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Lode line


When historic lead mine workings were found to exist beneath residential property in the Drake Gardens area of Tavistock, Devon, Hyder Consulting was called in to design an investigation and stabilisation scheme that would remove the risk of collapse and consequent damage to the properties.

Civil and ground-engineering contractor Forkers was awarded the contract to undertake the ground-treatment work, following a quality/price tender submission in September 2010.

The shallow workings, thought to date from 1857-1874, had resulted in the evacuation of some of the 14 affected properties.

The Land Stabilisation Programme, managed by the Homes & Communities Agency, provided gap funding for the project to deal with the effects of abandoned non-coal mine workings.

The scheme was carried out in two phases. The advanced works were conducted from November to December 2010, with the treatment phase in March to July 2011.

Advanced works

A range of intrusive investigation work was completed to confirm the extent and condition of the lead mine workings beneath the road, gardens and 14 residential properties. Works used rotary probing, dynamic probing, ultrasonic void surveys and trial pits.

Investigations also focused on locating a main adit and three mineshafts. A grouting trial was undertaken to prove the proposed stabilisation methodology while down-hole ultrasonic cavity surveys were performed to obtain data on the size and location of significant voids.

Charlotte Wheatley of Hyder Consulting explains: “We located some historical plans and sketches, which have been found to map the site quite accurately.

“The lead lode dips at approximately 60°, deepening eastwards against the topography, and was mined from a series of adits driven horizontally along the lode at depths of 22m, 44m, 73m and 95m (12, 24, 40 and 52 fathoms).

“The site is overlain by 5-7m of river terrace deposits, beneath which is a soft slate, within which the lead lode has been formed. The sub-crop of the lode runs close to or under several properties on the west side of Drake Gardens, resulting in the risk of void migration to the surface. There is a main shaft (Bedford Shaft) at the northern end of the site and two shafts in Drake Gardens.

“A drainage adit was conjectured to run north to south down the western edge of the site, and another cross-cut drainage adit was thought to run east to west; these adits being only 7-8m deep. The workings phase out to the north due to faulting,” adds Ms Wheatley.

Although early site investigations conducted by a local consultant had suggested there were significant voids, this was not borne out by the latest investigation work. There was significant micro-voiding of the soft and loose material filling the mined lode, and stabilisation work involved treating this very soft material.
Since the groundwater level is at 4m, there were initial worries that the grouting operation would result in flooding, so levels were monitored carefully during the works. Although water levels rose by up to 0.5m immediately after grouting, this quickly dissipated over the following 6-12 hours, so it did not present a problem.


Following analysis and interpretation of the investigation data by Hyder, details of the stabilisation scheme were confirmed.

Treatment has involved stabilising the shallow, upper 30m of the steeply-dipping lead-mine workings by drilling and grouting beneath the affected properties with a close grid of boreholes, and by using a grouting methodology to treat the broken and collapsed ground.

The drainage adits were investigated and treated where they affected the properties. Some 290 treatment holes have been completed, starting with a 3m perimeter line along the down-dip edge of the treatment grid.

The avoidance of services and utilities, including overhead electricity and telephone cables, required some boreholes to be repositioned. Trial pits and radar scanning was carried out to ensure all of these services were located prior to drilling.

The nature of the soft, broken material in the lode workings allowed less grout takes than expected, with 450m3 used.

Grout material storage and mixing was carried out in the site compound, located at the northern end of the site, away from the properties. The grout was a mix of sand and cement, with a small amount of bentonite powder, to produce a stiff substance for the compaction-grouting stabilisation process.

Trials were carried out to ensure the mix proportions, flow and pumpability achieved the specified strength, and to test the proposed grout mixing and pumping plant.

A Winget 400R discharged mixed grout into a Brinkman screed pump with 4m3/h capacity. Due to the distance, the grout was stage-pumped using a secondary agitator and screed pump, and then pumped directly into the treatment holes.

Due to the site’s confined nature and limited access (and with drilling carried out away from gardens, driveways and the cul-de-sac outside the properties), the scheme required the use of specialist rigs. Much of the treatment phase drilling has been to complete angled holes from around the properties to treat the workings below.

A number of specialist machines have been used, including rotary percussive Klemm KR904 drilling rigs and Klemm KR701 tracked mini-drilling rigs for work in the limited-access and confined space locations at the rear of the houses.

Drilling was carried out with waterflush, with the returned drill flush and cuttings pumped back to the compound for treatment. Rotary percussive steel casing of 100mm diameter was used to case off overburden and a 70mm-diameter hole was completed to the full treatment depth. An HDPE grout pipe was then placed to the bottom of the hole and the grout injected in ascending stages as the pipe was withdrawn.

Grout flow and pressure-gauging equipment allowed continuous monitoring of the grouting at each borehole, with the information digitally transferred to a data-logging system. This data was fed into a Rockworks 3D modelling software program to map the real-time progress of the work.

Community liaison
Due to the confined nature of the site and proximity of homes, this was a high-profile and high-impact local scheme, which relied heavily on extensive resident liaison and customer care. Well-attended public meetings were held before each phase, with the key to the success of the contract being the ability of the team to foster the co-operation of the public during the works.

The estate is a mix of private and rented homes. West Devon Council handled the early relationships, while Forkers was responsible for day-to-day liaison with residents. There were no major complaints. Every effort was made to ensure work was carried out around residents’ routines, particularly since homes had to be vacated whenever drilling was conducted close to or under them. In some cases, the council rehoused families in a local caravan site.

Forkers’ site agent, Tony Mellor, explains: “The stabilisation operation started in the south of the site and worked towards the north, with validation test holes following on behind. The time spent in individual properties varied. In the south, it was usually 3-5 days, but at the northern end of the street, where we also had the adit and shaft to treat, the work could take up to 2-3 weeks.”

Residents were given seven days’ notice by letter of any works near their home, explaining how long it would take. These letters were followed by a further update, 48 hours before work was due to start.

Reinstatement work began as soon as the stabilisation work was complete for each property; returning gardens, fences and planting to their pre-works state. Photographs were taken and surveys carried out in advance to allow for accurate reinstatements. West Devon Council homes will also be redecorated, and the affected section of the cul-de-sac and pavement will be resurfaced at the end of the project.

“Resident feedback on our performance has been very good,” comments Mr Mellor. “They were keen to have the work completed to ensure their properties are safe from collapse and saleable as well, but they also wanted the street to be clean, access to be maintained and the work to be carried out safely with minimal disruption – which we managed to do.”

The treatment works were complete by mid-July and reinstatement works will finish by the end of the month. A closing survey to sign off reinstatements and a CCTV survey of the sewers will also be carried out.
“It has been a very good team of people. Generally, the project has gone really well with no major hitches. Technically, it has been very interesting,” adds Ms Wheatley.

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