Tunnels In Soft Soils Research Articles

Probabilistic Seismic Risk Assessment of Metro Tunnels in Soft Soils

Tunnels are of significant importance in the sustainable development of global urban areas, particularly in metropolitan areas. It is of the utmost importance to evaluate the seismic performance of tunnels across a wide spectrum of earthquake intensities. In order to address this, our study presents a framework for the assessment of seismic risk in tunnels. This study employs the city of Shanghai’s urban metro tunnels as case studies. The nominal values of seismic risk for the three main damage states—minor, moderate, and major—were calculated. Furthermore, the influence of utilizing disparate fragility functions on expected seismic risk assessments was investigated. In this framework, the probability density functions of the different fragility curve models are employed to treat the probability values associated with them as random variables. This approach aims to facilitate the propagation of IMV in seismic risk assessments. The results demonstrate that the Bayesian framework efficiently incorporates the full range of input model variability into risk estimation. The findings of this study offer a foundation for decision-making processes, seismic risk assessments, and the resilience management of urban infrastructure.

Study on the Effect of Burial Depth on Selection of Optimal Intensity Measures for Advanced Fragility Analysis of Horseshoe-Shaped Tunnels in Soft Soil

Seismic intensity measures (IMs) can directly affect the seismic risk assessment and the response characteristics of underground structures, especially when considering the key variable of burial depth. This means that the optimal seismic IMs must be selected to match the underground structure under different buried depth conditions. In the field of seismic engineering design, peak ground acceleration (PGA) is widely recognized as the optimal IM, especially in the seismic design code for aboveground structures. However, for the seismic evaluation of underground structures, the applicability and effectiveness still face certain doubts and discussions. In addition, the adverse effects of earthquakes on tunnels in soft soil are particularly prominent. This study aims to determine the optimal IMs applicable to different burial depths for horseshoe-shaped tunnels in soft soil using a nonlinear dynamic time history analysis method, and based on this, establish the seismic fragility curves that can accurately predict the probability of tunnel damage. The nonlinear finite element analysis model for the soil–tunnel interaction system was established. The effects of different burial depths on damage to horseshoe-shaped tunnels in soft soil were systematically studied. By adopting the incremental dynamic analysis (IDA) method and assessing the correlation, efficiency, practicality, and proficiency of the potential IMs, the optimal IMs were determined. The analysis indicates that PGA emerges as the optimal IM for shallow tunnels, whereas peak ground velocity (PGV) stands as the optimal IM for medium-depth tunnels. Furthermore, for deep tunnels, velocity spectral intensity (VSI) emerges as the optimal IM. Finally, the seismic fragility curves for horseshoe-shaped tunnels in soft soil were built. The proposed fragility curves can provide a quantitative tool for evaluating seismic disaster risk, and are of great significance for improving the overall seismic resistance and disaster resilience of society.

Model tests and numerical simulations of deformation repair effect for operating shield tunnels under horizontal lateral grouting

To address the large deformation problem of operating straight-joint assembled shield tunnels in soft soil, horizontal lateral grouting is often used. Using an independently designed and developed lateral grouting test device, a horizontal lateral grouting model test was conducted to study the tunnel force state and deformation repair law under the action of lateral grouting. The mechanical responses of tunnel under single-hole grouting (SHG) and two double-holes grouting (DHG) cases were comparatively analyzed, and the test results were further validated via numerical simulations. The results indicate that several key indicators of tunnel behavior, including additional pressure, bending moment, radial displacement, convergence deformation, and ellipticity, exhibit a positive correlation with the grouting volume and a negative correlation with the distance to the grouting point when implementing SHG. Expect for radial displacement, all of these metrics caused by DHG were greater than those caused by SHG. The horizontal radial displacement increases under same-side DHG but decreases under different-side DHG. Interestingly, same-side DHG is more favorable for reducing internal force and repairing tunnel deformation, and effectively improves tunnel differentiated deformation state along the longitudinal direction. However, different-side DHG can provide a better effect for transverse convergence deformation repair under the premise of minimizing tunnel overall displacement.

