Excavation Surface Research Articles

Physical model test on the influence of the cutter head opening ratio on slurry shield tunnelling in a cobble layer

To study the influence of the opening ratio of a cutter head on slurry shield tunnelling in a cobble stratum, a slurry shield test system that could realize the action of a cutter head and the automatic circulation of the slurry was developed. The mechanical response of cobble stratum during excavation and the instability of the excavation face in the slurry shield tunnel was studied. Moreover, the influences of the cutter head opening ratio on the shield parameters, surface settlement and failure mode of the excavation face were investigated. Finally, the opening ratios adaptability for the slurry shield in the cobble strata was discussed. Results show that the smaller the opening ratio is, the smaller the active failure chamber pressure and ultimate support ratio will be, as well as the settlement area during active failure. With an increase in the opening ratio, the active failure mode of the excavation surface changes from local slip failure to integral slip failure, and the volume of failure soil increases. The cutter head thrust and torque decrease with an increase in the opening ratio. The increase in shield thrust induced by a low opening ratio can be attributed to three factors, i.e. the increasing contact area between the cutter head and the excavation face, a large friction resistance between shield shell and stratum and the slurry cake phenomenon on the face of the cutter head. A modified theoretical calculation method suitable for cutter head torque of slurry shield in cobble stratum was proposed based on the experimental results. Finally, the opening ratios adaptability of the slurry shield in the cobble stratum was obtained.

Comparison study of Excavation Stability using Different Tunnel Excavation Methods

For the safety and efficiency of tunnel construction, optimal schemes for tunnel excavation with different surrounding rock grades should be proposed to provide references for engineering construction. Through 3D numerical simulation, the mechanical properties of the surrounding rock during tunnel construction were studied by 3D numerical simulation. For different surrounding rock grades, three different excavation methods were simulated. A comparative analysis of ground settlement, surrounding rock displacement, and maximum principal stress caused by different construction methods was also performed. The results show that different construction methods have a small impact on the displacement of surrounding rock. When the surrounding rock is of grade I, the maximum vertical displacement of the excavation surface is less than 2 mm. Bench cut method can be used for excavation with fast construction speed. When the surrounding rock is of grade IV, the stratum settlement and excavation surface displacement are large. The Center diagram method can ensure the construction progress and minimize the vertical displacement of surrounding rock. When the surrounding rock is of grade V, the displacement caused by tunnel excavation further increases. The double-side drift method can be adopted to minimize the stratum settlement and surrounding rock displacement.

Borehole contraction and depth analysis based on Extended SMP criterion

The excavation of vertical boreholes is involved in several geotechnical engineering constructions. Earth pressure in the soil is released during the drilling process, resulting in soil stress redistribution and borehole contraction along the excavation surface. The stress and displacement responses tend to increase with soil depth, which may degrade the quality of hole formation and cause collapse. Therefore, the concerns of three-dimensional (3-D) stress and displacement states of the soil around the borehole should be addressed during the design and construction of associated engineering. This study proposes exact and simplified analytical solutions based on the Extended Spatially Mobilized Plane criterion to obtain a more accurate borehole contraction law and depth in drained soil. The 3-D stress and displacement values along the borehole depth are obtained by combining the derived plane solution of cylindrical cavity contraction with the classical earth pressure formula. The proposed solution can reasonably consider the influence of the intermediate principal stress and explicitly state its value. The contraction degree of the proposed solution is 7.76% less than the Mohr–Coulomb solution at a depth of 80 m with the specified soil parameters. A parametric analysis shows that the contraction will be reduced by 21.4% and 36.8% if the unloading ratio increases from 0.4 to 0.5 and 0.6, respectively. The borehole can be supported by a high unit weight of slurry in an elastic state. A decrease in soil shear strength impairs borehole stress state and hole formation quality, and internal friction angle affects more than cohesion in deep soil. The effect of dilatancy on borehole contraction is insignificant. Further, the proposed approximate solution, which ignores the elastic strain in the plastic zone, has a slight error in borehole displacement but is convenient for engineering calculation.

Simple Filter Cake Formation Test Method Using a Geotextile instead of the Sand Stratum

Slurries and filter cakes are widely used to support the earth pressure and water pressure of an excavation surface to maintain stability. Most major projects of slurry shield need to carry out filter cake formation tests to study the availability of slurry parameters. Nevertheless, it takes considerable testing and lengthy preparation of sand to simulate a stratum, therefore this paper examines a filter cake formation test method in which a geotextile is used to replace the sand stratum. Using the geotextile as a known filter medium, the quantitative relationship is studied between the membrane and the simulated sand, to examine whether the same experimental results can be obtained as using the simulated sand. A series of filter cake formation tests were carried out on four different sands and five different geotextiles with four different slurries and then the filter membrane properties were measured to explore the potential of the geotextile alternative for filter cake formation. The results show that the infiltration process of slurry in sand and geotextile was similar, and slurry infiltration occurred both with and without filter cake. 0.8 times apparent opening size of geotextile could be used to replace the characteristic pore diameter of the sand stratum (0.8AOS = D0) for the filter cake formation tests. The filter cakes on sand stratum and the substituted geotextile had similar properties, such as thickness and water content.

