Stability Of Excavation Face Research Articles

Centrifuge testing and numerical modeling of tunnel face stability considering longitudinal slope angle and steady state seepage in soft clay

Understanding the failure mechanism of a tunnel excavation face under complex hydrogeological conditions is a key challenge for geotechnical engineering. In this study, we investigated the excavation face instability induced by the longitudinal slope angle of the tunnel and steady state seepage flow. First, a fiber Bragg grating (FBG) system suitable for centrifuge model tests was proposed. The test apparatus was enhanced to investigate the influence of longitudinal slope angle on the active limit support pressure. Subsequently, a series of centrifuge model tests and Abaqus numerical simulation analyses were conducted to investigate the progressive failure mechanism of the excavation face. The experimental and numerical simulation results showed that the proposed geotechnical centrifuge FBG test system is fully feasible for multi-parameter monitoring of the centrifuge model. Under the assumptions of homogeneous and isotropic soil and steady state seepage flow, the seepage force in the limit support pressure is independent of the permeability coefficient. Moreover, the active and passive limit support pressures increase and decrease, respectively, with longitudinal slope angle, and the gap between them progressively decreases. Furthermore, under isotropic permeability, the soil internal friction angle, rather than soil cohesion, is responsible for the change in excavation face stability with longitudinal slope angle.

Excavation Face Stability of Shield Tunneling Closely Under Existing Tunnels: Particle Flow Simulation

When a new tunnel passes closely under an existing tunnel, it will induce ground subsidence and adversely affect the existing subway tunnel. Aiming at the construction form of shield passing closely under existing tunnel, a numerical model using two-dimensional particle flow was set up to study the failure form of the excavation face, the relationship between support force and excavation displacement, longitudinal deformation of the surface and the earth pressure distribution pattern around the existing tunnel. The results show that the support pressure of shield excavation face increases first and then decreases with the increase in excavation surface displacement due to the presence of existing tunnel. The maximum vertical settlement value of the ground surface appears in the range of 2D from the center of the existing tunnel. With the advance of shield tunnel, the earth pressure above the new tunnel segment increases gently. When away from the existing tunnel, the horizontal earth pressure above the tunnel increases gradually, and the earth pressure in the area ahead of the shield tunnel along the depth presents a significant nonlinear distribution. The research results can provide theoretical basis and early guidance for such construction in the future.

Failure Mode and Stability of Excavation Face on Shield Tunnel Undercrossing Existing Tunnel

The supporting pressure value of excavation face directly determines the stable state of excavation face, and its value will directly lead to instability of excavation face if the value is too small. When the shield is underneath the existing tunnel, special attention should be paid to the support pressure setting of the shield working face. When setting support pressure, the rigidity constraint of existing tunnel on surrounding soil should be fully considered. In this paper, we used ABAQUS software to analyse the failure mode of the soil around the existing tunnel due to the instability of the excavation surface caused by the small pressure setting of the excavation face, which is caused by the small pressure setting of the excavation face. By using the method of theoretical analysis, we optimized the prism in the traditional wedge model to chamfer platform with different opening angles to make it closer to the actual situation, and calculated the critical support pressure of shield tunnel face when it passes through the built tunnel. The research results can provide a reference for the effective value of support force of shield excavation face when the shield tunnel passes under the existing tunnel at a short distance.

Three-dimensional analysis of excavation face stability of shallow tunnels

Several studies have been focused on minimizing construction risks of underground works. However, additional studies regarding excavation face stability, which has great influence on global performance of shallow tunnels, will offer more security criteria to engineers. The aim of this work is to compare the results from an existing analytical and a numerical method in assessing the behavior of the excavation face subjected to different overburden depth and ground conditions. Typical parameters of shallow soils were considered for all simulations. The analytical method was applied to assess the excavation face pressure for different tunnel covers. Numerical methods were applied using ABAQUS 3D® finite elements software, which considered a continuous, homogeneous medium in a 6-meter-wide tunnel excavated at different depths, and assessed the need for support and stabilization pressure on the face. Furthermore, the influence of a well on the excavation line was also considered for such assessment. As result, excavation face was defined as stable by the analytical method; nevertheless, it was obtained a very low safety factor. On the other hand, results from the numerical method showed the need to apply pressure on the excavation face. Those limit support pressure values at tunnel face showed a direct correlation with the increase of cover-to-tunnel diameter ratio. Finally, the numerical method showed a more realistic result when compared with the analytical method.

