Numerical Investigation into the influence of bored piles on existing tunnels
Dr Felix Schroder, Imperial College, London
In the urban environment, deep foundations are often constructed in locations very close to existing tunnels. Many tunnels can often only tolerate minimal movements. Tunnel owners are concerned that the process of bored pile construction and/or the subsequent loading of the piles may cause intolerable movements or stress levels that might cause cracking of the tunnel linings. Over the last thirty years tunnel owners have developed restrictive guidelines based on their experience of the problem. In the research that forms the basis of this presentation the validity of these restrictive guidelines was investigated. In the first part of the presentation the influence of bored pile construction on existing tunnels is discussed. The biggest movements and stress changes during the construction of a bored pile in heavily overconsolidated soil are expected during the dry excavation of the pile bore. Three dimensional finite element analyses which assess the influence of the dry excavation of a pile bore on an adjacent tunnel are presented. The results from the three dimensional analyses are compared with results from an axially symmetric analysis in which no consideration was given to the presence of the tunnel. It is concluded that only very limited information on the influence of bored pile construction on existing tunnels may be gained from axially symmetric finite element analyses, and consequently three dimensional analyses are required. In the second part of the presentation three dimensional analyses of pile rows are used to justify a two dimensional plane strain simplification in which rows of piles are modelled as continuous walls. This plane strain approach is then used to investigate the influence of pile loading on existing tunnels. Two distinctly different configurations of the piles with respect to the tunnel are considered: in the first pile rows are located on both sides of the tunnel and in the second pile rows are only located on one side of the tunnel. In both cases extensive parametric studies are presented.
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