Tunnel Settlement Research Articles

Analysis of immersed tunnel settlement under complex load and multiple kinds of compound foundations

The silt typed soil thickness is up to 3~18m laying at bottom of foundation trench of ramp section at west island of SZ-Link and the surface silt is of extremely low strength subject to the influence of sand disturbance. The island cut and cover section tunnel foundation is improved with PHC and the settlement is relatively small. To accommodate the coordinated deformation of the immersed tunnel the soft foundation of the tunnel needs special treatment. Through options comparison and selection, the solution for foundation of tunnel element E1 is advance gravel cushion and compacted blocky stone layer and DCM outside the island and high pressurized jet grouting pile in the island. To accurately analyze the settlement characteristics of the element E1, determine its settlement amount and to achieve smooth rigidity transition of immersion tunnel and cut and cover section the settlement analysis of element E1 under complex loads and multiple kinds of foundations were carried out in combination with testing researches of all kinds of foundations. The primary research work comprises: (1) conduction of laboratory model experiment of gravel cushion, blocky stone compaction experiment, DCM laboratory triaxial test and provision of deformation computation parameters for given different foundation improvement solutions; (2) conduction of 3D finite element numerical analysis for element E1, analysis of immersed tunnel settlement deformation characteristics and provision of the law of E1 settlement distribution.

Characteristics, challenges and countermeasures of giant karst cave: A case study of Yujingshan tunnel in high-speed railway

Karst landforms were widely distributed all over the world, and they were mainly distributed in the Southwest and Southeast regions in China. However, tunnel construction in giant karst caves was rarely studied. In recent years, Yujingshan tunnel of Chengdu-Guiyang high-speed railway in Yunnan Province had encountered the largest karst cave in history of Chinese railway construction. This paper reported the hydrological and geological conditions of this case, as well as relationship between karst cave and tunnel, and compared it with the other three giant karst caves encountered in railway construction, and systematically analyzed the challenges it brought to tunnel construction, and put forward the corresponding countermeasures. Furthermore, in-situ monitoring results of the layered settlement of tunnel bottom and the crown settlement and horizontal convergence of tunnel in giant karst cave were analyzed. The results showed that the challenges mainly included: (a) huge scale of karst cave, (b) poor stability of cave wall, (c) poor stability and complex composition of filling content, (d) complex underground river and channels; the countermeasures mainly included: (a) backfill of cave, (b) roof reinforcement of cave, (c) both sides and the bottom reinforcement of tunnel, (d) diversion and closure of underground river, (e) bridge in the tunnel, (f) bench cut method. The feedback of in-situ monitoring results demonstrated the effectiveness of countermeasures. This case study can provide experience for construction of similar tunnels in giant karst caves.

Analysis and Prediction of Long-Term Settlement of Metro Shield Tunnel in Saturated Sand

Based on the existing monitoring data, this paper uses the matured data method to analyze, predict the long-term settlement of the subway shield tunnel, and comprehensively analyzes the long-term settlement of the subway shield tunnel in the saturated sand layer of Zhengzhou. Logical curve model, etc. is used to separately predict the long-term settlement of the Zhengzhou metro shield tunnel. Combining the prediction results to summarize the characteristics of the prediction model, a prediction model suitable for the long-term settlement of the subway shield tunnel in the sandy soil area represented by Zhengzhou is obtained, which can be used for the operation of the Zhengzhou subway tunnel. Management and similar engineering projects provide a certain reference basis.

Model Test on Ground Surface Settlement and Face Failure of Shallow Tunnel in Dry Sandy Stratum

Surface deformation and face failure may occur during shield tunneling in the shallow buried dry sandy stratum. The model shield machine is used to carry out the shield tunneling model test in dry sandy stratum. The surface deformation laws and the active failure process of the excavation face are analyzed by monitoring the surface deformation during advancing and face failure stages. It is concluded that the surface deformation curve is “M” shaped and asymmetrical about the tunnel axis. The surface settlement during face failure goes through two stages, and the surface collapse pit is approximately elliptical, whose center is located on the left side of the tunnel axis when the cutter head rotates clockwise.

