Study On The Effect Of Improvers On The Permeability Of Sandy Soil In Soil Pressure Balance Shield

With the stable development of China’s economy and society,urban rail transit,as an important part of urban public transportation,has achieved tremendous development in China in recent years.The earth pressure balance shield method is widely used in the construction of urban subway tunnels because of its advantages of fast construction speed,low labor intensity,low environmental impact,and economic construction.When the shield machine is digging in the water-rich sand layer and sand pebble layer,due to the low cohesion of the sand and sand pebble soil,the internal friction angle is large,the fluidity and plasticity are poor,the permeability coefficient is large,and the pressure balance of the soil silo is difficult to establish Moreover,under water-rich conditions,it is very easy to cause palm face instability,collapse and "surge" during the driving process.Therefore,it is necessary to adjust the permeability of the slag through the improvement of the slag to increase the slag’s impermeability and fluidity.In this paper,based on the section between Xuefu Road and Xinwang Road of Xi’an Metro Line 14,through data collation,laboratory tests,numerical simulations,etc.,the influence of the modifier on the soil permeability of the earth pressure balance shield is studied Research to provide reference for on-site construction.The main results are as follows:(1)Analyze and organize the micro-mechanism of foam and bentonite mud and the action mechanism of modifier to improve sand,which provides theoretical guidance for subsequent experiments and numerical simulation.(2)Considering the increase of soil impermeability and fluidity,using carboxymethylcellulose sodium,soda ash,caustic soda and other additives to optimize the experiment of foam and bentonite mud,and obtaining the optimal proportion of modifier under certain conditions Improve the performance of the modifier.The results show that sodium carboxymethylcellulose can significantly increase the half-life of the foam,while caustic soda,soda ash and sodium carboxymethylcellulose can increase the viscosity of bentonite mud and shorten the swelling time under an appropriate ratio.(3)The optimized modifier was used to conduct an indoor modified permeability test on the on-site sand samples,and the optimal addition ratio under different conditions was obtained,and orthogonal experiments were conducted to study the influence of various factors on the permeability of the sand.The results show that the modifier has better improvement effect with sand,and the optimized modifier has better effect.During the improvement,the mud injection ratio has the greatest impact,followed by the mud viscosity and foam volume fraction,while the foam injection rate has less effect.(4)By using FLAC3 D software to perform numerical simulation on the site conditions,comparatively analyze the changes of the hydraulic pressure distribution of the screw conveyor before and after the improvement,check the improvement effect of the slag,and optimize the improvement plan.