Predicting the settlements above twin tunnels constructed in soft ground

The preference in the UK, and also in some other countries, is to use twin bored tunnels for new transportation lines rather than single larger diameter tunnels. Depending on the distance apart for these twin tunnels, there will be some interaction with regard to the ground movements generated during construction. In clay soils, the disturbance due to the first tunnel construction lowers the stiffness of the soil in a zone above this tunnel and when a second tunnel is constructed, if the soil in the previously disturbed zone is disturbed again, it will move more than expected. Although the use of finite element analysis is becoming more prevalent for later stages of the design process, they are still relatively costly in terms of time for extensive use at the preliminary stages of design, or during the tendering stages of a project. The current alternative is to use a method based on the assumption that the settlements above a tunnel follow the shape of an error function curve, and in the case of multiple tunnels, simply add the individual settlement troughs together. This has been found to be very approximate in many cases and therefore the curvatures of the ground, and hence the strains induced into adjacent structures are poorly predicted. This paper presents a simplified method of adapting the settlement curve above the second tunnel to allow a more realistic prediction of the overall settlements above multiple tunnel constructions. The method is still based on the use of the error function curve and so can easily be incorporated into current software packages or spreadsheets. The method applies a modification factor to the soil within the zone of overlap between the predicted areas of disturbed soil above the tunnels. In clay soils it is shown that the ground settlements need to be modified by approximately 60% to take into account the prior disturbance from a first tunneling operation. Although there is only limited good quality published case history data related to multiple tunnel constructions, the paper describes the good agreement obtained when the method is applied to several published results. (A) "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.

  • Availability:
  • Authors:
    • CHAPMAN, D N
    • ROGERS, C D
    • HUNT, D V
  • Publication Date: 2004-7

Language

  • English

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Filing Info

  • Accession Number: 01011562
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD
  • Created Date: Dec 19 2005 3:17PM