Construction Mechanism And Control Technology For Shield Tunnelling Near The Interface Between Karst Caves And By Sandy Strata

This study investigates the failure mechanism in karst caves covered by sandy strata caused by shield tunnelling near the interface of the cave and the sandy strata,and to provide control measures for safe construction.The methodologies devised for theoretical analysis,numerical simulation and field investigation are applied.First,a method to estimate the grouting volume required to fill a karst cave along the tunnel line is proposed,based on fractal theory and field data from geological drilling and karst cave grouting.Secondly,a theoretical model that describes the exposure and destruction of a karst cave induced by shield tail grouting is established based on limit equilibrium theory,and an equation to calculate the critical shield tail grouting pressure is presented.The safe distance required between tunnel and karst cave to prevent excess deformation of the karst cave due to ground loss is then analysed using numerical simulation.Finally,safe construction parameters during shield tunnelling,and their influence on ground settlement are discussed with reference to a field case.The concluding remarks of this study are summarised as follows:1)A method for estimating the grouting volume of a karst cave is proposed.It is difficult to evaluate the volume of a karst cave due to its extensive distribution and irregular shape.To solve this problem,this study uses fractal theory to investigate the fractal properties of a karst cave according to field data from geotechnical drilling,and the fractal parameters obtained are used to calculate the volume of the karst cave.The karst grouting coefficient,the ratio of grouting volume determined in the field to the calculated volume of a karst cave,can be used to predict the total grouting volume of a karst cave adjacent to a tunnel based on its calculated volume.2)A model for the opening and destruction of a karst cave caused by shield tail grouting is developed,and the critical grouting pressure is calculated.During tunnelling near the interface of a karst cave and the sandy strata covering it,it is suggested that if the shield tail grouting pressure reaches a certain level,cracks near the tunnel would develop near the karst cave and sand would flow into the cave along with grouting slurry,which might lead to deformation or even destruction of the karst cave.In this study,a destruction model for a karst cave damaged by grouting is formed,based on limit equilibrium theory.The equation to calculate the critical grouting pressure is summarised by simplifying the grouting pressure as a circumferential uniform force and employing the theoretical solution proposed by Verruijt which applies complex function theory.It is concluded that the shield tail grouting pressure increases as the vertical distance between the tunnel and the karst cave increases,or the diameter of the tunnel increases,but the shield tail grouting pressure decreases as the depth of the tunnel increases.With the method above,the critical grouting pressure during tunnelling for Metro Line No.9 in Guangzhou was determined.3)A safe distance between the tunnel and a karst cave is proposed.A three dimensional model of the tunnelling process is established to analyse the deformation of a karst cave.i)When the karst cave lies below the tunnel,the vertical displacement of the top of the karst cave increases as the distance between the karst cave and the tunnel decreases,or as the diameter of the karst cave increases.The vertical displacement of the top of the karst cave was greater with shallow tunnels than those with deep ones.In the deformation,the karst cave was vertically compressed and expanded sideways.The deformation was more significant when there was a smaller distance between the karst cave and the tunnel,or with caves of greater dimensions.Also,the deformation was more significant with shallow tunnels than with deep tunnels.ii)When the karst cave was to the side of the tunnel,both the vertical displacement of the top of the karst cave,and its lateral displacement increased as the distance between the karst cave and the tunnel decreased or the diameter of the karst cave increased.The vertical displacement of the top of the karst cave and its lateral displacement decreased as the depth of the tunnel increased.The deformation of the side nearest the tunnel was greater than that on the side furthest from the tunnel.The deformation of the karst cave was more significant when there was a smaller distance between the karst cave and the tunnel,or with greater dimensions of the karst cave.The deformation became less noticeable when the depth of the tunnel increased.The safe distances between karst cave and tunnel were 1.9 times the diameter of the karst cave when the karst cave was below the tunnel,and 2.1 times the diameter of the karst cave when the karst cave was to the side of the tunnel.4)Based on data from Metro Line 9 in Guangzhou,the influence of geological conditions(the extent and size of the karst cave),tunnel geometry and tunnelling parameters on alignment deviation and surface settlement are summarised,and an environmentally friendly technique and appropriate parameters are proposed.The effects of the construction parameters are as follows.The thrust increased as the disk torque increased,and the advance rate increased as the screw rotation rate increased.The soil pressure could be controlled by adjusting the advance rate and screw rotation rate.During tunnelling at the interface of the karst cave and the sandy stratum,the vertical alignment should be lower than the designed alignment,and the operational parameters should be adjusted as appropriate according to the ratio of soft soil to rock at the excavation face,in order to avoid significant surface settlement.The measured surface settlement above the tunnel in the karst region was consistent with the settlement trough put forward by Peck,and the ground loss ranged from 1.1% to 2.5%.When the shield went through the karst cave and sandy strata,the surface settlement increased to a maximum of 85.8mm.A Back Propagation(BP)artificial neural networks model with 15 parameters related to tunnel geometry,geological conditions and shield operation factors is presented which is effective in the precise prediction of surface settlement.