Sandy Cobble Stratum Research Articles

Estimating Tunnelling-Induced Ground Deformation in Sandy Cobble Stratum Considering Rock Content Variation

Estimating Tunnelling-Induced Ground Deformation in Sandy Cobble Stratum Considering Rock Content Variation

Influence of synchronous grouting on surrounding sandy cobbles during shield tunnelling process: Insights from a scaled model test

Substantial soil deformation around tail-grouting activity is frequently observed during the shield tunnelling process in the sandy cobble strata, which is attributed to the convergence and potential collapse of the soils around the tail-void left behind the shield machine. The stability of the underlying strata is significantly impacted by synchronous grouting inside the tail-void. This study examines four-opening synchronous grouting, which is frequently used in shield tunnelling of metro-tunnels, to analyze the grout-spreading mechanism and soil displacement generated by the synchronous grouting in the sandy cobble layer. Laboratory tests are used to determine the characteristics of grouting mortar and soils in accordance with the similarity theory. The model experiment outcomes show that a rise in the GVR (Grouting-Volume Ratio) can cause the settlement at the ground surface. However, as the GVR improves, the pace of settlement growth diminishes. Additionally, the expanding cover depth of the tunnel contributes to the stability of the soil after synchronous grouting. As observed in the model test, when GVR achieves its maximum amplitude (i.e., GVR = 200 %), the grout mortar cannot spread to the crown of the tail-void, necessitating secondary grouting after the synchronous grouting during tunnelling practice. After grout hardening process, it was discovered that grout material above the tunnel waist was concentrated near the upper grout openings, implying a seepage spreading mode of the grout-mortar at these locations. In contrast, grout material below the tunnel waist was evenly distributed along the periphery of the tail void. The model test outcomes may be utilized as a guide for tail-grouting while shield tunneling in sandy cobble strata.

Investigation and measurement of old building response to the overlapped shield tunnel of multiple schemes in the sandy cobble stratum

Ground movements due to tunneling are becoming increasingly critical as buildings are located around construction sites. This study proposes a new combined reinforcement method using a foundation grouting oblique pipe roof. The former improves the bearing capacity of the subsoil, and the latter blocks the transmission of soil deformation, which weakens the influence of construction during overlapped tunnel under-crossing. Based on this new method, a case study of the shield tunneling response to an old building in Line 6 of China’s Chengdu Metro is presented. Additionally, three-dimensional numerical models without reinforcement, traditional foundation grouting reinforcement, and the new combined reinforcement schemes were compared. The numerical simulation performance was verified using a set of field instrumentation data, which demonstrated that the old building response to the overlapped tunnels was under control, and the maximum deformation, angular distortion, and principal tensile strain of the building were 5.25 mm, 5.10 × 10–6 rad/m, and 0.0081%, respectively. Compared with the traditional reinforcement scheme, the deformation, angular distortion, and principal tensile strain in the combined reinforcement scheme were reduced by 54.78%, 71.02%, and 70.22%, respectively. These results have important implications for the design and construction of shield tunnels and their response to old buildings.

Field monitoring of vibration characteristics during advanced ductule installation in sandy cobble stratum

