Within a decade it is hoped salmon will return to the River Aire in west Yorkshire. Yorkshire Water is rebuilding the waste water treatment works adjoining the village of Esholt - previously home to TV soap Emmerdale Farm. The £64M project will bring discharge water quality in the Aire up to standards required by the European Union's Freshwater Fish Directive, which comes into force in 2010.
Contractor Morgan Est won the design and build contract with consultant Carl Bro.
Lancashire-based groundworks contractor Pennine has won a £300,000 ground improvement sub-contract to insert a combination of vibro stone columns and vibro concrete columns across the site.
Under its six week contract, Pennine is installing 5,000 vibro stone columns across what was the sludge lagoon. A further 600 stone columns and a similar number of concrete columns will be installed elsewhere on the site in March.
Stone columns are created by vibrating aggregate into the ground. This displaces and compacts surrounding soil, increasing bearing capacity.
Material can be added until the required bearing capacity is reached. Columns also act as drainage routes for water.
The purpose of the ground improvement at Esholt is to improve the uniformity of variable soils before construction of an activated sludge plant, consisting of a massive reinforced concrete tank 204m long and 80m wide with perimeter walls 6.5m high.
Ground conditions on the site range from competent sand and gravel over coal measures and bed rock of variable quality to made ground and weak alluvial clay and sands and gravels.
Pennine senior engineer David Preece explains that the site had been used for waste tipping as well as a slurry lagoon. Ironing out variations in the bearing capacity of the ground was not the only challenge. The activated sludge plant also exerts a wide range of pressure over its footprint, ranging from 75kPa to 180kPa. Pennine was challenged to deliver the required bearing capacity across the site, while keeping total settlement to within 300mm, and differential settlement to within 1:500.
'During the detailed design stage, Morgan Est found a large, triangular concrete tank buried on the site, ' Preece says. 'We have absolutely no idea what it was used for, and at first thought we might be able to site the stone columns on top of the base. But further investigation proved that the ground underneath was too soft to support the weight of the new activated sludge plant. That left us with the only option of removing the whole thing.' Following onsultation between Pennine and consultant Carl Bro, land remediation was carried out before ground improvement work started.
Varying ground levels meant that some parts of the site had to be lowered by up to 6m while others had to be raised by 1m.
The consultation established that vibro stone columns could be used to support all parts of the sludge plant structure.
But it also revealed that further information was needed on the strength of the weakest clay soils and the organic content of the made ground.
Carl Bro arranged for the additional site investigation, which identied isolated areas of organic rich soils. These were removed ahead of the ground improvement work.
During installation of the vibro stone columns, displacement of surrounding ground takes place, and there is some mixing of the stone with the surrounding ground. Large quantities of organic material in the ground can cause problems as it loses volume as it decays, leading to potential movement.
Pennine's design allowed for wider spacing of stone columns in the most competent areas and a closer spacing of columns where the thickest deposits of weak soils were found.
Adaptation of the installation rig will enable Pennine to use it for the vibro stone and concrete columns.
Looking for economies, Morgan Est wanted to reuse good quality sands and gravels found on the site of the sludge plant elsewhere on the site. The company asked Pennine if it was feasible to replace it with lower grade materials excavated from elsewhere.
Pennine worked with Morgan Est's materials engineer and developed a method of selecting appropriate soils, placing them and validating the placement with appropriate testing.
Cone penetration testing was used across the site to verify bearing capacities after work was completed.