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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 81
PROCEEDINGS OF THE TENTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Paper 277

Applicability of Elastoplastic Modelling for Simulating Tunnel Excavation in Lightly Overconsolidated Clay

T.T. Abdel-Fattah+, A.M. Abdel-Rahman+, A.Y. Akl* and H.A. Hodhod*

+Department of Soil Mechanics and Foundation Engineering, Housing and Building Research Centre, Giza, Egypt
*Departement of Civil Engineering, Faculty of Engineering, Cairo University, Giza, Egypt

Full Bibliographic Reference for this paper
T.T. Abdel-Fattah, A.M. Abdel-Rahman, A.Y. Akl, H.A. Hodhod, "Applicability of Elastoplastic Modelling for Simulating Tunnel Excavation in Lightly Overconsolidated Clay", in B.H.V. Topping, (Editor), "Proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 277, 2005. doi:10.4203/ccp.81.277
Keywords: tunnelling, excavation, lightly overconsolidated clay, elastoplastic modelling, undrained condition, finite-element analysis.

Summary
This paper aims at examining the performance of the material constitutive models that are normally adopted in finite-element analyses used to predict the ground deformations associated with tunnelling in lightly overconsolidated clay. To do so, a tunnel construction problem with the following geometric properties is considered. The tunnel has a diameter of 6.0 m, and its centre is located 15.0m below the ground surface, with a ground water level located at the ground surface.

In order to study the effect of the range of the resulting plasticity on the ground behaviour, solutions are obtained for a number of overconsolidation ratios (OCRs) representative of normally to lightly overconsolidated clays. Analyses are performed for ground volume loss percentages (VLs) of 1% and 2.5%, which practically represents the ground loss associated with closed face shield tunnelling.

The results of analyses were obtained using the following constitutive models in addition to a linear elastic one:

  1. Mohr-Coulomb model and Druker-Prager model as representatives for linear elastic-plastic modelling,
  2. Egg Cam-clay model [1], and Modified Mohr-Coulomb model [2] as representatives for nonlinear elastic-hardening plastic modelling.

In addition to the above-mentioned models, some additional points found to be important for the practical use of the Mohr-Coulomb model and the Modified Cam-clay model. These points are summarised as follows [3]:

  1. In order to examine the effect of material dilation on the results obtained using the linear elastic-plastic models, solutions due to both associated plasticity and non-associated plasticity with (i.e., non-dialant material) are presented.
  2. Moreover, results due to the Cam-clay model(s) were obtained using both the soil angle of internal friction , as well as a reduced angle of internal friction . The reason for using the latter is that the stress path around the tunnel perimeter may be represented by triaxial extension for both the tunnel crown and invert, and by active compression for the tunnel springline.

The analyses were carried out using a plane-strain model under undrained condition. The results of analyses for the normalised settlement troughs and the horizontal movements at the ground surfaces are plotted for different OCR and VL ratios. These results are compared to the experimentally-based Gaussian distribution which represents the most reliable and accepted form in the literature.

The results of analyses showed that the effectiveness of the constitutive model adopted in the analysis is largely dependent on the amount of plasticity that is expected to develop. In the pre-yielding stage, the differences between the predictions obtained using the models considered are practically negligible. These differences increase if plasticity develops around the tunnel perimeter which can be due to either a low OCR or higher VL.

The results also showed that the use of the linear elastic-plastic models is not appropriate for the constitutive modelling of clayey soils. However, better predictions for the ground response due to excavation can be obtained using these models if a non-associated flow rule with is adopted. Moreover, these results showed that the Cam-clay models with a critical state parameter, , estimated using a reduced friction angle may provide better predictions for the ground response due to excavation.

References
1
S.J.M. van Eekelen and P. van den Berg, "The Delft egg model, a constitutive model for clay", G.M.A. Kuslers and M.A.N. Hendriks (eds.), DIANA Computational Mechanics'94, Kluwer Academic Publisher, pp. 103-116, 1994.
2
A.E. Groen, "Two elastoplastic models for the behaviour of soils", Technical Report 03.21.0.31.12, Delft University of Technology, 1995.
3
T.T. Abdel-Fattah, "Material modelling and 3-D finite-element simulation for shield tunnelling in soft clay", Ph.D. thesis, Cairo University, Egypt, 2004.

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