Optimal control for earth pressure balance of shield machine based on action-dependent heuristic dynamic programming

Earth pressure balance (EPB) shield has been widely used in underground construction. The excavation face stability is crucial to avoid the accidents caused by EPB shield tunneling, so that it is very important to propose an effective control method for the earth pressure balance in sealed cabin. Considering the problem that stable automatic control of the earth pressure in shield’s sealed cabin is difficult, an optimal control method of the earth pressure is proposed based on action-dependent heuristic dynamic programming (ADHDP), which can realize online autonomous learning and adaptive control in tunneling process. According to Bellman’s principle of optimality, the cost function with respect to the sealed cabin’s earth pressure is given. In addition, the action network and critic network of ADHDP controller are constructed. The critic network approximates the cost function and feeds error back into the action network. With the goal of minimizing the cost function, the action network utilizes the critic network’s error to optimize screw conveyor speed. The simulation results show that the earth pressure controller based on ADHDP can realize the earth pressure balance control, and the control process is steadier. Moreover, ADHDP controller has good dynamic performance and anti-interference ability.

Effect of Waterhead on Stability of Excavation Face of Slurry Balanced Shield

Effect of Waterhead on Stability of Excavation Face of Slurry Balanced Shield

Shield Tunneling in Rock–Soil Interface Composite Formations

With increased demand for the tunnel construction in rock–soil interface composite formations, the influence on surrounding environment especially the excavation face instability during construction and ground settlement in the long term has gained great attention. The researches about environmental disturbance by shield tunneling construction in single ground as the soil or rock conditions have been developed continuously. However, due to the complexity and uncertainty of the interaction between rock–soil interface composite formations and shield machines, works on these special conditions have not been carried out sufficiently. In this paper the theoretical, experimental and numerical researches on the excavation face stability and ground settlement are discussed while the in situ datum are used to support them. First, the typical projects in rock–soil interface composite formations are listed and the difficulties met are summarized. Second, the failure model of excavation face and support pressure from the tunneling shield in rock–soil interface composite formations are discussed. Then, a comprehensive survey of the factors of ground settlement during and after construction and some effective prediction models are made. Finally, the existing problems and directions for future research are introduced.

Evaluation of the Tunnel Face Stability through a Ground Stress Analysis with a Hemispherical Geometry Approximation

The evaluation of the stability of the excavation face is an important aspect in the design of a tunnel. When it is not possible to ensure excavation face stability in natural conditions, it is in fact necessary to intervene with remarkably costly reinforcement operations. The analysis of the stability conditions of an excavation face can be conducted, with a certain degree of detail, through numerical modelling. Simplified analytical models exist for shallow tunnels, but fewer are available for deep tunnels. One of the analytical methods most commonly used in the study of the stress conditions at the excavation face in deep tunnels is described in this study: The convergence-confinement method adapted to a spherical geometry. This method has here been extended to rock masses, which present a more complex rupture criterion (curved and not simply linear) than that of soils. The presented solution is of a finite difference numerical type. An extensive parametric analysis conducted on soils and rock masses has led to the estimation of the maximum lithostatic stress that still foresees the absence of a plastic zone around the hemisphere, which has been used to represent the excavation face. Therefore, this study makes it possible to obtain a preliminary estimation of the maximum depth of a tunnel in a certain type of soil or in a rock mass in which it is still possible to advance without the necessity of excavation face reinforcement operations. However, a more detailed and reliable analysis still requires more sophisticated instruments, such as numerical modelling.

Discrete element model for performance analysis of cutterhead excavation system of EPB machine

With the aim of probing for design theory of cutterhead excavation system of earth pressure balance (EPB) machine, the parameters representing the system performance are concluded firstly. Then a 3D model for cutterhead excavation system of EPB machine is presented with the discrete element method (DEM) software PFC3D, which is capable of simulating the tunneling ground in site, the machine structure and the excavation operation. The performance parameters indicating stability of excavation face, soil discharging rate, cutterhead system torque and cutter wear are measured by running the DEM code. The results obtained with the DEM model are accord with situ data. It indicates that the DEM model is a promising method replacing the field experiment to analyze the influences of the structural parameters on system performances, which are essential for structure optimization design of the cutterhead system of EPB machine.

The new clay mud and its improvement effects of tunnels

During tunneling process by earth pressure balance (EPB) shield, the strata containing large amounts of sand and gravel are often encountered. The excavated soil in the shield chamber has poor plastic flow and larger permeability, so it leads to the difficult construction of the EPB tunnel. In order to ensure tunneling advance successfully, the excavated soil must be improved by adding modified materials to change its physical and mechanical properties, and make it to have a good plastic flow state, low friction angle and low permeability. Soil improvement techniques can guarantee the stability of excavation face, achieve the balance advance of shield tunnel, reduce machinery load and the ground settlement, and improve the excavation speed. The existing mud used in engineering practice is the dispersing mud and water system whose compositions are bentonite and water. Both the composition and function are simple. The existing mud has various problems, such as large amounts of waste mud emission, difficulties in controlling mud indicators, large areas for mud treatment and large uses of new mud materials and environmental pollution. In this paper, a kind of environmentally friendly clay mud was developed, and the laboratory and field tests were conducted to verify its adaptability and superiority.

