Midland Metro undermined

Railway civil engineers, in the UK at least, have always been concerned about unknown or unpredictable hazards affecting their infrastructure. Buried abandoned mine shafts are a classic example, and there are well known photographs showing dramatic accidents caused by collapses under the tracks. Rail infrastructure operators are generally well aware of that particular risk and take appropriate measures.

The Midland Metro recently encountered a slightly different hidden danger that had a major impact upon operations and track renewals. On this occasion, the culprit was not a hidden shaft but the remains of an old mine working, believed to have been what is known as a bell pit.

Abandoned workings

Bell pits were used in the early days of the industrial revolution and probably a long time before that too. Each pit began as a short vertical shaft, dug from the surface down into the coal seam or other stratum to be mined. The miners then set about excavating the desired mineral from around the base of the shaft, working outwards and so forming a bell shaped excavation. The excavated material was hauled up the shaft to the surface for use in industry.

Clearly this was a dangerous business as the ground above the pit was often left unsupported, and it must have been a fine judgement deciding when to abandon a pit before it collapsed onto the miners!

In the Black Country, bell pits were commonly used to obtain coal or iron ore. When one pit was determined to be worked out or unsafe, another would be started nearby until no more valuable material could be safely and economically recovered from the site. It was quite usual to just walk away from worked out pits, leaving them to remain standing or to collapse, as nature and weather dictated. It was not common practice, at the time, to document the location of these pits, and even if it had been it is likely that such records would no longer be available today.

Modern consequences

Time to return to the present, and to the Midland Metro in the centre of Wolverhampton late last year, close to the stop known as ‘The Royal’, the last stop before the St. George’s terminus in the city.

As part of a project upgrading the Metro for the introduction of new and longer Urbos 3 trams from CAF, a new crossover had been laid in just south of ‘The Royal’. A life expired old crossover north of the stop was redundant as a result, and work began removing it and excavating out the foundation slab upon which it had lain.

All this was encompassed in a £4.5 million track improvement scheme lead by Centro, the region’s public transport co-ordinator. Main contractor for the works was Graham Construction, with specialist railway works undertaken by Rhomberg Sersa.

It was at this point that the project team got a nasty surprise. Paul Griffiths, metro programme director for Centro, recounted that, as the excavation proceeded, it became clear that under the slab was a lot of rubble, full of voids and unstable. Ground proving radar surveys had not detected this (probably because of the existence of the reinforced concrete track slab, it is thought), and neither had desk studies of existing records of the area and of the original Metro construction.

Reinstating the Metro over the top of this without proper remedial measures was clearly not an option. Whilst investigations were undertaken to establish the extent and depth of the rubble filled void, Centro began seeking advice about what they had found and how it might be made safe. Consultant AECOM was engaged to provide expert advice and Centro also consulted other metro operators, through the UITP and UK Tram.

The problem lay in an awkward location, since at the affected location the tramway runs between the two carriageways of the A41 Bilston Road between Steelhouse Lane and Hospital Street. The proximity of the highway restricted any engineering activities because of the risk of damaging the road. With the tramway shut, closing the road and further restricting options for commuters and other travellers was not an option.

Investigations eventually determined that the void extended to a depth of about 12 metres, covering an area about 54 metres long and 20 metres wide. With the size of the problem determined, activity shifted to consideration of possible remedial options.

What to do?

Amongst those considered was sheet piling right around the void, excavating the rubble fill and backfilling with suitable material, properly compacted. This was dismissed as too expensive and time consuming. A similar conclusion put paid to the option of bridging over the void with a slab carried upon bored piles inserted down to sound ground.

Other options considered and rejected included grouting up the existing fill, and the idea of laying in ballasted track over the rubble, accepting that regular track maintenance by tamping would then be essential for many years.

The solution eventually adopted involved covering the existing fill with a geotextile to prevent the migration of fines, then layers of geogrid separating layers of compacted hardcore, all supporting a 5 metre wide reinforced concrete track slab. AECOM recommended Wrekin’s E’GRID 3030L or an equivalent medium loading (100kpa or 12 tonnes axial load) biaxial geogrid.

The work obviously involved a certain amount of excavation of the existing fill so as to get the correct levels for the final slab, but this excavation was relatively shallow and presented no risk to the adjoining highway.