Settlement and load transfer mechanism of pile group due to side-by-side twin tunnelling

doi:10.1016/j.compgeo.2014.10.007

Computers and Geotechnics

Volume 64, March 2015, Pages 105-119

https://doi.org/10.1016/j.compgeo.2014.10.007 Get rights and content

Abstract

Development of underground transportation systems often involve twin tunnels, which may encounter existing pile groups during construction. Since many previous studies mainly focus on the effects of single tunnelling on single piles, settlement and load transfer mechanism of a pile group subjected to twin tunnelling are not well investigated and understood. To address these two issues, two three-dimensional centrifuge tests were carried out in this study to simulate side-by-side twin tunnels (excavated one after the other on both sides of the pile group) at two critical locations relative to the pile group, namely next to (Test TT) and below the toe of the pile group (Test BB). Moreover, numerical back-analyses of the centrifuge tests are conducted by using a hypoplastic model, which takes small-strain stiffness into account. Both measured and computed results show that the induced tilting of the pile group in Test TT is significantly larger than that in Test BB, with a maximum percentage difference of 120%. On the other hand, a slightly smaller (about 13%) settlement of the pile group is induced in Test TT, as compared to that in Test BB. This is because the pile group in Test TT is partially located within the major influence zone of tunnelling-induced ground settlement while the entire pile group in Test BB is bounded by the major influence zone of ground settlement. Two distinct load transfer mechanisms due to twin tunnelling are identified, i.e., the load in the pile group in Test TT transfers downwards from the pile shaft to the pile toe while the load in the pile group in Test BB transfers upwards from the pile toe to the pile shaft. Apart from load transfer along each pile, load re-distribution also occurs among piles during twin tunnelling. In both Tests TT and BB, axial load at pile head only reduces at a pile closet to the advancing tunnel face and the reduction is re-distributed to the other three piles. The load re-distribution among piles results in a maximum increase of axial force of 10% in Test TT.

Introduction

Tunnelling is an effective means to meet the rapidly increasing traffic demand in congested cities. Construction of tunnels is likely to encounter existing underground structures such as pile foundations. To understand influence of tunnelling on existing piles, a number of studies have been carried out based on field monitoring [39], centrifuge modelling [2], [15], [18], [23], [26], [27], numerical analysis and analytical analysis [4], [5], [12], [19], [20], [21], [28], [29], [41].

In spite of the numerous previous investigations on this subject, little attention was paid to twin tunnelling effects on pile groups, except the research work reported by Pang et al. [39]. They reported a field and numerical investigation on an existing pile group subjected to side-by-side twin tunnelling near its middle depth in Singapore clay. Ground settlement, distribution of axial force and bending moment in piles at the end of the first tunnelling were reported. However, tunnelling-induced pile responses such as settlement of the pile group, load transfer along each pile and load re-distribution among piles have not been the focus of their study. Ng et al. [36] investigated the responses of a pile group subjected to piggyback (i.e., vertically aligned) twin tunnelling based on centrifuge and numerical investigation. Compared to the piggyback configuration, it appears that the side-by-side (i.e., horizontally vertically aligned) twin tunnelling is more frequently encountered in practical engineering [11], [16], [31], [35], [37].

In view of the aforementioned issues, this study aims at investigating settlement and load transfer mechanisms of a pile group subjected to twin tunnelling. To achieve this objective, two three-dimensional centrifuge experiments were carried out to simulate in-flight advancement of side-by-side twin tunnels (one after the other) on both sides of an axially load 2 × 2 pile group in dry sand. The only variable that differed between the two tests was buried depth of the twin tunnels, which were located either next to (Test TT) or below the toe (Test BB) of the pile group. The centrifuge tests were back-analysed by three-dimensional finite element analyses, in which a hypoplastic constitutive soil model was employed. Measured and computed results are compared and interpreted, with particular attention paid to settlement and load transfer mechanism of the pile group due to twin tunnelling.

Section snippets

Test programme and setup

Fig. 1 illustrates schematic diagrams (elevation views) of the two centrifuge tests (i.e., Tests TT and BB). The model container had a plan dimension of 1250 mm × 1250 mm (i.e., 50 m × 50 m in prototype scale) and a depth of 850 mm (i.e., 34 m in prototype scale). The two tests were carried out at the Geotechnical Centrifuge Facility of the Hong Kong University of Science and Technology [34], at a centrifugal acceleration of 40g. Scaling factors between the model and the prototype are summarised in Table

Finite element analysis

To improve understanding of the two centrifuge tests, three-dimensional numerical back-analysis was carried out using the finite element program ABAQUS [10].

In addition to the two back-analyses, two supplementary numerical analyses (i.e., TT_D and BB_D) were carried out to investigate the distance (from the tunnel face to the location of pile group) required to achieve fully developed ground settlement in a greenfield condition. Moreover, another two supplementary numerical analyses (i.e., TT_B

Interpretation of measured and computed results

All measured and computed results are presented in prototype scale unless stated otherwise. For ease of discussion, the top of the sand bed in each test is taken as the reference datum (i.e., 0 m).

Summary and conclusions

In this study, two centrifuge model tests and three-dimensional finite element back-analyses were carried out to investigate settlement and load transfer mechanism of a pile group subjected to side-by-side twin tunnelling next to and below the toe of the pile group (Tests TT and BB, respectively). Based on the physical and numerical investigation, the following conclusions may be drawn:

(a)
In each test, ground movement induced by each tunnel occurs predominantly within a 60° wedge (inclined to the

Acknowledgements

The authors would like to acknowledge the financial support provided by the Research Grants Council of the HKSAR (General Research Fund Projects Nos. 617608 and 617511), the Quaid-e-Awam University of Engineering, Science & Technology, Sindh and Pakistan, the National Natural Science Foundation of China (Project No. 51408540) and China Postdoctoral Science Foundation (Project No. 2013M540494).

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Research PaperSettlement and load transfer mechanism of pile group due to side-by-side twin tunnelling

Abstract

Introduction

Section snippets

Test programme and setup

Finite element analysis

Interpretation of measured and computed results

Summary and conclusions

Acknowledgements

Tunn Undergr Space Technol

Soils Found

Tunn Undergr Space Tech

Tunn Undergr Space Technol

Soil Found

Comput Geotech

Tunn Undergr Space Technol

Struc Saf

Soils Found

Soils Found

Tunn Undergr Space Technol

Eurocode 7 part 1: geotechnical design: general rules, Final Draft prEN 1997-1

Pile responses caused by tunneling

J Geotech Geoenviron Eng

Graphical representation of constitutive equations

Géotechnique

Determination of parameters of a hypoplastic constitutive model from properties of grain assemblies

Mech Cohes frict Mat

Liquefaction and flow failure during earthquakes

Géotechnique

The coefficient of earth pressure at rest

J Soc Hungarian Arch nd Eng

Research Paper
Settlement and load transfer mechanism of pile group due to side-by-side twin tunnelling