Field measurements and numerical analysis of interaction between closely spaced bored tunnels
Chua, Heng Choon
Date of Issue2016
School of Civil and Environmental Engineering
The recent construction of the Circle Line (CCL) Mass Rapid Transit (MRT) Tunnels in Singapore has provided an opportunity to investigate the effects of interaction between closely spaced bored tunnels. The primary objectives of the research are two-fold. The first is to investigate the effects of interaction between closely spaced bored tunnels on ground surface settlements as well as the stresses and deformation induced in the tunnel lining of the pre-existing bored tunnel. This is achieved by analysing the field measurements from Contract 856 of Circle Line Stage 5, followed by conducting detailed two-dimensional (2D) numerical studies to investigate the ground surface responses and complex interaction mechanisms of closely spaced bored tunnels. The second objective is to propose a simplified methodology, empirical equations and design charts which will be practically useful to tunnel engineers in their preliminary design. Review of literatures relevant to the present research study was performed. Although a great deal of insights has been gained from the published literatures, the problems and shortcomings of some works were surveyed and summarised. The background of the Circle Line Contract 856 was introduced. The geological conditions as well as the engineering properties of soils and rocks encountered along the bored tunnel alignments were described in detail. Field measurements of ground surface settlements were classified according to groups of ground compositions. The interrelationship between ground surface settlements and Earth Pressure Balance Machine (EPBM) parameters was explored. Back analyses of the ground surface settlements for Circle Line Contract 856 using the 2D finite element program PLAXIS were carried out. Both the Mohr-Coulomb (MC) and Hardening Soil (HS) models were considered for bored tunnelling through stiff ground of Jurong Formation and mixed ground of Jurong Formation and Kallang Formation. The results demonstrated reasonably good agreement between the field measurements and the 2D finite element analyses. The HS model with its capability to model the non-linear stress-strain behaviour and also to take into account of both loading and unloading was found to be suitable for analysing tunnelling through stiff ground of Jurong Formation. A procedure that utilises the Excel spreadsheet built-in optimisation routine Solver to estimate the trough width parameter i and location of maximum surface settlement Loc Smax for single and closely spaced bored tunnels was developed. It is shown that the highly scattered field measurements were not completely in the range of the trough width parameter i proposed by Peck (1969). A comprehensive finite element study was carried out from which empirical equations were developed for estimating the trough width parameter i and location of maximum surface settlement Loc Smax for closely spaced bored tunnels with the same volume loss condition. The proposed empirical equations were found to be in good agreement with results of additional finite element analyses as well as with the results of finite element back analyses for selected field case studies. Further numerical parametric studies were then performed to investigate the interaction between closely spaced bored tunnels. The effects of tunnel geometric parameters as well as soil and lining deformation properties on the stresses and deformation induced in the pre-existing bored tunnel were studied. The results of the parametric studies indicated the complex interaction mechanisms governing the stresses and deformation induced in the pre-existing bored tunnel after construction of the second adjacent bored tunnel. The possibility of idealising the overall interaction mechanisms has been discussed. Based on the results obtained, design charts were proposed for estimating the induced incremental maximum bending moment as well as the vertical and horizontal diameter changes of the pre-existing bored tunnel after construction of the second closely spaced bored tunnel. The validity of the design charts was affirmed based on seven published case histories. For the more complex situation where joints are incorporated into the tunnel lining, a parametric study was carried out to assess the feasibility of simplifying the exact simulation. For all the cases analysed by the finite element approach, the stresses and total displacements generated by the 4-joint+key is close to the 5-joint for cases involving single bored tunnel. The same phenomena can be considered valid for interaction between closely spaced parallel bored tunnels. Further simplification of the 4-joint+key to non-jointed tunnel lining can be achieved by considering the effect of Ko. The present research has improved the understanding on the effects of interaction between closely spaced bored tunnels on the ground surface settlements and tunnel lining response of the pre-existing bored tunnel. The proposed methodology, empirical equations and design charts are useful as rapid and inexpensive tools for preliminary design considerations as well as for the interpretations of the field measurements.