Tunnel boring machines under squeezing conditions

Squeezing ground represents a challenging operating environment as it may slow down or obstruct TBM operation. Due to the geometrical constraints of the equipment, relatively small convergences of one or two decimetres may lead to considerable difficulties in the machine area (sticking of the cutter head, jamming of the shield) or in the back-up area (e.g., jamming of the back-up equipment, inadmissible convergences of the bored profile, damage to the tunnel support). Depending on the number and the length of the critical stretches, squeezing conditions may even call into question the feasibility of a TBM drive. This paper sets out firstly to give an overview of the specific problems of TBM tunnelling under squeezing conditions; secondly to analyse the factors governing TBM performance by means of a structured examination of the multiple interfaces and interactions between ground, tunnelling equipment and support; and thirdly to provide a critical review of the technical options existing or proposed for coping with squeezing ground in mechanized tunnelling.