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Fixing A Hole

Imagine dealing with a project that has hundreds of “clients” and the extent of the work is unknown. That was the challenge facing engineers after ground instability was found in a residential area in Reading. Claire Symes reports.

Little did the homeowners in Reading’s Field Road realise that a collapse caused by chalk mines below would result in more than a decade of investigation and over £10M of ground stabilisation work.

But work is now nearing completion.

Reading-based consultant Peter Brett Associates (PBA) was called in by Reading Borough Council (RBC) when the initial collapse happened (see box, overleaf) and has taken the lead on the project ever since.

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Work currently under way on site is the third - and hopefully final - phase of investigation and stabilisation in a bid to remove the blight from houses surrounding the initial collapse.

PBA is working with contractor Forkers on this third phase of the work that aims to follow up on phase two’s investigation of 250 properties in and around Field Road.

Work to stabilise 55 properties thought to be affected started in January this year and the £4.2M project is currently on schedule to be completed in late November.

The work is being funded by cash from the Land Stabilisation Programme of the Department of Communities and Local Government.

Bulk infilling

“The primary objective is to remove the blight placed on homes in the area by following up on the findings of phase two by confirming the presence of mine workings and stabilising the ground as necessary,” explains PBA associate Stuart Chandler. “Forkers is stabilising the ground with bulk infilling in voids and compaction grouting in areas of mined ground with design work being carried out as work progresses.”

According to Chandler, the findings of phase two suggested that there would be 17 areas of mine workings.

“The nature of the project made applying for funding complicated and tendering difficult,” says Chandler. “Although we had the results of the previous investigation to work from, we didn’t have a design or extent of works that we could use for funding applications - we had to use our experience of similar projects to build in the flexibility needed.”

“The risk involved for the contractor was obvious from the wide span of tender sums”

Chandler explains that the difficulty for the contractors bidding for the work wasn’t just the complexity of the scope of works but also the challenge of limited information regarding access and reinstatement requirements.

“The properties in the area vary in age from around the 1880s through to modern houses and flats,” he says. “This wide range in property types and the changes in site profile meant access could vary from simply removing and replacing a gate through to removing brick walls or craning rigs into position.

“The risk involved for the contractor was obvious from the wide span of tender sums,” says Chandler.

With more than 55 households requiring investigation and possible stabilisation, it was important to evaluate the bids for best value.

Forkers has taken the importance of community liaison seriously and Terence Forker is working as a full-time residents liaison officer on site for the contractor so that any problems or concerns can be quickly resolved.

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Despite the headline-grabbing start to this project, this third phase of work is being carried out without anyone having to leave their homes and with a high level of co-operation of the many residents impacted by the work.

Forkers is using inclined drilling techniques to probe under the perimeter of properties so it can carry out the work from the outside and avoid disruption to utilities and services. This is not only less disruptive for residents but also means the contractor can use larger rigs than it would have had it worked from inside the properties.

Different rigs

Nonetheless, Forkers has a range of different rigs available on site to ensure it can cope with access.

When GE visited the site, a large Casagrande rig was expected to join the fleet, which includes an 8.5t Klemm KR 904 geotechnical drill rig, 3.5t Krupp 30G tracked mini-rig and 2.5t Klemm KR701 tracked mini-rig.

“So far we have investigated 53 of the 55 properties scheduled for investigation in this phase of the works,” says Chandler.

So far 800 boreholes have been bored and 11 mine workings found and stabilised using 1,100t of grout.

“Forkers have a camera that can be lowered on a cable into boreholes in order for us to check the scale of any mines uncovered”

The voids are being filled with pulverised fuel ash (PFA) and cement mixed at a ratio of 5:1, while the compaction grouting is being carried out using a mix of sand, PFA, cement and bentonite at a ratio of 6:2:1:0.1.

“Co-operation and the skill of the drillers on this project has been key to the success of the work,” explains Chandler. “We are recording a variety of drilling parameters including rotation speeds, air pressures, penetration rates and flush returns, which has given us a clearer indication of the ground conditions.”

As areas of mine workings are discovered, the drillers have been liaising with PBA’s resident engineers Robert Foster and Nikki Brown and Forkers site team to adjust the design, as necessary.

“Forkers have a camera that can be lowered on a cable into boreholes in order for us to check the scale of any mines uncovered,” says Chandler.

According to Chandler, the partnership approach between PBA and Forkers’ site teams has played a large part in the project being on programme and on budget despite the high potential for unexpected ground conditions.

Field Road Timeline

Initially residents noticed a water leak in the road at 5am on 4 January 2000 and reported it to Thames Water.

The subsidence, which resulted in a 200m3 hole, occurred later that afternoon.

“It was quickly established that the collapse was due to mine workings and not solution features in the chalk. There was no history of mining known in the area before the collapse, so the hole was backfilled and an investigation launched,” says PBA’s Stuart Chandler.

The first phase of investigation used dynamic probing, which Chandler describes as “ideal for coping with the access issues”, along with light cable percussion boreholes, geophysics and window sampling.

The investigations expanded outwards from the initial area of collapse.

“Stabilisation began almost immediately using bulk infill for open tunnels and compaction grouting,” says Chandler. “The work continued for two years with some families out of their homes for the whole period.”

With the mine workings affecting such a large number of properties, funding the stabilisation work could have been tricky.

Fortunately the council took on the role of repairing anything below ground.

The investigations proved the initial findings that suggested the cause of the collapse was due to mine workings.

“There is very little or no virgin ground left in the area,” says Chandler.

It is estimated that the first phase of investigation and stabilisation cost Reading Borough Council £2.5M, but further work was needed in the area to remove the blight placed on houses outside the phase one area.

“Phase one focused on investigating the mine workings associated with the initial collapse in 2000.

For phase two further funding was made available through the Land Stabilisation Programme to enable a wider area to be investigated,” says Chandler.

The £3.4M investigation took two years to complete.

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