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Hidden depths

Geotechnical contractors are more involved than usual in below ground civils works for a waste to energy plant. David Hayward reports.

As process contractors this month replace geotechnical engineers inside a vast diaphragm walled hole in a Kent field, little remains visible of the innovative construction needed to complete the heavily propped cofferdam earlier this year.

'We anticipated straightforward wall panel construction through well compacted backfill, ' explains Rob Jackson, operations manager for geotechnical specialist Bachy Soletanche.

'But the fill proved loose and voided so we had to install a grout curtain around the panels to avoid losing bentonite support fl uid.'

The 60m long excavation is needed to house the boiler and furnace of a waste to energy plant being built on the outskirts of Maidstone by process contractor Lurgi for Kent Enviro power. The plant is designed to process up to 500,000t of Kent's annual domestic and business waste, converting it into a maximum 40MW of electricity per hour.

Main contractor Hochtief subcontracted design of virtually the entire cofferdam to Bachy Soletanche. Unusually, the foundation specialist's £4.5M contract included responsibility for the perimeter capping beam, prop system and a two-level base slab. Jackson claims that this freed the client from the major risk of interface clashes between different contractors.

Crucially, he says, having a greater than usual input into the sequencing of follow-on tasks allowed the design and site teams the opportunity to reprogramme and overlap key activities. By also having full design responsibility for this section of work, the company could repeatedly value engineer its operations to incorporate time and cost saving measures.

Such thoughts were far from Jackson's mind, however, when one of the company's computer controlled Hydrofraise diaphragm walling machines arrived to form the cofferdam's total 23m deep wall panels.

A high water table and deep excavation favoured the use of diaphragm walling rather than secant piling to construct the required watertight sides. And the hard 50MPa limestone and sandstone Hythe beds, lying an average 6m down, made Hydro fraise Evolution Two the obvious cutting tool.

The £1M machine's 12m deep frame, with its four lower 1m diameter cutting wheels, could drive through the rock at 8m 3/hr as its on-board computer steered the 45t frame downward, accurate to a 1 in 400 verticality.

The site is an old stone quarry, reinstated down to the rock with problematic loosely compacted, voided backfill. Fearing bentonite loss into surrounding voids, Bachy Soletanche initially used a lean mix concrete instead of bentonite as the panels' stabilising fluid.

Within days however, engineers had opted for the more effective solution of a protective grout curtain around the panels.

The 800 hole curtain was sunk close to the outside edge of the panels and extended through the backfill 8m deep to tie into the underlying rock.

The planned eight week diaphragm wall construction had taken nearer to 12 however, and Bachy Soletanche, anxious to recover the programme, revisited its then imminent base slab construction operation.

Reinforcement for the 1.5m thick slab had been designed as conventional loose-cut bars, bent and fixed insitu. But to win back lost time the subcontractor opted instead for Rollmat - a faster, prefabricated system.

This consisted of 5t of preassembled 1.2m high rebar cages sandwiched between prefabricated mats of top and bottom steel. Each of these mats had been formed of some 60 main longitudinal bars laid out in an assembly yard at 150mm spacings.

The reinforcement grid was held in place by light steel banding running laterally across the 12m wide mat and tackwelded to each bar. A couple of conventional fire hoses were laid out below the mat and incorporated into it as the bars were rolled into a tight 5t bundle for transport to site.

Here the bundle was positioned inside slab formwork and the hoses lightly inflated with air so the rebar mat could easily be rolled out like a carpet to form bottom steel.

Two 14m long Rollmat layers provided this lower reinforcement. The main 6m long prefabricated rebar cages were then placed over the mats and a third bundle rolled out on top of them to form the slab's upper steel.

Despite a higher initial cost, total reinforcement proved three times quicker to lay and needed a third the usual number of site steel fixers.

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