Sandy Cobble Stratum Research Articles

Investigating impacts of deep foundation pit dewatering on land subsidence based on CFD-DEM method

For a deep excavation in sandy cobble stratum, the loss of fine sand particles caused by dewatering generally leads to ground settlement. This paper first analyzed the rules of land subsidence caused by dewatering in the water-rich sandy cobble stratum by dewatering experiments. It is found that even after the groundwater level was restored, the land subsidence basically did not rebound, which shows that the land subsidence is consolidation settlement caused by the loss of fine particles. Then the coupled CFD-DEM numerical model was used to analyze the relationship between the fine particle loss, surface settlement and the grain size of filter media, dewatering rate. On this basis, the rules of land subsidence under the coupling effect of foundation pit dewatering and excavation were further explored by numerical calculation and field measurement. Foundation pit dewatering and excavation in the sandy cobble stratum is the key stage for land subsidence control, As the grain size of the filter media increases, the impact of dewatering on land subsidence gets stronger, therefore, the filter media grain size should be less 8 mm. This study is of great significance to the design of deep foundation pit dewatering in water-rich sandy cobble strata.

Study on the excavation disturbed zone during tunneling in sandy cobble stratum considering the material meso-structure

The excavation disturbed zone plays an important role in the evaluation of tunnel stability. The identification of the excavation disturbed zone is perceived as a main concern. Taking the material meso-structure into account, this paper investigates the construction mechanical behavior in heterogeneous sand cobble stratum and aims to precisely predict the excavation disturbed zone. Firstly, the meso-scale numerical models were used to simulate tunnel excavation in sandy cobble strata, in which the large-sized rock blocks and fine-grained soil matrix were treated as separate constituents. Then, the stress redistribution and deformation performance in the ground during tunneling were explored in detail. Subsequently, combined with the deformation performance characteristics, a method for identifying the excavation disturbed zone was proposed. The boundary of the excavation disturbed zone can be identified by the displacement mutation point of the monitoring paths. Finally, a parametric study was performed to understand the influence of key factors on the excavation disturbed zone, including the rock meso-structure parameters, the soil matrix properties, and the tunnel geometry characteristics. The simulation results indicate that both the rock content and rock size have a significant impact on the excavation disturbed zone, the strength parameters and lateral pressure coefficient have a marked effect, and the tunnel radius and tunnel burial depth have a negligible influence on the excavation disturbed zone under the given conditions when normalized.

Surface Settlement Induced by Slurry Shield Tunnelling in Sandy Cobble Strata—A Case Study

Surface Settlement Induced by Slurry Shield Tunnelling in Sandy Cobble Strata—A Case Study

Three-Dimensional Limit Equilibrium Solution of Minimum Support Pressure of Shield Tunnel Face in Sandy Cobble Stratum

The ground subsidence accident caused by the instability of shield tunnel face has been a major problem that puzzles urban subway construction in weak rock. Maintaining the stability of tunnel face is the key to ensure the safety of shield construction and reduce the environmental impact. In view of the shield with a diameter of 8600 mm in Chengdu metro line 18 through the sandy cobble stratum, it is more difficult to maintain the stability of the shield tunnel face than other strata. The characteristics of the sandy pebble stratum were taken into account firstly in this paper. The analytical equation was obtained by the limit equilibrium method. The optimization of the trapezoidal bottom has been improved based on the existing 3D trapezoidal wedge model. And the optimal solution of the ultimate support pressure was obtained. The influencing factors of support pressure are also analysed. When the tunnel diameter is fixed, the deeper the ground covering depth is, the greater the required limit support pressure becomes. But the ground covering depth has almost no effect on the ultimate support pressure even if the tunnel is deep. The limit support pressure is inversely proportional to the internal friction angle of the soil and is proportional to the diameter of the tunnel. Finally, theoretical limit support pressure can be also verified by the numerical simulation results and the field monitoring results. It provides a reference for the subsequent setting value of the soil warehouse pressure.