The sandy cobble stratum presents a high risk for underground tunnel construction due to its low cohesive properties and susceptibility to loosening and falling. The use of Advanced ductule for grouting reinforcement inevitably results in vibrations, and understanding how these vibrations propagate is crucial in selecting tunnel engineering support schemes and responding to accident risks. Based on a bored tunnel under construction in Xi’an, field vibration propagation characteristics testing were carried out for advanced ductile installation. The time-history response and frequency distribution characteristics of the vibration velocity within the tunnel face under sandy cobble stratum conditions were studied, and the law of vibration propagation attenuation within the tunnel face range was obtained. The results showed that: 1) During the conduit drilling process, the tunnel face mainly experienced vertical vibrations, with the horizontal velocity amplitude accounting for only 15%–20% of the vertical velocity amplitude. At a distance of 1.0 m from the conduit, the vertical velocity amplitude reaches 10.602 mm/s, and the vibration energy concentrates mainly in the frequency range of 150–250 Hz. At a distance of 1.5 m from the conduit, the bidirectional vibration velocity significantly attenuates; 2) The vibration characteristics within the tunnel face can be classified into three primary areas: “Loose and Falling” area, “Significant Vibration” area, and “Vibration Attenuation” area. Loose, falling and significant vibrations occurred mainly within a range of about 1.25 m around the conduit. 3) As the diameter of the conduit decreases, the amplitude of vertical vibration velocity decreases by about 20%. By reducing the design diameter of the advanced ductule in a reasonable manner, it is possible to effectively mitigate the impact of vibration caused by the sandy cobble stratum during installation. This can yield a positive impact, curtailing the occurrence of the tunnel’s collapse phenomenon and ensuring its stability.

Experimental study on stress distribution characteristics of a shield tunnel under passive failure

Urban rail transit construction will inevitably encounter tunnel working conditions with extremely complex surrounding environment and buried deep shield tunnel, and the stability of the tunnel surface cannot be ignored. A series of passive instability geotechnical model tests were carried out on the tunnel faces at different burial depths. The earth pressure, lateral earth pressure coefficient and soil arch area evolution law caused by passive instability of the tunnel face in sandy cobble stratum were studied. Once the preset position is reached, the rotation is stopped using the cutterhead and the screw excavator and the shield machine continues the jack, simulating passive instability of the tunnel face. The results show that the soil pressure in the horizontal earth pressure is significantly more affected by the passive instability of the tunnel face than the vertical direction. The soil in front of the excavation caused by the ejection of the shield machine is squeezed and moved forward, resulting in a decrease in horizontal earth pressure perpendicular to the direction of shield tunnel excavation. The results of the passive instability region obtained by numerical simulation are larger than those obtained by model experiments, because the numerical simulation method fails to consider the compacting effect caused by the extrusion of sandy cobble formations. It can be found that the uplift value of C/D = 1.0 is significantly greater than that of C/D = 1.5, which also confirms the conclusion that the instability area with a relatively small burial depth is larger and develops farther to the front of excavation.

Prediction of subsurface settlement induced by shield tunnelling in sandy cobble stratum

This study focuses on the analytical prediction of subsurface settlement induced by shield tunnelling in sandy cobble stratum considering the volumetric deformation modes of the soil above the tunnel crown. A series of numerical analyses is performed to examine the effects of cover depth ratio (C/D), tunnel volume loss rate (ηt) and volumetric block proportion (VBP) on the characteristics of subsurface settlement trough and soil volume loss. Considering the ground loss variation with depth, three modes are deduced from the volumetric deformation responses of the soil above the tunnel crown. Then, analytical solutions to predict subsurface settlement for each mode are presented using stochastic medium theory. The influences of C/D, ηt and VBP on the key parameters (i.e. B and N) in the analytical expressions are discussed to determine the fitting formulae of B and N. Finally, the proposed analytical solutions are validated by the comparisons with the results of model test and numerical simulation. Results show that the fitting formulae provide a convenient and reliable way to evaluate the key parameters. Besides, the analytical solutions are reasonable and available in predicting the subsurface settlement induced by shield tunnelling in sandy cobble stratum.

