Ground Loss Ratio Research Articles

Impact of Tunneling on Adjacent Piles Based on the Kerr Foundation Model Considering the Influence of Lateral Soil

The Kerr foundation model simulates the interaction between piles and soil. Considering the impact of lateral soil displacement on adjacent piles, the lateral displacement and bending moment of the adjacent piles caused by shield tunnel excavation are calculated in detail. Additionally, the reactions of groups of piles are obtained by focusing on the shielding effect of the piles on the soil displacement caused by shield tunnel excavation. The validity of the solutions is verified by comparing the calculated results with the boundary element program GEPAN. Additionally, adjacent pile lateral displacement and bending moment are compared, with and without considering lateral soil effects. Furthermore, this study investigates the influence of various factors, such as soil spring stiffness, pile–tunnel distance, ground loss ratio, and pile diameter on the pile group’s lateral displacement and bending moment. The research findings indicate that increasing the soil spring stiffness or the horizontal distance between the pile and tunnel can reduce the lateral displacement and the bending moment of the pile. On the other hand, as the ground loss ratio gradually increases, the pile lateral displacement and bending moment will also increase. However, when the diameter of the pile grows, the lateral displacement reduces, while the bending moment increases.

Effect of seepage on soil arching effect in deep shield tunnel

Shield tunneling and post-tunneling steady seepage are accompanied by stress and displacement variations, which could induce and influence the soil arching effect. Although there are many studies on the tunneling-induced soil arching effect, the research about the effect of seepage on soil arching effect is extremely lacking. In this study, a numerical model is firstly established and verified by field data. Then, a series of numerical models, whose simulation method of steady seepage is verified by adopting the conformal mapping technique, are established to study the soil arching evolution of deep-buried tunneling and post-construction steady seepage. The results indicate that seepage leads to an increase in effective vertical stress, which is consistent with the existing theory. The seepage weakens the soil arching effect resulting in the height of the arch zone reducing from 2.38D (D is the tunnel diameter) to 1.25D. The seepage leads to the further development of ground consolidation settlement, but the differential displacement in the soil mass decreases. The ground reaction curve in the steady seepage condition shows a bigger value than that after excavation. It is reasonable to control the ground loss ratio in the range of 0.5–1.0%, which can minimize overburden pressure with moderate ground deformation.

Visualization and digitization of model tunnel deformation via transparent soil testing technique

With the increasing demand for transportation infrastructure, the construction of urban tunnel systems linking different areas is indispensable. The deformation and the failure process during a tunnel excavation are essential concerns for geotechnical engineers. However, few studies have systematically investigated the effect of the ground loss ratio on tunnel deformation and the inequality between the ground loss ratio and the volume of the settlement trough. An experimental study using the transparent soil testing technique is performed herein for better visualization and digitization purposes. The three dimensional vertical and horizontal deformation patterns of a single tunnel are investigated for both the surface and the stratum right below considering different ground loss ratios. The relationship among the empirical constant of the settlement trough width, buried depth, depth–diameter ratio, and ground loss ratio is presented.

Numerical simulation on ground deformation characteristics during double-line shield tunnel crossing airport runway

Using Peck empirical formula and superposition method, the variation of ground settlement, uneven settlement and ground curvature caused by single-line and double-line shield tunnel are obtained. The finite element numerical simulation method is used to analyze the construction process of shield tunnel below the airport runway. Considering the existence of the runway structure and the successive excavation effects of the left and right line, the comparison of simulation and empirical formula is analyzed. Based on this, the deformation Characteristics of the runway is derived under different buried depths and different ground loss ratios.

Theoretical analysis of three-dimensional ground displacements induced by shield tunneling

In this study, an approach to estimate three-dimensional ground displacements induced by shield tunneling is proposed by using the superposition method, advocating consideration of the ground loss as well as the working loads during shield tunneling. The ground movements caused by different factors, i.e., applied support pressures, shield shell frictions and cutter head rotation, are investigated in detail, which can provide a better understanding of how to set the shield advancing parameters for ground disturbance controlling. The main advantage of this approach is that it can well reproduce the uplift and settlement characteristics of the ground. It is also of great significance for assessing the safety of closely spaced structures when the ground differential settlement is taken into account. To estimate the ground movements induced by twin tunneling, an additional ground loss ratio is introduced to describe the disturbance caused by the first tunneling. The proposed approach in this paper is tested against the field data at a construction site and provides a satisfactory estimate of the ground movement.

