Keywords: building response, tunnelling, monitoring, numerical simulation and soil-structure interaction.

One of the most important issues of tunnelling in urban areas is the assessment and how it impacts on buildings adjacent to the tunnel construction. Tunnelling in urban areas will suffer the same concern. Many researchers have focused on the assessment of the ground settlement above the tunnel and its effects on the settlement of adjacent buildings, particularly differential settlement.

The majority of the people in Salvador use the bus system, with 4 million trips a day, almost 98% made with the mass system transportation (400 lines and 2200 vehicles). For this reason, it was decided to build the Salvador Metro. In the demand study, a length of 1.2 km was chosen as tunnel, as a result of the high costs of the urban space and the preservation of World Heritage.

This paper will focus on a section of the tunnel that was excavated under a building constructed in the 1970s, near to Campo da Polvorá Station. The building has a height of 35 m. It is a reinforced concrete structure with 9 floors and a basement. The lift machine house and a water tank are located below the building. The tunnel axis crosses the garage ramp, 23 m below the street level. The building foundations are footings installed at a depth of 6 m. A complete instrumentation programme was implemented to measure building displacement, tunnel movements and water levels.

A soft ground tunnel was designed to be constructed with sequential excavation and flexible shotcrete lining, following the NATM philosophy commonly used in Brazil since 1970. NATM has become one of the preferred tunnelling methods in Brazil because of its flexibility to adapt to different soil conditions, contractual aspects and the required tunnelling equipment.

A complete instrumentation programme was implemented to measure building settlements, tunnel movements and water levels. Thirteen displacement pins were installed in the building. A fully instrumented section was installed in front of the building and also where either tunnel geometry or geologic conditions changed. It comprises the following instruments: six surface marks, two vertical single point extensometers, a piezometer and three convergence pins.

Ground control criteria, settlement through and interference with existing public utilities and buildings are the main concerns of underground works in urban areas. In Salvador, extensive three-dimensional finite element analyses were carried out to investigate the real ground conditions as a result of tunnel excavation using NATM. For this analyses the PLAXIS 3D Tunnel program (Brinkgreve & Vermeer [1]) was used. This is a geotechnical finite element package specifically intended for three-dimensional analysis of deformation and stability in tunnel projects.

Geotechnical applications require advanced constitutive models for simulations behaviour of soil and rock. In this study case, the elastic-plastic hardening-soil model was assumed for the soil behaviour. This is an advanced model for simulating the behaviour of different types of soil, both soft soil and stiff soils. The hardening-soil model, however, supersedes the hyperbolic model. It uses the theory of plasticity rather than the theory of elasticity and includes soil dilatancy and a yield cap, more details can be found in Schanz et al. [2].

The pattern of displacements at the roof tunnel was investigated. The structural behaviour was also investigated. The forecasted results for the structure were compared with the monitoring data. For the first column section, the predictions were bigger than the measured data. For the second column section, the settlements obtained from numerical simulation were quite well matched with monitoring data; however the angular distortions were not good. Therefore, the PLAXIS 3D - tunnel software could be used to model the maximum building response that occurred during the Salvador tunnelling.

1

Brinkgreve, R.B.J. and Vermeer, P.A., Finite Element Code for Soil and Rock Analyses. Plaxis Manual. Balkema, Rotterdam, Netherlands, 2001.

2

Schanz, T., Vermeer P.A. and Bonnier, P.G. The hardening soil model: formulation and verification, Beyond 2000 in Computation Geotechnics - 10 years of Plaxis, Balkema, Rotterdam, ISBN 90 5809040 X, 1999.

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