Influence of stone content on face stability for tunnels in sandy cobble strata

As a result of the complex geological conditions of sand-cobble stratum, various types of problems may arise during construction of tunnels crossing sand-cobble stratum, the most prominent of which is the stability problem of the face. In order to study the stability of the tunnel face in sand-cobble stratum, first, a continuous medium model of sandstone soil is established, and numerical simulation is carried out on the three-axis compression test of sandstone soil to analyze its micro-mechanical characteristics. Second, a calculation model of simulated sandstone strata excavation is established by FLAC3D finite difference software. Then, the safety factor K calculated by the strength reduction method is introduced as the evaluation index of the face stability, and the face stability of sand-cobble tunnels with a series of stone content, tunnel burial depth and tunnel diameter are analyzed. The results show that the macro stress-strain curve of sand-cobble soil in the three-axis compression test can be divided into linear stage, elastic-plastic stage and ideal plastic stage, and the shear strength of sand-cobble soil increases with the increase of stone content. In the process of tunnel excavation, the stability of tunnel face increases with the increase of stone content in the stratum; while the tunnel buried depth and diameter increase, the stability of tunnel face decreases. These results provide a reference for predicting the instability and failure of sand-cobble tunnels, which is critical for the construction and safety of tunnels in sand-cobble stratum.

Predictions of ground surface settlement for shield tunnels in sandy cobble stratum based on stochastic medium theory and empirical formulas

This paper focuses on the prediction of ground surface settlement induced by shield tunnelling in sandy cobble stratum. Based on the stochastic medium theory, an analytical solution to predict the surface settlement is developed considering the difference between soil and tunnel volume loss. Then, the effects of tunnel geometries, influence angle and volume loss on the characteristics of surface settlement are discussed. Through back analysis, a total of 103 groups of field monitoring data of surface settlement induced by shield tunnelling in sandy cobble stratum are examined to investigate the statistical characteristics of the maximum settlement, settlement trough width and volume loss. An empirical prediction is presented based on the results of back analysis. Finally, the analytical solution and empirical expression are validated by the comparisons with the results of model tests and field monitoring. Results show that the soil at ground surface has an overall dilative response for most of the shield tunnelling in sandy cobble stratum. In addition, the developed analytical solution is applicable and reasonable for surface settlement prediction. Meanwhile, the proposed empirical formula also shows good performance in some cases, providing an approach or a reference for engineering designers to preliminarily evaluate the surface settlement.

Assessment on the reverse circulation performance of slurry shield pipeline system assisted with CFD-DEM modeling under sandy cobble stratum

Reverse circulation of slurry shield pipeline system is one of the most effective method to solve slurry chamber clogging during shield tunneling in cobble stratum. In this paper, the assessment on reverse circulation performance of slurry shield pipeline system was performed using the computational fluid dynamics-discrete element (CFD-DEM) coupling approach. In the CFD-DEM coupling model, the shape and size of large irregular cobble particles were considered via 3D scanning technology. In addition, a parametric analysis was conducted, and the effects of the cutterhead rotational speed, slurry inlet velocity and rheological properties of slurry on the reverse circulation performance were investigated respectively. The results indicate that, the magnitude of impact on reverse circulation is in the order of slurry inlet velocity v > slurry plastic viscosity k > cutterhead rotational speed r > slurry yield stress τ0 > slurry power law index n among the working parameters of slurry shield and the Herschel-Bulkley (HB) rheological parameters of the slurry, and the time of reverse circulation is also of importance to the performance of the reverse circulation. Accordingly, a reasonable reverse circulation scheme was suggested, and applied to practical engineering. The findings in this study could provide some references for slurry shield pipeline system reverse circulation of similar projects in the future.

Experimental investigation of face instability for tunnels in sandy cobble strata

In order to investigate the influence of face instability for tunnels with different burial depths in sandy cobble strata on earth pressure and the instability region, geomechanical model tests and numerical simulations were performed. The continuous excavation method was adopted to reduce the pressure of the soil bin and restore the real engineering situation. Earth pressure in three directions of the observation section in front of the tunnel face was monitored during the tunneling of the shield. Evolutions of the lateral stress ratios at different stages were also investigated. The instability area of the shield tunnel face in sandy cobble strata with different burial depth ratios during the instability stage was obtained based on the change ratio of earth pressure and compared with existing researches. The earth pressure began to change when the excavation was one shield diameter away from the observation section, and when the excavation reached the observation section, the earth pressure decreased significantly. The burial depth of shield tunnel in the sandy cobble strata has a significant impact on the evolution of soil arch and the size of the failure area. The numerical simulation of the continuum medium cannot reflect the stress redistribution characteristics of the granular body like sandy cobble strata, and the failure area or stress disturbance area obtained by the model test is larger than the numerical simulation result. Existing methods have deviations in analyzing the failure area of shield tunnel face in sandy cobble strata. It provides not only guidance for shield tunnel excavation engineering in sandy cobble strata, but also a reference for the theoretical research on failure areas.