Prediction of Ground Settlement Due to Excavation of a Quasi-Rectangular Shield Tunnel Based on Stochastic Medium Theory

In this study, a method for calculating the ground loss-induced ground settlement during quasi-rectangular shield (QRS) tunnelling was investigated. Based on stochastic medium theory, an equation for calculating QRS tunnelling-induced transverse ground settlement was derived. A tunnelling face convergence mode coefficient, α, was introduced to make the derived equation applicable to various tunnelling face convergence modes. A modified equation for calculating the longitudinal variation in the ground loss ratio (e) was proposed to calculate e for any arbitrary cross-section in the tunnelling direction. On this basis, the proposed equation was longitudinally expanded, and an equation for calculating the three-dimensional (3D) ground settlement was obtained. A simplified model for additional grouting pressure was established, and the influence of additional grouting pressure on surface deformation was analysed. In addition, the proposed method was used to calculate the QRS tunnelling-induced ground settlement, and the results were compared with actual measurements. It is found that the predicted ground settlement obtained using the proposed method matches the actual measurements relatively well and reflects the 3D variations in the ground settlement. As α increases, the maximum ground settlement increases and the width of the ground settlement trough decreases, whereas the positions of the inflection points of the settlement curve remain unchanged. The horizontal distance between the inflection points is approximately twice the width of the tunnel.

Sensitivity analysis of ground settlement induced by pipe-jacking construction in soft soil stratum

Taking the ground settlement while pipe-jacking construction in soft soil as the engineering background, the effect of pipe jacking were studied by numerical simulation. It focused on the factors that mainly influenced surface settlement of ground including in-situ stress release rate (or ground loss ratio), chamber pressure, elastic modulus of soil, buried depth and diameter of pipe. That would provide theoretical basis for taking measures to reduce surface subsidence while jacking. The main conclusions were as follows: (1) The in-situ stress release rate and pipe diameter were proportional to the surface subsidence. (2) Chamber pressure, elastic modulus and buried depth were inverse proportional to the surface subsidence. (3) From the point of view of the influence on surface settlement, the sensitivity of pipe diameter and elastic modulus of soil was the greatest, followed by burial depth and chamber pressure, and the stress release rate was the smallest.

An analytical solution for the arching effect induced by ground loss of tunneling in sand

Because the earth pressure above tunnel crown was assumed uniform and the effect of particle flow of sand and ground loss were not considered, Terzaghi method generally produces discrepancies from model tests and field observations. To overcome this deficiency, the flow of sand particles above a tunnel was investigated firstly to define the shape and size of loosening zone by following the gravity flow principles. Then, by assuming non-uniform distribution of earth pressure and incorporating the factor of ground loss, a modified Terzaghi method has been developed. It can not only determine the loosening pressure on tunnel, but also the shape and size of loosening zone corresponding to ground loss and properties of sand particles. The research results reveal that the shape of loosening zone is ellipsoidal or a part of ellipsoid affected by the eccentricity and loosening factor of sand. The height of loosening zone increases with ground loss of tunneling, resulting in the propagation of loosening zone from deep in ground to ground surface. For most shield tunnels, with ground loss ratio less than 4.0%, the loosening zone is formed only at the upper tip of the ellipse and its shape resembles to a curved arch. According to the proposed method, the height of the loosening zone for shield tunnels in sandy ground is approximately 0.1D–0.7D (D, tunnel diameter) and the earth pressure is about 35–90% of the total overburden earth pressure for typical ground loss within the range of 0.5–4.0%. Via detailed comparison with model tests and Terzaghi method, the proposed method is proved to be closer to the model tests than the Terzaghi method. For the sake of convenience in application, simplified design curves for the height of loosening zone and earth pressure were plotted against ground loss, which is of great importance for engineers to set reasonable goal of ground loss in tunneling practice.

