A centrifuge modelling study of the response of piled structures to tunnelling

Tunnelling beneath piled structures may compromise the stability and serviceability of the structure. The assessment of potential structure damage is a problem being faced by engineers across the globe. This paper presents the outcomes of a series of geotechnical centrifuge experiments designed to simulate the effect of excavating a tunnel beneath piled structures. The stiffness and weight effects of piled structures are examined independently using aluminium plates of varying stiffness (`equivalent beam' approach) and the addition of weights supporteby aluminium piles. Greenfeld displacement patterns and results from pile loading tests are also provided. The variation of structure displacement profiles with plate stiffness, weight, and tunnel volume loss are used to illustrate the main effects of tunnel-pile interaction and the contribution of the superstructure to the global tunnel-pile-structure interaction. Results indicate that piles have a detrimental role in tunnel-structure interaction problems, whereas the superstructure stiffness and weight can, respectively, reduce and increase structure distor-tions and settlements. Finally, the potential for structural damage is evaluated by comparing structure and greenfield deflection ratios as well as resulting modification factors. The paper presents a unique set of results and insights which provide valuable guidance to engineers working across the ground and structural engineering disciplines

Tunnelling and its effects on piles and piled structures

Current needs for infrastructure and services in urban areas often require the construction of tunnels that may affect existing surface and buried structures. In general, the construction of new tunnels in the proximity of deep foundations raises concerns related to pile failure and associated structural damage (in both the superstructure and the foundation). Despite its practical importance, few studies have investigated the global tunnel-pile-structure interaction (TPSI) and, thus, engineers generally compensate for the lack of understanding with an overly conservative design approach. To provide insights into the interaction mechanisms of TPSI, this research used geotechnical centrifuge testing as the main investigation method to acquire data related to both greenfield tunnelling in sands and tunnel excavations beneath piles and piled buildings. In particular, a novel method was developed to study TPSI problems through the real-time coupling of numerical and centrifuge modelling, enhancing centrifuge modelling capabilities. Furthermore, empirical and closed-form solutions were used to study the tunnelling-induced displacement fields and simplified elastic analyses were used to provide insights into the global TPSI mechanisms. Results from the greenfield tests illustrate that ground movement prediction in sands is very complex because of soil arching effects and changes that occur as tunnels transition from relatively shallow to deep depths, resulting in highly non-linear displacement mechanisms. Results also illustrate the correlation between vertical and horizontal displacement mechanisms. In particular, the influence of soil relative density and volume loss on deformation patterns is highly dependent on the tunnel relative depth. To provide simple tools for engineering practice, empirical and closed-form solutions are proposed. Predicted ground movements provide sufficient accuracy for preliminary assessments, though limitations of these methods should be considered. The centrifuge tests on TPSI provide experimental evidence that tunnelling-induced pile displacements are affected by [i] pile installation method (displacement versus non-displacement piles), which affects the pre-tunnelling soil state and the distribution of loads between pile shaft and base, [ii] initial safety factor of the pile foundation, which is related to pile bearing capacity and superstructure self-weight, and [iii] superstructure stiffness and configuration, which results in pile load redistribution while minimising structural distortions. In addition, results show that potential for pile failure is a critical aspect for piles with relatively low initial safety factors and that pile failure may be prevented by a limited relative reduction in the pile load due to the superstructure. Finally, the importance of superstructure stiffness and self-weight on tunnelling-induced structural distortions is confirmed. Piled buildings respond critically to tunnelling beneath the pile tip depth in terms of flexural deformations. In general, it is shown that [iv] piles increase structural distortions compared to shallow foundations and that [v] the superstructure stiffness and self-weight decrease and increase the superstructure distortions resulting from tunnelling, respectively. Results are also evaluated within the modification factor approach; parametric analyses of elastic soil-pile-structure interaction are used to develop simple design charts that can be used to estimate horizontal strains and deflection ratio modification factors based on newly defined relative axial and bending stiffness parameters. The envelopes compare well with deflection ratio modification factors measured from centrifuge tests. Further research is needed to include the effects of soil plasticity, building self-weight, superstructure configuration and tunnel-structure eccentricity in these design charts. This dissertation highlights the improvements in the design of underground constructions that can be achieved by combining ground and structural engineering