Is a spatial investigation possible without long-distance refit/conjoin? Application to the MIS 11 lithic assemblage of levels E and J from La Cansaladeta site (Tarragona, Spain)

In this manuscript, we present the first systematic refitting results of the small-scale Middle Pleistocene (MIS11) rock shelter site of La Cansaladeta. The lithic materials that have been recovered from the archaeological levels E and J were the main study materials. These levels were investigated regarding spatial pattern analysis and analyzed with auxiliary methods such as quantitative density mapping demonstration and technological analysis of the lithic clusters. Thus, the spatial patterns of the two levels were compared and discussed, in terms of connections, clusters, and movement of the lithic elements. Undoubtedly, the well preservation of the archaeological levels offered a great opportunity for the interpretation of the spatial patterns in a high-resolution perspective. La Cansaladeta has not been paid attention adequately so far may be due to the small dimension of the excavation surface or to the scarcity of faunal record. Our results show that small-scale sites without long-distance refit/conjoin connections can provide significant spatial information. Indeed, if the sites have very well-preserved archaeological levels, the absence of long connections can be supported by the auxiliary methods.

Large deformation finite element analyses in TBM tunnel excavation: CEL and auto-remeshing approach

Analyzing tunnel excavation behavior involves large deformation issues. For this reason, large deformation analysis method is required to rigorously investigate the actual effect of excavation on surrounding ground. Two major numerical approaches capable of considering large deformations are applied to investigating the effect of tunnel boring machine excavation: coupled Eulerian-Lagrangian (CEL) and the automatic remeshing after small strain approach. The theoretical basis and application of the methods will be discussed before their relative performance is evaluated through the ground response due to TBM excavation. The ground response will be quantified by estimating the range of the excavation damaged zone (EDZ). By comparing the results, the range of the size of the EDZ will be suggested along the vertical and horizontal direction along the TBM advancement. Based on the computed results, it was found that the size of EDZ around the excavation surface and the tendencies was in good agreement among the two methods. Numerical results clearly show that the size of the EDZ around the tunnel tends to be bigger for harder rocks. The size of the EDZ is found to be direct proportional to the tunnel diameter, whereas the depth of the tunnel is inversely proportional due to higher confinement stress around the excavation surface.

Three-dimensional modeling of pendulum waves propagation under dynamic loading of underground excavation surface

The propagation of surface pendulum waves under nonstationary loading on the free surface of a cavity located in a block half-space is studied numerically. A mathematical model of transient viscoelastic deformation of the block medium is proposed. This model is based on the idea that dynamic behavior of the block medium can be approximately described as the movement of rigid blocks due to the compliance of interlayers between them. To describe the viscoelastic behavior of the interlayers, an internal friction model is used with the merit factor of the material as the determining parameter. The medium is modeled by a three-dimensional lattice of masses connected by elastic springs and viscous dampers in axial and diagonal directions. The influence of the merit factor and the cavity depth on the amplitudes of block velocities on the surface of the half-space is investigated for the longitudinal and Rayleigh waves.

Laboratory Measurement of Excavation Disturbance Effect Due to Trenchless Construction Using Distributed Fiber Optics Strain Sensing

Abstract Excavation disturbance induced by trenchless construction may lead to underground movement and deformation, which can be a potential hazard to the safety of overlying pavement structure. However, the development of a fiber optic–distributed sensing approach in recent years has provide an effective way for continuous measurement of soil strain. In this study, the mechanism of the excavation disturbance effect that is due to trenchless construction was investigated based on laboratory experiments. The sensing fibers were embedded within the overburden layer above the excavation surface to measure the distributed soil strain using Brillouin optical time-domain analysis. The relationships between ground settlements and soil strains were analyzed. The partition theory of excavation disturbance zone was verified based on test results. The influence of excavation on mechanical responses of overlying model pavement, including bending deformation and soil pressure, was measured and discussed. Laboratory test results indicated that soil strains were nearly symmetrically distributed around the excavation area, which was compressive close to the centerline and then became tensile on both sides of the excavation area. The width of the excavation disturbance zone derived from the soil strain measurements was found close to the calculation results based on the partition theory. Because of the uneven distribution of the support force that was caused by foundation settlement, the model pavement showed notable bending deformation after excavation. Therefore, the combined effects of traffic loading and excavation disturbance should be considered in the safety evaluation of trenchless construction.

