Model Test Study On Synchronous Grouting Mechanism Of Shield Tunnel In Sandy Gravel Stratu

Currently,there is little research on the process of mud filling the shield tail voids in the sand and gravel formations,the flow,diffusion,and distribution of mud in the shield tail voids,the evolution of mud pressure in the shield tail voids,and its influencing factors,as well as the effect of mud on ground displacement.Therefore,it is necessary to fully understand and master the diffusion mechanism of mud in the sand and gravel formations and the response of the formations to mud injection.In this paper,the orthogonal experimental method was used to study the effect of different proportions of cement,fly ash,fine sand,bentonite,and water on the characteristics of the synchronous grouting mud material for shield tunneling.Different proportions of mud were tested,and the initial viscosity,bleeding rate,setting time,consolidation shrinkage rate,and compressive strength of the mud were measured to investigate the influence of different material proportions on the synchronous grouting mud material characteristics of shield tunneling.Then,a model test method was used to design and build a synchronous grouting model test platform for shield tunneling based on the actual construction process of synchronous grouting for shield tunneling.The injection rate and tunnel depth were studied to investigate the diffusion mechanism of synchronous grouting mud in the sand and gravel formations and the response of the formations.The main conclusions obtained from the full text are as follows:(1)The characteristics of the sand and gravel stratum are loose and have poor selfstability.After the shield tail is extracted,the soil rapidly covers the lining of the vault,causing no grouting layer at the top of the tunnel.The soil above the vault collapses rapidly,and the soil below the vault continues to collapse.The diffusion of the slurry at the arch shoulder is mainly through infiltration and diffusion,and the diffusion form is semi-spherical.The slurry at the arch waist comes from the diffusion of the slurry at the arch shoulder,and the diffusion of the slurry at the arch waist is mainly through filling diffusion.The diffusion of the slurry at the bottom and foot of the arch is mainly through filling diffusion,and the trend of slurry pressure changes at different depths is not significant.(2)In the early stage of grouting,the upper soil layer of the tunnel collapses rapidly,and the thickness of the grouting layer on both sides of the arch shoulder is relatively large.Similar to the arch shoulder area,the thickness of the grouting layer on the arch waist will also increase with the increase of grouting rate.Due to the self-weight of the slurry and the squeezing effect of the surrounding soil,a large amount of slurry is concentrated at the arch foot position,resulting in a thicker grouting layer at the arch foot,while the thickness of the grouting layer at the arch bottom is relatively uniform.(3)When the shield tail passes,the maximum surface settlement occurs directly above the center of the tunnel,and some soil experiences self-locking effects,resulting in uneven surface settlement on both sides of the tunnel.The slurry mainly diffuses in a permeable form,and the diffusion process will cause sustained disturbance to the soil,leading to an increase in settlement.Continuous grouting at a high grouting rate helps to suppress the trend of surface settlement.Under the same grouting rate,deeper buried soil is more prone to form a soil arch effect,and the influence of surface disturbance is reduced,resulting in smaller surface settlement values.