Experimental study of face stability for shield tunnels in sandy cobble strata of different densities

This paper aims to study the influence of relative density on the working face stability of shield tunnel in sandy cobble stratum. A series of geomechanical model tests were conducted for shield tunnels excavating through loose sample (LS), medium dense sample (MS) and dense sample (DS), respectively. Relative densities of LS, MS and DS are 0.35, 0.55, and 0.75. The method of continuous rotation of spiral excavators while shield excavators stop tunneling is adopted to simulate the instability of tunnel face. Experimental results effectively reveal the variation characteristic of vertical and horizontal earth pressure and lateral pressure coefficient of shield tunneling. The evolution of the failure zone and soil arch height of strata with different densities was explored. The failure area and collapse range obtained from the model test were compared with the existing method. The effect of relative density on the horizontal earth pressure in the instability stage of the tunnel face in the sandy cobble strata is greater than that of the vertical earth pressure. In addition, increasing the relative density can effectively increase the lateral pressure coefficient as well as the stability of the tunnel face. The existing methods have deviations in analyzing the instability area of the tunnel face in the sandy cobble strata with different relative densities. The research in this paper provides a reference for the construction and theoretical analysis of sandy cobble strata shield tunnel engineering.

A meso-scale numerical modelling method for the stability analysis of tunnels in sandy cobble stratum

This paper presents a meso-scale numerical modelling method to investigate the stability of tunnels in sandy cobble stratum. Using Monte Carlo simulation, Python routine is implemented to randomly generate and filter the location information of blocks including center coordinates, sizes and orientation angles, then extract and import them into CAD modelling software in batches to mesh. The problem of overlapping detection for elliptical and ellipsoidal blocks is solved based on affine transformation, achieving higher efficiency and applicability than the existing methods. Then, using the proposed numerical modelling method, convergence-confinement analysis of circular tunnels with different cover depths is performed in plane strain condition. Meanwhile, face stability analysis of shield tunnels is carried out in three-dimensional condition. Results show that sandy cobble stratum may not be in accordance with the scale-independent soil-rock mixture. Blocks with small sizes are geotechnically significant and contribute much to the overall strength. When the cover depth ratio reaches a certain level, limit support pressure remains basically unchanged due to the strong arching effect above tunnel crown, while permissible support pressure increases approximately linearly to control deformation. Compared with the multi-scale model, hardening soil model considering the overall strength is suitable for the stability analysis of tunnels in sandy cobble stratum at the macro-scale with high precision, while the macro homogeneous model ignoring the presence of blocks provides a conservative (i.e. uneconomical) prediction for the design of tunnel stability.

Settlement and Stress Characteristics of the Ground in the Project of a Double-Line Tunnel Undercrossing an Airport Runway in a Sandy Cobble Region

The engineering technology of undercrossing an airport flight area is relatively mature; however, the use of shield tunnels crossing the operating airport flight area in high liquidity sandy cobble stratum has rarely been reported. To discuss the feasibility of a double-line shield tunnel undercrossing the airport flight area in a sandy cobble region. Based on the case of the Chengdu Metro Line 10 that undercrosses the Shuang-Liu Airport, which is located in sandy cobble region, the deformation and stress laws of airport runway pavement structures were investigated via a three-dimensional numerical model. Stratum displacement, ground settlement and pavement tensile stress under different tunnel depths were analysed. Then, the pavement tensile stress was taken as the safety evaluation index and the vulnerable area of the pavement structure was proposed. The results show that after excavation of the double-line tunnel, the maximum ground surface settlement occurred above the tunnel centreline and the ground settlement curves presented a V-type settlement trough instead of a W-type. The existence of a runway greatly limited the deformation of the surrounding soil; with increasing depth, the effect degree of the runway pavement on the soil settlement decreased. The most unfavourable region of the runway pavement structure under the influence of tunnel excavation was found. With increasing burial depth, the maximum settlement of the surface centre point decreased continuously. It is recommended that the tunnel burial depth not exceed 23.5 m in this project. According to the displacement and stress control limitation of the airport pavement, it can be judged that the shield construction method meets the structural stability requirements of the pavement. The calculated results provide a reference for the shield tunnel construction in the same geological condition areas.

