The design and construction of the Vila Madalena Station for the São Paulo Subway

Abstract

The Vila Madalena Station is part of the Green No. 4 Line of the Sao Paulo Subway. It was constructed underneath the Heitor Penetado Avenue, a densely occupied area of the city with high-rise buildings very close to the excavation limits. The Heitor Penetado Avenue is a major traffic artery with no parallel avenue nearby, through which traffic could be diverted. This was a major conditioning factor for the construction method, and NATM operations were adopted. Traffic was not disturbed by any surface excavation operation. Excavations for the running tunnels and platforms had 18.8 m width and 14.4 m height. They were excavated in a sequence of seven operations. A central pillar was cast at the center of a 9.1 m diameter pilot tunnel before excavating the side tunnels to create room for the platforms. Two tunnels crossed the main body of the station, the largest of which was the Mezzanine Tunnel with 6.86 m equivalent diameter. Two other tunnels crossed the station underneath. The largest one was the Exhaust Tunnel with 6.48 m equivalent diameter. The station was excavated through tertiary sediments of the Sao Paulo Sedimentary Basin. The ground mass mainly consisted of stiff clays and water bearing sand lenses and layers. Overburden at the west end of the station consisted of more than 20 m of sandy silty clay and clayey sand with SPT-N values in the range of 10-20 blows/ft. Settlement predictions for that favorable geotechnical condition led to low values of distortions and no restrictive measures had to be taken at the surface. Measured settlements confirmed predictions. Maximum settlement was about 16 mm and maximum distortion was 1:220. At the east end, the overburden was lower (10 m) and the presence of porous soil of colluvial origin (SPT - N values in the range of 2-5 blows/ft) led to higher settlement predictions. Pipes of two water mains running above would not be able to withstand distortions and a rigorous construction sequence had to be adopted in order to prevent damage. Bench excavation advanced in the opposite direction of the top heading excavation, which proved to be an efficient measure due to peculiar local conditions. Maximum settlement and distortion were 32 mm and 1:150, respectively. No damage occurred to the water mains or other utility lines. The most critical situation was the proximity (only 3 m) of the foundation of a 17-story high-rise building to the excavation of the intersection of two inclined tunnels for escalators and the Mezzanine Tunnel. The caisson base level was about at mid-height of the underground excavation. Building settlements and excavation stability were both serious concerns. Soil nailing and a careful excavation and shotcreting sequence were adopted and the maximum caisson settlement was successfully limited to only 5.9 mm. No damage occurred to the building either. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.