Centrifugal modeling on influence of superstructure construction of high-rise building on adjacent subway tunnel

With the urban development in China, high-rise buildings must be constructed adjacent to subway tunnels. The superstructure construction process directly affects adjacent tunnels. However, only a few studies have focused on this topic. In this study, a series of centrifugal model tests were conducted to investigate the impact of the superstructure construction process of high-rise buildings on the deformation of adjacent tunnels in soft soil. Three model tests were designed, considering the buried depth of the tunnel. A loading system was implemented to simulate the superstructure construction process with a centrifugal acceleration of 100 g. The results showed that the superstructure construction caused the adjacent tunnel to deform vertically and laterally owing to its downward dragging effect. The vertical deformation of the tunnel was approximately thrice the lateral displacement. As the buried depth increased, the superstructure construction changed the transverse deformation of the tunnel section from vertical to horizontal tension. The outcomes of the study will provide guidelines for the safe operation of subways.

Seismic performance and vulnerability analysis for bifurcated tunnels in soft soil

This paper investigates the seismic performance of complex comprehensive interchange underground structures. As an example, the seismic performance of Nanjing underground bifurcated tunnels constructed in soft soil is analyzed. A three-dimensional (3D) numerical finite element analysis model is established considering soil-tunnel interaction to analyze the seismic response characteristics and failure mechanism of the bifurcated tunnels. The results show that the tunnel damage degree is closely associated with the deformation response, and that the inter-story drift ratio of the lower layer is significantly greater than that of the upper layer. The pushover analysis method is used to obtain the seismic performance curve of the tunnel, and inter-story drift ratio limits are defined. The IDA procedure is then used to conduct multiple non-linear dynamic time history analyses based on the 3D finite element model, and IDA curve clusters are drawn with peak ground acceleration (PGA) as the earthquake intensity measures and the peak inter-story drift ratio as the damage measures of tunnels. The corresponding IDA percentiles lines are calculated, and the seismic fragility curves of the bifurcated tunnels are established. The post-earthquake functional failure probability of the bifurcated tunnels is obtained under different seismic fortification performance level.

Bayesian-based seismic risk assessment of shallow tunnels in soft soils

Bayesian-based seismic risk assessment of shallow tunnels in soft soils

Resilience of metro tunnel structures: monitoring deformation due to surrounding engineering activities and effect of remediation treatment

Long term performance of urban metro tunnels requires that the tunnel structure maintains not only a sound structural state, i.e., free from significant damages, but also a minimal change in the physical shape. Over-deformation can cause structural damage but more importantly can affect the safe operation of the metro lines, therefore is a primary target of monitoring and intervention where it becomes necessary. This paper presents an overview of recent developments in field monitoring and assessment of metro tunnels in soft soil concerning over-deformation. In particular, the over-deformation of shield tunnel rings due to nearby construction activities, especially deep excavation, and its rectification using grout treatment is discussed on the basis of relevant field experiment and numerical modelling reported in recent publications. Key considerations in the development of a Material Point Method (MPM) based numerical modelling framework for simulating explicitly the grouting operations are highlighted, and the general performance of the numerical model is examined with reference to the field test data and characteristic parametric observations. Further research needs in terms of more rigorous assessment and design of grouting treatment, in conjunction with the continued development of numerical modelling capabilities, are discussed.

The influence of pipe-jacking tunneling on deformation of existing tunnels in soft soils and the effectiveness of protection measures

Great attention must be paid to the safety and serviceability of existing tunnels when new tunnels are constructed nearby in soft soils. In this study, the influence of pipe-jacking construction on the Ningbo Metro Line 3 was investigated by field monitoring and numerical analysis. A Burger-creep visco-plastic (CVISC) model was used in the numerical analysis to examine the long-term deformation of the existing tunnels. The effectiveness of protection measures and improvement parameters were investigated. The monitoring results show that the deformation of existing tunnels caused by the jacking can be divided into four stages: (I) initial subsidence; (II) rapid rise; (III) deformation fluctuation, and (IV) slow rise. Compared with the Mohr-Coulomb model, the results of the CVISC model are more close to the monitored data. The numerical simulation shows that the maximum deformation of the existing tunnel is reduced by 11.66 and 8.07 mm under the protection measure of ground improvement and surcharge loading, respectively. The deformation of existing tunnel decreases as the elastic modulus and area of ground improvement increase. This study has reference value for the protection of existing tunnels in soft soils during adjacent construction.

