Geomechanics and Engineering, Vol. 14, No. 5 (2018) 479-489 DOI: https://doi.org/10.12989/gae.2018.14.5.479 479 Copyright © 2018 Techno-Press, Ltd. http://www.techno-press.org/?journal=gae&subpage=7 ISSN: 2005-307X (Print), 2092-6219 (Online) 1. Introduction Analysing the structure-foundation-soil interaction during tunnelling operation in a single coupled analysis is cumbersome. Realistic analysis of such systems can only be done using full, three-dimensional models that properly simulate the presence of existing structure, its foundation, appropriate material behaviour, and the tunnelling operation (Augarde et al. 1995, Chen et al. 1999 among others). In general, numerical modelling contributed greatly in understanding the performance of shield tunnelling (Katzenbach and Breth 1981, Clough and Leca 1989, Addenbrooke 1996). Previous works investigated the effect of several factors that affect the tunnel soil pile interaction (Poulos 1979, Lee et al. 1992, Loganathan and Poulos 1998, Chen et al. 2000, Loganathan et al. 2000, Loganathan et al. 2001, Lee and Ng 2005, Pang 2006, Lee and Chiang 2007, Cheng et al. 2007, Yang et al. 2011, Linlong et al. 2012, Lee 2013, Ng et al. 2013, Zidan and Ramadan 2016, 2015). Soil-structure interaction research studies attempt to simulate the behaviour of soil as it occurs in the field. To this end, few constitutive models are available: elastic, Corresponding author, Associate Professor E-mail: ahmedzidan@eng.bsu.edu.eg a Dean E-mail: osman-ramadan@eng.cu.edu.eg elasto-plastic (e.g., Mohr-Coulomb), hardening soil, etc. (Zidan 2012, Ardakani et al. 2014, Mohammad and Tavakoli 2014, Moradi and Abbasnejad 2015). Obviously, the accuracy of soil-structure interaction simulations is controlled by the adopted soil stress-strain relationship and the correctness of the assumed numeric values for the model parameters. This statement is further elaborated below. Samuel et al. (2013) compared the performance of braced excavation predicted by both the Mohr Coulomb (MC), and hardening soil (HS) constitutive models to the observed in-situ diaphragm wall deflections. According to their results, the HS model provided a competent result in comparison to observed diaphragm deflections, but the MC model significantly underestimated the diaphragm wall deflections. A similar outcome was reported by Kahlström (2013) who performed a comparison of the MC and the HS material models of Plaxis-2D in estimating the primary consolidation behaviour of soft clay. His results indicated that performing design with the MC material model is inadequate while the most accurate results, when compared to actual survey measurements, were achieved when computing with the soft soil material model. Furthermore, in their study of tunnel excavation in week rocks, Dong and Anagnostou (2013) stated that the MC yield criterion fails to map the non-linear stress-strain behaviour and the stress dependency of stiffness observed in triaxial testing on typical weak tectonized rocks such as kakirites. Besides, they reported that a modified HS model predicts the A hybrid MC-HS model for 3D analysis of tunnelling under piled structures Ahmed F. Zidan 1 and Osman M. Ramadan 2,3a 1 Department of Civil Engineering, Faculty of Engineering, Beni-Suef University, Salah Salem Street 62511Beni-Suef, Egypt 2 Structural Engineering Department, Faculty of Engineering, Cairo University, Giza, Egypt 3 Higher Technological Institute (HTI), 10th of Ramadan City, Egypt (Received June 9, 2016, Revised September 21, 2017, Accepted September 25, 2017) Abstract. In this paper, a comparative study of the effects of soil modelling on the interaction between tunnelling in soft soil and adjacent piled structure is presented. Several three-dimensional finite element analyses are performed to study the deformation of pile caps and piles as well as tunnel internal forces during the construction of an underground tunnel. The soil is modelled by two material models: the simple, yet approximate Mohr Coulomb (MC) yield criterion; and the complex, but reasonable hardening soil (HS) model with hyperbolic relation between stress and strain. For the former model, two different values of the soil stiffness modulus (E 50 or E ur ) as well as two profiles of stiffness variation with depth (constant and linearly increasing) were used in attempts to improve its prediction. As these four attempts did not succeed, a hybrid representation in which the hardening soil is used for soil located at the highly-strained zones while the Mohr Coulomb model is utilized elsewhere was investigated. This hybrid representation, which is a compromise between rigorous and simple solutions yielded results that compare well with those of the hardening soil model. The compared results include pile cap movements, pile deformation, and tunnel internal forces. Problem symmetry is utilized and, therefore, one symmetric half of the soil medium, the tunnel boring machine, the face pressure, the final tunnel lining, the pile caps, and the piles are modelled in several construction phases. Keywords: tunnelling; soil-structure interaction; piled structure; strain-hardening; finite element; three-dimensional analysis