Watkin Jones tackles Chester's Roman foundations

Used in Roman times as a store for brushwood and other wood fuels outside the city walls for local bakeries, Chester's Gorse Stacks is being prepared for its latest incarnation. Juliet Davies explains.

Chester, famous for its Roman walls and mock-Tudor Victorian rows, is also becoming a showpiece of modern architecture with the Chester Renaissance programme, a planned redevelopment of several areas of the city.

Gorse Stacks was named for its previous use for the safe storage of brushwood fuel for baker's ovens outside the city walls its most recent incarnation was as the city's main bus station.

Now on the 5,500m² site of Watkin Jones's mixed-use development, a massive excavation project is being undertaken to allow for three floors of underground parking.

But this is not the first time the ground here has been dug. The area is the site of a Roman quarry - the product of which can be seen throughout the city on the walls, the amphitheatre and other rediscovered foundations, from their occupation from 70AD to the 4th century. Although mostly backfilled, it was again plundered for any remaining sandstone by the Victorians.

Their highly efficient quarrying has removed all build-quality sandstone, so the current excavation to remove 50,000m³ of made ground and sandstone face will produce no new usable materials. The location means that no excavation material can be re-used on site - the fill material is to be removed to other locations, and the poor quality remaining sandstone will be ground to sand for re-use elsewhere.

Consulting engineer Chris Pratt, a director of Robert West Consulting, was in charge of the initial surveying and soil investigations, and aware of the site's history. "It unlocks the whole development - the ability to put an underground car park on the site," he explains. "It was feasible because so much of it was once a quarry. There's a large element of made ground in the middle, which has been excavated relatively easily, and we're enlarging that quarry so it's the key to a much wider redevelopment - that was why it was conceived."

Perimeter border

There was much investigation to be done to find out where the boundary of the rock face was before work started on site, as its perimeter is bordered by one of the city's main roads. "We had to satisfy Cheshire highways that we wouldn't undermine their road," says Pratt. "The awkward thing about it was that we never knew exactly where the old quarry was - the boundary of the old quarry went just outside our site boundary."

Self-supporting rock and a trench of made ground stretches beyond the site boundary and needed support along the lines that couldn't be dug any further out. "On the perimeter we've cast a reinforced concrete capping beam that's drilled into the head of the rock to stabilise the rock head at the face," explains Geraint Morgan, Watkin Jones's group commercial director. "Dawson Wam constructed the secant piling in two places - one on the border of George Street and another on the border of Victoria Road."

As Pratt adds: "It's Permian-Triassic sandstone - there's a belt of it across the country and it's generally good stuff. The Shropshire Union canal was cut directly into the sandstone centuries ago, and you can see it'll stand up by itself for hundreds of years more."

So excavating to such a depth, and in such close proximity, has had no detrimental effect on the project, or the canal. "We keep checking it - it's still full," laughs Morgan.

Determination of the perimeter meant that excavation could start in earnest. The made ground was a relatively easy job, although the sandstone varied greatly in strength. To break down the hard outcrops, Watkin Jones was able to use plant the Romans could never have dreamed of.

A Komatsu WS60 hydraulic traverse cutting unit for rock head trimming, a Daewoo LC-V long-reach rock breaker with a water-cooled pecker, and assorted 40t excavators filled more than 140 wagons a day with excavation material.

Original quarry

Even to the depths seen, the original quarry has not been fully dug out. The new structure's foundations will be built off the rock face on one side, and piled on filled ground on the other.

For the construction works, two Liebherr Luffing 160HC-L8/16 Litronic tower cranes are to be installed, chosen because of their suitability to working in extremely restricted spaces. To support the two jib cranes on the made ground, Bachy Soletanche has constructed CFA piling to the foundations.

When complete, 60,000t of made ground and 40,000t of sandstone and rock will have been removed. The last to be excavated is the rock used as an access ramp for all of the wagons and plant.

"We have 25% of the excavation remaining, which is effectively the ramp down into the hole. Once we've finished the piling and we've got the bases on one side, and the steelwork will be coming up, we'll be able to start excavating the ramp access," says Morgan. "The only problem is we've still got to work out how to then get the excavator out of the hole."

As Chester has been of historic significance over many periods, archaeologists were present throughout much of the excavation. They photographed and surveyed all of the rock face, and were able to identify actual tools that had been used by previous quarrymen. Large quantities of Roman pottery, animal bone and metal artefacts were found, as well as post-medieval pottery and glass.

Although no items of great historic importance were found, to those involved in construction the tool scars, the masons' marks, and the pockets cut in the quarry walls - in which sat the support for the wooden scaffolding - are a great reminder of how much our industry has changed, and of how many things have stayed the same.

Project factfile

• Project: Gorse Stacks
• Client: Gorse Stacks Developments
• Main contractor: Watkin Jones Group
• Subcontractors: Dawson Wam, Bachy Soletanche
• Specialist: Robert West Consulting
• Architect: Stephenson Belle