Evaluation and prediction of earth pressure balance shield performance in complex rock strata:A case study in Dalian,China

This research explores the potential for the evaluation and prediction of earth pressure balance shield performance based on a gray system model.The research focuses on a shield tunnel excavated for Metro Line 2 in Dalian,China.Due to the large error between the initial geological exploration data and real strata,the project construction is extremely difficult.In view of the current situation regarding the project,a quantitative method for evaluating the tunneling efficiency was proposed using cutterhead rotation(R),advance speed(S),total thrust(F)and torque(T).A total of 80 datasets with three input parameters and one output variable(F or T)were collected from this project,and a prediction framework based gray system model was established.Based on the prediction model,five prediction schemes were set up.Through error analysis,the optimal prediction scheme was obtained from the five schemes.The parametric investigation performed indicates that the relationships between F and the three input variables in the gray system model harmonize with the theoretical explanation.The case shows that the shield tunneling performance and efficiency are improved by the tunneling parameter prediction model based on the gray system model.

Experimental study on workability and permeability of sandy soils conditioned with thickened foam

Water spewing and muck plugging often occur during earth pressure balance(EPB)shield machines tunnelling in water-rich sandy strata,even though the conventional foam has been employed to condition sandy soils.In this study,a novel thickened foaming agent suitable for EPB shield tunnelling in water-rich sandy strata is developed.In contrast to conventional foam-conditioned sands,the thickened foam-conditioned sand has a low permeability due to the consistent filling of soil pores with the thickened foam,and the initial permeability coefficient decreases by approximately two orders of magnitude.It also exhibits a suitable workability,which is attributed to the enhanced capability of the thickened foam to condition sandy soils.In addition,the effect of concentration on the stability of the foam is explained by the Gibbs-Marangoni effect,and conditioning mechanisms for the thickened foam on sands are discussed from the evolution of foam bubbles.

Real-time analysis and prediction of shield cutterhead torque using optimized gated recurrent unit neural network

An accurate prediction of earth pressure balance(EPB)shield moving performance is important to ensure the safety tunnel excavation.A hybrid model is developed based on the particle swarm optimization(PSO)and gated recurrent unit(GRU)neural network.PSO is utilized to assign the optimal hyperparameters of GRU neural network.There are mainly four steps:data collection and processing,hybrid model establishment,model performance evaluation and correlation analysis.The developed model provides an alternative to tackle with time-series data of tunnel project.Apart from that,a novel framework about model application is performed to provide guidelines in practice.A tunnel project is utilized to evaluate the performance of proposed hybrid model.Results indicate that geological and construction variables are significant to the model performance.Correlation analysis shows that construction variables(main thrust and foam liquid volume)display the highest correlation with the cutterhead torque(CHT).This work provides a feasible and applicable alternative way to estimate the performance of shield tunneling.

Deviation correction strategy for the earth pressure balance shield based on shield–soil interactions

The control system presently used in shield posture rectification is based on driver experience,which is marginally reliable.The study of the related theory is flawed.Therefore,a decision-making approach for the deviation correction trajectory and posture rectification load for an earth pressure balance(EPB)shield is proposed.A calculation model of posture rectification load of an EPB shield is developed by considering the interactions among the cutter head,shield shell,and ground.The additional position change during the shield attitude correction is highlighted.The posture rectification loads and shield behaviors results can be solved by the proposed method.The influences of the stratum distribution(i.e.,bedrock height in the upper-soft and lower-hard strata)on shield behaviors and posture rectification loads are analyzed.Results indicated that the increase of pitch angle in the upper-soft and lower-hard strata causes a sharp rise in vertical displacement.The bedrock height increases the magnitudes of the required posture rectification moments when hr/D>0.5.For a tunnel with hr/D≤0.5,the variation of hr/D has little effect on the posture rectification moments.Finally,the posture rectifying curves based on the theoretical model are compared with the target ones based on the double circular arc interpolation method.The required results can be obtained regardless of the soil–rock compound stratum distribution.The maximum rectification moment in the rock layer is almost 12.6 times that in the soil layer.Overall,this study provides a valuable reference for moment determination and the trajectory prediction of posture rectification in compound strata.

Influence factors and calculation model of the adhesion strength of clayey soil for EPB shield tunnelling

When earth pressure balance (EPB) shield tunnels are constructed through clayey ground, the soil adheres to the cutter, cutterheadand chamber bulkhead due to the high adhesion strength between the steel and the clay, thus clogging the shield. To investigatethe influence of different factors on the adhesion strength, this study used montmorillonite, kaolin and mixtures of the two as testsoils. The adhesion strength between the steel and clay is determined with a customized rotary shear apparatus. The results showthat when the consistency index of the soil specimen is less than 1, the adhesion strength between the steel and clay increaseswith the consistency index. As the consistency index decreases, the effect of the normal pressure on the adhesion strength graduallyweakened. As the contact angle of the shear plate increases, thus reducing the hydrophilicity, the adhesion strength decreases. Whenthe soil specimens with different plasticity index values have the same consistency index value, the adhesion strengths are similar.The adhesion strength increases gradually with increasing surface roughness. Based on grey incidence analysis, the order of thefactors affecting the adhesion strength is as follows: normal pressure > consistency index > contact angle > plasticity index ≈ surfaceroughness. The normal pressure, consistency index and contact angle all have important effects on the adhesion strength betweensteel and clay. However, because there are no large differences in the contact angle among the metal materials, the approximateadhesion strength can be calculated by considering the effects of only the normal pressure and consistency index. Themeasures thatare effective for preventing EPB shield clogging are increasing the soil softness and decreasing the hydrophilicity of the cutterheadby applying new materials for the cutters and cutterhead.

Surface settlements induced by twin tunneling in silty sand

Surface settlements caused by twin tunneling are a major interest in underground space engineering.Most prevailing studies have investigated the surface settlements observed over twin tunnels constructed in clay.Yet,less attention is given to the twin tunnels exca-vated in silty sand,which is the focus of this study.A comprehensive field monitoring scheme was designed in the Hangzhou metro line six project to measure the surface settlements during twin tunneling by adopting earth pressure balance shields.The influence of four shield operational parameters,namely,face pressure,tail grouting pressure,penetration rate,and pitching angle on the surface settle-ments due to twin tunneling are explored.In general,the total settlement profiles observed over twin tunnels follow an inverted bimodal shape.The post-settlement observed over the 1^(st)tunnel due to the 2^(nd)tunnel excavation is non-trivial when the twin tunnels are spaced closely.Low face pressure and tail grouting pressure can induce excessive peak surface settlements during twin tunneling in silty sand.