The duisburg method of metro-construction, a successful application of the gap freezing method

Between March 1977 and August 1979 contract No.4 of the Stadtbahnbau (Metro-construction) in Duisburg was executed, making successful use of gap freezing.

The gap freezing was necessary because the Metro-tunnel is crossed by a groundwater stream (flow velocity up to 15 m/d) and it had to be assured that open cut construction of the tunnel was possible and that the original situation could be reinstated as far as possible after completion.

The Duisburg building ground also made a special construction method necessary. Ground strata: from surface to 2–4 m, civilisation deposits; from ? 4 m to ≈ 25–28 m below surface, glacial sand and gravel deposits, containing stones with a diameter > 20 cm and even boulders of 1 m³ and more; from approximately 28 m below surface, layers of Tertiary clay and silt; the groundwater table is ? 8 m below the surface, the stream flowing within the sand and gravel deposits from SE to NW (towards the Rhine).

Installing a groundwater barrier, for instance by erecting a continuous diaphragm wall enclosure, was already ruled out in early design stages as was the use of driven steelpiles.

At the inception of the design in 1974, it was decided first to carry out a measuring scheme to establish the groundwater flow velocity. This was followed by a large scale (1:1) trial freezing to ascertain the feasibility of the gap-freezing method.

When these experiments were scientifically valued it was established, that the risk involved was acceptable. The contract documents were prepared prescribing a combination of “cover and cut” with gap-freezing, which is tentatively called the “Duisburg method of Metro-construction”.

During the construction a large scale measuring and scientific research programme was carried out.