Soil Improvement Technology and Dynamic Control of a Large Diameter Shield in Sandy Cobble Strata

Soil Improvement Technology and Dynamic Control of a Large Diameter Shield in Sandy Cobble Strata

Study on the correlation of macro-fine parameters of surrounding rock in sand and cobble tunnel based on discrete element

As a mixture of soil and rock, the deformation of surrounding rock caused by tunnel excavation in the sandy cobble stratum is affected by the meso-structure of the stratum. In this paper, the mechanical and engineering characteristics of sandy cobble soil are studied in depth using a laboratory large-scale triaxial test and discrete element numerical simulation test, and the effects of main microscopic parameters on the characteristics of sandy cobble soil are analyzed, including the same specimen size, moisture content, compactness, confining pressure as well as loading method. The microscopic parameters such as contact modulus E c , friction coefficient f r , and stiffness bond ratio n of sandy pebble soil can be obtained. Meanwhile, the accuracy and uniqueness of microscopic parameters are determined using the response surface method. Moreover, the influence of different contact modulus and friction coefficient on the macroscopic mechanical properties of sandy pebble soil is quantitatively analyzed. The obtained conclusions are drawn as follows: the elastic modulus of sandy pebble soil decreases with the increase of water content. The peak value point of the stress-strain curve of sandy pebble soil increases with the increase of contact modulus and friction coefficient, respectively. The relationship between peak stress and the two types of fine-scale parameters can both be described by logarithmic functions. Compared to fine-grained soils, sandy pebble soils start to break down at about 3.5% of the axial strain, and the sandy cobble soil can still bear a certain load after reaching the peak strength. The results can provide a reference for the calculation and discrete element simulation of tunnels in sand and pebble strata.

Influence of relative density on deformation and failure characteristics induced by tunnel face instability in sandy cobble strata

This paper aims at influence of relative density on deformation and failure characteristics for a shield tunnel in sandy cobble strata. A series of model tests were performed for tunnels with different relative densities of 0.35, 0.55, and 0.75. The evolution law of the stratum deformation during the instability process of the tunnel face was analyzed. The soil disturbance range of tunnel face instability with different relative densities was explored. The results show that relative density has a significant impact on the extent of surface subsidence. The stratum deformation caused by the instability of tunnel face in the medium dense and dense sandy cobble strata only occurs significantly behind the tunnel face, while the stratum with loose sandy cobble has significant stratum deformation behind and in front of the tunnel face. The method of improving the relative density can be used to reduce the deformation of the sandy cobble strata.

A New Method for Piping Risk Evaluation on Unconfined Aquifers under Dewatering of Deep Foundation Pits

This paper presents a new method to evaluate the piping risk of unconfined sandy cobble aquifers during the dewatering of deep foundation pits. Firstly, the possibility of piping is judged according to the particle size distribution and pore diameter of the sandy cobble stratum. Secondly, the critical hydraulic gradients of particles with different sizes are calculated according to the pipe flow model. Thirdly, the hydraulic gradient around the foundation pit during the dewatering process is calculated by numerical method. Then, combined with the distribution of hydraulic gradient around the foundation pit and the critical hydraulic gradients of particles with different sizes, the range of moveable particles with different sizes in the surrounding stratum is evaluated. Finally, according to the sandy cobble aquifers conditions and the influence of particle movement on the stratum, the level of piping risk is determined. This method was successfully applied to evaluate the piping risk of Tawan Station in Luoyang Metro Line No.1, and according to the risk evaluation the pensile curtain was recommended for the retaining structure in this case. From the satisfied applications, this method is a reference for the evaluation of piping risk and selection of retaining structure when considering the impact of the dewatering in deep foundation pit with unconfined aquifers on the surrounding stratum.

