Resilience-based design for subway shield tunnels in soft soils: a perspective from the tunnel–soil system

Despite the fact that shallow tunnels have the benefits of low short-term construction costs and long-term operational costs primarily due to the shallow depth of the station boxes, the limited understanding of shallow tunnelling in soft soils is an obstacle to the development of shallow tunnels in urban areas. This study carries out a theoretical investigation of the effects of reducing the cover-to-diameter ratio C/D for shallow tunnels in soft soils. In stability analysis, the uplift, face stability and blow-out mechanisms are investigated. This study investigates interactions between the TBM and surrounding soil in tunnelling process, the stability of the TBM is not taken into account. The relationship between the C/D ratio and the required thickness-to-diameter ratio d/D as well as the required support pressures will be derived in various soils. Ranges of support pressures are also estimated for the TBM. Structural analysis is carried out for the variation of deformations and internal forces of the tunnel lining when reducing the C/D ratio. Since the conventional design models are not suitable in the case of shallow tunnels a new structural analysis model, which includes the difference between loads at the top and at the bottom of the tunnel, is proposed. Optimal C/D ratios with various d/D ratios for shallow tunnels in soft soils are also derived. With respect to ground movement analysis, this research investigates the areas affected by shallow tunnelling with a preliminary assessment of the risk of building damage by investigating surface and subsurface soil displacements. These areas are determined for different tunnel diameters in various soil types and are then compared to recent studies. The total volume loss is estimated at the tunnelling face, along the TBM, at the tail and includes long-term consolidation settlements. By combining empirical models from the literature and the proposed new models, the volume loss components are estimated both for short-term construction and for the long-term consolidation effects. This shows that a no volume loss is feasible in shallow tunnelling with careful control of the support pressure. The boundaries of the influence zones in shallow tunnelling are identified and discussed on the basis of various case studies. The effects of the soil parameters on the influence areas are also investigated. From these calculations, the limits and optimal C/D ratios for shallow tunnelling are deduced and recommendations and solutions for improving the shallow tunnelling process are proposed in this dissertation.

Deformation Response Induced by Surcharge Loading above Shallow Shield Tunnels in Soft Soil

For in‐service metro shield tunnels in soft soils, large circular deformations are a major concern because they usually lead to various problems, such as water leakage, joint openings, and concrete cracks. However, the monitoring of circular deformation depends mainly on manual surveying, and the automatic monitoring methods developed in recent years generally have low economic applicability and are not widely implemented. In this study, an automatic and cost‐effective system was presented to monitor circular deformation in shield tunnels by using only inclinometers. Experiments were conducted to prove the assumption that each segment can be regarded as a rigid body and to investigate the position of the joint rotation center. Then, a method for monitoring circular deformation based on the rigid body and plane section assumptions was proposed. The joint opening angle, maximum joint opening width, horizontal diameter convergence, and bolt strain were calculated from rotation angles of segments which can be monitored directly by inclinometer. A case study was conducted for a section of a metro shield tunnel with an ongoing pit excavation nearby. The rotation of segments was measured using MEMS inclinometers, and the data were transmitted using ZigBee and general packet radio service (GPRS) wireless communication technology. Results show that the proposed system could be implemented to improve transportation safety in relevant situations and similar conditions.

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.

Decision method on optimal time of preventive maintenance for metro shield tunnels in soft soils

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

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.

The impact of shallow cover on stability when tunnelling in soft soils

Deformation in settlement and grouting remediation of thickened larger-diameter metro shield tunnel in soft soil: A case study

<p>In recent years, China's construction demand for shield tunnel in soft soil continues to increase, and the control of ground settlement in tunnel boring process affects the safety of the tunnel itself and its superstructure directly. Paying close attention to controlling the strata loss and the ground settlement by multiple means is important to ensure construction safety. In this paper, the intelligent real-time monitoring system with dual-frequency ground penetrating radar (GPR) is used to detect the quality of back-fill grouting of shield tunnel, while monitoring points are arranged on the ground surface to acquire the settlement values in real time. The collaborative analysis of ground and underground monitoring results reveals the relationship between grouting and settlement values, and realizes the dynamic guidance on grouting operation, which helps to achieve the purpose of controlling ground settlement better. Last but not least, this paper proposes an outlook on a multiple-data fusion system based on cloud computing platform to adapt to more complex and multiple data in the future, so as to achieve the higher accuracy, efficiency and intelligence of monitoring data analysis.</p>

Behavior of shallow tunnel in soft soil under seismic conditions

A short but descriptive paper has been presented to understand various researches that have been made recently on shallow tunnelling in soft soils and the challenges faced during the construction. In a smart city, smart means of communication is vital, which should be safe and green mode of transportation. Due to the scarcity of space on the land surface of the urban and developed areas, there has been an increase of demand in the extensive use of underground structures. These constructions are often situated at shallow depths. Hence, there arises the demand for smart tunnels which requires a better understanding of various factors which influence the response of a shallow tunnel in soft soils is highly required. This understanding is enhanced with the help of the previous research papers on this specific issue which are a reliable and trustworthy source of information. Therefore, the sole purpose of this paper is to analyse various research works that have been accomplished in the field of shallow tunnelling in case of soft soils.

Improper synchronous grouting materials and construction parameters may cause the shield segments to float, resulting in tunnel dislocation, open joints, uneven deformation, and water leakage. This paper conducts tests on shield synchronous grout performance and presents a multi-objective optimization method for grout performance. This method considers initial setting time, shear yield strength, early compressive strength, and density as optimization performance indicators. An optimized grouting ratio suitable for tunnel anti-floating is recommended and verified. The corresponding construction parameters matching the grout performance are also discussed using numerical simulation methods. Results indicate that the performance of synchronous grout and grouting pressure, distribution, and shield advancing speed have the most significant impact on tunnel stability. A synchronous grout with ratios of water-cement, glue-sand, bentonite-water, cement-fly ash, and additive-glue of 0.602, 0.613, 0.267, 0.733, and 0.010 can obtain a shorter initial setting time and higher shear and compressive strength. Accordingly, the shield tunneling speed should not exceed 83.28 min/ring, ensuring the liquid grout length is regulated at 5 rings. Alternatively, grouting should be limited to the top portion of the segment, or additional grouting holes at high positions should be installed. The related studies can offer recommendations for shield tunnel construction in soft strata.

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.

In this research, typical deep circular tunnel sections in soft soils of Shanghai city are used as case studies to reveal the time-dependent fragility functions for deep circular tunnels in soft soils. The seismic performance of the tunnel lining is obtained based on a series of nonlinear dynamic analyses of the soil-tunnel system considering the effect of corrosion on the lining reinforcement. The aging effect due to corrosion of the reinforcement is taken into account by reducing the strength properties of the tunnel lining. Time-dependent fragility curves as a function of free-field Peak Ground Velocity (PGV) and service time t, are developed for minor, moderate and extensive damage states. Results show an overall increase of the seismic fragility for the deep tunnels over time, highlighting the important influence of aging effects on the performance of tunnels. The findings of this study provide an improved understanding of the response of tunnels in soft soils exposed to diverse hazards, and hence, facilitate the life-cycle seismic risk assessment and resilient designs of transport infrastructure.