Evaluation of ground loss ratio with moving trajectories induced in double-O-tube (DOT) tunnelling

This paper investigates the influence of moving trajectories on ground loss ratio (GRL) due to the double-O-tube (DOT) tunnelling method. DOT tunnelling has three moving trajectories: pitching, yawing, and rolling, which have different behaviours during tunnel construction compared with those from single circular shield tunnelling. These moving trajectories cause overexcavation during tunnelling. The calculation method of gap area between the DOT shield machine and linings is evaluated in this research. Based on the superposition concept, the modification equation of GLR is proposed, which takes both moving trajectory and grouting volume into consideration. A field DOT tunnelling case is analysed to determine the correlation between moving trajectories and ground settlement. The influence of tail grouting is discussed by adjusting the grouting volume in different periods. The finite element method is also employed by setting the modified ground loss ratio (GLR′) as the contraction increment of linings. Results from both the measured and simulated settlements verify the reasonability of the proposed equation and the effect of moving trajectories on ground loss.

Simplified solution for tunnel-soil-pile interaction in Pasternak’s foundation model

The current design practice for predicting the interaction mechanics for tunnel-soil-pile is generally based on Winkler’s foundation, which is subject to some important limitation, such as ignoring the continuity of the soil foundation. Furthermore, the current analytical studies are mostly employed the plane strain analyses and do not consider the influences of lateral soil displacements on pile behaviour. To improve the accuracy for the pile behaviour prediction induced by tunnelling, the analytical method should account for the effects of a number of parameters, such as the ground shearing displacements, and the influence of lateral soil displacements next to the pile. This paper focuses on a simplified solution based on Pasternak’s foundation model to predict the lateral displacements and internal forces of a single-pile and group-piles induced by tunnelling considering the effects of lateral soil displacements. First, the simplified solution of tunnel-soil-pile interaction, which reflects the influence of shearing displacements of foundation, is established on Pasternak’s foundation model. Second, the equivalent concentrated forces are supplied to the pile through the shear layer to consider the influence of lateral soils beside the pile. The validity of the solutions is verified by the boundary element program results, centrifuge test data, and field measurements. The calculated results are also compared with and without considering the effects of tunnel-soil-pile interaction. When the influences of lateral soil displacements are considered, the results are shown to be closer to the monitored in-situ data and the centrifuge test data. In addition, the influencing factors of a single-pile and group-piles displacements are also investigated, including the shear layer modulus, pile diameter, ground-loss ratio, pile-tunnel distance, and pile spacing. The influence of soil shear displacements on pile response cannot be ignored, and an error may occur when Winkler’s foundation model is used to solve this problem.

Deriving Spatio-Temporal Development of Ground Subsidence Due to Subway Construction and Operation in Delta Regions with PS-InSAR Data: A Case Study in Guangzhou, China

Subways have been an important method for relieving traffic pressures in urban areas, but ground subsidence, during construction and operation, can be a serious problem as it may affect the safety of its operation and that of the surrounding buildings. Thus, conducting long-term ground deformation monitoring and modeling for subway networks are essential. Compared with traditional geodetic methods, the Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) technique offers wider coverage and denser measurements along subway lines. In this study, we mapped the surface deformation of the Guangzhou subway network with Advanced Synthetic Aperture Radar (ASAR) and Phased Array Type L-band Synthetic Aperture Radar (PALSAR) data using the Interferometric Point Target Analysis (IPTA) technique. The results indicate that newly excavated tunnels have regional subsidence with an average rate of more than 8 mm/year, as found on Lines Two, Three, Six, and GuangFo (GF). Furthermore, we determined the spatio-temporal subsidence behavior of subways with PALSAR in delta areas using Peck’s formula and the logistic time model. We estimated the tunneling-related parameters in soft soil areas, which had not been previously explored. We examined a section of line GF, as an example, to estimate the ground settlement trough development. The results showed the maximum settlement increased from −5.2 mm to −23.6 mm and its ground loss ratio ranged from 1.5–8.7% between 13 July 2008 and 19 January 2011. In addition, we found that the tunnels in line GF will become stable after a period of about 2300 days in peak subsidence areas. The results show that the proposed approach can help explain the dynamic ground subsidence along a metro line. This study can provide references for urban subway projects in delta areas, and for the risk assessment of nearby buildings and underground pipelines along metro lines.

