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This paper proposes an analytical method based on drained solutions of cavity expansion and contraction in a unified clay and sand model to investigate tunnel-soil-pile interactions. Cavity expansion analyses are used to evaluate the effects of pile installation on ground stresses and to determine pile end bearing capacity and the distribution of shaft friction. Cavity contraction methods were adopted to replicate the tunnel convergence-confinement response using the singularity and image method for ground loss and ovalization of a shallow tunnel in a semi- infinite medium. A 2D model was developed which evaluates changes in mean stress and specific volume during pile installation and tunnel excavation. Outcomes from the developed analytical approach are compared against data from centrifuge tests in silica sand; results demonstrate that trends in pile load capacity degradation, mobilized safety factor, and tunneling induced pile settlement can be satisfactorily predicted for the case of a tunnel excavated beneath a pile with a constant service load. Criteria based on pile capacity, safety factor, and settlement are proposed which can be used to determine a critical tunnel volume loss or evaluate pile safety level. The paper contributes to the understanding of tunnel-soil-structure interaction mechanisms and provides an efficient means of conducting a preliminary risk assessment of tunnel-pile interaction.

Previous research on tunnelling beneath piles has assumed that caps (or other elements such as grade beams) above the pile head are elevated from the ground surface. This neglects the possible mobilisation of contact stresses beneath the foundation elements at the surface, which may occur as a result of pile settlements larger than ground surface movements caused by tunnelling. This paper presents a centrifuge study that compares the response of capped piles (where the cap is relatively stiff or rigid and is in contact with the soil) and simple piles (no cap) to tunnelling in dry silica sand. The paper includes results from 5 centrifuge tests, including both pile loading and tunnelling beneath piles. To consider a critical scenario, both simple and capped piles, subjected to a constant external load during tunnelling, are located directly above the tunnel. For strain measurement and estimation of internal forces, model piles are instrumented with fibre Bragg gratings (FBGs) along their shafts, while the external vertical and pile head settlement are also measured. The responses of simple and capped piles are compared by considering tunnelling-induced pile settlements and axial forces. Results show that simple pile settlements exceeded surface movements up to a point of pile failure (due to the loss of pile base capacity), whereas the cap restrained the pile settlement and satisfied equilibrium by mobilising the soil surface contact stresses (transferring the load from the pile to the cap). The paper highlights the role and potential benefits of considering the presence of pile caps in tunnel-pile interaction analyses.

In urban areas, the construction of tunnels and deep-excavations beneath and near to pile foundations can be detrimental for the superstructure and the foundation. A two-stage continuum-based nonlinear soil-pile interaction model is presented in this paper for predicting the axial and flexural response of piles affected by ground movements. The model accounts for the effects of near-pile non-linear (hyperbolic) soil stiffness degradation and unloading effects. The approach is used to analyse the relationship between the pile axial response (both displacements and internal forces) and greenfield ground settlements for purely-frictional and floating piles in uniform ground. Both displacement and non-displacement piles are analysed by applying appropriate pre-excavation loading sequences. Results demonstrate the influence of initial safety factor, installation method, and capacity distribution (between shaft and base) on pile settlements and on critical tensile axial forces (both in terms of magnitude and depth). Dimensionless design charts are provided to estimate pile settlements and critical axial forces for the case of greenfield settlements that either increase or decrease linearly with depth. These charts provide a rational and more general framework to describe excavation-induced effects on piles than empirical methods.

In urban areas, the excavation of tunnels beneath pile foundations can be detrimental both for the superstructure and the piles. An analysis method is presented in this paper that models the axial response of non-displacement piles to tunnelling by using a two-stage continuum-based nonlinear elastoplastic approach. This method accounts for the effects of tunnelling-induced ground movements, non-linear soil behaviour, and unloading effects. The approach is used to analyse the relationship between pile head settlement and greenfield ground movements for purely-frictional and floating piles with varying pre-tunnelling safety factor in a uniform ground. Results show that the influence of initial safety factor on the interaction level depth is significant and depends on the distribution of pile shaft capacity. Design charts are given that allow estimation of the tunnelling-induced settlements of non-displacement piles for a linear distribution of greenfield movements along the pile. These charts can also be used for deep-excavations.

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., Engineering and Physical Sciences Research Council [grant number EP/K023020/1, 1296878, EP/N509620/1]