Experimental Study On The Properties And Its Improved Test Of Sandy Cobble Stratum In Shield Construction

Sandy cobble,one kind of earth-rock aggregate in a general sense,mainly consists of boulders,cobbles and gravels.Sandy cobble stratum made of it is a typical stratum with mechanical instability.Essential features of such layer lie in the looseness of structure,binderless and size inequality.Thus,collapse easily happens in the excavation duo to the loss of stability.With the rapid development of subway business in China,sandy cobble will be met more in the future,which leads to great difficulties in the shield construction.In other words,high-strength of the cobble is apt to cause shield machine cutter to be frayed;accumulation of large size cobbles in the soil chamber shall generate skeleton effect,even puts the cutter out of operation;permeability of the stratum is nice,while water-holding capacity of it is poor,which causes water gushing in the outlet of the screw conveyor when shield tunneling machine meets water-rich sandy cobbles;poor liquidity of it generates mud cake in the cutter and soil chamber,affecting the balance of soil structure and leading to overexcavation.This study explored the relations between the mechanic characteristics and gravel content of sandy cobble from a typical foundation trench in Luoyang firstly by doing large direct shear test,slump test and permeability test in 10%,20%,30%,40%,50%,60%,70%,80% and 90% gravel content.The results show that both shear strength and internal friction angle of sandy cobble rise with the increase of the gravel content and the stress displacement curves in different gravel content demonstrate different non-linear relationship;slump and permeability coefficient also rise when the gravel content increases and both of them can not meet the demands of shield construction to the liquidity of sandy cobble.Then,the author does researches on optimization tests to the Ca-bentonite and Na-bentonite mud from a factory in Weifang of Shandong Province,measures and comparatively analyzes the time-changing viscosity of them under different clay-water ratios,making a conclusion that viscosity of Na-bentonite mud is better than that of Ca-bentonite mud when clay-water ratios of them are same;the best clay-water ratio of Na-bentonite mud is 1:10;viscosity of it rises with the increase of time and peak appears in the time between 18 h and 24 h,choosing 18 h as the time of mud expansion.At last,improved sandy cobble tests are made with Na-bentonite mud being infused to the sandy cobble in 20%,40%,60% and 80% gravel content;the clay-water ratio of Na-bentonite mud is 1:10 and the injection rate is 5%,10%,15%,20%,and 25% respectively.Slump test,large direct shear test and permeability test are proceeded to bentonite mud that is mixed well.Major findings are: firstly,the infusion of bentonite mud can effectively eliminate the separation of sand and cobble;secondly,with the increase of injection rate,both shear strength and internal friction angle of sandy cobble decrease significantly;slump increases and permeability coefficient decreases obviously,which meet the demands of shield construction;thirdly,the optimal injection rate is 17%,15% and 15% respectively of the sandy cobble in 20%,40%,60% gravel content.