Computer Vision-based Monitoring Method for Differential Settlement of Shield Tunnels

Shield tunnels in soft soils are inevitably subject to large differential settlement during construction and long-term operation, which can lead to various diseases and then impair structural safety and durability. Due to the inefficiency of traditional inspection methods, timely and accurate monitoring methods are necessary to ensure the safe operation and long-term maintenance of tunnels. In this study, a computer vision-based monitoring method for differential settlement of shield tunnels was developed and implemented. The monitoring area is located in an interval tunnel of a subway line in Shanghai during the freezing construction of its side channel. The influence of camera position’s micro-change was considered. The field equipment mainly consists of high-resolution industrial CMOS (complementary metal oxide semiconductor) digital camera sets, LED (light emitting diode) lights and power modules. The differential settlement relative to datum points are obtained and vertical displacement of one ring acquired by total station is compared with the proposed method to verify the feasibility of the monitoring system. The results indicate that the monitoring system is reliable and can be further applied.

Ground deformation induced by shield tunneling posture in soft soil

Constructing tunnels in soft soil using shield machines would induce soil deformation. Several factors, such as support pressure, ground loss, grouting pressure may affect the soil deformation. In engineering practice, the ground deformation could also vary with the posture of the shield machine. This study proposed an approach for estimating the soil deformation considering shield posture. Firstly, the shield-soil interaction displacement was obtained by considering shield geometry and its kinematic behavior, such as shield diameter and length, articulation, and shield deviation. Secondly, based on the elastic half-space theory, a calculation method for mapping the shield-soil interaction displacement to soil deformation was established. Finally, the proposed model was verified by the field measurement in the tunnel of Suzhou Metro line S1, and the results confirm that the postures of the shield machine in the soil influence the magnitude, distribution, and evolution of the soil deformation.

A compact blowout model for shallow tunnelling in soft soils

Tunnelling in soft soil conditions, especially with a shallow overburden, faces the risk of face instability due to blowout. Although several blowout models have been proposed to estimate the blowout pressure, mostly based on limit analysis or limit equilibrium, there is a significant gap between the allowable blowout pressures predicted by these models and the values observed in case studies, laboratory experiments and numerical simulations. This paper proposes a compact blowout model, which is more compact compared to a model proposed by Balthaus (1991). This new blowout model is able to predict blowout pressures more closely to the value observed by centrifuge testing, reduced scale experiments and case studies, whilst staying conservative. Its application on the Hochiminh Metroline No. 1 project in this study resulted in a smaller support pressure in the boring stage to avoid the occurrence of a blowout.

Research on Strata Deformation Induced by EPB Tunneling in Round Gravel Stratum and Its Control Technology

Recently, many studies have been conducted on the stratum deformation induced by earth pressure balance (EPB) shield tunneling in soft soil and sand. Movement laws vary largely among different strata. However, at present, relevant research mainly focuses on soft soil and sand, whereas little attention has been paid to the movement law of round gravel stratum with higher instability. In this study, a field monitoring test was carried out on the EPB shield machine when it passes through the round gravel stratum. Based on the analysis of monitoring results under different chamber earth pressures, thrust force, the torque of the cutter, grouting pressure, and grouting volume, the relationships between shield tunneling parameters and their influence on the disturbance of the surrounding soil mass were investigated. It was found that the surface deformation shape of the monitoring section of the south and north lines conforms to the Gaussian curve. The vertical deformation of the stratum at the tunnel axis is the largest. The maximum value is observed when the cutter head reaches the monitoring section. The horizontal deformation reaches a maximum value at the stage of the shield tail pass section. The strata deformation is not only related to the strata properties but also has a strong positive correlation with the shield tunneling parameters. The chamber earth pressure is the main factor affecting the stratum deformation before the arrival of the cutter head, and the grouting volume is the main factor affecting the strata deformation during the stage of the shield tail pass section.

Investigation of the pressure distributions around quasi-rectangular shield tunnels in soft soils with a shallow overburden: A field study