Evolution of temperature field and frozen wall in sandy cobble stratum using LN2 freezing method

There is high risk of water inrush and ground collapse when tunneling in sandy cobble stratum in confined aquifer. Among various methods, artificial freezing method is an effective method to pre-reinforce the soft ground to prevent water inrush and ground collapse during tunneling. This paper mainly focuses on application of artificial freezing method using liquid nitrogen in sandy cobble stratum in confined aquifer. The R1 metro tunnel in China’s Jinan city, a well-known City of Springs, is used as a case study. In order to prevent water inrush and ground collapse accidents in the shield machine reception process, liquid nitrogen is applied as a freezing method in the sandy cobble stratum near the reception shaft. First, the heat transition of water in soil associated with liquid nitrogen vaporization is analyzed. Then, a three-dimensional (3D) model is built and the freezing process is simulated. Considering the temperature change of gas nitrogen along the freezing pipe when flowing, the distribution of temperature field and the growth of frozen wall associated with freezing time is obtained. The numerical simulation results show that the freezing method using liquid nitrogen in the sandy cobble stratum can meet the requirement of shield machine reception in sandy cobble stratum rich in groundwater.

Ground collapse caused by shield tunneling in sandy cobble stratum and its control measures

This study investigated several ground collapses in sandy cobble strata induced by shield tunneling in two intervals of Lanzhou Metro Line 1 and proposed corresponding control measures. Ground collapses were classified into three types (A, B, and C). The distribution of low-compactness zones was the primary cause of the collapses that occurred after steady excavation (A-type), indicating that advanced grouting should be conducted beneath these zones. The grouting area should be a semi-circular ring in the transverse section, with a grouting ring height and thickness of 1.05 m and 0.51 m, respectively. A poor tunneling state caused by fluidity reduction in the cutter chamber was the main reason for collapses caused by restarting the shield after long-term shutdown (B-type). We suggest rotating the cutterhead every 12 h during long-term shutdown and supplementing the bentonite until the cutterhead torque falls below 3500 kN·m. Collapses caused by cutterhead blockage (C-type) were mainly due to encountering large boulders during tunneling. We suggest pulling the front shield back with hinge equipment when the cutterhead is blocked, with a backward distance greater than the penetration distance (35 mm) and a face pressure above the limit support pressure calculated using the method presented in this paper.

Model Testing on Failure Mechanism of Tunnel Face in Sandy Cobble Stratum

One of the universal problems in shield tunneling is face collapse caused by the sudden change of the soil in front of the tunnel face in weak sandy pebble stratum. In order to control the sudden change of the soil, the model test method is used to study the stability of the tunnel face in the sandy pebble stratum. The results indicate that (1) in sandy pebble stratum, the instability mode of the tunnel face can be divided into four stages of slow development stage, transition stage, rapid development stage and instability stage; (2) the influence of the ratio of the soil depth and the support pressure on the instability of the tunnel face can be divided into three stages of the insensitive stage, the sensitive stage and the failure stage; (3)finally, the instability failure shape of the sand–pebble stratum is upward developing in the lower part “chimney shape,” while the upperpart of the surface presents “spiral shape” subsidence and ultimately presents “crater shape”; (4) when excavation face is unstable, the surface horizontal settlement groove meets the normal distribution curve proposed by Peck and is similar to the settlement shape measured by the test. The study also found that the bamboo sticks with sand inserted in the soil in front of the tunnel face can increase the friction between the bamboo sticks and compact the soil, make the soil more prone to form soil arches, and effectively control the deformation of the soil in front of the tunnel face.

