The first known subsidence to hit the street occurred at Briars Lane Infant School in May 1978. A 3.6m by 4.6m diameter hole opened up in the school grounds and borehole investigations at the time suggested dissolution in the underlying chalk as the likely cause.
But further events in Briars Lane continued to cause problems for homes, pavements and gardens over the next three decades. Structural problems were sufficiently severe in places that in 1986 it was decided four pairs of semi-detached properties would be demolished.
Hertfordshire County Council and Welwyn Hatfield Borough Council were able to secure funding from English Partnerships to deal with the problems and instructed engineer Hyder Consulting to begin site investigation work in late 2004.
It drilled 45 boreholes as well as doing microgravity surveys in a six-month project to determine what was causing the problems. It also carried out a total of 1800 dynamic probes. All the investigations helped reveal the presence of up to 2m wide voids.
The work also helped establish whether there were any offshoot tunnels and gave a clear indication of where the perimeter should be for the follow-on bulk infilling work. Rig operators worked within circular perimeters around each of the houses in the areas affected by subsidence.
The results of the investigations showed mines appearing at depths of between 7m and 20m in low grade upper chalk. This is overlain by up to 5m of glacial clays, sands and gravels.
Site workers used CCTV cameras inside the boreholes that provided 360º images and revealed six large voids pointing to a network of mines. Hyder senior geotechnical engineer David Jones says these images showed the mines had suffered extensive roof collapses.
He says desk studies and anecdotal evidence suggest the chalk mines were in operation from the 18th century to the Second World War – some years after the site was developed for housing in the late 1930s.
Following discovery of the 2m to 4m high mines, comprising a series of galleries up to four levels, the two councils awarded a £4M contract to contractor Ritchies to treat the abandoned mines. This was based on Hyder's design and the consultant produced a 3D mine model to monitor the work.
Ritchies started work on site shortly after being appointed in July and is using a twofold approach – bulk foam infilling and compaction grouting – to stabilise the ground. The voids range in size from about 50m3 to 1000m3 but Jones says it is difficult to be sure how large they are because the mines and offshoot tunnels interconnect.
Gaining access to the site has been one of the biggest challenges the team has faced. The work has been made easier by the school closure (it was relocated 18 months ago), the temporary re-housing of some residents and the closure of the road at one end.
However, many residents have stayed put and causing minimum disruption to them has made it more difficult for site workers to stabilise the ground beneath their feet.
"There's a lot of plant working in a confined space. It would have been easy for us to do the work if we could have flattened the site," says Jones. "But it's not that simple because these buildings need to be re-used."
As a result Ritchies is using rigs that can stand away from the property while completing the work. Two of its own rotary drilling masts are attached to Hutte tracked excavators, which are supplemented by two Casagrande rigs. The masts have a long reach capacity to make it easier to get over obstacles such as garden fences.
Over 750 holes need to be drilled to complete the stabilisation work. Site workers pre-drill the between 100mm and 150mm diameter holes using flighted rotary augers before inserting by hand 3m lengths of plastic casing down to depth for the large open voids.
Ritchies site agent Andrew O'Donovan says the chalk is of such a consistency – similar to putty – that the holes stay open, but that the casing is necessary to help control where the grout goes. This ensures it does not get caught in the upper layers of soil before reaching the open voids.
For this part of the job a high cement grout was mixed with a protein-based foaming agent just before being pumped into the ground. This creates a lightweight fill material that has about one quarter of the weight of concrete. Site workers have completed the bulk infilling using a total of 1420m3 of grout – the equivalent volume of 18 double-decker buses.
The works are being staged to minimise disruption to residents, the idea being the less time they are out of their homes the better. Work is now continuing on the next portion of the contract – to compact the loose ground surrounding the voids.
The pressure ranges from 40bar to 5bar at the pump outlet as the hose is extracted from the ground. O'Donovan says Lutz monitors are being used at each of the injection points to monitor the pressure and flow of the grout.
The final part of the contract will follow on from this work and is again designed to cause minimise disruption for residents.
As well as being able to do injection work at a distance away from properties, the long-reach excavators will do about one-third of the injection grouting at an angle of 30º from the vertical.
These holes will reach further than the vertical ones – a maximum of 30m into the ground – to get to the right depth for compacting the loose material. This technique will allow site workers to compact ground directly beneath the homes without intruding on the properties themselves.
Monitoring on site is being done in a range of ways. To check the compaction grouting, site workers are doing more dynamic probing between injection holes to check it has sufficiently stabilised the ground.
In addition, two surveyors are on site to check level monitors that are placed around injection holes, ensuring there is no local ground heave that could be detrimental to surrounding homes.
Movement of the buildings directly affected by the works is being monitored by 57 optical survey points that are checked manually and 81 automated tiltmeters. These have been installed on 15 buildings and the latter have been set with alarm limits of 2mm and 4mm. At this point text message alerts are sent to on site and off site team members.
O'Donovan says the tiltmeters have registered some movements –usually in the region of 0.1mm – but adds that this is typically attributed to thermal expansion.
The work is due to be complete by the end of the year at which point a landscaping contract will follow to reconstruct paving and improve the look of the street.