Compaction on a huge scale at an old colliery requires a fast, thick-layer technique. Damon Schünmann went to Scotland to see it in action.
Describing the Heartlands regeneration project in West Lothian, east Scotland as big doesn't do it justice. The quantity of earth that is being excavated, relaid and compacted is simply enormous.
To put it into perspective, if the entire fill from Iceland's giant Kárahnjúkar Dam, 8.5M. m3, (GE September) and the 11.5M. m3 of dredged material from Sweden's Gothenburg harbour scheme (GE October) were added together, the total would still be about 5M. m3 short of that being worked at the defunct Polkemmet coal mine site.
Coal production from the mine started in the 19th century. When it closed in 1985, the 5.85km 2site became derelict, littered with old infrastructure, mining spoil 'bings'- one of which is burning, pithead works and burnedout cars (GE July 2003). It posed a clean-up problem that West Lothian council was unable to tackle alone.
Enter the Ecosse Regeneration consortium (Kelvin Homes, Samuel Trust and the Royal Bank of Scotland) with a viable proposal.
Crucial to the work is opencast extraction of saleable material. Before development, J Fenton & Son (Contractors) will remove 1.4Mt of coal and 0.5Mt of fireclay from the ground that also consists of boulder clay, sandstone and shale.
A job of this scale requires some serious earthmoving kit and J Fenton is using 100t payload Caterpillar 777D trucks to excavate 25M. m3 of material over four years.
The contractor is excavating to depths of about 40m as it chases the coal seams. This work produces huge quantities of excavated material and large scale compaction is needed to a standard that will support the infrastructure for the planned regeneration.
Subcontractor Landpac Ground Engineering won the £2M contract to compact the north end of the site, and is creating an engineering platform to support 2,000 homes and an industrial site that it is hoped will bring £500M into the region.
The South Africa-based company claims it is the biggest scheme of its kind under way in Britain. Managing director of the UK operation, Dermot Kelly, says: 'Production is at a very high rate. Our contract is for 8M. m3 over a two year period.'
Landpac is using its patented high-energy impact compaction (HEIC) system to cope with the quantity of material. Kelly says: 'J Fenton is placing material in 1.2m layers at a rate of 100,000m3 a week, which is colossal.'
One advantage of working on such a large site is the flexibility in where material can be reused. Less desirable material can be put in areas where little or no compaction is needed.
The HEIC system involves towing three or five-sided drums, weighing 10t to12t, which deliver 75kJ into the ground per drum rotation from 150mm to 230mm drop heights. Kelly says HEIC can improve the ground's load-bearing qualities more rapidly than more conventional methods because the impact rollers travel at 8km/h to12km/h over comparatively thick layers of placed material.
But the clever part is Landpac's use of its continuous impact response system (CIR) to monitor soil response in real time. Data is received by an accelerometer attached to the axle that connects the twin drums, and is used to plot a map of the entire rolled area. This is coordinated by GPS to produce a picture of site compaction that can be compared with readings taken by conventional means.
Landpac managing director Charles Davis says: 'CIR gives a relative measure but we can't say so many Gs equals so much strength [in the soil], so conventional testing such as plate loading can give an absolute number that our system can spread over the site.'
The combination of impact compaction and the CIR system has distinct advantages over other technologies, he believes.
'Vibro rollers tend to monitor improvement in 300mm layers, but ours uses deceleration monitoring to a greater depth.'
The company says its system can detect at depth, exposing any hidden problems below thinly compacted layers, and also shows distinct demarcation around areas that have previously been worked.
The maps show three coloured categories of compaction. Red is weaker areas, yellow denotes intermediate areas and green is the firmest. But just because an area is shown in red does not necessarily mean it needs further work.
Consultant WA Fairhurst & Partners can then do a conventional test from which it can assert that the rest of the site will be as good or better than the red area. Davies believes this gives added assurance when certifying the site.
Fairhurst senior environmental geologist Andrew Nunn says: 'The compaction trials we conducted at the start of the job gave us the confidence to reduce the amount of insitu testing.'
Fairhurst senior engineering geologist Alan Blair adds: 'We're at the cutting edge of compaction using thick layers instead of thin.
Highway compaction specification is not compatible with this size of project and the GPS and CIR has halved the cost of compliance testing, such as sand replacement and nuclear density testing.
'The results are outstanding; this could be the way forward for this type of bulk earthwork and remediation project.'
The consultancy is using geographic information software to store, manage and display site data including the CIR information and insitu testing results. This gives it a tool for certification and validation for future clients, so for a given set of coordinates Fairhurst can provide data in a format they can access themselves.This conforms to best industry practice instead of the client just receiving a paper sign-off.
At the time of GE's visit, protracted heavy rain had been hampering earthworks and compaction, but the different geology across the site meant delays were minimised.
Kelly says: 'The boulder clay comes out at less than optimum moisture content which means that we can compact it straight away.
But in wet weather, when surface moisture is a problem, we can concentrate on the harder areas of sandstone and shale.'
Landpac project manager Simon Dix adds: 'When the weather is good we can complete a hectare a week; we're doing 10 to 30 coverages a day.'
Contractors used a 2m boulder clay cap to tackle the hydrogen sulphide-producing burning bing, but it continues to smoulder. The solution will involve taking it out and laying it in 300mm to 400mm layers before compacting and again capping it with boulder clay to extinguish it.
Even though the colliery waste contains about 11% to 12% coal which could feed the fire should it combust, compaction should reduce air voids and prevent mass re-igniting because the layers are so thin.
Plans for the south side of the site include two international standard golf courses, a driving range, a clubhouse complex and the protection of an area of marshland in the central west area. Less compaction in this area will therefore be needed.
Work on the golf courses should start in spring 2005 with house building beginning in May 2005.