Experimental Study of the Influence of Excavation Surface Stability and Sand Flowability Caused by Dense Slurry–Earth Pressure Balance Shield Tunneling in Silty Sand Stratum

Silty sand is highly permeable and has a large internal friction angle. To ensure the stable and efficient construction of the Earth pressure balance (EPB) shield in silt stratum, it is necessary to reduce the disturbance to the stratum during tunneling and reduce the risk of spewing and blocking during soil discharge. Thus, a new type of dense slurry‐EPB shield construction technology is proposed. Using a custom‐made test device, the consolidation tests were conducted on sand after mixing it with different amounts of slurry to study the change in the pore pressure in the mixed soil and sand stratum and analyze the stress transmission rule. In addition, slump tests were conducted on the mixed soil to study the change in the fluidity. Based on the laboratory test results, a field investigation was carried out to study the effects of the amount of slurry addition on the pore pressure in the stratum, the driving parameters, and the sand’s flowability during shield tunneling. The results show that when the amount of slurry reaches a certain value, the filter cake effect occurs. The greater the amount of slurry added during the shield tunneling, the smaller the rate of change of the pore pressure in the stratum. When the amount of BC2 slurry added reached 8 m3 per ring, the rate of change was reduced by 57%, and the slump value improved by 93%. The type of filter cake of the DS‐EPB shield was only internal filter cake during tunneling, which reduced the disturbance of the excavation face and improved the flowability of the sand. These results are of great significance to the application and promotion of DS‐EPB shields in sandy cobble strata.

Three-Dimensional Discrete Element Analysis on Tunnel Face Instability in Cobbles Using Ellipsoidal Particles.

Soil disturbance has always been the major concern in shield tunneling activity. This paper presents the investigation on the micro-scale responses of the soils during shield tunnel excavation in sandy-cobble stratum. The code paraEllip3d is employed in discrete element method (DEM) analysis in which the soils are mimicked as an assembly of ellipsoids. Triaxial tests on the micro-scale responses of cobbles are carried out using the materials sampled from the tunnel face during construction period, and corresponding DEM simulations are performed to calibrate the micro parameters for the ellipsoids. On this basis, the face instability process during the shield tunneling in cobbles is studied using 1 g model test as well as corresponding DEM simulation. The micro-scale responses of cobbles are investigated by triaxial test as well as corresponding DEM simulations. Multiple material responses are discussed in the DEM simulations, including the stress–strain relationship, the contact distribution, and the force chain evolution in the elementary and model test. Finally, the mechanism of tunnel face instability in cobbles are discussed on the basis of aforementioned investigations.

Conditioning experiment on sand and cobble soil for shield tunneling

Sand and cobble soils have a large internal friction angle and poor fluidity. Many problems have been encountered during shield tunneling in sandy cobble stratum, such as cutter head wearing, difficult soil discharge, and excessive thrust force. In this study, foam or foam and bentonite slurry were tested to improve the soil properties. First, the optimum modifier percentages, namely, those of the foaming solution concentration and the bentonite slurry soil–water ratio, were determined; the optimum values were 3% and 1:10, respectively. Second, various percentages of cobble contents (CCs, i.e., 20%, 40%, 60%, and 80%) were added in the sand and then subjected to large-scale indoor shear test and slump test by incorporating foam. Experimental results indicate that as the foam injection ratio (FIR) increased, the foam soil content wrapped around the sand and cobble soil also increased, the separation of cobble from sand weakened gradually, the shear strength nonlinearly decreased, and the slump gradually increased. To satisfy the shear strength and fluidity requirements in shield tunneling, the FIR of sand and cobble soil with CCs of 20%, 40%, and 60% was determined. The sand and cobble soil with a CC of 80% was further improved by mixing foam and bentonite slurry, thereby obtaining the best modifier ratio suitable for shield construction. Finally, the results of the indoor test were further verified through the monitoring and analysis of the tunneling parameters and earth pressure before and after the conditioning of the sand and cobble soil during the shield tunneling. These results can be of great importance in solving of the problems encountered in shield tunneling in sandy cobble stratum.

