With a history of glass and metal manufacture dating from the late 1600s, a tight plot on the banks of Bristol's Floating Harbour called for careful piling design and installation. Marcus Brierly reports.
More than 300 years ago, Cheese Lane in Bristol bordered the River Avon and was the setting for many industries which used the waters as a coolant, for transport and for waste disposal.
The 9m height difference between high and low tides was tough on the quay and on the boats, and a ship strong enough to withstand the rigours of entering, docking and leaving the port was said to be 'shipshape and Bristol fashion' The Floating Harbour, created in 1803 by damming the Avon at Cumberland Basin and Temple Meads, provided a section of river free from tidal change, and a harbour ships could use without risk of grounding. Today this still stable waterfront is the centrepiece of Temple Quays, the new corporate centre of Bristol.
Tower Wharf in Cheese Lane is a new sixstorey complex of offices with a semi-basement car park being developed by HBG Properties on an plot with a long and productive past.
By 1673 two limekilns had been built on the half of the site nearest the river, while four tenements set in gardens occupied the rest.
A glasshouse owned by Sir Abraham Elton was built in 1710 and for the next 100 years crown glass and bottle glass were manufactured there.
Various 'poor houses' are recorded on the northern side of the site at this time, but with the sale of the works in 1809 the glass cone was demolished and spelter, copper and wire began to be manufactured in a new building.
During the 19th century production gradually shifted to lead sheet and pipe manufacture.
In 1862 the property was bought by Sheldon, Bush and Patent Shot Company.
Production of lead shot continued on the site until 1994 when the factory closed. The surviving shot tower, built in 1968, is now a grade II listed building.
Given the site's history, it is not surprising the environmental report commissioned by the developer revealed contamination of soil and groundwater which had to be contained to protect future occupants.
The site is underlain by numerous foundations and supports for lead furnaces, machine bases, boiler houses and a weighbridge. Consulting engineer Buro Happold undertook extensive desk research of the site before developing the design.
Because of the site's historical significance, Bristol and Regional Archaeological Services had a continuous watching brief throughout removal of spoil.
The rough, undulating site was underlain by up to 1.5m of fill, becoming deeper towards the water's edge. It consisted of stones, brick, slag, clinker, bitumen, road stone, limestone, coal, mass foundation remains, timber and metal fragments.
The fill was contaminated with significantly elevated concentrations of metals, non-metals and organic contaminants. A well about 4.5m deep containing free phase oil was also discovered. Perched groundwater about 2m depth was reported to be, at least in part, significantly contaminated with hydrocarbons.
Under the fill lie estuarine alluvial deposits extending a substantial depth below the former river bed. Below these there is a variable band of river terrace gravel, between 1m and 3m thick. Triassic Redcliffe Sandstone bedrock is between 11m and 13m below the surface.
Beneath the estuarine deposits is a sub-artesian aquifer which is not contaminated and may be hydraulically linked to the river.
The site presented significant difficulties in terms of obstacles to boring and prevention of the migration of contamination between groundwater bodies and soils.
Contractor Simplex Foundations was asked to consider the implications for installing piles for the new building and to provide a design solution.
In preparation for piling, main contractor HBG Construction excavated up to 1.5m of fill to remove the major surface contamination and to level the site.The oil-filled well was treated with bentonite and back-filled with concrete.
With the further problem of a heavy weight exclusion zone up to 4.5m from the Floating Harbour wall, Simplex decided to use a cranemounted Soilmec RT3S rotary bore rig on an IHI800 base unit to install not only the six piles on the river front but for the rest of the project.
The 105 piles for the contract are socketed up to 7m into the sandstone, with average lengths of about 18m. Temporary casing was used to 12m depth to eliminate the possibility of transferring contamination from the two water bodies and to seal off soil-based contamination.
The piles are formed by auger boring through the fill and then inserting the casing, which is drilled through the alluvium and gravel until it hits the sandstone, sealing off any contamination. The rock socket is then formed, before concreting of the pile and removal of the casing. Casing will be permanent in the six piles next to the harbour wall.
Once piling is finished, the floor of the semibasement will be cast to seal off any potential contamination pathways to the building to protect end users, a requirement of the Environment Agency.
Simplex also installed a kingpost perimeter wall on three sides of the site, bored to between 7m and 9m and formed using 11m to 12m long universal columns.
Noise and vibration were other issues that Simplex had to take into account - GWR Radio has its broadcast studios next door.
Tower Wharf, designed by architect the Alec French Partnership, should be shipshape by September next year. It will offer 6,638m 2of office space and feature an impressive glass atrium entrance on Cheese Lane, plus a colonnaded public walkway along the waterfront.