The first paper, entitled ‘CTRL North Downs Tunnel’ was by Watson, Warren, Eddie and Jager, representing the various companies making up Rail Link Engineering and Eurolink. The paper, presented by Watson and Eddie, gave an introduction to the design, procurement and construction of the tunnel. The authors provided a brief overview of the Channel Tunnel Rail Link (CTRL) project and then described the tunnel in some detail.

The single bore, twin-track rail tunnel is to be used by trains travelling at up to 300km/h. The tunnel will be 3.2km long with a cross section of 166m2 and will be the largest rail tunnel ever constructed in the UK. Preliminary results of hydrofracture and creep testing of the Middle Chalk were discussed and a new approach to real time interpretation of tunnel monitoring data was introduced.

The speakers said that the CTRL is the most significant railway infrastructure project to be built in the UK since construction of the Channel Tunnel over a decade ago. The method of construction requires initial support to be provided by a conventional shotcrete primary lining, locally reinforced by a combination of mesh, lattice girders and rockbolts. An in-situ secondary lining provides long-term support to the tunnel for the 120 year design life.

The design of the portals to blend with the landscape was described and details of the site investigation results provided. The tunnel is being driven in the Lewes, New Pit and Lower Chalk, with generally good quality material. The authors recounted how hydrofracture tests had been performed to obtain the in-situ stress of the chalk and how the design had been refined. This value engineering exercise resulted in significant savings in the cost of the final lining.

The second paper, ‘Tunnelling using the prevaulting support system in chalk for the Ramsgate Harbour Approach Road Tunnel’, by Crow & Newman described the first UK use of the Perforex prevaulting system. The tunnel fulfills a strategic requirement to gain access to the port, avoiding the town centre. The authors explained that the 800m long, 13m diameter tunnel was being driven through the Upper Chalk in the south east of England with overburden depths of only 5-16m beneath residential properties.

Prevaulting system

The tunnel, now nearing completion, is being constructed by a JV of Taylor Woodrow and Perforex with designer Kent County Council Engineering consultancy (now Babtie), supported by Howard Humphreys and the Dr Sauer Company. The main interest of the paper was the use of the Perforex prevaulting system. The machine and its chain saw-like slot excavation arm was described in detail, along with the method of filling the slot to provide ground support before excavating the face. It was explained that in less favourable ground the face was also supported with fibreglass dowels and, if necessary, shotcrete.

The benefits of the system for this particular tunnel centred on the security provided by the system when driving the tunnel beneath housing with low cover. Results of extensive monitoring both in the tunnel and on the surface were summarised and it was stated that all movements were within the designed limits and very close to those predicted.

The paper also extolled the virtues of partnering as a way to carry out tunnelling works and the presenters concluded by saying that the scheme had demonstrated how tunnelling could overcome demanding environmental and financial constraints to provide transport access to the expanding port of Ramsgate.

The third paper, ‘CTRL Contract 430: Ashford tunnels design, development and implementation’, was presented by the author, H Roscoe. This paper outlined the project to build two cut+cover tunnels in the centre of Ashford, 570m and 422m in length. The author described the way in which the field test results were being applied in the works now under construction and illustrated the way in which co-operation between the partners was producing further efficiencies. The work is being undertaken by Kvaerner Construction to a design by Rail Link Engineering. Value engineering workshops have resulted in a number of cost effective changes being made that include:

  • Bottom up rather than top down construction

  • Changes to the slab/wall connections

  • The use of temporary de-watering to reduce the number of propping levels

  • The use of permanent props for temporary support

The author described the last two changes in detail and explained the benefits to the contract. He summed up by saying that, at the time of writing, the preliminary earthworks for the box construction were well advanced and over 1200 piles had been bored. The results of the field tests performed at the start of construction are being applied to the design of both the permanent and temporary works. Box design has been developed to suit the preferred construction logistics, making the structure quicker and safer to build. Roscoe did, however, conclude by saying that more could have been achieved if a longer lead period had been available, allowing the ideas resulting from value engineering to be fully developed.

The themes common to these three papers was the use of field geotechnical results to refine design parameters and the partnering arrangements used on the contracts. The full text of the papers will be available with the final conference summary.