Shield Equipment Optimization and Construction Control Technology in Water‐Rich and Sandy Cobble Stratum: A Case Study of the First Yellow River Metro Tunnel Undercrossing

The Lanzhou Yellow River Tunnel is the first metro shield tunnel that undercrosses the Yellow River in China. It was completed after successfully overcoming several construction challenges, including strata with a high proportion of large‐sized sandy cobble stratum being saturated, large‐scale sand pits along the bank of the Yellow River, and a combination of boulders and erratic blocks of rock. Given the difficulties in constructing the tunnel, this paper summarizes the scheme employed to transform the cutter and cutter head design to be applicable to sandy cobble stratum and the key technology used to form the slurry film to facilitate crossing the sand pits in a systematic way. The transformation scheme primarily involved the addition of an adjusting device to control the aperture ratio of the shield on the cutter head, a protective device for the hob hub, and a protective device for the piston rod in the oil cylinder of the crusher. The implementation of measures mentioned above guarantees the safe completion of the tunnel, which can provide a reference for similar projects.

Lagged settlement in sandy cobble strata and earth pressure on shield tunnel.

Lagged settlement is a typical accident induced by shield construction in sandy cobble strata. This paper analyzed the process and mechanism of lagged settlement, results show that all phases are in accord with the characteristics of ellipsoid theory of particle flows. Based on this theory, a method for calculating coefficient of lateral earth pressure and loosened earth pressure is proposed in this research. For the coefficient of lateral earth pressure, the boundary of loosened ellipsoid is divided into two parts, the arch zone and the excavation zone, and the lateral pressure coefficients are derived respectively according to the stress state. For loosened earth pressure on tunnel, the Terzaghi earth pressure theory and Protodyakonov earth pressure theory are adapted in different conditions according to the state of loosened cobble soil. Theories developed in this study can be applied on determination of shield excavation parameters, as well as calculation of loosened earth pressure and control of tunnel support.

A multi-scale analysis method for the simulation of tunnel excavation in sandy cobble stratum

Sandy cobble stratum is a kind of heterogeneous geological body. A further study on the safety of wall rocks and underground structure demands developing suitable analysis method to simulate tunnel excavation in such a formation. Macro-scale simulations assume the sandy cobble soil to be homogeneous in numerical models. They get more efficient computing, but have difficulties in capturing enough information about the meso-scopic fields needed. Meso-scale simulations explicitly represent the individual components of the heterogeneous internal material structure in numerical models, e.g. the shape and the spatial distribution of rocks. They achieve high accuracy, but large computational cost would be needed. To reduce the numerical effort with the precision guaranteed, a multi-scale analysis method for simulating tunnel excavation in sandy cobble strata is proposed. In this method, the numerical model is divided into two regions, involving the meso-scale region and macro-scale region. The meso-scale region is a critical sub-domain disturbed by tunnel excavation, in it, the rocks and soil are considered as separate constituents. In the macro domain, the sand gravel soil was regarded as homogeneous materials whose effective material parameters were determined using an equivalent homogenization method. Three critical issues in the multi-scale method are explored, including: (i) the determination of meso domain and macro domain, (ii) the determination of rock maximum size that can be homogenized in the meso domain and iii) the determination of effective material parameters of soil-rock mixture during the equivalent process. By comparing with simulation results predicted by macro-scale and meso-scale simulation methods, the validation of the proposed multi-scale simulation method was carried out. The result indicates that the proposed multi-scale analysis method has a high efficiency without loosening accuracy on the simulation of tunnel excavation in sandy cobble stratum.

