Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Wand wishes settled

Developers are under increasing pressure to build on contaminated brownfield sites. Judith Cruickshank reports from one site where cement stabilisation has made the difference between commercial success and failure. Pictures by David Jones.

If you wanted to find a project to exemplify the challenges facing the construction industry of tomorrow -and the solutions - HBG Construction's Axis Park project in Langley, Berkshire, could fit the bill nicely. Contaminated by unwelcome nasties like oils and asbestos, the 22ha brownfield site is being transformed into a modern business park - thanks to thousands of tonnes of cement.

Lying alongside the M4 close by a number of residential developments, the site was most recently the location of a Ford Iveco truck plant. During the Second World War and for some years afterwards it housed an Air Ministry factory.

There is no reliable history for the site, says HBG Construction's director of technical services Richard Hare-Winton. The site investigation carried out by WS Atkins prior to award of the contract to HBGC revealed significant contamination. Of necessity, the Atkins investigation had been carried out with the old buildings still standing. It was therefore a fair bet that demolition would uncover a few surprises.

HBG moved on site in the spring of 1998 and demolition began immediately. Contamination soon proved to be more widespread than predicted. There was asbestos in the factory buildings and some buried asbestos was also discovered. And almost inevitably, given the site's long industrial history, demolition revealed uncharted underground storage tanks

Ground conditions were made ground overlying terrace gravels. The water table is about 3m below ground level - though this can vary with levels of rainfall. There are also some perched water tables. Much of the contamination was due to oils, and some had leached into groundwater.

Maximum re-use of materials was a key factor in HBG's plan. The conventional solutions - hauling away the demolition waste, followed by stripping of the topsoil, excavation of contaminated areas and importation of new material were ruled out. For a number of reasons - cost, time, disturbance to local residents, to name only a few, lorry movements would be kept to a minimum.

Bio-remediation of excavated oil-contaminated soils and groundwater treatment took place concurrently with demolition. A full testing and reporting system was set in place to demonstrate that clean-up had been achieved to the satisfaction of MEPC's independent adviser White Young Green.

Material from the dismantled structures was crushed on site, steel taken away for re-use and the amount of material which had to be taken off-site for disposal represented only a tiny percentage of the whole.

At the end of this process HBG was faced with a site which consisted of varying depths of original fill, some areas of undisturbed ground and some where soil had been cleaned and replaced to a fully engineered specification. The proposed layout of the site meant that the buildings would straddle all these areas. And in the case of at least one tenant, requirements concerning settlement and in particular differential settlement, were more than usually demanding.

The tenant concerned is the Post Office which will occupy the largest building at Axis Park. The Worldwide Advanced Network Distribution (WAND) building will be equipped with the very latest in electronic mail handling equipment. Fit-out is rumoured to be costing in the region of £100M.

Faced with conditions which could reasonably be described as 'challenging', HBGC adopted a range of techniques to suit the ground conditions and the demands of the new buildings. Initially soft areas of made ground and deep loose fill were dynamically compacted by Keller to a performance specification.

Foundations for the structural frames are generally shallow bases founded on the natural gravels, but piled foundations were selected for the heavily loaded WAND superstructure.

Norwest Holst is currently installing 450 CFA piles to a depth of 23m through the made ground and gravels and into the London clay. Cement-based soil stabilisation techniques were selected to treat the majority of the rest of the site to provide suitable foundations to the internal ground slabs and external pavements. Lime stabilisation is used only on the wet areas.

Stabilisation is being carried out by specialist contractor McArdle Stabilisation under a series of lump sum contracts. The company currently runs two Wirtgen WR2300 recyclers - a tangible and costly sign of its belief in the potential of the technique. With a considerable bank of experience the company was able to work with HBG and with its own consultant, Babtie, to produce practical and efficient schemes for Axis Park.

First action was to confirm the findings of the site investigation with a series of slit trenches across the now cleared site. The areas of made ground were excavated and screened to remove coarse and unusable materials and the exposed formations tested and proved. In the area of the 40,000m2 WAND building the plan calls for treatment to a depth of 200mm in two stages. In the initial stage 2% of cement is added to improve the 200mm soil depth. A second stabilisation pass then adds 8% cement to the upper 100mm depth.

The initial pass is intended to provide a platform for the piling operation. When piling is completed McArdle will carry out the second pass giving a strength of 2N/mm2.

The design of the Wirtgen recycler ensures that the elements used in the stabilisation process are added in the correct proportions. The amounts are pre-set and addition is computer controlled and automatically adjusted according to the speed of the machine.

An automated spreader deposits a 50mm layer of cement. Then follows the water bowser pushed, via a rigid towbar, by the recycler, which mixes the soil/cement/water combination to the required depth with the teeth on the rotating drum. Depth of mixing is also pre-set.

A Caterpillar 14G grader fitted with laser control fine-grades the surface to the required levels. The area is then compacted using McArdle's 25t Hamm vibratory roller. This roller size is essential to produce a good result, says McArdle director John Thompson. The number of passes varies with the type of soil being treated but a battery of tests ensures the required strength is achieved.

Just how accurate a finish is achieved using this mix of machinery can be judged from the fact that following the first stage of improvement, the area of the WAND building varied just +/-10mm from datum. And even at this stage, before final strengthening, the surface appears to be standing up well to trafficking by Norwest's piling rigs and the visiting truckmixers.

In theory the Wirtgen recycler and its attendant plant could handle up to 3,500m2 a day and given the wide open spaces of the Axis Park site you might expect that the machine would simply work its way from one end to the other at full speed. But nothing is ever that simple, and McArdle's team is working in bites around piling operations, erection of the first building and construction of the gravity-fed drainage system.

In total more than 90% of the site will be improved, using 1,000t of Buxton lime to dry and stabilise the wet areas and 7,000t of cement to strengthen the ground and contain any residual contamination. This will allow construction to go ahead on a clean, stable site and has saved the cost of thousands of lorry journeys, not to mention the pollution they inevitably cause and the annoyance to the surrounding population.

Given the need to build on brownfield sites it seems fair to conclude that there will be ever more call for cement-based stabilisation techniques and that McArdle's investment in a second recycler will be fully justified.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.