Stability Of Excavation Face Research Articles

Study on the Influence of Walnut Shell Coarse Particles on the Slurry Permeation and the Air Tightness of Filter Cake.

Slurry shields rely on the formation of a compact filter cake to maintain excavation face stability and ensure construction safety. In strata with high permeability, significant slurry loss occurs, making filter cake formation and air tightness maintenance challenging. In this study, light organic walnut shell was selected as an additive coarse particle material for slurry. Slurries incorporating two types of coarse particles, sand and walnut shell, were prepared, and tests on slurry permeation and air tightness of the filter cake were conducted in three different strata. The results indicate that the addition of coarse particles effectively improves filter cake formation and air tightness in high-permeability strata. It is essential to use graded particles in highly permeable strata, with controlled maximum and minimum particle sizes. As the content of coarse particles increases, the air tightness of the filter cake initially increases and then decreases. Notably, the air tightness of filter cakes containing walnut shell is superior to those containing sand. Replacing sand with walnut shell as a slurry plugging material enhances filter cake quality in high-permeability strata. For highly permeable strata with a permeability coefficient greater than 1.0 × 10-3 m/s, an addition of 30 g/L to 40 g/L is recommended.

Model Design and Application for Excavation Face Stability in Upward Shield Tunneling

Model Design and Application for Excavation Face Stability in Upward Shield Tunneling

Stability analysis of tunnel face considering the shape of excavation and reinforcement measures

In mountain tunnels, a series of advanced reinforcement measures are often employed to maintain the stability of the tunnel face, particularly in challenging geological conditions. However, there is a lack of design theories regarding the advanced support structure for uniquely shaped tunnel faces, necessitating further exploration. Based on the limit equilibrium method, this paper presents a derivation of the mechanical model for analyzing excavation face stability considering shape changes and bolts. Additionally, a design theory for horizontal glass fiber bolts in longitudinally inclined tunnel faces is established. The reinforcement effect of the pipe shed is further considered. Finally, the proposed model is validated, and parameter analysis is conducted to guide design practices.

Analysis and prediction of the river levee settlement derived from shield tunneling considering the excavation face stability

Analysis and prediction of the river levee settlement derived from shield tunneling considering the excavation face stability

Numerical Investigation of the Effect of Longitudinal Fiberglass Dowels on Tunnel Face Support in Layered Soils

Tunnel face extrusion rigidity is an important factor for solving stress–strain problems in loose ground conditions. In previous studies, the effect of horizontal and vertical soil layering on tunnel excavation face stability in the presence of longitudinal fiberglass dowels has not been studied. Therefore, in this study, the effect of fiberglass dowels on the stability of the tunnel face in layered soil has been investigated. In this matter, the best dowel arrangement for minimizing the excavation face extrusion in the case of two-layer soil (horizontal or vertical) has been focused on. For this purpose, firstly, a 3D numerical model was validated based on field data provided previously, and then a 3D numerical tunnel was developed in FLAC3D, adopting the Mohr–Coulomb failure criterion. In continuation, the effect of tunnel diameter, initial pressure ranging from 0.5 to 1.5 MPa, and different placement angles of fiberglass dowels ranging from 0 to 9 degrees, with respect to the tunnel longitudinal axes on the tunnel face extrusion, have been investigated. In the case of horizontal layering, the results showed that the maximum extrusion rate is significantly increased where the elasticity modulus of the soil is reduced. In addition, comparing the maximum extrusion in vertical and horizontal layering, it was found that its value in the horizontal mode is much higher than in the vertical. Additionally, the extrusion of the tunnel face has changed significantly due to an alteration in the initial stress. Finally, it was discovered that tunnel face extrusion is not significantly affected by altering the angle of the fiberglass dowels.

Intelligent decision method for stability assessment of shield tunnel based on multi-objective data mining.

