Choice of TBM type for mountain tunnelling under very poor geological conditions: Hybrid, Slurry, Epb, Dsu, Convertible Tbms
Rock tunnelling under mountains, high covers and in very poor geological conditions might cause major problems to the advance of a TBM, especially in the most critical conditions. To improve the capability of the TBMs to cope with the adverse conditions typical of these applications, the traditional open gripper TBMs evolved to Double Shield in the seventies and to Double Shield Universal at beginning of the 21st century. In parallel the technological improvements and innovations in soft and mix ground tunnelling of the EPB and Slurry TBMs in the last 30 years, extended the capability to control face instabilities and water inflows under a wider range of geologies. Further to the above, in an attempt to combine in one single TBM the capability to work with different operating modes, several hybrid or convertible TBM types have been developed. Nowadays several different TBM types are available in the market and, understandably, the choice of the best type for a specific application became quite complicated. This choice is critical and does impacts the entire tunnel design (typical section, support, lining, alignment, schedule) as well the project cost and construction risks. In an attempt to provide a basic guideline to select the best TBM types for the different possible applications in mountain tunnelling, this paper describes the main design features of the different types of TBMs available on the market and their preferred field of application. The above based also on several recent applications under critical geological conditions.On these logistically demanding projects, the site teams and subcontractors are under tremendous pressure to ensure each installation is successfully executed within the tight timeframe allotted.