Tunnelling and its effects on piles and piled structures

Current needs for infrastructure and services in urban areas often require the construction of tunnels that may affect existing surface and buried structures. In general, the construction of new tunnels in the proximity of deep foundations raises concerns related to pile failure and associated structural damage (in both the superstructure and the foundation). Despite its practical importance, few studies have investigated the global tunnel-pile-structure interaction (TPSI) and, thus, engineers generally compensate for the lack of understanding with an overly conservative design approach. To provide insights into the interaction mechanisms of TPSI, this research used geotechnical centrifuge testing as the main investigation method to acquire data related to both greenfield tunnelling in sands and tunnel excavations beneath piles and piled buildings. In particular, a novel method was developed to study TPSI problems through the real-time coupling of numerical and centrifuge modelling, enhancing centrifuge modelling capabilities. Furthermore, empirical and closed-form solutions were used to study the tunnelling-induced displacement fields and simplified elastic analyses were used to provide insights into the global TPSI mechanisms. Results from the greenfield tests illustrate that ground movement prediction in sands is very complex because of soil arching effects and changes that occur as tunnels transition from relatively shallow to deep depths, resulting in highly non-linear displacement mechanisms. Results also illustrate the correlation between vertical and horizontal displacement mechanisms. In particular, the influence of soil relative density and volume loss on deformation patterns is highly dependent on the tunnel relative depth. To provide simple tools for engineering practice, empirical and closed-form solutions are proposed. Predicted ground movements provide sufficient accuracy for preliminary assessments, though limitations of these methods should be considered. The centrifuge tests on TPSI provide experimental evidence that tunnelling-induced pile displacements are affected by [i] pile installation method (displacement versus non-displacement piles), which affects the pre-tunnelling soil state and the distribution of loads between pile shaft and base, [ii] initial safety factor of the pile foundation, which is related to pile bearing capacity and superstructure self-weight, and [iii] superstructure stiffness and configuration, which results in pile load redistribution while minimising structural distortions. In addition, results show that potential for pile failure is a critical aspect for piles with relatively low initial safety factors and that pile failure may be prevented by a limited relative reduction in the pile load due to the superstructure. Finally, the importance of superstructure stiffness and self-weight on tunnelling-induced structural distortions is confirmed. Piled buildings respond critically to tunnelling beneath the pile tip depth in terms of flexural deformations. In general, it is shown that [iv] piles increase structural distortions compared to shallow foundations and that [v] the superstructure stiffness and self-weight decrease and increase the superstructure distortions resulting from tunnelling, respectively. Results are also evaluated within the modification factor approach; parametric analyses of elastic soil-pile-structure interaction are used to develop simple design charts that can be used to estimate horizontal strains and deflection ratio modification factors based on newly defined relative axial and bending stiffness parameters. The envelopes compare well with deflection ratio modification factors measured from centrifuge tests. Further research is needed to include the effects of soil plasticity, building self-weight, superstructure configuration and tunnel-structure eccentricity in these design charts. This dissertation highlights the improvements in the design of underground constructions that can be achieved by combining ground and structural engineering

Protecting surface and buried structures from tunnelling using pile walls: a prediction model

When tunnelling poses excessive risks for buildings and buried foundations, a pile row barrier may shield the existing structure from ground movements. This paper presents a three-dimensional linear elastic prediction method to evaluate the protective action of pile walls against surface and subsurface ground movements due to new tunnels, both directly behind the wall as well as within the entire ground. Analyses are carried out to evaluate the vertical and horizontal movements of the ground and the pile wall as the result of soil-pile row interaction. New factors that quantify the wall efficiency in reducing settlements and deflections behind the wall are proposed; the results indicate that the effectiveness of the pile wall at reducing horizontal displacements is limited. Subsequently, predictions are compared against field and numerical data to demonstrate that the elastic solution is applicable, particularly for small ground losses. Finally, the barrier efficiency in reducing settlements is discussed comparing pile walls and diaphragm walls

Centrifuge and real-time hybrid testing of tunnelling beneath piles and piled buildings

Tunnels are constructed increasingly close to existing buried structures, including pile foundations. This poses a serious concern, especially for tunnels built beneath piles. Current understanding of the global tunnel-soil-pile-building interaction effects is lacking, which leads to designs which may be overly conservative or the adoption of expensive measures to protect buildings. This paper presents outcomes from 24 geotechnical centrifuge tests that aim to investigate the salient mechanisms that govern piled building response to tunnelling. Centrifuge test data include greenfield tunnelling, pile loading, and tunnelling beneath single piles and piled frames, all within sand. The global tunnel-piled frame interaction scenario is investigated using a newly developed real-time hybrid testing technique, wherein a numerical model is used to simulate a building frame, a physical (centrifuge) model is used to replicate the tunnel-soil-foundation system and structural loads, and coupling of data between the numerical and physical models is achieved using a real-time load-control interface. The technique enables, for the first time, a realistic redistribution of pile loads (based on the superstructure characteristics) to be modelled in the centrifuge. The unique dataset is used to quantify the effects of several factors which have not previously been well defined, including the pile installation method, initial pile safety factor, and superstructure characteristics. In particular, results illustrate that pile settlement and failure mechanisms are highly dependent on the pre-tunnelling loads and the load redistribution that occurs between piles during tunnel volume loss, which are related to structure weight and stiffness. The paper also provides insight as to how pile capacity should be dealt with in a tunnel-pile interaction context

