Geosynthetics: Covering coal legacy

Oct 26, 2023

A complex remediation project at a Walsall industrial site has opted for the less common solution of using high strength geogrids over historical coal mine shafts.

Property developer HBD, part of the Henry Boot group of companies, received planning consent for its Phoenix 10 development from Walsall Council in April 2021. The 18ha brownfield site is between Junctions 9 and 10 of the M6.

HBD then appointed John F Hunt Group's regeneration business (John F Hunt Regeneration) to prepare the site for the construction of a new 58,000m2 industrial and business park. Tailor Engineering is in turn involved as specialist element designer, and Forkers as a ground engineering contractor.

The site has a long industrial history of coal mining, iron smelting and brick making. In 1917, the James Bridge Copper Works was constructed and remained operational until 1999. It was demolished soon after and the area has been used for domestic and commercial waste.

The site is not dangerous to the health of people living or working nearby. Nonetheless, a programme of intensive remediation and ground engineering work has to be completed before construction can begin. This includes addressing issues with groundwater, metals and foundry waste, and areas of infilled land and made ground.

John F Hunt Regeneration ground engineering director Adam Fisher says: "We’re in the local community here; there are sports pitches, a school on our doorstep, and there are commercial and residential areas."

John F Hunt will thus carefully treat areas of contamination and stabilise the numerous mine shafts in preparation for construction. It began work on site in early 2022.

"This is a huge site with a mining legacy, and there has been quarrying as well. Despite its great location, it remained undeveloped for a very long time because it's challenging and costly," Fisher adds.

"We are preparing the site for a large employment scheme. It's next to the M6, so it benefits from fantastic access to motorways and is perfect for that sort of development."

A programme of remediation and ground engineering work has to be completed before construction can begin on site

John F Hunt's scope of work, jointly with Forkers, involves drilling and grouting to help stabilise the mine workings associated with 69 shafts. It currently has 12 drilling rigs on site.

"We’re stabilising the shaft and mine workings from the current ground surface while the landfill is still in place," Fisher explains.

"Once we’ve finished the grouting, we excavate down to expose the shafts, and underneath [future] buildings we’re putting in reinforced concrete shaft caps. But underneath every other part of the site – covering about 25 shafts – we’re introducing geogrid reinforcements."

The first of the high strength geogrid shaft caps in an area of proposed hard standing and open space was installed towards the end of September.

"We’re right in the heart of this work at the moment. In conjunction with that we’ve got a massive earthworks undertaking, because we’ve got to take out all of the landfill – screening, processing and validating it," Fisher notes. "We’ve also got groundwater that we’re treating. There's impacted groundwater within the mine workings. So, we’re pumping the water, treating it and using it in the stabilisation – it's going into the grout. And that's a way we’re creating betterment in the water environment."

The Walsall Canal passes through the project site. John F Hunt, with help from Tailor Engineering, will also restore the canal banks and make sure that they are stable for the life of the development.

"The amount of ecological benefit from canals is staggering. The site is part of an industrial area and running through the middle you’ve got this waterway, and there's reeds and wildlife, which are significant," says Fisher.

"There was talk of sheet piling, but the canals and rivers just don't like hard engineering of that nature if it can be avoided. So, we’re trying to divert away from sheet piling and come up with something softer. We’re just starting now to do some further investigations." The geotechnical solution could include driven tipping plate soil anchors and a vegetated slope with geogrids.

Temporary works are needed to support part of the site which borders onto the M6. A number of services also need diverting.

There is currently a 12m high raised slope that forms part of the landfill.

"We’re taking it all out and then we’re going even deeper. When we come back up, we’re going to be putting in a big, reinforced soil berm or bank, and there's going to be a water attenuation feature in front of that," Fisher explains.

"So, there will be geogrids for traditional reinforced earth and GCL [geosynthetic clay lining] of the attenuation feature."

High strength geogrids are used to span voids above the mine shafts

The project includes a wide array of materials, from ground improvement GCLs for containment to geogrids for soil stabilisation.

Tailor Engineering director Nicola Brusa says: "Some of the elements of the ground improvement are quite niche. We are using the BS 8006 Part 1 [code of practice for strengthened/reinforced soils and other fills] for designing all of the high strength uniaxial geogrids to span voids [above mine shafts]."

Tailor Engineering has also used this solution over mining works in karstic areas and weak chalk on projects across the UK and Europe.

"For any areas that could be under threat of collapse following the formation of a void, a high strength geogrid reinforcement placed at the base of an embankment is now considered an accepted foundation engineering technique," Brusa explains.

"The geogrid reinforcement is designed to prevent catastrophic collapse of the embankment and to prevent unacceptable surface deformations which may occur during the design life of the structure.

"High strength geogrids are designed for a 100 year to 120 year design life. Generally, this method is used for infrastructure embankments over soft soils or supported by piles. However, geosynthetics have been used to span features within housing developments located in former quarries or historical mines."

He adds: "This solution of spanning voids with geogrids is not new; it's been used since the 1970s. But it's a very niche application in the sense that not many contractors, engineering firms or projects are considering this as a sustainable way of stabilising a shaft."

Using high strength geosynthetics instead of traditional solutions could also offer sustainable and economic benefits and protection against unexpected collapse caused by subsidence.

"The geogrid we are using is not a standard one either; we’re talking about high strength 1,300kN to 1,600kN geogrids," Brusa notes.

The geogrid chosen for the project has a polyethylene coating to provide high resistance to installation damage.

This is because a geogrid manufactured from a polymer, such as PVA (polyvinyl alcohol) or uncoated PET (polyester) geogrid/geotextiles, could suffer a reduction in strength due to hydrolysis in ground with a high water table.

The alternative to the geogrid solution would be to use reinforced concrete. However, according to Fisher, the reinforced concrete component could be 10 to 15 times more expensive. In addition, the geosynthetic option has a smaller carbon footprint.

"Sustainability certainly plays a massive role when you’re using this technique," Brusa adds, "and I would also point out that, there is not only economic and financial benefits for using it, but also from a health and safety point of view, because you don't need to have any shutters or any reinforced concrete works going on, avoiding mobilisation/demobilisation of specialist machinery."

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