ACCIONA Construction has had to overcome important technical and logistical difficulties in excavating the Santoña-Laredo undersea tunnel that forms part of the project to improve the sanitation infrastructure of the Santoña marshlands in Cantabria
This is a vital project for eliminating discharges and restoring water quality in an important area of the Cantabrian coast, which is extremely valuable due to its biological productivity and its role as a hibernation habitat and a transit point for numerous bird species.
ACCIONA Construction, subcontracted by TRAGSA, is constructing a 1.4 km undersea tunnel to carry wastewater between Laredo and Santoña. It is also carrying out the works to prepare the launch shaft (the future pumping station of the undersea tunnel on the Santoña side) and the exit shaft (the entry works for the undersea tunnel from Laredo), including lowering the level of the water table in the backfill area of the launch shaft.
The undersea tunnel will have a 3.5 metre internal diameter and be lined with 25 cm thick lightly-reinforced and fibre-reinforced concrete rings. When complete, the tunnel will house two pipelines to carry the wastewater, each with a diameter of one metre. The tunnel will be excavated using a Herrenknecht Mixshield-type tunnel boring machine (TBM).
The lack of space, the geological characteristics of the ground and local weather conditions are just some of the challenges that ACCIONA Construction has had to face during the excavation of the tunnel.
The space for assembling and launching the tunnel boring machine was extremely limited, both on the surface, due to its location in a residential zone, and inside the launch shaft. The limited space inside the shaft was divided into two separate levels, one at a depth of 17 metres and the other at 36 metres, making the works even more challenging. Moreover, the project had not planned for an auxiliary gallery, constructed using conventional excavation methods, to completely assemble the tunnel boring machine in a horizontal position.
All of these factors made excavating the tunnel a major technical and logistical challenge and required the team to design a very unusual TBM assembly plan.
The Mixshield TBM was initially assembled vertically, called an “umbilical assembly”, using the space available on the two levels inside the shaft, with drilling of the tunnel starting from this position. The machine’s components were then assembled in their normal horizontal configuration once the excavated tunnel became long enough.
To be able to carry out the works in the launch shaft in the tunnel pumping station on the Santoña side, the water table had to be kept at 17 metres below the surface of the shaft. Given the location of the shaft, right on the coast, a water drainage and pumping system was installed and continuously operated.
The tunnel’s location, in a marine environment with high levels of humidity and salinity, presented additional challenges. To overcome these issues, macro-synthetic fibre was chosen to reinforce the concrete of the lining ring segments, in addition to a light conventional reinforcing structure, thereby preventing potential weakening of the concrete caused by corrosion typically associated with marine environments.
The solutions proposed by ACCIONA Construction for this socially-important project for the Cantabrian coast will be applied to future wastewater, irrigation or underground pipeline projects in which space restrictions call for alternative assembly procedures to be designed. Similarly, the use of materials based on concrete reinforced with macro-synthetic fibre represents a new and efficient alternative solution.