A Displacement-Based Theory for Predicting the Support Force on the Shield Tunneling Surface in Sandy Soil Layers

Due to the poor stability of the loose sandy soil layer, if the support force is not properly controlled during the construction process of the shield tunnel using the earth pressure balance method, it is easy to cause the ground to collapse or uplift. Therefore, understanding the support force of the excavation surface of shield tunneling in sandy soil layer is very vital to ensure the stability of the excavation surface. Firstly, it is assumed that the damaged soil is a three-dimensional wedge and a modified three-dimensional wedge in the active and passive failure modes, respectively. The shallow soil pressure theory and the soil plastic limit equilibrium theory are derived by analyzing the stress distribution on the damaged soil. The equation for revealing the inner essence between the support force of the shield excavation surface and excavation surface displacement under the condition of sand-covered soil is used. Secondly, the numerical simulation method analyzes the displacement of the excavation surface when the support force changes under different working conditions, and the relationship curve between the excavation surface support force and the shield tunneling displacement is obtained. The comparison and analysis between the numerical simulation calculation and the theoretical analysis indicate that the deduced calculation equation for the excavation surface support force based on the displacement earth pressure is reasonable.

Seismic Behaviour of Excavations Reinforced with Soil–Nailing Method

Soil nailing is an in-situ soil reinforcement technique that is used to enhance the stability of land slopes, retaining walls and excavations. This technique involves the installation of closely spaced and slender reinforcing solid bars, into the pre-drilled holes, which may be different in terms of length and angle of drilling. In this paper, the effects of length and angle of installed bars on the seismic behavior of excavations are investigated. For this purpose, a parametric finite element study was performed and dynamic time-history analyses were carried out using Tabas (Iran, 1978) and Manjil (Iran, 1990) earthquake ground motion records. The results of the investigation demonstrate that the length, angle, and distance of the nailing bars influence the acceleration response as well as the vertical and horizontal displacements at the excavation surface during excitations. Furthermore, it is shown that the overburden weight, soil type, and earthquake acceleration influence the seismic stability of soil–nailed walls and the amplification factor. In the final part, a case study of soil–nailed wall constructed for pit stability in “Tehran’s Tohid Saraye-Mahalle” is investigated.

Deformation and Numerical Simulation Analysis of Deep Foundation Pit Excavation of Nanjing Yangtze River Floodplain Metro Station

Based on the foundation pit excavation construction of a subway station under the soft soil layer of the Nanjing Yangtze River floodplain, Numerical simulation of foundation pit construction process is carried out to study the deformation law of surrounding soil and surrounding structure during foundation pit construction. The research shows that the established finite element numerical model can better predict the structural and stratum deformation caused by the excavation of the foundation pit; the lateral displacement of the ground wall and the ground settlement behind the wall based on different constitutive models are different, but the overall trend is the same. The lateral displacement of the diaphragm wall presents a “bulging” pattern. The surface settlement around the pit shows a settlement trough at a certain distance from the connecting wall. The horizontal displacement of the diaphragm wall increases with the increase of the excavation depth of the foundation pit, and the maximum horizontal displacement occurs above the excavation surface. The conclusions obtained have certain guiding significance for subsequent construction and parameter optimization.

A 3DEC Numerical Analysis of the Interaction Between an Uneven Rock Surface and Shotcrete Lining

Rock tunnels excavated using drilling and blasting technique in jointed rock masses often have a very uneven and rough excavation surface. Experience from previous studies shows that the unevenness of a rock surface has a large impact on the support effect of shotcrete lining. However, clear conclusions regarding the effect of 2D and 3D uneven surfaces were not obtained due to limited studies in the literature. The numerical analyses reported in this paper were made to investigate the influence of the surface unevenness of a circular tunnel opening on the support effect of shotcrete using a 3D numerical code (3DEC). The models were first calibrated with the help of observations and measured data obtained from physical model tests. The influential factors were investigated further in this numerical study after calibration had been achieved. The numerical analyses show that, in general, the unevenness of a tunnel surface produces negative support effects due to stress concentrations in recesses (compressive) and at apexes (tensile) after excavation. However, shotcrete sprayed on a doubly waved uneven surface has better support effect compared to shotcrete sprayed on a simply waved tunnel surface. The development of shear strength (specifically frictional strength) on the uneven interface between the shotcrete and the rock contributes to this effect, in the condition where bonding of the shotcrete does not work effectively. The interface is a crucial element when the interaction between the rock and shotcrete is to be simulated. When an entire tunnel surface is covered by shotcrete with high modulus, more failures will occur in the shotcrete especially when rock surface is uneven. Based on the numerical model cases examined, some recommendations on how to incorporate tunnel surface conditions (2D or 3D unevenness) in the design of a shotcrete lining are given.