Due to improper operation of the shield construction process and unknown geological surveys, shield construction faces many risks in passing through complex strata, among which the excavation face instability is the most serious, potentially leading to disastrous accidents. To address these issues, this research focuses on the limit support pressure and the excavation face stability in the soil when crossing the Yangtze River. First, an analytical formula for the limit support pressure of the excavation face is established through the wedge model. The support safety coefficient is used to assess the excavation face stability quantitatively. Then the rough set algorithm is used to analyse the sensitivity of each index to establish the reduced evaluation index system for the excavation face stability. The back propagation (BP) neural network is used to train the learning data, and a neural network evaluation model with a prediction error of 5.7675 × 10-4 is established. The prediction performance of BP is verified by comparison with the TOPSIS prediction model and the cloud model. The evaluation method proposed in this paper provides an essential reference for evaluating the underwater shield tunnel excavation face stability. This article is part of the theme issue 'Artificial intelligence in failure analysis of transportation infrastructure and materials'.

Numerical investigation and prediction of the excavation face stability for river-crossing shield tunneling: An intelligent prediction model for limit support pressure

The instability of the shield tunnel's excavation face, particularly underwater tunnels, is one of the most hazardous factors in subsea tunnel excavation projects. Therefore, an accurate prediction of the limit support pressure to maintain the excavation face's stability is essential to minimize the possible risk of damage. This study proposed an intelligent prediction model to address these issues. First, the analytical formula of the limit support pressure of the excavation face was established through mechanical analysis to give the mechanics criterion for active instability of the excavation face. Subsequently, the numerical simulations were conducted to reveal the evolution mechanism of the excavation face instability. The result revealed that the buried depth of the tunnel, water depth, driving speed, internal friction angle, water content, cohesion, and support safety factor were the main factors affecting the limit support pressure. Furthermore, the support vector machine (SVM) model was established for predicting the limit support pressure. The prediction results agreed well with the actual data, thereby indicating the feasibility and convenient implementation of the SVM predictor. The proposed model was validated as an effective method for predicting the limit support pressure for further engineering applications.

Numerical study on the hydrodynamic properties of bentonite slurries with Herschel-Bulkley-Papanastasiou rheology model

During the construction of slurry shield tunnels, bentonite slurries play a significant part in maintaining excavation face stability where suitable slurries infiltrate into the stratum to form an impermeable filter cake. To enhance filter cake quality, accurate prediction of the hydrodynamic properties of bentonite slurries is always required during slurry parameters design. The main purpose of this study is to investigate the flow characteristics of bentonite slurries with different polymer contents using computational fluid dynamics (CFD) simulation. Mixtures of sodium bentonite powder, carboxymethyl cellulose (CMC), and water in different ratios were prepared, and a series of indoor rheological tests were conducted. Then an optimized numerical method for slurry flow simulation was developed by incorporating the Herschel- Bulkley-Papanastasiou (HBP) rheology model with the finite volume method (FVM). The proposed model was implanted in OpenFOAM, and then applied to theoretical cases of non-Newtonian plane-Poiseuille flow for model verification. Results show the theoretical and simulation results were in good accordance, with a successful description of the shear thinning characteristics of fluids and no divergency during the numerical solution procedure. Based on it, CFD simulations were implemented to investigate pipe flow characteristics of bentonite slurries with different CMC contents. Results indicate when the inlet velocity is constant, the decrease of pipe diameter can cause a more inhomogeneous flow and the growth of pressure drop; rises in rheological parameters can all increase the energy consumption and reduce the infiltration distance. When the yield stress is 25 Pa, the pressure drop reaches a maximum value of 5.47 kPa, compared with 2.69 kPa for slurries with a yield stress of 5 Pa. Furthermore, the infiltration law of bentonite slurries in pipes essentially conforms to a non-Darcy infiltration with an initial hydraulic gradient i0, which has a positive relationship with yield stress of fluids. i0 increases from 0.082 to 0.414 after adding 0.3% CMC into bentonite slurries. This research can provide significant guidance for the establishment of slurry infiltration theories and relevant numerical simulations.