Monitoring, modelling and prediction of segmental lining deformation and ground settlement of an EPB tunnel in different soils

In this paper, the deformation characteristics of tunnel lining structures in different soils are studied by theoretical analysis, numerical modelling and field monitoring. The deformation of segmental lining was monitored by distributed optical fiber sensors in the tunnels of Foshan Metro Line 2 during EPB shield tunnelling. The structural deformation characteristics, including the longitudinal strain and curvature, the circumferential and longitudinal joint deformation, the longitudinal settlement, and the circumferential convergence of the tunnel under different ground conditions and at different depths are analysed based on the monitoring data. The optimized conjugate beam method is developed to analyse the lining deformation and the ground settlement. The PLAXIS 3D finite element model is utilized to study the effects of soil properties (cohesion and friction angle), tunnel depth and soil reinforcement on the tunnel lining deformation and ground settlement. Numerical modelling is further applied to simulate and predict the lining deformation of Foshan Metro Line 2 tunnels, which are compared with the monitoring data. The prediction results can effectively help to reduce the risks of large deformation and settlement in weak and soft grounds.

A simplified two-stage method to estimate the settlement and bending moment of upper tunnel considering the interaction of undercrossing twin tunnels

With the expansion of existing networks of underground system, many metro tunnels are excavated in a pair undercrossing existing tunnels. The new excavations will affect the stress distribution and settlement behavior of the existing tunnels, particularly when the clearance is small. Generally, the construction of the second undercrossing tunnel induces more settlement in the upper tunnels compared to that caused by excavating the first one, due to the interaction between the new excavations. The degree of interaction makes it difficult to predict the settlement of existing tunnels. In this paper, a simplified two-stage analysis method considering the interaction between the undercrossing twin tunnels is proposed to estimate the induced response of existing upper tunnel. In the first stage, the greenfield settlement caused by the first excavation is estimated using Peck’s empirical function, and that caused by the second excavation is modified with a modification factor to consider the influence of the first tunnel. In the second stage, the additional stress calculated using the greenfield displacement is imposed on the existing tunnel resting on Pasternak foundation model. The calculated settlement profile and bending moment of the existing tunnel using the two-stage analysis method are compared with those obtained from finite element analysis and historical field monitored data. The effects of the characteristic parameter of Pasternak foundation model, the spacing between the new twin tunnels, and the modification factor of greenfield settlement, on the settlement profile and bending moment of the existing tunnel are also investigated.

Influence of Shield Tail Clearance on Surface Settlement during Shield Construction in Soft Soil Area

The shield construction process of shield tail gap caused by formation damage, the formation damage caused by the settlement is as long as the part of tunnel subsidence analysis, therefore in the finite element software analysis of shield construction settlement value, the shield tail clearance simulation is an important link in the entire model, this article analyzes three common finite element simulation method, shield tail clearance displacement convergence method based on non-uniform distribution and actual matches are higher than layer such as generation method and stiffness discount subtraction. Finally, the displacement convergence method is used to simulate the grouting effect of shield tail on the surface settlement.

Effect of Grouting Reinforcement on Settlement of Existing Tunnels: Case Study of a New Crossing Underpass

In the construction of the new shield tunnels crossing existing tunnels, grouting rings are often installed in the existing tunnels for protection. To study the effect of grouting rings on the settlement of existing tunnels, the variations of soil properties and relevant parameters before and after the grouting are analyzed. By considering the effect of the grouting ring on the reduction of the stress and the reinforcement of the surrounding foundation soil, the stress reduction model of the heterogeneous soil layer and the model of the tunnel settlement calculation are proposed. The formula for the reduction of the stress after passing through the grouting ring is deduced. Moreover, by comprehensively taking the influences of the shield excavation and grouting ring into account, the formula for calculating the vertical displacement of the existing tunnel caused by the excavation of the new tunnel is derived. Further, the impacts of the length and the thickness of the grouting ring on the settlement of the existing tunnel are studied. The results of the theoretical calculation method proposed herein are well consistent with the data computed by the finite element model, and both follow a relatively similar change trend. According to the obtained results, installing the grouting ring on the existing tunnel can effectively reduce the tunnel settlement. Finally, the results demonstrate that increasing the length and the thickness of the grouting ring can strengthen the protection of the existing tunnel, but the effect gradually decreases.

