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Slab solution for main line mine moves

Construction of a piled slab should solve subsidence problems that have plagued a section of the East Coast Main Line near Edinburgh for more than 120 years.

The Newcastle to Edinburgh section of the East Coast Main Line is one of the most dramatic stretches of railway in the British Isles. Much of it hugs the clifftops of the Northumberland and Scottish Borders coastline.

As trains approach Edinburgh, the line cuts in from the coast on to flatter, gently undulating ground. Surprisingly, the geotechnical challenges are probably greater here than along the clifftops.

In particular, subsidence of a 250m section at Wallyford has been causing intermittent problems for more than a century, and especially so over the last decade.

Since early 1999, subsidence risk has reduced train speeds to only 60km/h for passenger services and 30km/h for freight. Given that Wallyford lies within a long section of straight track which should allow speeds of 200km/h, the restrictions have made it impossible for trains at run to line speed over the last two years. This is inconvenient, but the real issue for Railtrack Scotland is ensuring the situation does not deteriorate.

The problems stem from the fact that the site is underlain by unrecorded shallow stoop and room mine workings in the Great Seam Coal, which are typically 2. 5m thick.

Subsidence occurred in the 1880s although no further movement was recorded until 1992 when a 2m diameter, 6m deep crown hole developed immediately next to the track. Smaller crown holes appeared in 1994.

Later that year grouting consolidated the mine workings over a 250m length of the track.

But despite these operations, further subsidence occurred last year.

Railtrack Scotland called in geotechnical contractor Ritchies and consultant Donaldson Associates to investigate.

The study included investigation of the ground conditions, assessment of the reasons for ongoing subsidence and evaluation of solutions.

Donaldson Associates set about developing a detailed ground model, explains the company's Willie Cannon. Ground conditions were found to be between 4m and 8m of very loose and loose silty sands overlying a thin and intermittent layer of glacial till and rockhead.

The Great Seam Coal is 8m beneath the railway with a dip of about 16degrees oblique to the track.

The rocks overlying the Great Seam generally comprise weak mudstones and siltstones with some sandstone units towards the down dip end of the site.

Strata were generally in an advanced state of collapse with many small but poorly connected voids throughout the collapse zone.

Occasionally some open 'rooms' remained, with an average void height of 2. 5m.

Significant groundwater fluctuations proved something of an enigma and are believed to have had a significant influence on the ongoing movements. Groundwater level varies seasonally by 3m, with the highest level within the track ballast. A slight artesian pressure was also recorded from piezometers installed within the workings in the Great Seam.

In 1994, grout was injected through inclined holes at atmospheric pressure. This was to avoid causing heave in the track, which remained in use throughout.

'Low grout takes recorded over much of the area of consolidation confirmed the generally advanced state of collapse, ' says Cannon.

'However, the subsequent ongoing movement recorded in 1999 was considered to be attributable to the number of voids which were not consolidated by the grout due to the lack of interconnectivity in the collapsed strata.

'Large grout takes were recorded in some areas although it appears the grout may have been subsequently washed out by the groundwater, leaving the intermittent open rooms found in the recent investigations. '

Donaldson identified two distinct zones: an eastern zone where the strata had generally collapsed, and due to poor interconnectivity regrouting was not considered feasible, and a western zone where the workings were generally open allowing regrouting.

Focusing now on the more problematic eastern zone, Railtrack organised a technical brainstorming workshop with Donaldson Associates, Kvaerner Construction and Kvaerner Cementation Foundations to discuss several options including consolidation by dewatering, permanent diversion of the track and a structural solution to support the track over the collapsed strata.

In October 1999 Railtrack entered a partnering arrangement with main contractor Kvaerner Construction for the preferred option - a piled ground slab and temporary diversion of the railway. Kvaerner Construction subsequently appointed Donaldson Associates as civil and geotechnical designers with WS Atkins undertaking permanent way design.

The temporary track diversion was a major operation in itself, as it is also positioned over the shallow mine workings. The rationale was to put the diversion on a concrete ground slab designed to span across crown holes of the size known to have previously developed at the site.

Foundations for overhead power gantries were also oversized so as to remain stable should a crown hole develop.

The diversion is 300m long and trains will continue to reduce their speed to 60km/h, since it forms a relatively sharp curve in the track.

Slab ends have been formed perpendicular to the track alignment and transition zones formed by a Tensar geogrid reinforced stone layer avoid twist faults as the track passes from the old to new formation.

Kvaerner started work on the diversion in January this year and completed the task on schedule to allow transfer onto the new alignment during a 54-hour possession over the Easter holiday weekend.

Attention is now on the permanent pile supported ground slab. Piles are installed through the overburden and rock strata and socketed into the pavement of the Great Seam.

The slab is designed to act as a bridge with no reliance on the subgrade for support with the piles designed to work through up to 5m high voids. Downdrag loads due to any further collapse and railway loadings were included in pile design.

Evaluation of the pile type was carried out by Donaldson Associates, Kvaerner Construction and Kvaerner Cementation Foundations, which resulted in selection of a 219mm diameter, 12. 5mm thick steel casing and a 170mm diameter, up to 3. 5m deep rock socket.

Pile testing before Christmas confirmed the drilling technique through the collapsed strata and the adequacy of the rock socket. Test piles were reinforced with a Dywidag anchor bar to provide a free length to transmit the test load to the rock socket where the design loading is to be carried.

Piles were tested to the required loading of 1000kN with settlements of less than 4mm under the proposed working load. Armed with this information, Donaldson determined the pile layout with 2m spacing both longitudinally and laterally. This gave four piles across the slab and 78 rows along it. Due to the dip of the Great Seam the piles extend from 9. 5m to 27m.

Additional piles for overhead gantry and signal bases give a total of 336 piles.

Production piles will be reinforced with a single 40mm diameter GEWI steel bar, except where loads are heaviest, in which case a 50mm bar in the rock socket will be coupled to a 40mm bar within the cased length. Piles are designed as composite section with the casing, grout and reinforcement all acting together to provide the structural capacity.

Railtrack was particularly concerned over the performance at either end of the piled slab and so Donaldson designed 'zones of transitional stiffness' to ensure a smooth ride between the existing ground and the relatively stiff piled slab.

Transition design comprises a zone of stone columns overlain by a 550mm deep Tensar geogrid reinforced stone platform and a tapered concrete slab. The slab is omitted from the design after 7. 5m and the stone columns stopped after 12. 5m with the geogrid platform being gradually phased out over a further 15m. There is no structural connection between the transition zone and the main slab.

Work since Easter has concentrated on stone columns for the transition zones.

Kvaerner Cementation Foundations began piling in mid-May using three rigs to install the 336 piles over eight weeks. Slab construction will start by the middle of this month and finish by the end of August. Kvaerner Construction will hand back the track in late September, allowing trains to run at a top speed of 200km/h for the first time in two years.

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