The Brasília underground transportation system comprises 6.5 km of shallow tunnel excavated in a soft red soil known as porous clay that overlies harder residual soils. The tunnel diameter is 9.6 m. Settlement observations indicated that surface settlements were two to three fold the initially predicted value, although no indication of excavation instability was observed. Settlements reached, at one section, 500 mm without failure. Another striking feature was settlement amplification between the top of the excavation and the surface by a factor that averaged 1.2 but reached up to 4. This occured because of the collapsible nature of the porous clay, which presented a considerable reduction of volume as the tunnel face advanced. This paper describes tunnel design, construction, and instrumentation; and summarizes geology and soil properties from in situ and laboratory tests. Field measurements of settlements and horizontal displacements are described and analysed. The main cause of the large settlements was collapse of the porous clay structure. Key words: tunnelling, porous clay, settlements, collapse.
Surface settlements caused by mechanised tunnelling in hard rock with heterogeneous tunnel face conditions – Empirical and numerical calculations using the example of Boßler Tunnel
