Analysis of ground collapse caused by shield tunnelling and the evaluation of the reinforcement effect on a sand stratum

doi:10.1016/j.engfailanal.2020.104616

Engineering Failure Analysis

Volume 115, September 2020, 104616

https://doi.org/10.1016/j.engfailanal.2020.104616 Get rights and content

Highlights

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Four reasons for the collapse of sandy ground by shield tunnelling are analyzed.
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The stratum before and after reinforcement have been evaluated.
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The tunnelling parameters before and after reinforcement have been evaluated.

Abstract

Based on laboratory and field tests for a cross-river highway shield tunnel in China, the causes of ground collapse by shield tunnelling in a water-rich fine sand stratum were analyzed. To ensure the safety of subsequent shield tunnelling, reinforcement treatment was carried out, and then the reinforcement effect was comprehensively evaluated. The results showed that the fine sand properties, slurry circulation quality, support pressure set and excess pore water pressure fluctuation are the main reasons for the collapse of sandy ground. The comparative analysis of the permeability coefficient of the stratum and tunnelling parameters of the shield indicated that the grouting reinforcement effect on the fine sand stratum was obvious and that the disturbance to the stratum can be effectively controlled.

Introduction

During the construction of a cross-river shield tunnel, the soil above the tunnel in the floodplain near the river usually features a shallow water table in sandy stratum [1]. Compared with clayey stratum, the mechanical properties of the sandy stratum are more complex and less stable: (1) The strain localization. The strain localization on a shear band induced by postpeak softening is one of the most important deformations and strength characteristics of sand [2], which was verified by triaxial tests [3], [4], [5]. (2) Seepage mechanism. The model tests in a centrifuge [6] and three-dimensional stress-pore pressure coupled numerical simulation [7] showed that the seepage in sandy soil is one of the most important factors leading to ground instability. In addition, the water content in a sandy soil had a significant effect on both the profile and magnitude of the settlement of the ground surface [8].

Many scholars had performed much research on ground settlement caused by shield tunnelling, which can be summarized as empirical formula, theoretical predictions, model tests and numerical analyses. By analyzing a large number of measured data, Peck proposed a calculation formula for predicting surface settlement [9]. Many scholars had modified this formula to adapt to different geological conditions, including sandy soil [10], [11], [12]. According to the different deformation characteristics of soil, different theoretical analyses, such as elasticity, elastic–plastic and viscoelastic-plastic methods, were adopted to study soil deformation caused by shield tunnelling [13], [14], [15], [16], [17], [18]. Due to the variability in construction conditions and the complexity of soil conditions, there is no very accurate theoretical solution to date. Using the centrifuge model test, the ground settlement caused by shield tunnelling can also be simulated and predicted [19], [20], [21]. However, due to its high cost and limited applicability, the development of this method is limited.

The numerical simulation can take the types of influencing factors into full consideration and reflect the nonlinear characteristics of the soil, making the calculated results more accurately. Recently, the finite element method has been developed from simple stress fields to multifield coupling simulations [22], [23], [24]. When shield tunnelling in water-rich sand stratum, the variation in the pore water pressure will influence the seepage state and change the physical and mechanical properties of the soil [25]. Instability is easily generated during excavation, and even ground collapses may occur [26]. The finite element method was utilized to calculate the required support pressure and the seepage force when groundwater flows toward the tunnel excavation face. [27]. Compared with the dry formation, the support pressure required for tunnelling in the water-rich sand stratum increased remarkably.

To ensure the safety of shield tunnelling in shallow overlaying water-rich sand stratum, an effective method to prereinforce the stratum is by grouting [28], including reinforcement from the ground surface [29] and from the tunnel [30], which can effectively improve the shear resistance and bearing capacity of the original sand layer. The pregrouting is also of paramount importance in controlling groundwater inflow in tunnelling situations in highly permeable water-bearing ground. However, there are few researches results on how to evaluate the effect of the grouting reinforcement.

The studies above mostly analyze the ground settlement caused by shield tunnelling from a single factor or a few factors. However, the settlement is the result of comprehensive action, and affected by various construction aspects. Based on laboratory and field tests for a cross-river highway shield tunnel in China, the controlling construction aspects are discussed, and the effect of fine sand stratum grouting reinforcement is comprehensively evaluated by comparative analysis of the permeability coefficient of the stratum and tunnelling parameters of the shield.

