Parameters identification: back analysis case study of a shallow tunnel in Oporto, Portugal

The back analysis of tunnels behaviour allowed by the information obtained by later observation assumes a remarkable importance to the study of this kind of infrastructures. In the scope of the project of tunnels one often uses the numerical modelling as a means to achieve credible solutions. The process needs the definition of constitutive laws to simulate the behaviour of the materials, which have to be based on the geotechnical parameters, in order to make the approach as real as possible. However, the parameters denote almost always a very high level of incertitude. It is precisely in this matter that back analysis can establish an important "bridge" between modelling and the material, or in other words, between mathematics and nature. Thus, it provides the possibility to correct the parameters previously deduced from geological analysis, directly from field observations, and besides that, it is also useful because it allows one to verify if those models are adequate to each particular case to be analysed. The identification of some of the geotechnical parameters of a ground massif of residual soil of granite affected by the excavation of a shallow roadway tunnel located in the city of Oporto in the north of Portugal, was performed. Its small depth and excavation under some old buildings made the need for control of surface behaviour relevant. The process was supported on field measurements and the algorithm applied was based on the finite element method. A plain strain state was assumed in a two dimensional analysis and two elastic models, one linear and the other hyperbolic, were used to simulate the mechanical behaviour of the soil. The three dimensional effect of the excavation of the face of the tunnel was simulated considering the confinement-convergence method. Back analysis was performed resolving the inverse problem, with minimization of the differences between the observed displacements and those numerically evaluated. The obtained parameters were then compared with the results of the characterization tests. The results clearly proved the existence of a high degree of interdependency between the parameters analysed, which were the deformability modulus of the granite residual soil, E, and its coefficient of earth pressure at rest, K0. The results also showed that the relationships between the values of those parameters and the ratio of the deformability modulus increasing with depth were unequivocally linear. The back analysed situations seem to be frankly compatible with the values of the coefficient of earth pressure at rest deduced for this material by Viana da Fonseca (1996) from self-boring pressuremeter tests. For these cases the deformability modules could be represented by the segments illustrated in the figure aside, whose framing with the values obtained using a correlation of the SPT results, is quite interesting. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.

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  • Authors:
    • MOREIRA, C
    • Almeida e Sousa, J
  • Publication Date: 2004-7

Language

  • English

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  • Accession Number: 01011675
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD
  • Created Date: Dec 19 2005 3:24PM