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When the shield method is used in the upper soft and lower hard strata, it is very difficult to successively predict the ground settlement of a double line shield tunnel. By considering the Xuewa section of the Qingdao Metro, this paper describes the establishment and correction of the Peck equation for predicting the surface settlement of a double shield tunnel. The modified Peck equation was validated using numerical simulation and field monitoring methods, and the influence of the earth chamber pressure and grouting pressure on the surface settlement was analyzed. It was found that the modified Peck equation can better analyze the law of ground deformation caused by shield construction in the upper soft and lower hard strata. The ground settlement can be reduced by appropriately increasing the earth chamber pressure and grouting pressure in the numerical simulation. The results obtained by this study can provide a scientific reference for similar tunnel construction in relevant areas in the future. [ABSTRACT FROM AUTHOR]

The conventional method of urban subway tunnel excavation using a double-shield tunnel boring machine (DS-TBM) typically involves a two-stage backfilling process behind the segment wall using pea gravel and grouting. However, this method has inherent drawbacks such as uneven backfilling of pea gravel and delayed grouting, which can lead to structural issues such as segment dislocation, damage, and groundwater leakage during tunnel excavation. To address these problems, this study proposes a technical theory of synchronous backfilling self-compacting concrete backfilling material (SCCBM), considering the limitations of the conventional DS-TBM backfill method. This theory uses a specially designed wear-resistant seal plate suitable for a DS-TBM shield tail. The optimal proportion of SCCBM for backfilling the segment wall was determined based on Okamura's empirical method using anti-dispersion powder, fine stones (1–5 mm particle size), and coarse stones (5–10 mm particle size) as the basic materials. The SCCBM was designed and evaluated using various test methods, including the mini-slump, standard slump flow, standard L-box, and improved L-box tests. The engineering characteristics of the backfill material, such as initial setting time, blocking ability ratio, stone rate, and axial compressive strength, were considered during the evaluation. The results demonstrate that all indicators meet the standards for backfilling behind subway tunnel segment walls. Furthermore, an on-site synchronous backfilling SCCBM experiment was conducted to validate the feasibility and advantages of the new synchronous backfilling technology. The experiment involved observing the complete backfill process and monitoring the vertical displacement of the segment. Through these observations, the practicality and benefits of the proposed synchronous backfilling technology were confirmed. • A novel synchronous backfilling technology using self-compacting concrete in double-shield TBM tunnel was proposed. • The optimal proportion of self-compacting concrete backfilling material (SCCBM) using empirical methods was determined. • The feasibility and advantages of the proposed synchronous backfilling technology was confirmed by field data. [ABSTRACT FROM AUTHOR]

Synchronous grouting has a very important effect on shield tunnel, which directly affects the stability of excavation face and the safety of segment structure. Testing the setting time and early strength of mortar is conducive to timely control the surface settlement and prevent the segment from floating. The flow table test can ensure that synchronous grouting can fill the excavation gap of shield machine to the greatest extent. Oedometer test can obtain physical and mechanical indexes such as elastic modulus, Poisson's ratio, compression and drainage of slurry at different ages. The test results showed that the use of well-graded, small-sized machine-made gravel (0–5 mm) and natural sand (0–2 mm) with high-efficiency additives promoted the formation of early strength in grout and reduced the probability of plugging in the grouting pipe at the tail shield. The oedometer test results indicated that the quantity of water drained from a grout mixed for more than 2 h was stable and the grout deformation within 1 h of compression accounted for 80% of the total deformation. The Young's modulus of the grout reached 10 MPa when the grouting pressure was > 0.2 MPa, meeting the requirements for field injection. In this study, an efficient synchronous grouting mixture proportion suitable for calcareous sand layer is proposed, and the performance of slurry in fresh state is obtained through a variety of tests, which can be widely used in shield tunnels in similar strata. [ABSTRACT FROM AUTHOR]

The excavation by earth pressure balance (EPB) shield could cause stress fields varied, which is harmful to adjacent existing structures. Thus the effect of excavation clearance of the EPB shield on the settlement of existing structures should be taken into account. The transit tunnel from Meicun to Shangmeilin of Shenzhen Metro Line 9 constructed by the EPB shield with mid-shield-grouting was investigated. The deformation of existing tunnel is analysed by three-dimensional (3D) finite element methods (FEM). It is found that the settlement of overburden soil is obviously influenced by the structure of the existing tunnels through the plastic development of the soil. The rate of settlement is remarkable when the shield effect zone approaches the existing tunnels so that timely mid-shield grouting is an efficient method of controlling the settlement induced by the excavation clearance. However, the grout used in this method should be with the properties of short hardening time, large elastic modulus and low shear strength. Different standards of settlement control should be employed with respect to different underground tunnels, in which minimum elastic modulus of the filler that is usually determined by fitting function. This can be taken as a guide rule of the selection of material parameters in the project. The simulation results are in good agreement with the settlements measured for the existing tunnels and the ground surface. The parameter sensitivity analysis by means of the Spearman’s correlation coefficients shows that the elastic modulus of filling material significantly influences on the settlement of the existing tunnels and the ground surface. [ABSTRACT FROM AUTHOR]