Impact of close-proximity shield tunnel construction: optimisation measures and case study

The construction of new tunnels in close proximity to existing tunnels or structures often adversely affects those nearby structures, presenting a significant challenge for tunnel engineers. This study explores various impact mitigation measures for close-proximity shield tunnelling in soft ground. Four countermeasures are investigated: deep soil mixing, diaphragm walls, ground freezing, and pre-stressed lining. The mitigation measures are compared in terms of technical performance, using numerical simulations to analyse the resultant tunnel lining principal stress and horizontal deformation, as well as carbon footprints. The key findings indicate that the pre-stressed lining provides the most effective mitigation for controlling tensile cracks in the tunnel lining. These four mitigation measures have limited influence on reducing horizontal deformation of tunnel lining, which is caused by the tunnel construction. Regarding carbon dioxide emissions, the pre-stressed method results in the lowest carbon dioxide emissions and is regarded as the most environmentally friendly measure.