Residents in part of Hemel Hempstead have been living with the risk of ground collapse from historic chalk mining for 12 years but work is now underway to remove the problem. Claire Symes reports.
Working for just one client can have its challenges but Bam Ritchies effectively has multiple clients to manage the expectations of on one of its current grouting projects. Although the work is being funded by the Homes and Communities Agency (HCA) and the client is the local council, Ritchies has direct responsibility to work with the householders affected by the work.
Nonetheless, many residents in Hemel Hempstead’s Highbarns estate are very relieved that Ritchies is on site and the work is now underway. The work will end a 12 year period of watching and waiting since the first ground collapse from historic chalk mining occurred in the front garden of one property.
Under its contract with Dacorum Borough Council, Ritchies is using a combination of bulk infilling and compaction grouting below 38 properties, the road and some public areas in Highbarns to a remedial programme designed by Hyder.
“The houses on the estate were mostly built in the late 1950s and early 1960s but there was no indication of a problem in the area until the front garden of the house at 5 Pond Road collapsed in 2001,” says Bam Ritchies contracts manager Andrew O’Donovan.
Emergency repairs were carried out and an initial investigation into the problem was funded by the council but a further collapse in Highbarns in 2007 added emphasis to the need to remediate the area. Emergency grouting was also undertaken at that stage, however, it has taken until now for remedial work to be undertaken for the wider area due to the complexities of different house ownerships and insurers - even lack of insurers in some cases, due to the ongoing problems.
“The priority was the open mine workings, which were identified geometrically by laser surveys, and these have been stabilised using bulk infilling”
Andrew Morris, Bam Ritchies
It is situations such as this where the HCA steps in and the work at Hemel Hempstead has been part funded by HCA landholdings and part funded with investment under the HCA’s Land Stabilisation Programme.
The ground investigations after the first collapse showed that the problems were caused by chalk mining below the site in the late 1800s and early 1900s with the material used to make lime for agriculture or for the local paper making industry. The location and extent of the mine, and partially collapsed ground, were identified using microgravity surveys, probing and boreholes.
“The chalk at Highbarns is is overlain by clay with flints,” says Morris. “The top 3 to 4m of chalk is very weak.”
Since the initial collapse, other properties have shown evidence of the effects of ground movement with internal cracks being the most common issue. The road through Highbarns has also been closed to traffic since the first collapse as there was concern over how the weight of traffic would impact on the mined areas.
The remedial programme of work was designed by Hyder but it was Ritchies approach to the work that helped the company win the work.
Part of the solution included working with a newly opened Hanson concrete plant to supply the grout ready mixed to the site, rather than mixing material actually on site. “We have hired our own 100t silo on the Hanson site for the limestone dust we’re using on the scheme, as well as one for the cement,” says O’Donovan. “The Hanson facility is very hi-tech so the mixing quality is high and the material is brought to site by Hanson’s trucks.”
Around 10 trucks a day have been delivering the grout to two central pumping stations and grout is then piped directly to where the drill rigs are working.
According to Bam Ritchies geotechnical engineer Andrew Morris, avoiding mixing grout on site has helped to minimise disruption for local residents, many of whom have been able to stay overnight in their properties while work has been carried out.
The contract was awarded in December 2012 with work starting on site in January. Morris is confident that the grouting work will be completed in August.
The treatment varies across the site depending on whether the ground below has been identified as a void or partially collapsed, marginal ground but the overall area covers just over 60,000m2 and directly affects 38 properties around Highbarns. The site was sub-divided into seven areas depending on the extent of the mining and to help Ritchies plan the phases of work.
“The area of priority was the open mine workings, which were identified geometrically by laser surveys, and these have been stabilised using bulk infilling,” says Morris.
“People living in properties affected by the open voids were evacuated during the remedial works as the bulk infilling could have induced collapse,” says Morris. “We developed a contract with each householder to agree the action plan for their property and setting out what was going to be done, when and the timescales involved.”
Generally it took two weeks to treat each property and the council offered people affected discounted accommodation during the work.
People living in properties affected by partially collapsed ground were asked to vacate their properties during working hours only - from 8am to 6pm - as the risk of collapse during the work was significantly smaller. Ritchies still developed a contract with each of these householders to agree how the work would proceed and these discussions resulted in some work being re-programmed to accommodate the needs of residents.
Ritchies also has a liaison officer working on the project so that residents have a contact to talk to if they have any concerns.
In total, Ritchies has used around 1200m3 of cement grout with a strength of 1N to back fill open voided areas. O’Donovan has said that this volume was in line with expectations and this phase of work was completed at the end of February. Work is now progressing on the compaction grouting of the partially collapsed ground.
Most of the compaction grouting is being carried out using inclined 125mm diameter boreholes with lengths of up to 35m to reach under the properties and minimise internal work. The bulk infilling was mostly carried out using 125mm diameter vertical boreholes to depths of up to 25m below ground level.
To minimise the need to remove fences and walls for access, Ritchies has used a variety of different rigs from compact mini piling rigs to an excavator-mounted drill.
“We are using a packer system to isolate the area to be treated and Jean Lutz monitoring equipment to check the volume and pressure of the grouting,” says O’Donovan. “Once we reach 10bar at full depths, the packer is raised by 2m and the process is repeated.”
According to O’Donovan, the grout takes for the compaction grouting part of the work have been higher than expected when GE visited site. “But we are still focusing on the worst areas,” he says and expects the final figure to be around 1,600m3 for the compaction grouting work.
When the work is complete, Ritchies expects to have drilled 900 boreholes - 800 for the compaction grouting and 100 for the bulk infilling - with a linear length of 16,000m.
Grouting is not the end of the story though - Ritchies will also have to undertake around 7,000 linear metres of probing to validate the grouting work. Some has already been completed and the results so far have been good.
“We have been working with Hyder to feed information back into software developed by Rockware to monitor the progress,” says Morris. “The GIS Rockwork program uses the laser surveys of the mines before the work and overlays the grout volumes delivered onto the site plan. This gives an instant indication of whether what is happening on site fits the model the scheme is designed to and has allowed us to change hole spacings and lengths as a result.”
Indications so far are that the planning put in beforehand and checking process in place are helping to ensure that the problems experienced by Highbarns residents will soon be a thing