Limit face pressure analysis of an EPB shield under suffusion conditions

This study investigated the excavation face stability of an Earth-Pressure-Balanced (EPB) shield tunnel under suffusion conditions using a finite difference method. Based on the seepage erosion tests on gap-graded sand, the suffusion process was properly simulated by the gradual variations in porosity, fines content, permeability coefficient, and dry density. The variation in eroded soil strength was based on the coupling effect of fines content and relative particle roundness according to a series of tests on binary mixtures. A prism-wedge equilibrium model was revised to obtain the limit support pressure of the excavation face under suffusion conditions. It was found that the amount of fine particle loss and relative particle roundness were two key factors in determining the limit support pressure, and the decrease in seepage force resulting from the increase in eroded soil permeability contributed to a smaller limit support pressure than those under seepage-only conditions. The limit support pressure under seepage-only conditions is relatively conservative and is adequate to maintain face stability.

Critical support pressure of shield tunnel face in soft-hard mixed strata

Support pressure is a key factor to the stability of excavation face during shield tunneling, which further imposes a safety issue on surrounding buildings and facilities. However, several studies have been reported on critical support pressure in mixed strata regarding twin-tunnel excavation. This paper aims to obtain the critical support pressure based on minimizing the influence on the surrounding environment induced by shield tunneling in soft-hard mixed strata. A series of three-dimensional finite element models are established to analyze soil and lining responding with different support pressure caused by twin tunnels excavation. The results reveal that the ground surface settlement and lateral displacement of soil are sensitive to the change of support pressure, especially under clay-dominated conditions. Increasing support pressure slightly changes the lining movement and location of maximum stresses of the existing tunnel due to adjacent tunnel excavating. According to this study, the upper and the lower critical support pressures are 1.43 and 1.06 times of lateral earth stress at tunnel crown, respectively, which are also compared with the limit support pressure and measured results from a shield tunnel in the literature. The critical support pressure has great significance for the stability of the excavation face and minimizing the influence on the surrounding environment caused by shield tunnel excavation.

Parameter analysis of excavation face stability of shield tunnel under high water pressure seepage

When a river-crossing shield tunnel is excavated in saturated soft clay strata, the fluctuation of the water level, the tunnel buried depth, and the physical properties of soil have a significant influence on the tunnel face stability. In this study, based on the basic theory of fluid-solid coupling of the surrounding rock soil, the fluid-solid coupling calculation method and the stability criterion of the tunnel face were used. The failure modes, deformation characteristics, and fluid-solid coupling effect were systematically investigated under different water levels, buried depths, diameters, and soil permeability coefficients when the excavation face was unstable. The results show that with the changes of water level, buried depth, and tunnel diameter, the collapsed body presents a logarithmic spiral distribution as the underwater shield tunnel face is unstable. The limit support force of the excavation face increases linearly with the rise of the water level, and the seepage force greatly influences the limit support force. For the deep-buried shield tunnel, the limit support force and maximum horizontal displacement of the excavation face change slightly due to soil arch effect. The soil arch effect can be used as the deep- and shallow-buried boundary of an underwater shield tunnel. As for the underwater shield tunnels with different diameters, the instability of excavation face first occurs at the lower part of excavation face. The face stability of underwater small-diameter shield tunnel is better than that of large-diameter one. In addition, the soil permeability coefficient has a weak effect on the tunnel face stability.

Stability Analysis of Shield Inclined Tunnel Faces under the Change Effect of Soil Heterogeneity and Pore Water with Buried Depth

The purpose of this work is to look at how soil heterogeneity and pore water pressure impact shield inclined tunnel excavation face stability as burial depth varies. The calculation model of excavation face stability was established, and the supporting pressure of maintain the excavation face stability was solved via the upper bound method. The results showed that the soil heterogeneity coefficient and pore water pressure coefficient had substantial effects on the supporting pressure. The location of failure face and failure range of shield inclined tunnel under varied heterogeneity coefficient and pore water pressure coefficient were presented. In addition, the boundary depth of shallow tunnel under the influence of soil heterogeneity and pore water was obtained, which supplemented the relevant tunnel code.

