high shear strength under unsaturated condition.However,with the increase of saturation,its shear strength will decrease sharply.However,due to the lack of comprehensive understanding of softening property of granite residual soil,the engineering safety problems occur.Therefore,it is of great significance to study the unsaturated strength of granite residual soil.In this paper,the granite residual soil of a foundation pit in Shenzhen area is taken as the research object.The soil water characteristic curves of remolded granite residual soil is measured.In order to get the rule of the drop of strength with the increase of saturation,unconsolidated undrained and consolidated undrained tests of remolded granite residual soil with different saturation are set.Moreover,the relationship between strength and matric suction is obtained based on the soil water characteristic curve.On this basis,the discrete element model of granite residual soil is established.Combined with laboratory test results,the main meso-parameters(particle friction coefficient and bond strength)are gotten.In addition,the relationship between the saturation and the main meso-parameters,the influence of mesoparameters on macro shear strength is studied by using the discrete element software PFC3 D.Furthermore,the mechanism of softening property of granite residual soil is discussed.The main research conclusions are as follows:(1)The rate of decay of the granite residual soil used in this paper is very high.The primary mineral components of the samples are kaolinite and quartz.The content of coarse particles and the plasticity index is high.The high content of the coarse particles results in the widely distribution of macrovoid in the soil,which leads to small air entry value and residual humidity.(2)The result of the triaxial test shows that: The increase of saturation will lead to the decrease of cohesion and internal friction angle of granite residual soil.In the unconsolidated undrained test,the variation of saturation has a greater influence on the internal friction angle.However,the change of saturation has a greater influence on cohesion in consolidated undrained test.According to the soil water characteristic curve,the matric suction can improve the strength of soil obviously.But the contribution of matric suction to soil strength decreases gradually with the decline of saturation.(3)The discrete element model in good agreement with the physical test results show that the model has certain rationality.It can be used to explore the microscopic mechanism of unsaturated granite residual soil damage.Based on the model above,a series of granite residual soil consolidation undrained test results show that the increase of soil saturation weakens the coupling effect between soil particles,resulting in decreased soil cohesion and friction angle.