Study On The Collapse Mechanism Of Karst Soil Cave Under The Action Of Precipitation Funnel

Karst disasters are widely developed in China.With the rapid development of human activities,karst surface collapse disasters have caused more and more serious economic losses and security risks to human beings.Existing studies have shown that a considerable part of karst collapse is triggered by the mutual drive of underground water and gas,which in the final analysis is caused by the change of underground water level,and its essence is the migration of water in soil body.The collapse is often prone to occur during or after the steep rise of water level under long-term low water level.At the same time,soil caves of different sizes and uneven distribution are often developed underground in karst areas.Therefore,on this basis,the influence of dry-wet cycle on the stability of soil caves is studied.The work is completed and the results are as follows:(1)By studying the previous studies and a large number of engineering examples,the preliminary geological conditions for the development of karst collapse disasters are summarized,including space conditions,material conditions and hydrodynamic conditions;Several typical stratigraphic structure models of the overburden and the development models of karst fractures in the lower bedrock are summarized.According to the engineering practice,the research object is the typical karst geological model of the single water-blocking overburden and the uniformly distributed pores in the lower bedrock.The collapse mechanism and development process of different types of karst collapse are analyzed,which provides theoretical basis for the study of soil cave stability in karst area.(2)Based on the typical karst geological model mentioned above,the model test of karst soil cave collapse is designed and studied.The results show that the rapid fluctuation of water level has a great influence on the stability of the soil cave under the influence of hydrodynamic action and dry-wet cycle action,but no collapse occurs.However,when the water level rose and fell for a long time,the soil moisture content decreased significantly,and the range of dry-wet cycle was obvious.After the water level rose again,the overburden instability soil cave collapsed,and the long-term dry-wet cycle had a great impact on the stability of the soil cave.(3)According to the phenomenon of model test,a number of factors affecting the stability of the soil tunnel are summarized,such as the size of the soil tunnel,the porosity of the soil,the amplitude and number of dry-wetting cycles,and the design test is conducted to study each influencing factor respectively.The results show that the size of the soil cave is the main factor affecting the stability of the soil cave.The larger the size,the worse the stability of the soil cave and the more prone to collapse.The larger the range of dry-wetting cycle is,the more significant the dry-wetting cycle effect of soil is.The higher the degree of micro-cracks in single cycle soil is,the worse the stability of soil cave is.The larger the porosity of soil,the more obvious the seepage of water in soil and the worse the stability of soil.The number of wetting and drying cycles amplifies the cave-causing effect of the above factors.Specifically,with the increase of wetting and drying cycles,the shear strength of soil decreases,and the structural damage of soil accumulates gradually,which leads to collapse after reaching the critical state.(4)Through the finite difference numerical calculation method,a single factor simulation scheme for the amplitude of dry-wetting cycle,soil porosity and number of dry-wetting cycle is designed.The results show that the law of stress field and displacement field during the model calculation is similar to the model test results,and the model parameters are reliable.The larger the porosity of soil,the larger the range of dry-wetting cycle,the more the number of dry-wetting cycle,and the worse the stability of soil cave.The influence of soil porosity mainly exists in the first drying-wetting cycle,and the amplitude and frequency of drying-wetting cycle continue to exert and accumulate their influence with the test progress.With the increase of drying and wetting cycles,the plastic failure zone develops first near the soil hole and gradually radiates upward to the surface.Finally,it reaches through the surface of the overburden and causes the collapse.Figure 65 table 17 reference 76...