Slipform Tunnelling (Engineering - Ground Engineering)

The intention of this project is to develop a new method of tunnel construction which will bring together materials, machinery, and methods which will halve the cost of tunnel construction and deliver substantial improvements to the safety, sustainability (CO2 emission reduction), programme and will provide for durable, resilient, maintenance-free assets.

The concept is loosely based upon a technique known as the Extruded Concrete Lining (ECL) system which was first developed in the 1970s. ECL produces a monolithic lining by extruding concrete between the excavated surface and an inner formwork. Since its introduction, it has been used to construct a small number (< 20) of tunnels around the world, with varying degrees of success. Whilst the basic concept is sound, limitations in materials, machinery, and method have greatly hampered the adoption and efficiency of ECL.

Advances in tunnelling technology and materials engineering in the last ten years mean that it is now feasible to contemplate combining emerging technologies into a single system which has the potential to be truly disruptive in the tunnelling and underground space sector. Taking a holistic approach to all aspects of tunnel construction the project will explore critical areas for improvement including:

Elimination of the Inner Form
The process to date has required the erection of a temporary inner form. This process is similar to the traditional ring building approach in tunnelling and is a time-consuming stop-start process. The project will investigate the feasibility of continuously slipforming to replace the inner form. The slipform would be an integral part of the tunnelling machine and be articulated to facilitate accurate alignment control on curves.

Replacement of Concrete manufactured from Portland Cement The manufacture of Portland Cement is reported to be responsible for 8% of the Earth's CO2 emissions. Alternative materials such as Geopolymer Concrete would be ideally suited to Slipform Tunnelling and would reduce CO2 emissions by over 80%. Furthermore, the lining material would be self-healing to enhance watertightness and durability.

Reinforcement Systems
When first developed, the ECL method utilised traditional steel reinforcing bars. These were slow to erect and created a long-term corrosion risk. High and Ultra-performance fibre reinforcement systems added to the Geopolymer Concrete remove the need for rebar, and would deliver exceptional structural performance, long-term durability and would be perfectly suited for use in Slipform Tunnelling.