THE FIRST four sections of the massive Herrenknecht Mixshield tunnelling machine, which will excavate a 1.6km long stormwater tunnel in Hastings, East Sussex, arrived by sea last
The cutting head and three machine 'cans' make up main body of the machine. Equipment was landed on the beach, and transferred to lorries to complete its journey to the drive shaft. This involved travelling through the hilly town centre - requiring the temporary removal of street furniture and traffic lights. The components were lowered into the 18m deep shaft, from where it will begin tunnelling. Once assembled, the TBM will be 7.5m diameter, 55m long and weigh around 580t.
The Mixshield method is a variant of slurry tunnelling. It uses a bubble of compressed air just behind the cutting head to maintain constant pressure in the slurry at the tunnel face and allows excavation through variable ground from competent sandstone to an uncemented sand, such as that found in Hastings Beds.
Construction of the Hastings tunnel will be hampered by a high water table and high ground-
water pressures which could lead to running sands in the uncemented areas. Head design will incorporate roller cutters, point attack picks and drag
picks to excavate the various material.
The stormwater tunnel forms part of Southern Water's £100M investment in Hastings and the nearby town of Bexhill. It varies between 20m and 60m depth and will store stormwater during heavy rain and pump it to a new treatment works before being discharged to sea through a new outfall.
It is hoped the tunnel will reduce the risk of flooding to properties during periods of heavy rainfall and improve bathing water quality off the resort's beaches.
The work forms part of Miller Civil Engineering's £43M design and construct contract for the stormwater improvements.