Client praises 'professional manner'

A May Gurney team has successfully completed the challenging task of exposing and diverting a potentially fragile live 500mm water main - located adjacent to other live services and obstructions - within a six-hour time slot.

The task was carried out as part the £11m second phase of carriageway dualling to the A256 west of Sandwich, for Kent County Council. The council has since gone on record in praise of the professional manner in which May Gurney overcame the many challenges, and completed the task with the minimum of disruption and in a safe and efficient manner.

The A256 crosses an existing bridge over a major flood relief channel, and a second road bridge was required - but there were two major obstacles to overcome.

First, the existing control sluice had to be replaced further downstream with a new structure; and second, the location of the proposed new bridge was occupied by 3 x 33Kv cables, BT network fibre optic cables, 2 x sewer rising mains, and a 500mm diameter strategic water main feeding the reservoir serving the Isle of Thanet.

From the early planning stages it was clear that the water diversion required careful consideration. The connection points were located in the centre of the existing carriageway, and the reservoir only had six-hour peak usage capacity. To complicate matters further, Southern Water later revealed that the water main had suffered previous structural failures due to corrosion at the joints.

The project team devised a two-phase plan of action.

Phase 1 was to locally expose the live joints individually (by remote suction techniques) and install encapsulating collars, which were anchored to cast in-situ concrete foundation slabs. Sections of the exposed pipe were also carefully measured and ultrasonic testing carried out, in order to maximise knowledge and minimise future risk. The excavations were then backfilled while the remaining lengths of new pipe were laid back to the points of connection.

Phase 2 commenced with the excavation and installation of the major cofferdams (exposing the joints prepared in phase 1) and placing a concrete base slab - to prop the cofferdam and house the anchorage points for the new main. This last point was necessary because of the need to carry out the changeover within the six-hour period; works outside that window would be subject to mains pressure, necessitating the requirement for restraint systems for the existing and the newly commissioned pipework.

Works commenced at 6.00am on 26 October - and 5 hours and 50 minutes later Southern Water supervisory staff confirmed that the diverted main was satisfactory and re-charging could commence.

The main contract is due for completion in late summer 2007.