Model Test on the Passive Failure of Slurry Shield Tunneling in Circular‐Gravel Stratum

AbstractTo study the passive failure behavior of slurry shield tunnels in the circular‐gravel stratum, a set of slurry shield test devices capable of cutter cutting and slurry automatic circulation is designed and developed. The effects of the cover‐span ratio on the evolution process of ground surface displacement, the distribution of earth pressure and the passive failure mode of the face are discussed. The results show that the ground surface is gradually uplifted with the increase of slurry chamber pressure, and the amount uplifted decreases with the increase of the cover‐span ratio. The earth pressure near the face during testing experienced the growth stage, the rapid decrease stage, and the stable stage. The passive failure of slurry shield face experiences excavation face stability stage, slurry film splitting stage and splitting through stage in sequence, and the ultimate supporting force of passive failure of excavation face increases with the increase of the cover‐span ratio. The essentially of the passive failure is a kind of slurry splitting phenomenon in which slurry splits the slurry film and then splits the soil layer. When a passive failure of the excavation face occurs, there is a mud channel formed by the slurry split stratum in front of the excavation face. When the cover‐span ratio is relatively small, the channel extends to the ground surface and forms a hole whose size is basically the same as the diameter of the shield. The hole morphology gradually changes from an ellipse to a circle with the increase of the cover‐span ratio.

Experimental Study on the Characteristics and Formation Mechanism of Dynamic Filter Cake for Slurry Shield Tunneling

The key to guaranteeing excavation face stability in slurry shield tunneling is the formation of an impermeable dynamic filter cake. At the same time, the effect of the cutter head and rotation speed should be taken into account. We studied the characteristics and formation mechanism of the dynamic filter cake using a newly developed experimental apparatus. The experiment results show that the hysteretic infiltration zone appeared in the curves of stepped loading filtration while the cutter head was rotating, and the volume of water filtration increased by 11.2% compared to when the cutter head stopped. The higher the rotation speed was, the lower the conversion rate of the effective stress was. Under the same rotation speed, the formation time of the 6-cutter arm was almost 5 s slower than that of the 5-cutter arm. As the cutter arms and the rotation speed increased, the stratum’s electrical conductivity increased and stabilized at a distance of 20 cm from the cutter head. The filter cake transited from ‘filter cake plus an infiltration zone’ to ‘an infiltration zone without a filter cake’ with the increase of the rotation speed. The thickness of the dynamic filter cake was smaller than that of the static filter cake, the thickness of 10 groups decreased significantly, and the average thickness decreased by 76.15% at 1.0 rpm. The mesoscopic formation process of the dynamic filter cake can be divided into six stages. This study revealed the slurry penetration mechanism and filter cake characteristics present under cyclic damage by the shield cutter head to the filter cake and soil and provided theoretical support on how to maintain the stability of the excavation face during slurry shield tunneling.

Analysis of Face Stability Based on Incomplete Soil Arching Effect

The excavation face stability is a critical issue in the analysis of the tunneling disturbance to the surrounding environment, in which the soil arching effect is a phenomenon that cannot be ignored. Based on the wedge-prism model, this study proposed a slice method to calculate the limit support pressure, taking into account both the vertical soil arching effect of the prism and the horizontal arching effect of the wedge. In addition, the rotation of the principal stress was introduced to improve the lateral stress ratio on the vertical sliding faces including the wedge and the prism, in which two new trajectories of the major principal stress were recommended, based on a previous numerical study. Furthermore, the contribution of the incomplete soil arching effect was discussed in this study, assuming that there was a function between the effective internal friction angle of the soil and the tunnel cover depth. Through the comparison of the results with theoretical and numerical models, the improved model was validated and showed agreement with them. Finally, parameters related to the limit support pressure were thoroughly investigated to provide a better understanding of the proposed model.

