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

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

Performance-based seismic design of flexible-base multi-storey buildings considering soil–structure interaction

A comprehensive parametric study has been carried out to investigate the seismic performance of multi-storey shear buildings considering soil–structure interaction (SSI). More than 40,000 SDOF and MDOF models are designed based on different lateral seismic load patterns and target ductility demands to represent a wide range of building structures constructed on shallow foundations. The cone model is adopted to simulate the dynamic behaviour of an elastic homogeneous soil half-space. 1, 5, 10, 15 and 20-storey SSI systems are subjected to three sets of synthetic spectrum-compatible earthquakes corresponding to different soil classes, and the effects of soil stiffness, design lateral load pattern, fundamental period, number of storeys, structure slenderness ratio and site condition are investigated. The results indicate that, in general, SSI can reduce (up to 60%) the strength and ductility demands of multi-storey buildings, especially those with small slenderness ratio and low ductility demands. It is shown that code-specified design lateral load patterns are more suitable for long period flexible-base structures; whereas a trapezoidal design lateral-load pattern can provide the best solution for short period flexible-base structures. Based on the results of this study, a new design factor RF is introduced which is able to capture the reduction of strength of single-degree-of-freedom structures due to the combination of SSI and structural yielding. To take into account multi-degree-of-freedom effects in SSI systems, a new site and interaction-dependent modification factor RM is also proposed. The RF and RM factors are integrated into a novel performance-based design method for site and interaction-dependent seismic design of flexible-base structures. The adequacy of the proposed method is demonstrated through several practical design examples

Estimation of inelastic displacement demands of flexible-based structures on soft soils

This study aims to develop efficient tools for performance-based seismic design of soil-structure interaction (SSI) systems on soft soils. To simulate the SSI effects, linear and nonlinear 'equivalent fixed-base single-degree-of-freedom' (EFSDOF) oscillators as well as a sway-rocking SSI model were adopted. The nonlinear dynamic response of around 10,000 SSI models and EFSDOF oscillators having a wide range of fundamental periods, target ductility demands, and damping ratios were obtained under a total of 20 seismic records on soft soil sites. Based on the results of this study, a practical method is developed for estimating the base shear and maximum displacement demands of a nonlinear single-degree-of-freedom structure on soft soil deposits. In the proposed procedure, the effect of frequency content of ground motions is considered by normalising the period of vibration by the spectral predominant periods, while the nonlinear EFSDOF models are used to improve the computational efficiency

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

Geocellular railway drainage systems: physical and numerical modelling

The importance of resilient railway infrastructure is paramount when considering the increased likelihood of extreme weather and flash flood events in coming years. One of the main causes of instability of railway tracks is excess water in the trackbed, particularly when it is at or above the interface of the ballast and subgrade. Conventional drainage systems are susceptible to clogging and deterioration. Resilient track drainage systems should therefore have sufficient capacity to allow water to dissipate quickly, but they should also be designed to ensure long-term operation with minimal or easily performed maintenance. This paper presents results from an investigation of a potential new railway drainage system using geocellular components. In the paper, the development of a large scale physical model is described which represents a full scale unit cell of a sleeper-to-sleeper track substructure. The physical model includes ballast and subgrade layers, under-track and lateral drainage systems, rainfall simulation, and instrumentation. Results demonstrate the relative hydraulic response of the drainage system with and without the geocellular components. The paper also describes the development of a numerical model of the track subgrade and drainage system, which was first calibrated and verified using experimental data from the physical model, then extended to study the effect of certain parameters on the hydraulic response of the railway track. Results indicate that the under-track geocellular drainage system offers potential benefits in terms of maintaining a lower water table level within the subgrade as well as in aiding the migration of fines out of the ballast

Elastic-brittle-plastic behaviour of shale reservoirs and its implications on fracture permeability variation: an analytical approach

Shale gas has recently gained significant attention as one of the most important unconventional gas resources. Shales are fine-grained rocks formed from the compaction of silt and clay sized particles and are characterised by their fissured texture and very low permeability. Gas exists in an adsorbed state on the surface of the organic content of the rock and is freely available within the primary and secondary porosity. Geomechanical studies have indicated that, depending on the clay content of the rock, shales can exhibit a brittle failure mechanism. Brittle failure leads to the reduced strength of the plastic zone around a wellbore, which can potentially result in wellbore instability problems. Desorption of gas during production can cause shrinkage of the organic content of the rock. This becomes more important when considering the use of shales for CO2 sequestration purposes, where CO2 adsorption-induced swelling can play an important role. These phenomena lead to changes in the stress state within the rock mass, which then influence the permeability of the reservoir. Thus, rigorous simulation of material failure within coupled hydro-mechanical analyses is needed to achieve a more systematic and accurate representation of the wellbore. Despite numerous modelling efforts related to permeability, an adequate representation of the geomechanical behaviour of shale and its impact on permeability and gas production has not been achieved. In order to achieve this aim, novel coupled poro-elastoplastic analytical solutions are developed in this paper which take into account the sorption-induced swelling and the brittle failure mechanism. These models employ linear elasticity and a Mohr–Coulomb failure criterion in a plane-strain condition with boundary conditions corresponding to both open-hole and cased-hole completions. The post-failure brittle behaviour of the rock is defined using residual strength parameters and a non-associated flow rule. Swelling and shrinkage are considered to be elastic and are defined using a Langmuir-like curve, which is directly related to the reservoir pressure. The models are used to evaluate the stress distribution and the induced change in permeability within a reservoir. Results show that development of a plastic zone near the wellbore can significantly impact fracture permeability and gas production. The capabilities and limitations of the models are discussed and potential future developments related to modelling of permeability in brittle shales under elastoplastic deformations are identified

Centrifuge Study on the Influence of Tunnel Excavation on Piles in Sand

© 2020 American Society of Civil Engineers. Tunneling-induced ground movements can affect the equilibrium state of an existing pile, causing uneven settlement among pile groups and damage to connected structures. This paper presents results from five centrifuge tests aimed at evaluating the load redistribution mechanisms that occur within piles located close to tunnel excavation. Two main mechanisms are studied: first, those related to ground displacements and stress relief related to tunneling and, second, those related to pile head load changes caused by connected superstructures (accomplished using a hybrid centrifuge-numerical modeling method). A novel fiber Bragg grating sensor system was used to measure shaft shear stresses along model piles. Results are used to quantify the relative impact that these two mechanisms have on pile load redistribution during tunnel volume loss. In addition, posttunneling pile loading tests were performed, with results indicating that tunneling-induced ground volumetric strains could influence the posttunneling loading response of piles