Physical Model Test On Xi'an Land Subsidence Study

Land subsidence is an environmental geological phenomena of regional ground elevation reduced caused by the soil of surface crust compression under natural and man-made factors and it is a permanent loss of environment and resources which is non-compensation. Land subsidence is a kind of geological disasters with the characteristics of generate slowly, long duration, wide affected area, complex genetic mechanism and difficult to control, Which pose a threat to resource utilization, environmental protection, economic development, urban construction and people's lives. Since the seventies of the last century, direct economic losses due to the fissures and land subsidence in Xi'an area reached four billion yuan. So the study of the causes and the disaster-causing mechanism of land subsidence in Xi'an has an important meaning.This paper introduces the research Status at home and abroad, the main model, disaster-cause of land subsidence and land subsidence overview in Xi'an, especially the xi'an land subsidence mechanism test, that is the test of xi'an land subsidence physical model and the test of infiltration and consolidation under high-pressure. Discussing the land subsidence mechanism through the test can obtain the following conclusions:1,The result of the Physical model test showed that the stress and deformation of the soil has a good relationship. In the periods of groundwater table ascending, the sand strata exhibit elastic behavior. Whereas, in the period of descending the sand strata exhibits elasto-plastic or elastic behavior. Through the test, we found that when silty clay layer release water and compact along with water pressure reduce, the deformation has a significant lag effect and the lag time extend with the degree of consolidation increasing.2,Through the physical model test, we found that during silty clay layer release water and compact, when increase the water pressure of the aquifer, the deformation obviously slowed down and become stable, that is , it is feasible in theory to control land subsidence in Xi'an by reducing the groundwater exploitation to recover the water level.3,Through the test of infiltration and consolidation under high-pressure, we get the relationship curves at different depth (axial compression) between time and sand deformation which caused by pumping (pressure reducing) and the permeability coefficients at different void ratio(e-k curve), that quantifies sand sample permeability characteristics at different densities, provide the basis for the calculation of soil strata deformation with variable parameters.4,Through the test of infiltration and consolidation under high-pressure, we found the permeability coefficient of the sample mutate (reduce suddenly) when the axial compression increase from 4MPa to 5MPa. Through the particle analysis, we found that the grade of the sand sample has changes, binocular microscope and scanning electron microscopy photographs revealed the reason of the change of permeability coefficient pictures and grade is that part of particle be crushing or fracturing. It shows that sand sample deformation obtained under the conditions of high stress is considerable.