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A Welsh shopping centre development is getting geosynthetic protestion against potential mine crown failures. Damon Schunmann went along to see a solution unusual for the UK.

Site preparation for a new out-of-town shopping centre is underway at a former brickworks in Merthyr Tydfifil, South Wales.

The main contractor for the £12M earthworks, Edmund Nuttall, also has to deal with coal and ironstone tips and abandoned mine workings.

It has awarded a £3M mine infilling subcontract to its geotechnical division Ritchies.

The client is Charles Robertson Developments, owner and operator of the Trago Mills store chain in Devon and Cornwall.

When GE visited at the end of July, the 54ha site had been profiled into distinct tiers similar to paddy filds. A boating lake is under construction at the highest level and there will be a petrol station area on a middle level. The store and main car park footprints were clear on a lower tier.

'We have grouted under the footprint of the store, the petrol station and the boating lake as these were three critical areas where the risk of ground movement and crown holes coming to the surface was unacceptable, ' says Nuttall contract manager Mike O'Neill (see box).

Another part of the site is getting different protection against the possibility of future crown holes reaching the surface.

'It would have been massively expensive to drill and grout everything, so we will have geogrid under the car park, ' says O'Neill. 'The design risk assessment said there was less chance of a crown developing there because of the reduced loading, but the geogrid will span any hole, preventing a collapse while remedial measures are carried out.'

Naue Geosynthetics was called in on a £110,000 contract and its inhouse consultant BBG did a detailed design, tweaking the conceptual design by project manager and consultant C D Gray Associates to see which geogrid should be used.

'The aim is to prevent a situation with a bus in a hole, ' says Naue Geosynthetics managing director Chris Quirk.

The original plan was for two layers of polypropylene geogrid.

Engineers wanted a material capable of bridging a 3m diameter crown hole for 1000 hours. But BBG thought polyester was more appropriate for this kind of work.

The Naue team says this is because polypropylene tends to stretch under stress, whereas polyester is a glassy plastic not prone to deforming.

'We went from two layers to one as we are using a more efficient grid, ' says Quirk. He says the switch was unbiased because the company manufactures both kinds of grid, and claims changing to one polyester layer saved about £118,000.

'Polypropylene is actually a cheaper plastic but it's less efficient when deformation is a design consideration, ' he says. 'If you are just improving the strength of a soil for a road or car park, then deformation isn't a design parameter and we would go with polypropylene.

'But when deformation is an issue we would always go with polyester and because you need less of it, it becomes a cheaper solution.

The design factors of safety for creep for polyester is 1.53 but for polypropylene it is 2.5.'

BBG came up with a uniaxial design featuring a Naue 60kN uniaxial geogrid, Secugrid 60/20 R6, which will direct the loading from vehicles up and down rather than going out radially. This meant geogrid overlaps along the longitudinal edges could be reduced to 300mm, saving a significant amount of material. At the ends of the rolls, the transverse joints have a 1.7m overlap.

The grid's 60kN longitudinal strength bars do the work; all the 20kN bars have to do is keep the 60kN ones in position.

Nuttall is placing the Secugrid near the surface with a 300mm type I site-won sub-base on top.

Roads through the site will get the same geogrid treatment as the car park.

'We are the only company that makes prestressed bars of polyester welded together, ' says Quirk. 'This makes it a much more efficient geogrid than any other manufacturing process.

Professor of civil engineering at University of Strathclyde, Alan McGown, dubbed them 'the third generation of geogrids'.'

'Our polyester grids are all welded whereas some of our competitors use extruded polypropylene or HDPE which is not as ef cient [on a job like this] because of creep issues. Others use woven polyester grids. But each company tends to make one family of plastics so they have one horse for all races. We have both uniaxial and biaxial grids, both in polyester for where stress and strain is an issue, or cheaper polypropylene where it isn't. We feel we can discuss the technicalities of both rather than just one type of plastic.'

McGown says geogrids are not often used to bridge mine voids in the UK (where they are normally used for reinforcement) but that it is common practice in Europe, which he attributes simply to a different design philosophy.

BBG designed the grids at the Trago Mills car park to have a maximum deflection of 500mm over an anticipated 3m void, which is the normal expectation of a crown diameter in this situation.

However the Naue team says crown hole sizes are a big unknown, with designers picking 3m based on past experience.

Cardiff-based CD Gray Associates is allowing for a small amount of deformtion over a crown hole as some deformation is actually required to mobilise the strength in the geogrid.

For the lining of the boating lake, on the site's highest tier, subcontractor Keytec Environmental put in both a geomembrane and geotextile protector. It rst placed 150mm of colliery shale, with a Nico ex 300 overlaying this to protect an 800 impermeable membrane on top. Over this went 300mm of puddle clay.

All work for the current phase should be complete by the end of this month, but there will be a delay before store building begins as the client has yet to let the contract.

Mine infilling

'The old brickworks site has also been extensively mined over the last few centuries, ' says Nuttall contract manager Mike O'Neill. It also features both coal and ironstone seams.

'It was derelict when we came to site in February last year and still had the foundations from the old brickworks.

'The first job was to do a series of boreholes.

We had old maps showing the adits and shafts, so we did a drilling investigation to see the orientation of the adits. We could then fill them with pea gravel as grouting wasn't necessary because most were on the edge of the site, ' O'Neill says.

For the second phase of mine infilling, Ritchies brought in four Boart Deltabase drill rigs and a Casagrande M9, all equipped with hydraulic top hammers, and built a grouting station.

'We've drilled over 110km in 4000 holes and put in over 15,000t of grout, ' O'Neill says.

'Another issue is we have the River Taff on the east side of the site. When the A470 was built in the 1970s, also on the east side, there were problems with grout getting into the river, so we had a high degree of discussion with the Environment Agency for methods of working to make sure this did not happen again.

'We filled voids going down to 20m because the ground investigation showed the biggest voids were 2m [from floor to ceiling].

'You need a one to ten or greater ratio of solid material above a given void to mean it is unlikely that a crown hole will propagate to the surface.

'We have also moved 1.3M. m 3 of material for the earthworks operation that is predominantly colliery spoil. Once we started the excavation we were exposing fines that would be picked up by rainfall and discharged into two brooks, Swansea Road and Heol Gerrig.

'To prevent this we had a surface water management plan approved by the Environment Agency and we intercept all the surface runoff and direct it into temporary lagoons. It is treated there with occulent to encourage the suspended solids to separate and we allow the clear water to discharge into Heol Gerrig brook.

'As construction continues the water is diverted to other [catchment] areas around the site.'

Ritchies' grouting work finished in late spring.

Nuttall's geogrid work is due to complete this month.

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