Modelling the behaviour of a retaining wall in residual soils for a cut and cover construction of a deep station in Metro do Porto

Abstract

This paper presents a re-appreciation of the ground characterisation and the criteria to select the most representative geomechanical parameters to consider in a numerical model to predict the behaviour of a retaining wall of a deep excavation in highly weathered granite rock masses and residual soils. This study was focused in the construction of a deep station of Metro do Porto, which involved a cut and cover solution, with unusual proportions (in plant and in depth), built in the typical Oporto's granite weathered profiles, being the excavation conducted with retaining walls consisting of multi-anchored concrete piles. Specific sections were carefully instrumented, due to the presence of historic buildings in the vicinity. The definition of representative model parameters was based on precise laboratory tests over high quality soil samples, including oedometer and high-precision triaxial tests. Geotechnical and geological characterisation of all the area for the original design, was initially based on in situ tests, such as SPT and rock masses classification, and on the local experience on this type of ground. After this construction, the assumptions of parameterisation, using a constitutive modelling based on new laboratory tests over high quality block samples, allowed a reanalysis of the assumptions on the design phase. A back-analysis of monitored displacements and forces during the construction was made, assuming the designed structural solutions, which were in fact implemented in construction, but considering the new approaches on the definition of the geomechanical parameters for the prevailing weathered rock masses, necessary for the numerical simulation based on the commercial software Plaxis®, using the Mohr-Coulomb and “Hardening-Soil” models. Some specific changes of the constructive sequence during the excavation and activation of supporting system were attained, by looking at the information found during the construction. The results of this parametrical re-approach and analysis of the singularities of highly weathered granite and corresponding residual soils masses for modelling of retaining walls of large excavations are discussed.

Keywords:

• residual soils
• multianchored retaining walls
• constitutive models
• triaxial test

Acknowledgements

The authors wish to express their gratitude to “Metro do Porto, SA” and '”Normetro, ACE” for their authorisation to publish data from the Porto Metropolitan Project. A special thanks is also due to the design team Pedro Carvalho and Daniel Sousa from “Viaponte, SA”, as well as Eduardo Marques, who has collaborated in the geological model. This research was developed under the activities of CEC, FEUP, namely the PTDC/ECM/099475/2008 project, financed by European Community (QREN/UE/FEDER), through the Operational Program for Competitive Factors – COMPETE.