Adoption of size effect combined with Winkler elastic foundation beam theory in section shield tunnel under underground space development

It aims to discuss the adoption value of combining the size effect and Winkler elastic foundation beam theory in the large-section shield tunnel under the background of underground space development. Two kinds of soil layers of silty clay and sand pebble are simulated. The difference of settlement, longitudinal deformation, and earth pressure of large-section shield tunnel in the simulated soil layers is analyzed. The longitudinal design model of shield tunnel based on large-section and compound stratum is constructed according to Winkler’s elastic foundation beam theory and the longitudinal equivalent continuous model of shield tunnel. The results show that the tunnel diameter has an exponential relationship with the effective rate of longitudinal stiffness and equivalent longitudinal stiffness. The overall longitudinal settlement curve of the tunnel in the compound stratum presents the “S” shape. The maximum differential settlement of the tunnel is −55 mm. The soil pressure around the tunnel from high to low is arch bottom, left arch waist, right arch waist, and arch top. The maximum settlement of the tunnel in the silty clay and sand pebble is −6.08 mm and −15.85 mm, respectively; the maximum uplift of the arch bottom is 11.65 mm and 17.74 mm, respectively; the earth pressure around the arch top is 2.184 MPa and 4.22 MPa, respectively; and the earth pressure around the arch bottom is 2.236 MPa and 3.88 MPa, respectively. At the boundary of the two strata, the maximum earth pressure around the top of the arch is 4.56 MPa, and the maximum earth pressure around the bottom of the arch is 4.17 MPa. It suggests that the longitudinal design model of shield tunnel with large-section and compound stratum is successfully constructed according to the size effect combined with Winkler elastic foundation beam theory, which provides a reference method for the design of the tunnel structure with large cross-section.

Deformation behavior monitoring of a tunnel in its temporary shoring demolishing process using optical fiber sensing technology

For tunnels with large cross-sections, demolition of the temporary shoring of the tunnel can pose a very serious threat to the safety of the structure; Therefore it is very important to ascertain the deformation behavior of the tunnel in this stage. In this study, a tunnel settlement inversion model, based on the strain measurements of distributed optical fiber, has been proposed and the feasibility of the model has been verified by laboratory tests. Based on field observations, the strain distribution characteristics of the optical fibers and the variations in the mechanical properties of the tunnel were analyzed, taking the demolition process into consideration. The results of the monitoring indicated that demolition of the bottom shoring will cause large and uneven crown settlements; this is because this step will weaken the vertical supporting force at the bottom of the arch. Besides, continuous removal of the bottom shoring of the tunnel can result in large settlement and deformation of the crown, which should be pay attention in this stage. The settlement curve of the tunnel has also shown that the strain distribution of the optical fibers was able to reflect the performance of the tunnel in the demolition process of its temporary shorings.

Early warning limits for uneven settlement of metro tunnels based on vehicle–track interaction simulations

Metro shield tunnels in soft soils often exhibit uneven settlement, which causes leakage, cracking, dislocation, and bolt failures. Excessively uneven settlement affects the safety and durability of a tunnel structure and aggravates the track irregularity, thereby impeding operational safety, ride comfort, and steady function. As ride safety and comfort are more sensitive than tunnel structure safety to uneven settlement, they can potentially be used as indexes for the early warning of uneven settlement. Accordingly, a 20 m long metro rail car model with 10 degrees of freedom was established, and its behaviors were analyzed under different settlement amplitudes and running speeds. The results indicated that the rail car operational comfort index was more sensitive than the safety index to uneven settlement. An early warning value was then determined on the basis of the comfort index ranges for settlement length and vehicle speed, enabling the development of an early warning strategy for uneven settlement. The proposed strategy was subsequently applied to a case study, which monitored the uneven settlement of an interval of a metro shield tunnel using inclinometers.

Modification of Peck Formula to Predict Ground Surface Settlement of Twin Tunnels in Low Permeability Soil

Accurate prediction of surface settlement induced by tunnel excavation is significant for preventing damage to existing structures under complex geological conditions. The Peck formula is currently considered as an efficient solution for surface settlement prediction. This paper proposes a modified Peck formula considering geological conditions to improve the accuracy of surface settlement prediction of twin tunnels. The asynchronization of the sinking rate and stability of the vault settlement and surface settlement within the river‐affected area may attribute to the groundwater drawdown caused by cofferdam construction on the river. A modified Peck formula is put forward with soil permeability and width‐controlling parameters involved. There is a small settlement at the center of the twin tunnels, making the settlement trough upward buckling, which is like a “W” shape. This situation can be accurately predicted by the modified formula with a significantly increased adjusted R‐square. The modified formula can accurately predict the surface settlement of tunnels excavated in low permeability soil layers with a permeability coefficient between 10−4 cm/s and 10−7 cm/s, especially in the groundwater drawdown environment. The reliability of the modified Peck formula is verified by other cases in Nanjing and Singapore.

