A specially devised dimpled sheeting was installed in the Gotthard Base Tunnel to collect seepage and crack water over large areas.
One of the most important prerequisites for the operational safety and longevity of a tunnel relates to ensuring that harmful underground water is kept out of the inner lining and the traffic zone. As a consequence effective and long-lasting drainage layers are particularly significant. Seeping underground water is also collected extensively and transferred to a 600 mm diameter drainage pipe at the base of the tunnel vault. Towards this end a plastic dimpled sheeting specially devised for the purpose produced by the Dörken GmbH & Co. KG Herdeke/D is placed directly onto the stabilising and filtering shotcrete layer. It is intended to prevent the plastic sealing membrane as the actual tunnel seal being affected by pressing water.
Approval Test for Sealing Systems
In conjunction with the construction of the 2 Swiss AlpTransit Base Tunnels, currently being produced, the difficult marginal conditions resulting from high overburdens and rock temperatures as well as the long service life pose extreme demands on the waterproofing and drainage of the tunnel structures. The danger scenarios relating to underground water on the one hand and the high requirements placed on the materials that are used on the other prompted the client to call for an approval test procedure for sealing systems. During the execution of these extensive tests it was shown that given the state of the art from 1996 corresponding standard waterproofing systems failed to fulfil the demands for the AlpTransit Base Tunnels. However within the scope of the approval test it turned out to be possible to develop such systems even further thanks to the findings obtained so that verification of suitability could be attained for this high requirement profile as well. During the development and execution of the test method it also became evident that it is not simply the quality of the materials employed that counts but that the seal and drainage must be regarded as a system taking the on-site conditions into consideration.
Highly alkaline, acidic and hot
As a result Dörken entered a system partnership with a sealing membrane producer in order to come up with a joint solution, which was certificated and approved by the AlpTransit Gotthard AG.
The most important condition was: the drainage sheets that were used had to possess a service life of 100 years – even given extremely unfavourable marginal conditions, for the highly alkaline seepage and crack water is in some cases acidic and up to 45 ° C hot owing to geothermic influences. Consequently a high-grade chemically stabilising drainage sheeting was developed specially for the Gotthard Base Tunnel. In addition Delta-AT 1200 is extremely robust and stable, possesses 1,200 g/m² basis weight and a compressive strength of 950 KN/m².
Strict Specifications for Development and Production
The new dimpled sheeting underwent an extremely strict approval and test procedure during the development, harmonisation and modification stages. For instance the sheets were aged over a 24-month period; in the process they were stored in hot, 0.5 % sulphuric acid at 50 °C and in hot water enriched with oxygen at 70 °C and then again tested. The sheets were also manufactured in compliance with the highest standards. Samples were taken on a regular basis during production and examined by means of an Oxidation Induction (OIT) test to make sure they fulfilled the determined recipe and quality. In addition rolls were taken from the current production run and examined by accredited Swiss test labs such as the Technotest AG and the EMPA on behalf of the Swiss construction management to ensure they complied with the technical specifications. First after the Swiss test labs had approved the quality was it possible to supply the batches produced for the drainage sheeting on the tunnelling site.
Since 2009 a further high-grade drainage sheeting has been installed in the Gotthard Base Tunnel. Delta-AT 800 is a slightly lighter version of Delta-AT 1200. It was developed for areas, where the requirements are generally not quite so high. Although the sheeting is equally durable thanks to the application of the same high-grade recipe, its basis weight is “only” 800 g/m² and its compressive strength 650 KN/m³.
By May 2011 more than 500,000 m² of Delta-AT 1200 and 350,000 m² of Delta-AT 800 have been incorporated in the Gotthard Base Tunnel.