Development mechanism of covered karst collapses induced by groundwater drawdown
With continuous development of society and economy,acceleration of urbanization,increase of water consumption in industry and agriculture and improvement of people's life,the interference and destruction of environment caused by human activities and engineering construction are increasingly serious,and the number of karst collapses is rising day by day.As a major type of geological disaster in karst areas,karst collapses threaten local people's life and property because of their suddenness and covertness.Therefore,it is of great significance to study the induced factors and development mechanism of karst collapses.Since the construction of the diversion tunnel of Chemabi reservoir in 2017,a large number of ground collapses have occurred in Malong district of Qujing City.The field geological investigation found that there is a Quaternary overburden layer with sandy clay-clay-sand structure in the study area in which occur strong karst development and frequent groundwater activities.Karst collapses in the study area are mainly distributed near rivers.In this study,we took karst collapses in the east of Malong district,Qujing City as the research objects.On the basis of fully understanding the geological conditions,hydrodynamic conditions and geological prototypes of collapses in the study area,we took the collapses of multi-structure overburden as examples,among which we selected a typical karst collapse as the prototype.Then we scaled down the geological prototype in equal proportion under the principle of similarity.Meanwhile,we prepared the materials similar to the physical parameters of the soil samples from overburden layers of the geological prototype,and constructed a geophysical model consistent with the prototype in the laboratory.Subsequently,according to the water discharge of the construction near the subsidence area and the data of observation well,we estimated the rate of groundwater drawdown,and monitored the pore water pressure in the overburden layer,the vacuum pressure in the karst cave and the cumulative displacement of the overburden soil.Finally,we carried out the experiments of karst collapses triggered by the groundwater drawdown under two typical working conditions.According to the experiment data,we put forward the collapse mechanism of the overburden layer,and the evolution characteristics of karst collapses with permeable layer-aquiclude-permeable layer.The results show as follows:(1)Negative pressure zones in covered layers and karst caves may occur because of groundwater drawdown.Due to the air recharge caused by relatively closed aquitards and the water-holding effect of the soil in overburden layers,the pore water pressure in overburden layers and vacuum pressure in karst caves will increase to the maximum with the fluctuation of groundwater discharge.The faster the rate of groundwater drawdown becomes,the greater the growth rate and peak of pressure will be.(2)According to the evolution characteristics of pressure and the deformation quantity of overburden layer,the collapse evolution can be divided into 3 stages:the stage of development of soil caves with rapid pressure increase but small displacement of overburden surface,the stage of soil cave expansion with the reducing growth rate of pressure but a rapid increase of displacement of overburden surface and the stage of instability and collapse of overburden layers when both pressure and displacement of the overburden surface reach the maximum.At the first stage,upper aquitards and middle strong permeable layers,i.e.the soil above the water level,are subjected to the erosion of pore water and the vacuum suction effect of pore water pressure generated in the negative pressure zone within the overburden layer.At the second and third stages,the vacuum pressure in the karst cave plays a leading role in the vacuum suction erosion of the entire overburden soil.(3)The rapid decrease of groundwater level is the main factor leading to the collapses in the study area.When the rate of groundwater drawdown is relatively small,the overburden soil may undergo minor deformation but will not collapse.However,when the rate of groundwater drawdown in Malong district reaches 0.1 cm·min-1(1.44 m·d-1),the vacuum pressure can provide the force of vacuum suction erosion.Together with osmotic pressure,this force is large enough to generate collapses.The research can provide reference for the prevention and early warning of collapse disaster in Malong district.
karst collapsephysical model experimentmulti-structuregroundwater drawdowncollapse mechanism
国家科技期刊平台
NSTL
万方数据
