Interaction between tunnelling and groundwater

Abstract

Tunnelling beneath the groundwater table causes changes in the state of stress and the pore water pressure regime. In such tunnelling problems, there are three important issues that have to be addressed during design and construction stages including construction, stability, and environmental issues. First, water inflows during tunnelling significantly hamper the tunnelling works, thus resulting in a global increase in the construction costs. Second, as the stress-strain-strength characteristics of the surrounding ground are governed by the effective stress, the change in the pore water pressure regime during the tunnelling process can affect the short- and long-term tunnel stability. Third, the direct environmental consequence of water inflows during tunnelling is the drawdown of groundwater level in the surrounding aquifer. The short- and long-term drawdown can affect vegetation, groundwater supply and chemistry. The related ground subsidence occurring as a result of the reduction in water pressures in the soil layers can damage structures. A parametric study using a 3D finite-element model was performed on a hypothetical tunnelling situation of which a 10-m-diameter horseshoe shaped tunnel is constructed 25 m below the ground level in the ground condition frequently encountered in Seoul, Korea. The ground considered consists of 5.0 m of miscellaneous non-cohesive fill/alluvium material. Underlying the fill/alluvium layer is a 15-m-thick decomposed granite soil layer followed by a 20-m-thick moderately weathered granite rock layer through which the tunnel is excavated. Below the weathered rock layer is a solid rock layer. For simplicity, the groundwater table was assumed to be located at the ground surface level. In regard to the post-tunnelling groundwater flow regime, a drawdown condition with no recharge at the ground surface during the tunnelling process was assumed. The results indicted that the drawdown of groundwater during tunnelling commences well before the tunnel face approaches a given section, and that the post-tunnelling pore water pressure regime around the tunnel depends significantly on the lining permeability. In addition, the relative lining permeability with respect to the ground permeability has a more pronounced influence on the lining response than the post-tunnelling groundwater level. Also revealed was that soils around the tunnel experience stress paths that are quite different from those without the coupled behavior. Perhaps the most important conclusion is that results of analyses for tunnelling cases similar to those considered in this study can be far from realistic when the stress-pore pressure coupled behavior is not considered. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.