The magnitudes and distributions of the pressures around a tunnel lining are the most significant variables in shield tunnel design, as they determine the safety of the lining structure and influence the investment cost of the tunnel project. The grouting process has a negligible effect on the pressures exerted on the shield tunnel segmental linings when the boring machine is drilling. However, improper grouting control might result in large bending moments, local damage of segments and leakage at joints, leading to a reduction of the linings’ safety, durability and serviceability. On the other hand, more and more special-section shield tunnels are developed to solve the problems associated with infrastructure construction under complex environmental conditions. However, previous studies on the pressure distribution around tunnel linings all focused on circular tunnels. Few research about the loads during construction on special-section shield tunnels was conducted. In this study, three in-situ monitoring tests for the total pressure distributions of shallow buried quasi-rectangular tunnels in soft soils were carried out, which is deemed to be the first field study for special-section shield tunnels. Different grouting situations were adopted and the temporal and spatial distributions of the lining pressures during and after the construction were obtained. When the lining segments are pushed out of the shield tail, the adverse pressure distributions caused by grout ejections were observed which are typical non-uniform and unsymmetrical. The effect of different grouting situations attenuates quickly in the longitudinal direction within a range of 9 rings. Additionally, the influence of the following back-up equipment and transportation carriages on the pressures outside the linings were first observed and studied by means of these field tests. The outside pressures fluctuated in different stages with the coincident pace of construction activities and performed as a counterforce and to resist the inside loads, which was different from the active loads exerted on the lining when the segmental rings were pushed out. From the long-term perspective, the pressure distributions in different tests tend to stabilize at a similar state. The influence of the grouting process or other construction loads on the lining pressures needed about 50 days to be relieved. A comparison between the measured results and the theoretical long-term pressures was conducted and the applicability of the proposed load distributions in the design model for quasi-rectangular tunnels was demonstrated. Additionally, a pressure mode combining cosinusoidal pressures and theoretical long-term pressures is proposed to predict the grouting pressures when lining segments are pushed out of the shield.

Modal analysis of subway tunnel in soft soil during operation

Modal parameters are of great significance in civil engineering because they can characterize the properties of structures and be used for vibration control and structural health monitoring. Subway tunnels are long linear truss structures combined with the mutual coupling of the surrounding soil. Therefore, the operational modal analysis of a mutual coupling tunnel is complicate, as is the modal identification of shield tunnels in a time–frequency domain, and these are hot civil engineering topics. Using the shield tunnel of Shanghai metro line No.12 project as a case study, we carried out the vibration response monitoring of a subway tunnel during operation and presented methods to identify structural modal parameters. The modal parameters of lower vibration modes were estimated using response measurements. Modal frequencies and shapes were identified with high precision and accuracy using the orthogonal polynomial clustering algorithm under hammer excitation conditions and the autoregressive-moving-average model under ambient excitation conditions. The dynamic behavior of a mutual coupling tunnel presented obvious low frequency characteristics, and the first 9th order mode frequencies were less than 100 Hz. The diagonal values of the modal assurance criteria were all greater than 0.85. The modal parameters can be used for the health monitoring of operational subway tunnels.

Dynamic Response of Underground Tunnel in Soft Soil under Surface and Subsurface Explosion

Dynamic Response of Underground Tunnel in Soft Soil under Surface and Subsurface Explosion

Intelligent Prediction of Multi-Factor-Oriented Ground Settlement During TBM Tunneling in Soft Soil

Tunneling-induced ground surface settlement is associated with many complex influencing factors. Beyond factors related to tunnel geometry and surrounding geological conditions, operational factors related to the shield machine are highly significant because of the complexity of shield-soil interactions. Distinguishing the most relevant factors can be very difficult, for all factors seem to affect tunneling-induced settlement to some degree, with none clearly the most influential. In this research, a machine learning method is adopted to intelligently select features related to tunneling-induced ground settlement based on measured data and form a robust non-parametric model with which to make a prediction. The recorded data from a real construction site were compiled and 12 features related to the operational factors were summarized. Using the intelligent method, two other features in addition to cover depth–pitching angle and rolling angle–were distinguished from among the 12 feature candidates as those most influencing the settlement trough. Another new finding is that advance rate does not emerge in the top 10 selected models from the observational data used. The generated non-parametric model was validated by comparing the measured data from the testing dataset and performance on a new dataset. Sensitivity analysis was conducted to evaluate the contribution of each factor. According to the results, engineers in general practice should attend closely to pitching angle during tunnel excavation in soft soil conditions.