Application of artificial neural network for prediction of flow ability of soft soil subjected to vibrations

Vibrations induced by the operation of underground trains result in certain changes in the flow characteristics of the underground soft soil, which may lead to problems like ground settlement and damages of subway tunnels. In this study, an improved drag-sphere device is implemented to investigate the flow ability of soft soil subjected to vibrations, and the experimental results indicate that vibrations with high frequencies and low confining pressure enhanced the flow ability of soil samples. Then an artificial neural network (ANN) model is developed based on the obtained experimental data to predict the soil viscosity, where the genetic algorithm (GA) is implemented to optimize the weights and biases in the network. Specifically, by comparing the simulated results with experimental data, the optimal topology, training algorithm, and transfer functions are selected for the proposed model, and the model predictions are in high agreement with the experimental data, which denotes the proposed ANN model is accurate and reliable. Moreover, an analysis on the contributions of each input reveals that the water content affects the soil viscosity most while the frequency has the least impact for a single factor, which is in correspondence with the fact that the flow ability of soft soil is mainly affected by the geological conditions and its natural properties.

Development of Optic‐Electric Hybrid Sensors for the Real‐Time Intelligent Monitoring of Subway Tunnels

The settlement and deformation monitoring of subway tunnels had difficult in long‐distance and real time measurement. This study proposed an optic‐electric hybrid sensor based on infrared laser ranging technology and cable‐sensing technology. The working principle, hardware layer, design details, laboratory calibration and field validation were presented and discussed. The optic‐electric hybrid sensor implemented the real‐time intelligent analysis modulus for the whole system which could analysis the measurement errors and improve the accuracy. The laboratory calibration tests were carried out and the results shown that the hybrid sensors had measurement resolution of 1 mm with the maximum measurement range of 100 m. A remote real‐time intelligent monitoring system is established based on the hybrid sensors. The system contains an edge computing module, real‐time communication module and warning light signal with three colors. The stability of data acquisition and transmission of the intelligent control monitoring system under long‐term conditions was examined. Test results shown that the system was quite stable for the long‐term measurement. The whole system was verified in a constructing subway tunnel of Wuhan Metro Line 8, China. According to the field monitoring results, the deformations and the state of health safety of the tunnel was evaluated. The results of this study could provide useful guidance for tunnel deformation monitoring and has great practical value in civil engineering.

Experimental Study on Stiffness Softening of Soil‐Rock Mixture Backfill under Metro Train Cyclic Load

Due to the thick soil layer, short backfill time, and low degree of consolidation of the soil‐rock mixture backfill in Chongqing city, metro train tunnels passing through this type of strata are prone to large settlements during operation, which greatly affects the stability of the tunnel and the safety of metro train operations. In response to this problem, the dynamic triaxial test of the soil‐rock mixture backfill under cyclic loading was carried out to study the dynamic characteristics of the soil‐rock mixture backfill under cyclic loading. The effect of initial consolidation degree, effective consolidation confining pressure, and rock content on the stiffness softening of soil‐rock mixture backfill was analyzed. The results show that the initial consolidation degree, effective consolidation confining pressure, and rock content are all important factors affecting the stiffness of soil‐rock mixture backfill under cyclic loading. As the number of cycles increases, the lower the initial consolidation degree and effective consolidation confining pressure, the faster the attenuation of the softening index, and the larger the amplitude. As the rock content increases, the softening index increases and the stiffness of the backfill changes from softening to hardening. Based on the test data, the softening‐hardening model of the soil‐rock mixture is established, which is in good agreement with the field test results. This study can provide a reference for predicting and controlling the postconstruction settlement of the metro tunnel in the soil‐rock mixture backfill.

Research on displacement prediction of shield tunneling through existing tunnel based on LSSVM

The construction scale of the shield tunnel underpass is expanding day by day. In order to study the safety influence and deformation control of the shield tunnel underpass on the existing tunnel, the LSSVM model is established. Based on the collected soil storage pressure, foam volume, simultaneous grouting volume and other six shield construction parameters and corresponding sample data of the tunnel bottom displacement, the horizontal displacement and settlement displacement of the existing tunnel bottom caused by the approach construction are predicted. Taking a subway project as an example, the research results show that the prediction model of shield tunneling under the existing tunnel bottom level and settlement displacement has strong generalization ability and rapid and accurate prediction effect. This method can provide reference for similar projects.