An in-tunnel grouting protection method for excavating twin tunnels beneath an existing tunnel

A section of closely spaced twin tunnels at Shangmeilin station on Shenzhen metro line 9 was excavated by the shield tunneling method. Part of this section lies closely beneath the tunnel servicing Metro line 4. Due to restrictions on ground surface to control the extra settlement of the existing tunnel in the area, an in-tunnel grouting protection method was adopted in combination with the shield method. This method greatly reduced the effect on the existing tunnels when building the twin tunnels underneath. The settlement of and stress on the existing tunnels associated with the construction of the new tunnels were systematically monitored. Because of the twice under-crossing process, the excavation of twin tunnels had a superposed influence on the existing tunnel and surrounding soil. The behavior of the overlying tunnel was analyzed based on instrumentation records obtained from the project. The settlement profile of the existing structure displayed a “V” shape after the first under-crossing but a “W” shape after the second. The hoop stress of the existing tunnel induced by shield tunneling below had beneficial effects on the stress state of the tunnel structure. By contrast, the longitudinal stress exerts a bending moment on the existing tunnel, with a large tensile stress that had adverse effects on its structure. Based on the monitored data, three deformation modes are proposed to describe the behavior of the existing tunnel. In addition, the differences between the two under-crossing processes are discussed; both the ground loss ratio and width parameter of the settlement trough in the second profile were larger than those of the first profile due to the decrease in soil stability.

Observation of Ground Movement with Existing Pile Groups Due to Tunneling in Sand Using Centrifuge Modelling

In developing urban areas, there are unavoidable interactions between existing structures and new construction such as tunnelling through pre-existing pile foundations. An accurate prediction of the ground deformation due to tunnelling is necessary to assess potential damage to those existing structures. Two dimensional tunnelling model tests were carried out to study the movement of both soil surface and subsurface layers of dry Toyoura sand with two pile groups of different length at both sides of the tunnel. The model pile group has four piles with spacing and diameter ratio of 5. The tunnelling process was simulated by reducing the diameter of the model tunnel with initial diameter of 70 mm for various ground loss values at 100g centrifugal acceleration. The induced soil movement and displacement of pile foundation were observed using the particle image velocity technique and displacement transducers. The test cases were conducted at two different tunnel cover depth and diameter ratios (C/D = 1.5 and 2.5) and three different horizontal distances between the pile groups and the tunnel. Tunnel machine showed good agreement of soil movements with the results from real site construction at small ground loss ratios. The cases with the pile group induced larger maximum settlement in the vertical direction than the cases without piles. There was an effect of the pile groups on the ground movement at the location more than five times pile diameter away from the piles in the longitudinal direction.

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.

Centrifuge investigation into the effect of new shield tunnelling on an existing underlying large-diameter tunnel

A centrifuge model test was carried out to investigate the effect of new shield tunnelling on an existing underlying large-diameter tunnel. Three construction steps of a new shield tunnel were simulated in the test. Soil excavation with ground loss and grouting in each step was simulated by discharge and injection of a dense solution. The vertical displacement and the longitudinal stress of the existing tunnel, as well as the pore water pressure at its spring line, were measured. The vertical displacement and the longitudinal stress of the existing tunnel increase as a result of the excavation of the new shield tunnel. However, the vertical displacement and the longitudinal stress decrease when grouting is injected for the new tunnel. The vertical displacement and the longitudinal stress exhibit an approximately linear change with increases in ground loss ratio and grouting ratio of the new shield tunnel. In the test, the heave zone of the existing tunnel is within 39m of the axis of the new shield tunnel. The variation in pore water pressure at the spring line of the existing tunnel is low during construction of the new shield tunnel. Grouting is an effective measure to mitigate the responses of the existing tunnel.

Surface settlement caused by twin-parallel shield tunnelling in sandy cobble strata

City metro tunnels are usually constructed as twin-parallel tunnels and their adjacent construction may lead to surface deformation, affecting the surface environment and the safety of the tunnels. Due to its strong dispersion, sandy cobble strata can be easily disturbed by shield tunneling. Based on the project of the Chengdu Metro Line 1, field and model tests were carried out to study the surface settlement caused by shield tunneling in sandy cobble strata by measuring surface settlement curves, ground loss ratios and construction influence zones. The discrete element method (DEM) was used to study the factors affecting the formation of ground arches in sandy cobble strata at the microscopic level. Results show that the shape of the surface settlement curve in sandy cobble strata is different from that in soft soil. The buried depth and clear spacing of the two tunnels had a significant impact on the formation of ground arches.