PSO-based Elman neural network model for predictive control of air chamber pressure in slurry shield tunneling under Yangtze River

Abstract The excavation face stability is crucial for safety and risk management in slurry shield tunneling, especially for the river-crossing tunnel. To avoid face collapse or blow-out, shield operators need to keep air chamber pressure balanced using their own experience, which would be difficult, discontinuous and less reliable in the process of construction. Considering the disadvantage of the manual control process, this paper presents a predictive control system for air chamber pressure in slurry shield tunneling using Elman neural network (ENN) model. It mainly contains a theoretical model, an ENN predictor and an ENN controller to set optimal control parameters automatically tracking the desired air chamber pressure. Moreover, to improve the learning capability of ENN model, a particle swarm optimization (PSO) algorithm is implemented. This system has been tested with collected data of slurry shield operation parameters in the Yangtze riverbed metro tunnel project in Wuhan, China. Analysis revealed that the predictive control system using PSO-based Elman neural network model in this paper has the potential for enhancing face stability in slurry shield tunneling.

Study on Soil Improvement Technology of EPBS Tunnel

The tunnel construction often encounters the problem of adaptability of sand-gravel formation and earth pressure balance shield (EPBS) construction, which has brought new technical problems and challenges to those who are participating in the construction project. In order to ensure the normal operation of shield construction, the soil must have ideal plasticity and liquidity, and low permeability to intercept groundwater. Therefore, we must adopt the soil improvement techniques ,namely, to inject the modified materials (mud, foam, polymer, etc.) into excavation face and earth pressure tank(if necessary, to the screw conveyor) to ensure the stability of excavation face, achieve EPBS driving, and help to reduce mechanical load, reduce land subsidence, and increase driving speed at the same time. The ground improvement technique is an important part of the EPBS method. The quality of the application of soil improvement technology is very important to maintain the stability of excavation face and the screw dump device’s dumping smoothly. It also has a significant impact on the shield machine’s functioning safely,economically and efficiently.

Slurry Shield Tunnelling in Clayey Soils: Typical Problems and Countermeasures

The distinguishing characteristics of clay are high cohesion and low permeability. For slurry shield tunnelling in clayey soils, favorable aspects are that slurry cake with low permeability can be established more easily and effectively at the excavation face thanks to clay’s high cohesion, which is advantageous for stability of excavation face, and long stand-up time of clay offers enough time for backfilling of the tail void before collapse of surrounding soils, thus lessening ground volume loss. However, some typical problems are encountered due to clay’s high cohesion and low permeability. One primary problem is the clogging of slurry pipeline, once in case of which, slurry pressure will fluctuate severely, thus inducing unstable condition at the excavation face. In extreme cases, the pipeline bursts and soils at the excavation face collapse towards the cutterhead for immediate drop of slurry pressure. Another common problem is clay’s adhesion to the cutterhead, which weakens the excavation efficiency of cutterhead and limits advance rate of the shield machine. Tunnelling will inevitably disturb surrounding soils and excess pore water pressure occurs. In clayey soils, due to clay’s low permeability, it usually takes quite a long time for the excess pore water pressure to disperse completely. The consolidation settlements associated with pore water dispersing account for a large percentage of the total settlements. Accumulated ground settlements threaten structures and pipelines nearby. For these problems encountered during slurry shield tunnelling in clayey soils, both preventive and counter measures are put forward in detail in this study. The proposed measures can be used as a reference to avoid, mitigate and deal with problems encountered during slurry shield tunnelling in clayey soils.

Experimental Research on the Engineering Properties of Foam-Improved Red Clay Soil

Red clay soil is widely distributed in south China, the microstructure of red clay soil was studied applying scanning electron microscopy (SEM), and the X-ray diffraction analysis (XRD) test found that a large number of swelling inducing minerals, for example, montmorillonite, illite-montmorillonite or chlorite-smectite were contained in the red clay soil. Shield tunneling in this kind of stratum is prone to arising “cake” and “arch” phenomena and it prone to lead screw conveyor device unsmooth dumping, so soil improvement measures must be taken. Foam as the most advanced soil conditioner has been widely used in shield construction. Using self-developed foam agent, experimental research on foam conditioning red clay soil was carried out, test results show that foam can not only significantly reduce the soil shear strength, but also can greatly enhance the soil's compressibility and fluidity, which is significant for the smooth dumping and excavation face stability maintenance.