Method of Calculating the Vertical Displacement and Additional Stress of Existing Tunnels under the Influence of Grouting Rings of New Tunnels

In order to reduce the disturbance to adjacent tunnels caused by tunnel crossing, the existing tunnels are often reinforced by setting grouting rings along the pipe piece ring. In this context, the volume expansion of the grouted soil has to be taken into account, and a mechanical model of the volume expansion of the grouting rings is proposed here to study the effect of the grouting rings of new tunnels on the additional stress on the existing tunnels as well as their vertical displacement. The additional stresses on and the vertical displacement of the axis of the existing tunnels caused by the expansion of the grouting rings were deduced based on the stochastic medium theory. The influences of various volumetric expansion rates (Q) of the grouting rings, different lengths of the grouting rings, and different tunnel crossing angles on the settlement of and the additional stress on the existing tunnels were studied. The results demonstrate that the grouting rings can effectively reduce the impacts of the additional stress and the settlement deformation on the existing tunnels. The results of the tunnel settlement obtained from the calculation method proposed in this paper are in good agreement with the measured engineering data.

Field Test Optimization of Shield Tunnelling Parameters Undercrossing an Existing High-Speed Railway Tunnel: A Case Study

An existing high-speed railway tunnel undercrossed by shield tunnelling is a high-risk project. Selecting reasonable shield tunnelling parameters is key in controlling the deformation of existing high-speed railway tunnels. In this paper, a Metro Line 3 undercross of an existing high-speed railway tunnel project is taken as the research background. First, a shield section with similar geological conditions as the undercross section is selected as the test section. Then, reasonable shield tunnelling parameters are determined by monitoring the settlement values of surface and ground structures in the test section. Finally, combined with the monitoring results of vertical soil displacement, the reliability of the shield tunnelling parameters is verified. After the completion of shield tunnelling in the test section, there is no void behind the shield tunnel wall, and the monitoring results of the vault settlement and segment posture in the tunnel meet the control values in the specification, which further support the rationality of the tunnelling parameters. In the process of shield tunnelling the undercross of the existing high-speed railway tunnel, the monitoring results of overall tunnel settlement, bottom uplift point settlement, track settlement, and track height difference meet the control values in the specification, which effectively avoids the adverse impacts of the shield tunnelling undercross of the existing high-speed railway tunnel and ensures the safety of the existing tunnel.

Forecasting maximum surface settlement caused by urban tunneling

In this article, maximum surface settlement (MSS) of urban tunnels was investigated on the basis of three operational parameters of tunnel width, tunnel depth, excavation method, as well as three soil parameters of cohesion, friction angle and elasticity modulus. Seven intelligent methods of long short-term memory (LSTM), deep neural networks (DNNs), K-nearest neighbor (KNN), Gaussian process regression (GPR), support vector regression (SVR), decision tree (DT), and linear regression (LR) were used to perform investigation. The intelligent methods were studied on the basis of 300 datasets accessed from 8 urban tunnels in Iran. Two cross-validation methods of hold-out and 5-fold were utilized for analyzing the prediction results. Finally, the DNNs method with R2 = 0.9939 and RMSE = 3.396301689 mm in the hold-out cross-validation mode and R2 = 0.9937 and RMSE = 2.199337605 mm in the 5-fold cross-validation mode, was recommended and suggested as the best prediction method for MSS.

New Method of Monitoring Tunnel Feet-Lock Pipe (TFP) Mechanics Using Fiber Bragg Grating (FBG)

Abstract Tunnel feet-lock pipe (TFP) is one of the most effective measures used to control tunnel settlement and is widely used in tunneling with soft ground or weak rock mass. Mechanical analysis is an important method to optimize TFP design and construction, and its results can be used to evaluate the structural stability of the tunnel. Although the traditional direct resistance method (DRM) can measure the mechanics of TFP, it is not suitable for harsh environments (humid environments, chemical etching, and electromagnetic field noise) or long-term monitoring. In this article, a method to measure the mechanics of TFP by using Fiber Bragg grating (FBG) is proposed. The strain and bending of TFP monitored by FBG is used to analyze the mechanics of TFP. In order to verify the feasibility and accuracy of the FBG method to measure the mechanics of TFP, an experimental scheme was designed to compare DRM and theoretical analysis. A supporting device and loading device were developed for the experimental scheme. Compared with the theoretical value, the error of strain and bending measured by FBG was stable within 5 με, the error of strain measured by DRM was approximately 15 με or even 20–30 με, and the error of bending measured by DRM was 10–15 με or even 25 με. Therefore, the precision of measuring the mechanics of TFP by using FBG is higher than by using DRM.