Contrast Analysis on Deformation Characteristic of Urban Subway Tunnel Excavation by Numerical Simulation and Field Measurement

Under the particular geological conditions of clay and sandy soil interbed, through numerical analysis and field monitoring, the surface settlement, horizontal deformation and the stress distribution were analyzed after the tunnel excavation in a metro station in Beijing, China. There are differences of surface deformation whether the backfill grouting behind arch during tunnel excavation. While the backfill grouting behind arch, the ground surface settlement and the horizontal displacement reach 9.36 mm and 2.16 mm respectively. But while no backfill grouting behind arch, the ground surface settlement and the horizontal displacement are 14.51 mm and 8.85 mm respectively. The maximum ground settlement induced by tunnel excavation is directly proportional to ground loss ratio. When the ground loss ratio is 1%, the result is well consistent with the field measurement value and the numerical simulation result.

3D Analytical Solution of Soil Deformation Induced by Shield Tunnelling Construction

Three-dimensional (3D) analytical solution of soil deformation induced by ground loss in shield tunnelling construction was researched. It is put forward that the ground loss ratio is not a fixed value, but changes in driving direction. The calculation formula of ground loss ratio in driving direction was deduced. Based on two-dimensional (2D) analytical solution of uniform ground movement model of shield tunnelling, the three-dimensional analytical solution of ground deformation induced by ground loss is deduced. The settlement in vertical direction and the displacement in lateral horizontal direction at any point can be calculated; and the method is only applied to the construction phase. In analytical calculation: the predicted soil displacements are in good agreement with the measured values, and the method is easy to use; the closer the soil to tunnel is, the faster the lateral horizontal displacement changes; the extent of change of lateral horizontal displacement in longitudinal direction is smaller than displacement in lateral direction.

An analysis method for the influence of tunneling on adjacent loaded pile groups with rigid elevated caps

AbstractTunnel excavation could influence the working performance of adjacent pile foundations. How to accurately predict the tunneling‐induced pile responses is becoming an important issue in urban areas. Previous methods tend to neglect the influence of working loads on the tunneling‐induced pile responses. In this paper, a two‐stage analysis method is proposed to study the behavior of pile groups with rigid elevated caps subjected to tunneling‐induced ground movements, in which the pile–pile interaction, coupling of longitudinal and lateral deformation, and influence of working loads are considered. This method is validated by comparing the results with those from a general pile analysis program, GEPAN, centrifuge test data and field measured data. Thereafter, a series of parametric studies is carried out. The results show that the influence of working load on tunneling‐induced pile responses is significant. Three different features of the pile responses will take place for different depth ratios. With the increase in soil stiffness, the tunneling‐induced axial force increases linearly, whereas the bending moment increases nonlinearly. For soils with high stiffness, the maximum bending moment increases linearly with the ground loss ratio within the range of 0–5%; but for soils with low stiffness, this tendency becomes obviously nonlinear. Copyright © 2010 John Wiley & Sons, Ltd.

Responses of single piles to tunneling-induced soil movements in sandy ground

The responses of single piles under various working loads to nearby tunneling were investigated using centrifuge model tests. First, the tunneling-induced soil movements and the tunnel stability in saturated sandy ground were examined. Two instrumented piles with penetration depths of 27 m were located either side of, and at various distances from, tunnels embedded at depths with various cover-to-diameter ratios, and used to measure the bending moments and axial forces at various depths for various ground loss ratios during tunneling simulations. The test results show that in the case of shallow tunneling near a long pile the unit skin frictions on the pile from the tunnel axis to an elevation of 1.5 tunnel diameters above the tunnel axis rapidly decrease with increases in the ground loss ratio. A significant degradation of the end bearing capacity results in a large settlement of the pile if the pile tip is near the tunnel. The depth ratio was found to be a significant influence on the bending moment profiles along the piles, but both the depth ratio and the working loads on the pile head determine the axial load profile and the pile settlement. A mechanism for pile load transfer during new tunneling is proposed to enable construction engineers to prevent structure failure in piles and excessive pile settlement.