Study On Soil Improving Techniques With High Water-Pressure In Rich-Water Content Sand Stratum For Earth Pressure Balance Shield

It is difficult for urban underground space engineering to construct EPB shields in sand,gravel,cobblestone and other strata.On the one hand,this type of stratum has the characteristics of large porosity and high permeability.If the stratum contains groundwater at the same time,it is easy to cause gushing due to pressure on the excavation surface.Besides,instability and settlement were caused by excavation disturbance.On the other hand,because the soil improvement technology used by the shield tunneling in such formations will affect the parameters such as earth pressure,cutter head torque,and propulsion speed,thus normal shield tunneling would be affected.At present,the research on soil improvement of shields in such strata is still insufficient.The selection and proportion of improved materials lacks theoretical guidance,there are no relevant evaluation standards,and most of them are based on field experience,so unnecessary cost increases or construction risks were often caused.Therefore,this paper relies on the background of the shield tunneling project of Ha～ Da section of Harbin Subway Line 2.In view of the problems existing in the construction of shield tunnels with high pressure and water-rich sand,mainly includes:the theoretical conditions for the occurrence of gushing,based on resistance to gushing the occurrence of soil improvement ratio,the meso-mechanism of soil improvement,and the effect of soil consolidation on cutter head torque were studied.Theoretical research on earth pressure balanced shield gushing was carried out,a simplified mechanical model of gushing was established,and the relationship between soil permeability coefficient and water pressure and flow velocity was studied through a numerical calculation model,and the critical conditions for shield gushing to occur were determined.In the laboratory,experiments were carried out on soil improvement ratio based on foam,bentonite mud,and polymer materials.Combining with the results of gush theory research,an improvement ratio suitable for the soil in the stratum was determined,and the micromechanism of the interaction between the improved material and the soil was analyzed.Triaxial consolidation undrained tests and friction tests were performed on the soil before and after the improvement.The changes in the shear strength and friction coefficient of the soil under the pre-consolidation at different times were studied.The cutter head torque was calculated by the performance index,and the effect of soil improvement on torque reduction and the effect of consolidation time on torque were obtained.The main research contents and results are as follows:A simplified mechanical model of the soil in the screw conveyor and a simplified model of groundwater flow in the shield were established.The effects of the soil properties on the establishment of the pressure balance and the occurrence of the gush were analyzed.The purpose of soil improvement in shield tunnel construction of highpressure water-rich sand layer is put forward.The groundwater flow law in the chamber and screw conveyor were studied,and the critical mechanical conditions for the occurrence of gush in the shield were proposed.A numerical gushing calculation model was established to analyze the influence of soil permeability coefficient before and after improvement on the calculation of groundwater pressure and velocity in the model.And the soil critical permeability coefficient was obtained.In the laboratory,a variety of materials such as foam materials,bentonite mud,polymer materials,and modified water glass were studied in a single or composite manner to improve the soil body.The test evaluates the performance of the improved soil and analyzes the feasibility of the ratio through numerical calculation.Finally,the soil improvement materials and suitable ratios for the construction of high-pressure waterrich sandy were determined.The composite ratio materials were sodium-based bentonite mud and CMC(carboxymethyl cellulose).The results of indoor ratio research theoretically meet the requirements of anti-gush,and have been successfully applied in the project site,which has been verified by actual engineering.Based on the slump test results,an improved flow plasticity evaluation standard for high-pressure water-rich sand layers was proposed.A 3D reconstruction model of unimproved remolded soil and improved soil was established by computer tomography technology,and the sample was divided into several layers in the vertical direction.Through the analysis technology of threshold segmentation,according to different grayscale and color gamuts,the particle part and pore part of the soil were divided,the effect of porosity by soil improving was analyzed.The mechanism of soil improvement on porosity and internal pore channels was studied based on scanning electron microscope scanning experiments,and the meso-effect mechanism of the improved material on soil particles’ encapsulation,cohesion and agglomeration was revealed.Based on the problem of abnormal cutter head torque caused by the downtime during the EPB shield construction,triaxial consolidation undrained shear tests and friction tests were carried out on the remolded soil and the improved soil.The improved soil was preconsolidated at different times to simulate and study the effects of different downtimes on the soil in actual engineering.The test obtained the mechanical properties of the soil under different pre-consolidated time.Based on this,the influences of the shield cutter head torque on the soil improvement and the pre-consolidated time were revealed.According to the results of the torque study and the actual measured data in the field,the optimization measures were proposed which to prevent the increase of cutter head torque during the downtime of the shield.