Predicting the settlements above closely spaced triple tunnels constructions in soft ground

In the UK the competition for underground space in the urban environment is increasing. In order to fulfil the requirement for improved transport links in London an increasing amount of tunnel constructions have been undertaken. The new tunnel constructions can disturb existing underground structures (i.e. pipes, tunnels and piles) and cause the movements in overlying buildings. For these reasons, methods for predicting movements above new tunnels are vital to practicing tunnelling engineers. This paper refers to previous work undertaken by the authors into the development of a modification factor to adapt empirical predictions based on the Gaussian curve equation for predicting vertical displacements above twin tunnels. It is shown how this method can be extended to incorporate the problem of triple tunnel analyses. The results are compared with those from a non-linear finite element analysis and sub-surface case history data collected during the construction of the Heathrow Express Tunnels in London. The results show a considerable improvement compared to the 'unmodified' empirical method. The role of construction sequence and the subsequent settlement profiles are also examined in the paper.