Laboratory Test Research On Friction Reduction Of Soil-cement Mixing Pile In Sandy Gravel Stratum

When the soil-cement mixing pile is constructed in sandy gravel stratum,it is easy to cause difficulty in mixing and serious wear of the drilling tools,the reason is that the sandy gravel stratum has high shear strength,strong water permeability,and the performance of cement slurry is not suitable for formation requirements.In this paper,the performance of cement slurry is optimized,and then the friction reduction of cement slurry on sandy gravel before and after optimization is compared and analyzed.Finally,the relevant working conditions are simulated by numerical method.The main conclusions are as follows:(1)The rheology and water loss resistance of cement slurry have an important influence on the difficulty of mixing construction in sandy gravel stratum.The traditional cement slurry used in practical projects has low viscosity and poor resistance to fluid loss,so it is necessary to optimize the cement slurry.(2)A new type of cement slurry with low fluid loss and moderate viscosity was prepared by adding cement admixtures for experiments.the ratio is: 1.0 water-cement ratio + 2% bentonite + 1.6% PVA + 2.5%sodium tetraborate/PVA.The apparent viscosity of the cement slurry is30.8m Pa·s,the plastic viscosity is 24.5m Pa·s,the dynamic shear force is6.4Pa,and the fluid loss is 12.8m L.The filter cake is thin and smooth.(3)The internal friction angle and fluidity of cement slurry and sandy gravel mixed soil samples were measured by direct shear test and slump test.The internal friction angle decreases first and then increases,the fluidity is good,but the water retention is poor;the internal friction angle of the soil with the new cement slurry is decreased,and the fluidity is slightly smaller,but the water retention is good.An experimental device is designed to characterize the mixing difficulty and wear performance of sandy gravel,and the effects of different cement slurries on the stirring torque and wear performance of sand and pebble soil samples are measured.Through experiments,it is found that with the increase of cement content,the fluctuation range of soil mixing torque tends to decrease.After optimization,cement slurry is more effective than pure cement slurry after adding cement content of 10% and 12% to soil,which significantly reduce the stirring torque of the sandy gravel.With the increase of stirring speed,the fluctuation range of sample stirring torque value decreases.When the cement content is 4% to 8%,the average torque value of sample mixing increases with the increase of mixing speed;At 10% and 12% cement content,the average torque value of sample mixing decreases with the increase of mixing speed.After the optimized cement slurry is added to the soil sample according to 10%cement content,the wear performance of the soil sample can be significantly reduced.(4)The discrete element numerical simulation test of cement slurry and sandy gravel mixed soil sample is carried out,and it is found that the simulation results are close to the laboratory test.According to the actual construction parameters,different blade types,drilling speed and blade inclination angle are set for simulation.It is basically consistent with the actual construction.On this basis,the influence of blade inclination on stirring is predicted,and the results show that when the blade inclination is 25°,the stability during stirring is better,and the increasing trend of stirring torque is smaller.(5)According to the analysis and series of test results,it is suggested that during the actual construction of the cement-soil mixing pile in the sand and pebble soil layer,the rheology and water loss resistance of the cement slurry should be optimized;the thin cement slurry can be injected first and then the admixture is gradually added to increase the slurry viscosity and dynamic shear force,thereby reducing the stirring torque;the stirring torque can be reduced by appropriately increasing the stirring speed and reducing the drilling speed;the three-blade drilling tool can be selected and the blade inclination angle can be appropriately increased to reduce the difficulty of stirring construction.