Development of coupled centrifuge-numerical modelling: investigation of global tunnel-building interaction

There is an increasing demand for underground space in urban areas for infrastructure development. This has resulted in tunnel construction taking place in close proximity to buried infrastructure and building foundations. Various studies have considered the effect of tunnel construction on buildings; however the global tunnel-ground building interaction problem is still not well understood. This is due partially to the fact that the available modelling tools do not accurately replicate the global behaviour of soil-structure domains. This research aims to enhance physical modelling capabilities by coupling centrifuge and numerical techniques. The research focuses on tunnelling beneath buildings which are founded on piled foundations. In this paper, the proposed method and the developed equipment are presented. The expected outcomes of this research will provide a better understanding of complex tunnel-ground-building interactions which will help to improve the design approach of tunnels beneath buildings

Development of coupled centrifuge-numerical modelling: investigation of global tunnel-building interaction

There is an increasing demand for underground space in urban areas for infrastructure development. This has resulted in tunnel construction taking place in close proximity to buried infrastructure and building foundations. Various studies have considered the effect of tunnel construction on buildings; however the global tunnel-ground building interaction problem is still not well understood. This is due partially to the fact that the available modelling tools do not accurately replicate the global behaviour of soil-structure domains. This research aims to enhance physical modelling capabilities by coupling centrifuge and numerical techniques. The research focuses on tunnelling beneath buildings which are founded on piled foundations. In this paper, the proposed method and the developed equipment are presented. The expected outcomes of this research will provide a better understanding of complex tunnel-ground-building interactions which will help to improve the design approach of tunnels beneath buildings

Centrifuge and real-time hybrid testing of tunnelling beneath piles and piled buildings

Tunnels are constructed increasingly close to existing buried structures, including pile foundations. This poses a serious concern, especially for tunnels built beneath piles. Current understanding of the global tunnel-soil-pile-building interaction effects is lacking, which leads to designs which may be overly conservative or the adoption of expensive measures to protect buildings. This paper presents outcomes from 24 geotechnical centrifuge tests that aim to investigate the salient mechanisms that govern piled building response to tunnelling. Centrifuge test data include greenfield tunnelling, pile loading, and tunnelling beneath single piles and piled frames, all within sand. The global tunnel-piled frame interaction scenario is investigated using a newly developed real-time hybrid testing technique, wherein a numerical model is used to simulate a building frame, a physical (centrifuge) model is used to replicate the tunnel-soil-foundation system and structural loads, and coupling of data between the numerical and physical models is achieved using a real-time load-control interface. The technique enables, for the first time, a realistic redistribution of pile loads (based on the superstructure characteristics) to be modelled in the centrifuge. The unique dataset is used to quantify the effects of several factors which have not previously been well defined, including the pile installation method, initial pile safety factor, and superstructure characteristics. In particular, results illustrate that pile settlement and failure mechanisms are highly dependent on the pre-tunnelling loads and the load redistribution that occurs between piles during tunnel volume loss, which are related to structure weight and stiffness. The paper also provides insight as to how pile capacity should be dealt with in a tunnel-pile interaction context

Mixed empirical-numerical method for investigating tunneling effects on structures

The assessment of potential for building damage due to ground displacements caused by tunnelling is a global issue being faced by engineers. There is a two-way interaction between tunnelling and existing buildings; tunnel construction affects a building by inducing displacements in the soil underlying its foundation, and buildings influence tunnelling induced displacements via their weight and stiffness. Numerical analyses are widely used to investigate tunnelling and its impact on structures, however numerically predicted ground displacements are generally wider and shallower than those observed in practice. This paper presents a two-stage mixed empirical-numerical technique to estimate the effect of building stiffness on ground displacements due to tunnelling. In the first stage, greenfield soil displacements are applied to the soil model and the nodal reaction forces are recorded. In the second stage, the effect of tunnelling on a structure is evaluated by applying the recorded nodal reactions to an undeformed mesh. Results from conventional numerical analyses of the problem are compared against those obtained using the mixed empirical-numerical approach. Results demonstrate the importance of imposing realistic inputs of greenfield displacements when evaluating structural response to tunnelling