Development of a multi million pound extension to a Middlesbrough hospital has led to a seven hectare site being treated with an unusual combination of ground improvement techniques. Ground bearing capacity has been enhanced using quicklime to dehydrate the soil and a capping layer containing incinerator ash has been specified to protect the ground against water ingress.
Works were completed in April 2008 by the engineering specialist Roger Bullivant Ltd’s newly formed ground improvement division EcoSmart. The division was established in November last year to deliver a full package of ground improvement systems, from site clearance to foundation installation.
The £1.3M, 11 week contract in Middlesbrough has been carried out to provide a durable platform on which the hospital extension and its foundations can be built. Soil stabilisation with quicklime has been adopted as a cost effective alternative to improving the ground using vibro compaction techniques.
At the start of works in January, vegetation was removed from what was a sloping site and topsoil stripped and stockpiled for landscaping, before heavy plant set to work reprofiling 30,000m3 of stiff clay. Redundant foundations buried within the ground and concrete hardstandings were broken up, moved to a stockpile and fed into a mobile crushing plant for reuse on site.
A fleet of milling machines was then used to rotovate an application of quicklime or cement into the ground, after which a 19t roller compacted the processed material. Modified soil was compacted with a minimum of eight passes to build up the level of stabilised ground in 300mm deep layers.
EcoSmart operated a fleet of ‘fully integrated’ milling machines, which are designed to deposit and immediately mix an application of quicklime or cement into the ground, rather than spread it over a wide area for processing at a later stage. Use of this type of plant helps to reduce dust associated with spreading quicklime or cement. “We are working close to a live hospital so it was very important to keep dust levels down to an absolute minimum,” says EcoSmart director Seamus McNulty.
Soil samples were tested for moisture content and strength every day of the contract by a team of laboratory technicians based on site; and results were verified by an independent materials specialist.
“Everything was reused and no materials were removed from site,” he adds. “This was a huge cut and fill exercise and the addition of quicklime or cement increased the CBR (Californian Bearing Ratio) of the ground to an average of 30%, which is suitable for the two storey construction that will follow on this site.”
The newly stabilised soil was sprayed with a bitumen coating to provide an initial waterproofing barrier, before a recycled aggregate capping layer was constructed on top to a depth of 100mm. The aggregate introduced to site was a type one equivalent Incinerator Bottom Ash Aggregate (IBAA), containing no virgin material and supplied from household waste incineration plants at Huddersfield and on Teesside. Around 75% of raw material fed into the plant can be reused as a recycled aggregate sub base for a wide range of applications, including use under clause 803 of the Specification for Highway Works. Around 9600t of IBAA was supplied for the hospital construction contract. The material was recovered from the incineration process by sustainable materials supplier Ballast Phoenix and applied by EcoSmart to clause 803.
EcoSmart’s national operations manager Simon Wilson says: “Use of recycled rather than virgin aggregate for the capping layer reduced the quantity of material brought to site by 3,000t because the recycled material has a far superior spread rate compared with virgin aggregates. This is the first time we have specified aggregate formed from crushed household waste on one of our contracts and in doing so we managed to reduce the number of lorry movements to site from 630 to 480.”
Use of recycled aggregate in place of virgin limestone on the contract is thought to have saved 2.3ha of habitat from being quarried and cut CO2 emissions by 40%.
New legislation introduced on 6 April 2008 could encourage the greater use of recycled aggregates as a means of protecting recently improved ground. Known as ‘Site Waste Management Plans’, the initiative places a legal requirement on all projects in England worth over £250,000 to make detailed records of what happens to waste. The legislation should help the construction industry to make better use of resources and maximise the value of its wastes.