The mechanism of hysteretic ground settlement caused by shield tunneling in mixed-face conditions

Shield is used more and more widely, such as coal mine roadway, hydropower tunnel, traffic tunnel and so on. But Tunneling with a tunnel boring machine (TBM) may cause inevitable ground subsidence. Though over the world numerous researches have been conducted for surface settlement induced by TBM in soft ground, the researches of surface settlement induced by TBM in a sandy cobble stratum are limited and a comprehensive study of the mechanism of delayed settlement induced by TBM in a sandy cobble stratum is unavailable. A ground stable state or surface settlement can be determined based on real-time monitoring data for surface dynamic subsidence. Subsequently, TBM tunneling parameters can be adjusted to accommodate various geological conditions. A sand-pebble-soil matrix is a typical heterogeneous material. The macro-mechanical performance of this matrix significantly differs from any material. In a general situation with a low water level and minimal disturbance, a stratum can stabilize by itself for a long period of time. Considering the characteristics of the stratum, ground loss can be divided into two phases: immediate settlement, which tends to stabilize, and delayed settlement, which tends to occur in sand-cobble strata, where settlement develops at a much slower rate than in single-medium strata. Monitoring data is not sufficient to guide the construction in the case of delayed settlement. Cobble-soil matrix can be treated as a spatial structural system that is constituted by single granular soil, aggregates of granular soil and pebble grains. Based on Particle Flow Code in 2 Dimensions (PFC2D), the mechanical characteristics of the matrix and the TBM tunneling process were numerically simulated. Movements of the pebble grains were traced and recorded in real time. The model addressed the mechanism of surface collapse from the perspective of mesomechanics. According to the model, a matrix formed self-stabilizing arch that overlies an underground cavity seems gradually wear out with expanding the cavity and eventually penetrating to the ground surface. The law of ground movement and the formation mechanism of ground subsidence in TBM advancement were investigated. The main factors that affect surface subsidence are the speed of advancement, the underground water level and the supporting period. In the numerical analysis, the surface-loss lag was reproduced and the field monitoring data were verified. The findings of this study provide a new method for investigating ground subsidence in similar strata.

Study on Mechanical Characteristics of Tunnel Support System in Sandy Cobble Stratum

Because the sand gravel stratum with loose structure, good gradation, easy disturbance, poor stability, deformation and other characteristics of long duration, large deformation and collapse failure phenomenon is extremely easy to cause in the tunnel construction, leading to a primary support structure for strong deformation or even destruction, serious impact on the tunnel construction and safety, is one of the main problems encountered in tunnel construction. The paper based on the engineering background of Gongboxia sand gravel stratum tunnel, studied the Sandy pebble tunnel primary support stiffness and the influence of strength characteristics, also mechanical impact characteristics of secondary lining and timing. By studying the mechanical behavior of tunnel support system, the paper puts forward the parameters of the initial support of the sand gravel tunnel, and makes clear the relationship between the stress of the supporting arch and the stress of the lining and the timing of the two lining. The research results can provide supporting design reference for sand pebble stratum tunnel and similar projects.

Study on Landslide Mechanism and Treatment Technology in Tunnel of Sandy Cobble Stratum

The steady ability of sandy pebble stratum is poor, during the tunnel construction, landslides occur frequently. In order to clarify the mechanism of landslide in sand gravel tunnel and put forward effective treatment technology, the paper takes the Gongboxia tunnel, which located in Qinghai Province, as the engineering background. In view of the landslide in tunnel, the influence factors and formation instability model are analyzed, also the comprehensive treatment technology of landslide is put forward. The results show that the geological factors are the main factors of tunnel collapse, design factors and construction factors contributed to the collapse of the tunnel collapse to a certain extent, the interaction of multiple factors; sand gravel stratum collapse mechanism belongs to loose medium caving, the instability mode is gravity collapse; based on the analysis of tunnel collapse mechanism. Put forward the "dense row of short pipe shed, three bench reservation core soil + locking anchor pipe, grouting backfill, the temporary support" comprehensive treatment technology, through on-site data reflect, have very strong practicability and pertinence. The research results can provide the material of engineering analogy for the design and construction of similar projects, and have important reference significance.

