City Research Online

Abstract

The rising population in urban environments comes with an associated demand for increased public transport. Due to the level of surface congestion an often utilised solution is to construct rapid transit systems within tunnels. Any sub-surface construction will generate ground movements which have the potential to cause damage to existing surface and sub-surface structures. Urbanisation and congested cities have driven the need for accurate predictions of tunnelling-induced settlements and has produced many publications (e.g. Peck, 1969; Cording & Hansmire, 1975; Clough & Schmidt, 1981; O’Reilly & New, 1982; Attewell & Yates, 1984; Cording, 1991; Mair et al., 1993 and Mair & Taylor, 1997). Largely, however, these empirically based prediction methods are concerned with single tunnel greenfield, arrangements. Generally, mass rapid transport systems comprise of a pair of tunnels constructed within relative close proximity. This is known as twin-tunnel construction. A number of case studies have shown a relative difference in the settlements due to each tunnel construction (e.g. Cooper et al., 2002; Cording & Hansmire, 1975 and Nyren, 1998). These were further investigated by numerical studies which support these observations (e.g. Addenbrooke & Potts, 2001 and Hunt, 2005). Analyses that use isotropic linear elasticperfectly plastic soil models have tended to produce wider surface settlement troughs then observed by the Gaussian distribution (Mair et al., 1981). It is that clear valuable insight could be gained from a physical model based study. Therefore, a series of plane strain centrifuge tests was carried out investigating twin tunnelling-induced settlements in overconsolidated clay. Apparatus necessary to perform these tasks required a significant amount of time to develop and was relatively complex. The main variables were the spacing between the tunnels, both horizontally and vertically, and the magnitude of volume loss. The tests were conducted at 100g where the cavities represented two 4m diameter tunnels at (usually) a depth of 10m at prototype scale. The tests utilised novel apparatus designed during the research to enable the simulation of the construction processes related to volume loss in separate sequential tunnels.

The results presented are in regards to the prediction of ground movements in the plane perpendicular to advancing tunnels and the significant findings of the research are as follows: -

1. Single tunnel surface and sub-surface settlement troughs are well represented by Gaussian distributions, however, the twin-tunnelling predictions can be improved by modifying the settlements solely due to the second tunnel construction.

2. The magnitude of volume loss from the new tunnel construction had increased due to the presence of the first tunnel. This effect was lessened by larger spacings between the tunnels.

3. Second tunnel settlements can be predicted using equations by Peck (1969), O’Reilly & New (1982) and Mair et al. (1993) but with the modifications. The surface and sub-surface settlement distributions towards the existing tunnel were observed to be wider than a single tunnel.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > Engineering Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses

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