Experimental Study And Constitutive Model Of Completely Decomposed Granite

Completely decomposed granite(CDG) is abundant in South China and used widely as a geo-material in local construction projects. However, due to the lack of understanding and research of the completely decomposed granite, there have been many failures in construction projects, resulting in a substantial loss of lives and damage to property. Therefore, it is urgent demand to research the CDG soils.In this paper, the completely decomposed granite in Shenzhen was studied, comprehensive physical property tests and triaxial tests were performed. Based on the results of tests, a modified Structured Cam-Clay model was proposed for the undisturbed CDG and a bounding surface model was proposed for the reconstituted CDG. This study is helpful to engineering practice to some extent.Laboratory physical properties tests included the specific gravity test, moisture content test, atterberg limits test, compaction test, particle size distribution test, mineralogical analyses and chemical analyses were preformed. According to the results, quartz and kaolinite are dominated in CDG soil. The soil is poorly graded and nonuniform, it covers a wide range of particles with various sizes. The CDG soil could be denominated as the silty clay sand and the degree of weathering is completely decomposed.The triaxial tests included isotropic compression test, conventional drained and undrained compression test for undisturbed CDG; isotropic compression and swelling test, conventional drained and undrained compression test, constant p’ compression test for reconstituted CDG, considering the effect of the anisotropic compression and different over consolidation rations(OCR) to the undrained shearing behaviors; isotropic compression test, conventional drained compression test for cemented CDG.It can be seen that the mechanical behaviors of reconstituted CDG can be described by the critical state soil mechanics theory. Therefore, the modified Cam-Clay model was firstly used to simulate the mechanical behaviors of reconstituted CDG. The comparison between the simulated curves and the test curves revealed that the model has several disadvantages: 1) the plastic deformation taken place in the swelling process cannot be simulated; 2) the excess pore water pressures and the phase transformation in the conventional undrained compression test cannot be simulated well; 3) the volume change in the conventional drained compression test under highly compression pressure cannot be simulated well.Therefore, based on the critical state theory and the bounding surface elastoplasticity theory, a bounding surface model was proposed in q-p’ plane. The state parameter ? was introduced to considering its’ effect on the dilatancy. The code of model was compiled in matlab. The comparison between the simulated curves and the test curves revealed that the bounding surface model has several advantages compared with the modified Cam-Clay model: the plastic deformation taken place in the swelling process can be simulated well; the excess pore water pressures and the phase transformation in the conventional undrained compression test can be simulated well; the stress-strain relationship of the conventional undrained compression test under different OCR could be simulated to some extent, the simulated curves were closer to the test curves than the simulated curves of the modified Cam-Clay model.As seen, for reconstituted CDG studied in this paper, the bounding surface model proposed is better than the modified Cam-Clay model on simulating the mechanical behaviors of the soil.Both the undisturbed CDG and the cemented CDG are structured soil, with the initial soil structure. So in this paper, the Structured Cam-Clay model, which has been used widely, was firstly adopted to simulate the mechanical behaviors of the undisturbed CDG and the cemented CDG. The comparison between the simulated curves and the test curves of the conventional drained compression test revealed that the model has several disadvantages: 1) the peak deviator stress and the dilatancy under lower consolidation pressure cannot be simulated well; 2) the volumetric strain calculated by the model under higher consolidation pressure was larger than that measured in the triaxial test, resulting in larger shear strain under the flow rule of the model. Further, in the plastic deformation stage, a small deviator stress increment would result in a large shear strain increment. The simulated deviator stress curve was far diverged from the test curve.Therefore, based on the Structured Cam-Clay model, the yield function and the plastic volumetric strain were modified properly in this study, and the modified model in this paper was named “modified Structured Cam-Clay model”. The code of model was also compiled in matlab. The comparison between the simulated curves and the test curves revealed that the proposed model has several advantages compared with the Structured Cam-Clay model: the peak deviator stress and the dilatancy under lower consolidation pressure can be simulated well; the simulated deviator stress curve and volumetric curve under higher consolidation pressure were closer to the test curves.It can be concluded that, as for the structured completely decomposed granite studied in this paper, the proposed model is better than the Structured Cam-Clay model in simulating the mechanical behavior of the CDG.