Unmapped mine shafts have the potential to reveal surprises even once a location is known, as one in Gateshead is currently demonstrating
Gateshead is a well-known risk area for shallow, unmapped coal mine workings, but when the known worked seams are more than 146m below a site, few would expect a shaft location to be revealed at the surface. Nonetheless, that is exactly what happened below some properties in Miller Street in early 2016. Two years on, work to remediate the problem is nearing completion – despite a few surprises.
Difficult access, risk of further collapse and the need to buy the properties and partially demolish them has taken some time. And that has not been the end of the challenges at Miller Street. The first treatment hole undertaken by Soil Engineering Geoservices for the Coal Authority suggested the presence of a shaft extending to 24m but a second treatment hole revealed a much bigger problem.
Initial estimates suggested that the shaft was 24m deep from early ground investigations and information gained from the first treatment hole. The hole went to 29m to prove there was no false bottom created by collapsed debris. The second treatment hole revealed a very different picture – a shaft to 71.8m below ground level.
“The shaft is actually stepped and narrows at the first seam,” explains Soil Engineering Geoservices business development manager Simon Baxter.
“The experience at this site does demonstrate the importance of not relying on a single borehole to treat a shaft. It also shows why grouting from the base up on several locations is a reliable approach to completing an adequate treatment.”
Despite the scale of the shaft – which was not shown on any Coal Authority plans – there was only minor cracking in the buildings above it. The only indication that there was an issue was a depression that appeared in the ground surface in February 2016.
“We received a call on our helpline and when we visited the site, we could see there had been a collapse,” says Coal Authority principle project manager Mick Owens. “The depression in the ground was only a few metres across, but we were confident it was the result of an unrecorded shaft even before we started the ground investigation.
“There are lots of shallow mine workings in the area that date back to before the 1900s that are not mapped.”
Residents in the rental properties were evacuated and Soil Engineering undertook the ground investigation work under its framework agreement with the Coal Authority.
The ground investigation involved vertical boreholes to define the exclusion zone and angled holes to gain some understanding of the collapsed shaft. Limited access and the fact that conditions in the shaft were too poor for the use of cameras meant that only limited information was available before treatment began.
Owens says this is where the benefit of the Coal Authority’s framework agreements comes in as it has allowed the contracts for treatment to be flexible to accommodate a developing understanding of the problem.
“It was clear that there was a void space above the coal seam,” says Owens.
The ground investigation was completed a few months after the initial collapse, but the planning for the treatment was a lengthy process, which involved the Coal Authority buying the affected properties from the landlord.
Given the instability of the ground, demolition of extensions to the houses had to be carried out brick by brick which also took time.
The design for the grout treatment was undertaken by Coal Authority engineers and focuses on bulk infill to avoid the need to cap the shaft.
Work on site started in late April and was scheduled to finish in early June. However, the deeper than anticipated shaft means that the work is now expected to take a further four weeks.
The planned solution involved three treatment points within the shaft itself that will use bulk infill techniques. Twenty were also inserted in the area around the shaft to 30m depth. They will use a compaction grouting approach to manage relaxed ground around the shaft.
Access has been a major challenge. Fortunately the houses are at the end of a terrace with alleyways to the side and rear, but the extensions on both properties have had to be demolished to allow the stabilisation to be undertaken.
The risk of further collapse means that all shaft grouting had to be undertaken from a T-shaped platform. This is bespoke designed by WM Services to span the 13.9m diameter exclusion zone. The design was not straightforward as the ground slopes, and there is limited access to lift the structure into place. The risk of ground collapse and the sloping site also had to be carefully considered, which is why the project is using two structures in a T-shape to span the collapse.
“The first treatment hole showed that the bottom 5m to 6m was voided,” says Soil Engineering Geoservices contracts manager Peter Stoddart.
The ground conditions at Miller Street are stiff clay to rock head at 10m with the first coal seam encountered at 24m.
“We’re using 2m lengths of casing and casing to the bottom of the treatment hole, then lifting the casing in 2m stages and grouting to achieve a pressure of 2bar before lifting again.”
In total 64t of grout was placed in the first treatment hole.
Total grout take on the scheme at the start was estimated to be around 200t. At the time of grouting the first treatment hole, Stoddart commented that it was taking a lot of grout but that there would be “better understanding with the second hole”.
He was right – the second treatment hole – just 0.5m from the first – extended to 77m and has so far taken over 90t of grout but had not yet been completed at the time of GE’s visit. The additional depth of the shaft has led to the number of treatment holes being increased from three to seven.
Stoddart says that a test hole will also be completed before work starts on the 20 bores around the shaft.
The high grout takes have been a challenge, again due to the limited space on site where the team only has space to store 26t of cement at a time. It has used a 410 colloidal mixer to produce the grout on site.
The plan had been to use a 6:1 PFA grout although the UK’s move away from coal powered energy means it is becoming difficult to source PFA so the work at Miller Street is being undertaken using neat sulphate resistant cement.
While the scale of the shaft may be greater than anticipated, Owens believes that it is an isolated one and that neighbouring properties should not have the same issues.
The overall cost of the work is estimated to be in the region of £600,000 but that includes buying the properties. The Coal Authority hopes to recoup the cost of the house purchase at the end of the work by selling it back to the landlord.