Muck Problems in Subway Shield Tunneling in Sandy Cobble Stratum

Abstract Experiments aiming at the muck improvement for Chengdu’s subway shield tunneling in water-rich sandy cobble ground initially finds out the kinds and ratio of muck improver suitable for the this strata, and through the field application determines the reasonable ratio of muck improver for shield tunneling in sandy cobble ground, providing a reference to other muck improvement for shied tunneling in similar ground. And the earth pressure balanced shield is widely used in tunnel engineering, with its characteristics as wide adaptability, safe operation, fast driving speed, low environmental noise and low cost. Taking Chengdu Metro Line as the background, aiming at the difficulty faced by EPB shield tunneling in water-rich sandy cobble ground, and based on the explorations and researches of the predecessors and scholars, engineering and technical personnel, the present paper combines with engineering examples to make a systematic summary and research on key construction technology of EPB shield tunneling through sandy cobble ground from the aspects of EPB shield adaptability, , structure and transformation of cutter and helical unearthed wares, settlement control method, and muck improvement measures. It has certain reference value and guiding significance to the selection of shield machine and the reconstruction of shield machine in complex geological condition.

New challenges in construction mechanics of urban rail transit

This paper aims to raise concerns about construction mechanics. Firstly, operating railway transit at present and scales of ongoing rail transit projects are briefly introduced. Then, major constraints of urban rail transit construction are analysed in two aspects, including complexed geology conditions and strict environmental requirements. Difficulties caused by geology conditions contains excess settlement and overall failure in very soft soil, the delayed ground collapse and cutter wear of shield tunnelling in sandy cobble stratum, construction safety threats in complex stratum, water bearing stratum and gas-bearing stratum. About environmental restrictions, difficulties of constructions crossing existing railways, buildings and cultural relics are summarised. These difficulties challenge construction mechanics in failure behavior descriptions and calculation precisions. In the end, this paper considers that breakthrough in construction mechanics should be seek in mechanical models, shakedown solutions subjecting slight dynamic load, and multi-factor forecasting models.

Impact of shield tunneling on adjacent spread foundation on sandy cobble strata

The section of shield tunnel of the Chengdu Metro line passes primarily through sandy cobble strata. There are many buildings with spread foundations along the lines. Shield tunnel construction will disturb the ground, causing displacement or stress to adjacent spread foundations. Based on the similarity theory, a laboratory model test of shield tunnel driving was carried out to study the influence of shield tunnel excavation on the displacement of adjacent spread foundation. The results show that foundation closer to the tunnel has greater displacement or settlement than that further away. The horizontal displacement is small and is influenced greatly by the cutting face. The displacement along the machine driving direction is bigger and is significantly affected by the thrust force. Settlement occurs primarily when shield machine passes close to the foundation and is the greatest at that time. Uneven settlement at the bottom of the spread foundation reaches a maximum after the excavation ends. In a numerical simulation, a particle flow model was constructed to study the impact of shield tunnel excavation on the stresses in the ground. The model showed stress concentration at the bottom of the spread foundation. With the increasing ground loss ratio, a loose area appears in the tunnel dome where the contact force dropped. Above the loose area, the contact force increases, forming an arch-shaped soil area which prevents the loose area from expanding to the ground surface. The excavation also changed the pressure distribution around spread foundation.

Structural Optimization of Shield Machine Cutterhead Applied in Sandy Cobble Stratum

With the aim of probing the optimal cutterhead structure for sandy cobble stratum in Beijing metro, four typical cutterhead structure are presented. The load on the cutterhead is calculated. The finite element models of cutterhead with thickness of 350 mm, 400 mm and 500 mm for each structure style are established respectively. By analyzing the stress distribution for each model, we find that the maximum stress of spoke-style cuttehead are lower than that on the board-style cutterhead; the structural properties of cutterhead with straight corbel are better than that with oblique corbel. Finally, the optimal cutterhead structure and size for sandy cobble stratum is selected.