Section snippets

Project overviews

The cross-river tunnel project includes two separate tunnels; the northern tunnel is 1615 m long, and the southern tunnel is 1423 m long. Shield tunnelling starts from the launching shaft on the east bank of the river and reaches the receiving shaft on the west bank after crossing the river. A single-lining type tunnel is adopted with a 11.3 m outer diameter, 10.3 m inner diameter, 50 cm thickness and 2.0 m ring width of the segments. The shield tunnel is mainly located in the stratum of a

Physical and mechanical properties of the fine sand

The main cause of sand layer collapses is the low cohesive and weak cementation ability, which leads to the loss of clay particles with the flow of groundwater and weaken the sand layer until it collapses [31]. Four sandy soil samples taken from the collapse location were used to perform physical and mechanical tests, including specific gravity tests, grain subdivision tests, void ratio tests, direct shear tests and triaxial compression tests. The grain size distribution, natural void ratio,

Reinforcement for fine sand stratum before shield tunnelling

As no other structures to be protected located nearby, there was no plan to reinforce the strata at the flood plain for both northern and southern tunnel at the preliminary design stage. After the collapse occurred in front of the northern tunnel, dense sandy soil mixed with slurry was used to backfill the collapse cavity, and compaction was carried out after backfilling to improve the self-stability of the backfilled soil. However, large settlement still occurred in the subsequent tunnelling

Evaluation of the grouting reinforcement effect on the fine sand stratum

The grouting reinforcement effect refers to the strength improvement in the composite soil. After grouting, the engineering characteristics of the fine sand should be improved: the void ratio and permeability coefficient should be reduced, the compactness, cohesion and internal friction angle of the ground soil should be increased. The permeability is directly related to the compressive strength and pore water pressure of the soil, the decrease of the soil permeability is an important

Conclusions

Based on a cross-river shield tunnel project in China, the causes of ground collapse by shield tunnelling in a water-rich fine sand stratum are analyzed, with laboratory tests, site investigation, field monitoring and numerical method. Properties of the fine sand stratum, slurry circulation quality, support pressure and pore water pressure fluctuation are considered as the main reason for this collapse. To ensure the safety of subsequent shield tunnelling, reinforcement treatment is carried

Declaration of Competing Interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgments

This work was sponsored by the National Natural Science Foundation of China, China (No. 51708564, 51678578, 51978677), China Postdoctoral Science Foundation, China (No. 2019T120768, 2018M633223), Guangzhou Municipal Science and Technology Project, China (No. 201804010107), Guangdong Basic and Applied Basic Research Foundation, China (2020A1515011271). The authors are grateful to these institutions for their support.

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The interaction between closely-spaced twin tunnels has emerged as a prominent topic in tunnelling engineering, especially under complicated geological conditions like high water pressure and deep burial depth. This paper proposes a novel monitoring technology that utilizes MEMS multi-parameter fusion sensors in conjunction with wireless signal transmission technology to track the structural response of the existing tunnel during the construction of closely-spaced cross-river twin tunnels. The experiment enabled prompt deployment of monitoring devices without disrupting the construction process, and the investigation meticulously monitored the additional stresses, tilt angles, and relative displacement on the former tunnel lining. The results indicate that: (1) The progression of the additional stresses in former tunnel is classified into five stages, corresponding to the advancement of later tunnel's construction and dependent on the position of its heading face. (2) The intensity and spatial distribution of additional axial and circumferential stresses are non-uniform and depend on the proximity to the later tunnel. (3) The degree of change in the tilt angle is directly proportional to the distance from the later tunnel. (4) The deformation of the circumferential joint exhibits oblique uphill deformation with upward deflection at the lower-middle segment and downward deflection at the top, elliptical deformation with radial convergence and vertical expansion, and nearly no rotational deformation in the circumferential direction. The deformation of the longitudinal joint is manifested as an upward displacement of the upper segment blocks in the excavation direction, with radial convergence and leftward horizontal displacement, resulting in circumferential compression. (5) There is a remarkable correlation between the additional stress, tilt angle, and deformation characteristics, with the three factors changing in concert. (6) Secondary grouting exerts a significant influence on the stress and deformation of segments, and despite being localized, it may cause additional damage to the tunnel structure. This paper provides a dependable and efficient approach to monitoring the performance of twin tunnels in complex geological situations. Furthermore, the results highlight the interaction mechanism between twin tunnels and the combined effect of structural response, providing guidance for similar projects.
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View full text

Analysis of ground collapse caused by shield tunnelling and the evaluation of the reinforcement effect on a sand stratum

Highlights

Abstract

Introduction

Section snippets

Project overviews

Physical and mechanical properties of the fine sand

Reinforcement for fine sand stratum before shield tunnelling

Evaluation of the grouting reinforcement effect on the fine sand stratum

Conclusions

Declaration of Competing Interest

Acknowledgments

Tunn. Undergr. Space Technol.

Soils Found.

Sust. Cities Soc.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Comput. Geotech.

Tunn. Undergr. Space Technol.

J. Clean Prod.

Tunn. Undergr. Space Technol.

Strain softening and shear band formation of sand in multi-axial testing

Geotechnique

Behaviour of sands under generalized drainage boundary conditions

Can. Geotech. J.

An experimental study of failure and softening in sand under three-dimensional stress condition

Granul. Matter

Ground movement and tunnel stability when tunnelling in sandy ground

J. Chin. Inst. Eng.

Tunnelling-induced ground settlements in a groundwater drawdown environment – A case history

Tunn. Undergr. Space Technol.

Deep excavations and tunnelling in soft ground

Analysis of undrained soil for tunnelling-induced ground movements in clays

J. Geotech. Eng.

Investigation of deformation and collapse behavior of circular lined tunnels in centrifuge model tests

Can. Geotech. J.