Estimation of the excavation damage zone in TBM tunnel using large deformation FE analysis

This paper aims to estimate the range of the excavation damaged zone (EDZ) formation caused by the tunnel boring machine (TBM) advancement through dynamic three-dimensional large deformation finite element analysis. Large deformation analysis based on Coupled Eulerian-Lagrangian (CEL) analysis is used to accurately simulate the behavior during TBM excavation. The analysis model is verified based on numerous test results reported in the literature. The range of the formed EDZ will be suggested as a boundary under various conditions – different tunnel diameter, tunnel depth, and rock type. Moreover, evaluation of the integrity of the tunnel structure during excavation has been carried out. Based on the numerical results, the apparent boundary of the EDZ is shown to within the range of 0.7D (D: tunnel diameter) around the excavation surface. Through series of numerical computation, it is clear that for the rock of with higher rock mass rating (RMR) grade (close to 1st grade), the EDZ around the tunnel tends to increase. The size of the EDZ is found to be direct proportional to the tunnel diameter, whereas the depth of the tunnel is inversely proportional to the magnitude of the EDZ. However, the relationship between the formation of the EDZ and the stability of the tunnel was not found to be consistent. In case where the TBM excavation is carried out in hard rock or rock under high confinement (excavation under greater depth), large range of the EDZ may be formed, but less strain occurs along the excavation surface during excavation and is found to be more stable.

Analysis of Stress and Deformation Characteristics of Deep‐Buried Phyllite Tunnel Structure under Different Cross‐Section Forms and Initial Support Parameters

Deep‐buried soft rock tunnels exhibit low strength and easy deformation under the influence of high ground stress. The surrounding rock of the soft rock tunnel may undergo large deformation during the construction process, thereby causing engineering problems such as the collapse of the vault, bottom heave, and damage to the supporting structure. The Chengwu Expressway Tunnel II, considered in this study, is a phyllite tunnel, with weak surrounding rock and poor water stability. Under the original design conditions, the supporting structure exhibits stress concentration and large deformation. To address these issues, three schemes involving the use of the double‐layer steel arch to support, weakening of the steel arch close to the excavation surface, and weakening of the steel arch away from the excavation surface to support were proposed. Using these schemes, the inverted radius was varied to explore its influence on different support schemes. For simulation, the values of the inverted radius selected were as follows: 1300 cm, 1000 cm, and 700 cm. The proposed support plan was simulated using FLAC3D, and the changes in the pressure between the initial support and surrounding rock, the settling of the vault, and the surrounding convergence were investigated. The numerical simulation results of monitoring the surrounding rock deformation show that the double‐layer steel arch can effectively reduce the large deformation of the soft rock well. When the stiffness of one of the steel arches was weakened, the support’s ability to control the deformation was weakened; however, it still showed reliable performance in controlling deformation. However, changing the radius of the invert had an insignificant effect on the deformation and force of the supporting structure.

Experimental Study on the Formation and Characteristics of Mud Filtration Cake in Large‐Diameter Slurry Shield Tunneling

Multiple soil layers may be exposed simultaneously on the excavated surface of a large‐diameter slurry shield. To study the formation and characteristics of mud filtration cake on the excavation surface during large‐diameter slurry shield tunneling, penetration tests of mud slurries in different soils were carried out using a self‐made device, and the microstructures of different mud filtration cakes were observed using scanning electron microscopy. The test results showed that there were three categories of filling forms for mud slurries permeating the soils: mud filtration cake, mud cake + permeation zone, and permeation zone; correspondingly, there were three types of filtration loss, which was mainly affected by the specific gravity of mud slurry. Finally, the porosity and the fractal dimension for the pore area of the mud filtration cake were calculated, and it is found that the fractal dimension of pore area is beneficial to classify the type of mud filtration cake.