Short-Term and Long-Term Displacement of Surface and Shield Tunnel in Soft Soil: Field Observations and Numerical Modeling

Constructing subways in soft soil strata by shields may cause large displacement of ground and tunnels during and after construction. To evaluate the corresponding short-term and long-term displacement, this paper presented a three-dimensional numerical model based on the project of Suzhou rail transit line S1. This model adopted the modified Cam-clay model to simulate the behavior of soft soil and can consider the grouting parameters and the water leakage of the assembled segment lining. In addition, an on-site monitoring project was implemented, and the field observations were compared with the simulation results. Finally, the sensitivity of key parameters was carried out by the established numerical model. The results indicated that the grouting volume, the thicknesses of soft soil under the tunnel, and the tunnel leakage conditions have a significant impact on the ground and tunnel settlement. Notably, serious tunnel leakage will cause large long-term consolidation settlement, and the increasing thicknesses of soft soil under the tunnel does not increase the long-term settlement of the stratum and the tunnel when the tunnel meets the secondary waterproof requirements, but does increase the corresponding short-term settlement.

Characterization of Underlying Twin Shield Tunnels Due to Foundation-Excavation Unloading in Soft Soils: An Experimental and Numerical Study

Excavation near or above existing shield tunnels often results in adverse impacts on tunnel stability. To ensure the serviceability of existing tunnels, this paper presents experimental and numerical studies with reference to a foundation pit case history excavated above twin-tube shield tunnels in soft soils. The experimental tests were firstly applied to study the deformation characteristics and structural response of the shield tunnels. Thereafter, an extensive numerical investigation was performed to determine the influence of some factors such as cover-to-excavation depth ratio, length-to-depth ratio, and unloading ratio on tunnel displacement behaviors. It was demonstrated that the tunnel heaves as the excavation proceeds, and heaves and horizontal displacements reach their maximum values when the excavation is finished. The earth pressure around the tunnels is symmetrically distributed in a gourd shape, with a larger reduction at the tunnel crown and invert and a smaller reduction at tunnel side walls. Additionally, the earth pressure at the tunnel crown and invert changes more significantly than that at other parts. The tunnel moment increment is significantly affected by the tunnel excavation depth. The axial force at or near the side walls of the tunnel is the most sensitive to the unloading effect induced by the excavation activity.

Fragility assessment of tunnels in soft soils using artificial neural networks

Recent earthquakes have shown that tunnels are prone to damage, posing a major threat to safety and having major cascading and socioeconomic impacts. Therefore, reliable models are needed for the seismic fragility assessment of underground structures and the quantitative evaluation of expected losses. Based on previous researches, this paper presented a probabilistic framework based on an artificial neural network (ANN), aiming at the development of fragility curves for circular tunnels in soft soils. Initially, a two-dimensional incremental dynamic analysis of the nonlinear soil-tunnel system was performed to estimate the response of the tunnel under ground shaking. The effects of soil-structure-interaction and the ground motion characteristics on the seismic response and the fragility of tunnels were adequately considered within the proposed framework. An ANN was employed to develop a probabilistic seismic demand model, and its results were compared with the traditional linear regression models. Fragility curves were generated for various damage states, accounting for the associated uncertainties. The results indicate that the proposed ANN-based probabilistic framework can results in reliable fragility models, having similar capabilities as the traditional approaches, and a lower computational cost is required. The proposed fragility models can be adopted for the risk analysis of typical circular tunnel in soft soils subjected to seismic loading, and they are expected to facilitate decision-making and risk management toward more resilient transport infrastructure.

Protective effect of partition excavations of a large-deep foundation pit on adjacent tunnels in soft soils: a case study

The safety of metro tunnels is seriously threatened when protective measures are not well organized during adjacent foundation pit excavations in soft ground. To date, the protective effects obtained from adopting partition excavations and a deep diaphragm wall in large-deep excavations have not been fully understood. In this study, 3D numerical analyses incorporating the hypoplastic constitutive model, which can take into account the small strain stiffness of soil, were performed to reproduce tunnel responses subjected to lateral excavations. The mechanism of crack development in tunnel lining, the protective effects of partition excavations, and diaphragm walls are investigated in sequence. The obtained results indicate that crack development in tunnel lining is mainly dominated by lining deformation in the longitudinal direction, instead of deformations in the radial and circumferential directions. Through firstly excavating the nearest excavation to the tunnel that is small in size, the tunnel displacement and bending moment are reduced, since the system rigidity of the nearest excavation increases significantly after applying supporting structures. By contrast, such reduction is found to be detrimental when the nearest excavation is large in size and is performed at the end. This indicates that a comprehensive consideration of both the order and the size of partition excavations is important. The inhibiting effect on the tunnel displacement caused by increasing the depth of the diaphragm wall is found to function nonlinearly, depending highly on its referred depth. Additionally, this effect is more effective than increasing the width of the diaphragm wall under constant volume conditions.