Experiment Study Of Shear Properties Of Completed Decomposed Granite

Granite is abundant in southeast China. Under the action of weathering, it decomposes gradually, turning into completely decomposed granite(CDG) which has special properties. The objective of this study is to understand the physical properties and mechanical behavior of CDG, so as to avoid engineering accident.In this paper, a series of tests on shearing properties of CDG sampled from a certain foundation pit in Shenzhen are set, including drained and undrained consolidated tests, to study the stress-strain response, shear failure characteristics, and the structural change law in the loading process. Comparing with the analytical results, the differences of physical and mechanical properties between completely decomposed granite and granite residual soil are studied. The main contents include:1. Void ratio of CDG in this paper is relatively large, and its average water content is not high, while the content of coarse particle is relatively high, so that the CDG belongs to liquid-limit clay soil(classification method of hydroelectric department) or silty-clay sand(comprehensive classification method).2. Results of drain and undrain consolidated triaxial tests as follows:(1) Original completely decomposed granite has relatively strong structural properties, but the structural properties is weaker as the increasing of confining pressure.(2) Because of the existence of structural properties, the cohesion of original soil is higher than which of re-compacted soil.(3) Original soil doesn’t occur obviously single shear zone, But a complex failure pattern which’s soil surface is uneven; while re-compacted soil occurs dilatancy.3. The change law of original soil’s structural properties during shearing reflects by “ structure contribution rate”:(1) Structural properties of original soil decrease with the increasing of consolidation press until completely destroyed.(2) Structural properties of original soil is through the process of reaction gradually to damage. “ Structure contribution rate” in peak can be used to measure the size of structural properties.(3) Regarding the start point while the “configuration contribution rate” curve decreased as the sign of the beginning of strain localization. The greater the confining pressure, the strain localization occurs earlier, and allways before the peak shear strength of undisturbed soil.(4) The failure rate of soil structure is slowed under the undrained condition, but also weaken the play of structural nature.4. Comparison of the completed decomposed granite and granite residual soil.(1) The average water content, void ratio, liquid limit and plasticity index of CDG are smaller than granite residual soil. And the content of coarse particles in CDG is more than the residual soil.(2) By the comparison of “configuration contribution rate”, the structure of CDG is stronger than the residual soil.(3) Undisturbed CDG shows higher strength and shear modulus than the residual soil under the same confining pressure. For the reconstituted soil, the stength of CDG is higher than the residual soil under the drained condition. However, it is on the contrary under the undrained condition.(4) The factors that effects the failure pattern of soil obviously contains the content of coarse particles in the soil, the void ratio, the confining pressure and the drainage conditions.(5) The content of sand shows obvious effect on the failure pattern of the residual soil. But for the completed decomposed granite, the content of sand shows little effect.