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Micro diversion

Last autumn's low rainfall has been a blessing to a £5M flood alleviation scheme in Sussex.

Client and contractor for a microtunnelled water line in Sussex are sighing with relief that 2001's autumn rainfall was low. It gives them time to rescue a tunnel machine and re-start a 190m long drive stalled by tough angular cobbles.

Rainfall and high groundwater would have caused the local Environment Agency to worry anyway. The twin pipejacked tunnel and a second pair to follow nearby, are key elements in flood alleviation for the historic county town of Chichester.

'We had record flows in the Lavant river in late 2000, ' says Andrew Gilham, the EA's project manager for the scheme. 'At one point this little, ephemeral, chalk stream was carrying over 8m3/s, well beyond the capacity of the city centre culverts.'

Slightly lesser flows in 1994 had caused widespread damage and only heavy emergency pumping and temporary diversion piping to another watercourse prevented a second disaster last winter.

Studies in the mid-1990s concluded that overflow should be diverted into the upper catchment of a neighbouring river, the Forebridge Rife.

'There is evidence that the original watercourse may have gone that way but was diverted by the Romans' says Gilham.

An overall £5M scheme will include improvements along that river to cope with additional flows to Pagham harbour. Work for a £1.6M emergency connection between the two rivers, the first phase of the scheme, has been brought forward.

'It uses an existing offtake through a historic mill on the Lavant, ' says Gilham. 'A new control structure will tap excessive flow and discharge it through twin tunnels 190m long to an existing flooded gravel pit.'

From the other side of the pit, a second pair of tunnels - also 1.2m diameter - will run obliquely for 150m under the A27 to a second gravel pit, from where connections will be made into the Forebridge.

Microtunnelling was an almost inevitable choice. The first tunnels run from the mill and a cluster of hotel buildings to pass under the Chichester ring road roundabout and the industrial estate. Working space is tight and there are services throughout the route, says Gilham, including fibre optic cable and major gas and electricity lines. The disruption caused by open cut would have been considerable.

'And the Highways Agency is not at keen on cut and cover. It insisted on a bored tunnel solution for the A27, ' says Rupert Clubb, the EA's Sussex area flood manager.

Bolton contractor AE Yates is working in a partnership arrangement with the EA and consultant Binnie, Black & Veatch (BBV). The aim was to finish in March with the thought that, once holed through, even half-completed tunnels would serve to carry overflow pipes if water levels rose suddenly.

But the drive has proved tough. Yates and the designer agreed a scheme for an Iseki TCC 1450 Unclemole to drive back from the gravel-pit side where access was easiest.

The spacious sheet-pile starter pit will be concreted into an outlet structure later when a small temporary bund into the gravel pit will be removed. The bund serves meantime as a reaction wall for the pushing jacks.

As the Iseki got under way, it rapidly began to show rising torque at the cutter head. Eventually the machine could not move forwards.

'We think the problem is from a combination of cohesive ground and the high angularity of flint cobbles, ' says Andrew Row from the consultant. The tunnel line is on a sector where chalk detritus meets alluvial deposits. Flint, which is brittle but very hard, is being crushed by the machine but jagged fragments then lock together.

Fortunately the Iseki stopped beneath an open area and was rescued via a 6m deep investigation pit, dug in December.

Partnering has now come well into play, says Clubb. He is pleased at the way it has worked to prevent conflict. Costs will be argued later.

Yates, which is well experienced in microtunnelling, will now bring in a Herrenknecht machine. This is more powerful than the Iseki, but, more importantly, it introduces bentonite slurry, used for spoil removal, further forward in its mechanism, rather than after the cobble crusher as in the Iseki. It is hoped that earlier lubrication in the spoil mix will allow the machine to get through.

The Herrenknecht also has hydraulic drive, comments Yates site manager David Atkinson, rather than the electric Iseki, which may help.

Clubb is optimistic that not too much time has been lost, particularly as a second Herrenknecht machine will soon join the first. He thinks an April finish is still feasible and points out that because this was an accelerated phase, the overall scheme should finish by year end as planned.

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