Tuesday August 3 1999 saw the breakthrough of the Herrenknecht Mixshield (S-128) on the 1.6km long, 6.5m diameter stormwater storage tunnel in Hastings, UK. The $68.8m tunnelling project was completed in two years by contractor, Miller Civil Engineering for Southern Water as part of the $167m Hastings Bathing Water Improvement Scheme.
The Mixshield (May 98) was chosen to excavate the tunnel as the local geology comprises complex, variable ground, including estuarine deposits; peat beds; uncemented water bearing sands; clays; mudstones; and limestones.
A shared risk contract was negotiated between Miller and Herrenknecht. Major fault zones and fissures across the tunnel route caused air leakage, which resulted in the face sometimes being difficult to seal. Hydrophilic polymers and bentonite were used, followed by grouting to obtain early strength.
Lining consists of rings incorpor-ating seven precast reinforced concrete segments which were placed by a vacuum erector system. The segments are fully gasketed and are tapered. The total number of rings used between the drive and reception shafts came to 1066, the date of the Battle of Hastings!
The 1.6km long tunnel will intercept the sewers and provide storage for 52 000m³ of stormwater until conditions permit it to be pumped to a treatment plant before discharge into the sea.