The Dynamic Response and Failure Model of Thin Plate Rock Mass under Impact Load

The layered rock mass widely exists in mining, construction, transportation, and water conservancy projects, and the damage phenomena of plate crack and spalling often occurs in the process of coal and rock dynamic disaster in deep mining. Therefore, the rock mass nearby excavation surface is usually considered to be composed of layers of thin plate rock mass to reveal the damage and failure mechanism of rock mass. In the whole dynamic process of mining and coal and rock dynamic disaster, rock mass would bear the dynamic disturbance from mine earthquake, and at present, the mechanical characteristics of rock mass are mainly studied under static load, while dynamic mechanical response characteristics and the mechanisms of dynamic damage, failure, and disaster‐causing are still unclear. This study mainly focused on the dynamic response characteristic and failure mechanism of rock mass based on a rectangular thin plate model. The frequency equations and deflection equations of the thin plate rock mass with different boundary conditions (S‐F‐S‐F, S‐C‐S‐C, and C‐C‐C‐C) were established under free vibration by the thin plate model and the dual equation of the Hamilton system, and the deflection equations under impact load were derived based on the Duhamel integral. And then, the effective vibration modes of the thin plate rock mass with different boundary conditions and their natural frequencies were obtained by Newton’s iterative method. Based on the third‐strength theory and the numerical simulation results by LS‐DYNA, the maximum shear of the effective vibration modes and the processes of damage and failure under impact load were analyzed. The research results showed that the initial position of damage and failure may be determined by effective vibration mode with the lowest frequency; the develop tendency of which by the combined actions of other effective vibration modes and the effective vibration modes with lower frequency could have greater influence on the process of damage and failure of the thin plate rock mass, which are beneficial to revealing the mechanism of coal and rock dynamic disaster.

Study on the relationship between the direction of air flow and the dust control effect in integrated excavation surface

In order to improve the dust control effect of integrated excavation surface, FLUENT was used to simulate the relationship between different inlet air flow direction and working face dust concentration distribution to study the relationship between inlet air flow direction and dust control effect of integrated excavation surface. In combination with field tests, the corresponding wall air duct was selected and processed and applied to engineering practice. The results show that different air distribution directions have different influence on the dust control effect of working face. When the Angle of radial air outlet is 0°, the air flow directly acts on the transport belt to form secondary pollution, and the dust control effect is not good. The best dust control effect is 15° or 30°.

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.

Experimental Research on the Impact of Temperature on the Adhesion Characteristics of Soil-Structure Interface

Under the working condition of mud cake, the continuous action between the shield cutter head and the soil on the excavation surface can generate high temperature in the process of shield tunneling and excavation, which changes the characteristic of the adhesion between the soil and the cutter head to intensify the phenomenon of making a mud cake on cutter, finally leading to a vicious circle. To study the effect of temperature on the characteristics of the adhesion of the soil on the surface of the structure, the soil adhesive situation and adhesion force at different interface temperatures were tested through a self-made experiment device. According to the result, it was indicated that the moisture content has a significant effect on the adhesion force of the soil, and the adhesion force firstly increased and then decreased with the increased of the moisture content and reached the maximum value near the plastic limit moisture content. The adhesion force changes very gently when the interface temperature is low. When the temperature reached 50°C, the adhesion force continues to increase as the interface temperature continuously increases except for the soils with high moisture content; moreover, the interface temperature has a great influence on the content of soil adhered on the structure surface. As for the soil with moderate moisture content ( ω = 21.21 ~ 31 %), this content of the adhered soil increased exponentially with the increase of interface temperature; this content firstly decreased and then increased when the moisture content was high. When the soil was dry, there was almost no adhered soil on the surface and the interface temperature had no effect on the adhesive situation. By comparing and analyzing the adhesion state of the soil on the surface of the structure, the influence of temperature on the adhesion characteristics is mainly reflected on the variation of the soil moisture content within the influence range of the interface, the variation of the energy required for the destruction of the adhesion interface, and the change of the location of the weakest antistripping plane induced by both before. This research can better understand the law of formation and development of mud cakes and provided a new idea of solving the problem of mud cakes on the cutter head.

Formation process of paste HDN slurry in gravel stratum

At present, due to the complex geological conditions and long-distance drive, scheduled and unscheduled cutter head intervention maintenance during shield tunneling has become a major innovation in shield engineering in China. Therefore, paste HDN has been successfully applied to cutter head intervention under pressure, but corresponding research work on its film-forming mechanism and process is lacking. Film-forming experiments and theoretical analysis of the paste indicated that the film-forming process of the paste during gravel-stratum formation can be divided into two stages: Seepage blocking of the forming pore diameter and forming of a mud cake on the excavation surface. The seepage process was completed almost instantaneously, and the forming process of a 5–10 cm mud cake required 1–2 days. In addition, a mud diffusion model was established for investigating the mud infiltration process. The results of the aforementioned experiments were applied to the cutter head intervention of Guangzhou Metro and Lanzhou Metro, and this application ensured the efficient and safe completion of the intervention task.