A hybrid experiment/theory method for soil conditioning in sandy cobble strata with large cobbles and boulders

A hybrid experiment/theory method for soil conditioning in sandy cobble strata with large cobbles and boulders

Characteristics of Ground Surface Settlement of Double-Line Adjacent Metro Construction in Sandy Cobble Stratum: A Case Study of Beijing Airport Line

As a long-term urban transportation infrastructure, the stability of metro construction is very important. With the continuous expansion of urban metro network, metro adjacent construction is more and more frequently encountered. In this paper, taking the double-line adjacent construction problems encountered during excavation of Beijing New Airport Line metro in sandy cobble stratum as an example, the surface settlement caused by excavation of single-line metro and double-line adjacent metro is studied by computed tomography scan, electron microscope scanning, laboratory mechanical tests, orthogonal inversion and numerical simulation and other methods. The results show that 1) the sandy cobble particle size and sandy cobble distribution of Beijing sandy cobble samples are relatively dispersed, and the sandy cobble particle size is from 3 to 20 cm, of which the largest proportion is the sandy cobble particle size from 5 to 8 cm. 2) The internal friction angle of Beijing sandy cobble samples with different moisture content is from 27° to 40°, which is much larger than that of other soil samples. Even at the post-peak stage, the shear stress of some sandy cobble samples does not gradually tend to be gentle, but continuously increases or has secondary peaks. 3) For the excavation of double-line adjacent metro, the maximum settlement difference and the impact range of excavation are proposed as indicators to investigate the degree of interaction of the adjacent metro. 4) The results of numerical orthogonal analysis show that the width of surface settlement zone increases significantly with the increase of tunnel diameter and spacing when excavating adjacent metro in sandy cobble stratum, but the rate of increase with tunnel diameter (from 6 m to 14 m) almost reaches 2 times of the rate of increase with the spacing (from 3 m to 15 m). 5) The maximum settlement value of sandy cobble stratum increases with the increase of tunnel diameter and decreases with the increase of spacing. The orthogonal linear analysis shows that the influence of tunnel diameter on the maximum settlement value is much greater than the spacing. The maximum settlement increases with the increase of tunnel diameter, which is about 3.6 times of that with the decrease of spacing.

Experimental study of the comprehensive technology of grouting and suspension under an operating railway in the cobble stratum

The construction technology of shield tunnelling under an operating railway in an adverse stratum is complex and risky. The existing research results mostly focus on the settlement control of the tunnel under the existing structure, but they are not adequate to provide a systematic reference for shield tunnelling under an operating railway with strict requirements on the settlement control. Based on the background of the Luoyang Rail Transit Line 2 shield passing through the Luoyi Railway, the comprehensive technology of grouting and suspension (CTGS) has been proposed, and field test and numerical simulation are used to study the effects of its application. The results show that the comprehensive technology of grouting and suspension has a significant effect on the rail settlement control of shield tunnelling under the operating railway in a cobble stratum. Using the grouting method to grout subgrade soil can improve the strength of the overlying soil layer. Using the suspension method to suspend the railway tracks can directly transfer the load to the pile foundation to avoid the subgrade bearing excessive load. CTGS combines the grouting method with the suspension method, which can be used as the preferred technology to control the influence of subgrade soil settlement on railway track surface. Compared with the similar technology, although the project cost is slightly higher, CTGS significantly ensures railway safety during the construction process, which can help keep the railway operation free from interference during the construction period and has obvious comprehensive benefits. CTGS has an important reference value for the safety control of shield tunnelling construction with the risky source under adverse geological conditions such as a sandy cobble stratum.