Excavation Face Stability of Slurry Shield Tunnel in Saturated Sand Layer

Excavation Face Stability of Slurry Shield Tunnel in Saturated Sand Layer

Coordinated optimization control of shield machine based on dynamic fuzzy neural network direct inverse control

During shield machine tunneling, the earth pressure in the sealed cabin must be kept balanced to ensure construction safety. As there is a strong nonlinear coupling relationship among the tunneling parameters, it is difficult to control the balance between the amount of soil entered and the amount discharged in the sealed cabin. So, the control effect of excavation face stability is poor. For this purpose, a coordinated optimization control method of shield machine based on dynamic fuzzy neural network (D-FNN) direct inverse control is proposed. The cutter head torque, advance speed, thrust, screw conveyor speed and earth pressure difference in the sealed cabin are selected as inputs, and the D-FNN control model of the control parameters is established, whose output are screw conveyor speed and advance speed at the next moment. The error reduction rate method is introduced to trim and identify the network structure to optimize the control model. On this basis, an optimal control system for earth pressure balance (EPB) of shield machine is established based on the direct inverse control method. The simulation results show that the method can optimize the control parameters coordinately according to the changes of the construction environment, effectively reduce the earth pressure fluctuations during shield tunneling, and can better control the stability of the excavation surface.

Theoretical Analyses of the Stability of Excavation Face of Shield Tunnel in Lanzhou Metro Crossing beneath the Yellow River

Abstract Based on an actual engineering case, this study focused on the limit support pressure, instability mechanism, and instability failure mode of the excavation face in the sand pebble stratum...

Physical model tests and discrete element simulation of shield tunnel face stability in anisotropic granular media

The stability of excavation face in shield tunneling plays a key role for construction safety. The ignorance of soil anisotropy in most previous studies would induce inaccurate stability assessment. This paper studies the failure of shield tunnel face in cross-anisotropic granular media by physical model tests and discrete element simulation. Model tests were carried out on the tunnel face stability in anisotropic granular media, and initial anisotropy was generated by controlling the long axis of non-spherical particles. By conducting image analysis on the picture taken by HD camera, the failure mode of tunnel face was obtained. It consists of a sliding wedge and an overlying loosen area, and the inclination angle of sliding wedge varies with the bedding plane. The variation in limit support pressure with the intersection angle of the shield tunneling direction and the soil bedding plane was obtained. Discrete element simulation was further employed to study the tunnel face stability in cross-anisotropic granular media; the microscopic parameters were calibrated by fitting against the particle drop test and repose test. Clump particle consisted of three identical ball was used in the simulations, and its long and short axes were in accordance with rice particles. The obtained varying characteristic of limit support pressure with intersection angle from simulation is consistent with the test results, and the obtained failure mode is also similar to that of physical model test. The principal stress distribution at failure state was analyzed in the discrete element simulation, and the change of major principal stress direction from vertical to nearly horizontal in the loosen area clearly shows the formation of soil arches.

Influence of Forepoling Umbrella on the Settlements Induced by Shallow Urban Tunneling

In this study, the influence of a forepoling umbrella system to control ground surface settlements induced by shallow tunneling in urban areas under dynamic railway traffic loads was investigated. 2D and 3D numerical models of the Zarbalizadeh-Maddah underpass tunnel were developed to evaluate excavation face stability and ground deformations. The 2D modeling results indicate stable conditions of the tunnel due to its well-designed support systems. Based on a sensitivity analysis performed in the 2D modeling, cohesive strength and elastic modulus of the equivalent element to the forepoling zone, show the greatest effect in controlling ground surface settlement. In spite of the stable condition of the tunnel, surface settlement obtained from the 2D numerical modeling results in a relatively higher value than instrumentation data (around 43%), recorded during tunnel excavation. In order to more accurately investigate the effect of forepoling on surface settlement, a 3D simulation was performed, with consideration of the Longitudinal Deformation Profile of the tunnel face to better estimate stress relief for different stages of excavation. The results displayed a lower value of surface settlement compared to the 2D model, and in good agreement with the instrumentation data. Therefore, a forepoling pre-support system could have a pivotal role on the control of ground surface settlement by developing the radial and longitudinal arching around the excavated opening of a tunnel in shallow soft ground under high dynamic loading.