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Sinkhole signal

Foundations Geosynthetics

A new geosynthetic warning layer system will ensure the safety of high-speed trains as they pass over collapsed mine workings in Germany, reports Max Soudain.

Germany's largest reserves of lignite, or brown coal, are found in the area around Leipzig, Halle, Bitterfeld and Lausitz in former East Germany.

From the 1950s extensive opencast mining of the near-surface coal deposits supplied the region's electricity power stations and heavy industry.

Since German reunification in the early 1990s, mining has been greatly reduced. But it has left a terrible legacy of environmental damage: large areas of wasteland, huge spoil tips and heavy contamination of soils and groundwater.

Mining of lignite dates even further back than the 1950s. At Gr÷bers, a small town between Leipzig and Halle, lignite was extracted underground at the Clara Verein mine until the 1930s.

A major railway node is now being built here as part of Deutsche Bahn's (German Railways) extension of the highspeed Line 8 between Berlin and Nürnberg. The junction is on a new loop from Erfurt to Bitterfeld to include Leipzig and its new international airport on the high-speed route.

The first stage of the scheme involves construction of 23km of line east from Gr÷bers to Leipzig, via the airport. This line will cross the Halle-Leipzig railway at Gr÷bers, where up to seven tracks will run side by side, including those for freight and local surface trains. The 3km node includes several crossover points, a new station and a road underpass.

Part of the junction lies directly over the Clara Verein mine. Here the 5m to 8m thick lignite seam lies 30m to 70m below ground in Tertiary deposits. It was mined in extraction chambers, typically 4m by 4m and up to 5m deep, accessed by vertical shafts from ground level and by tunnels connecting chambers at depth. When mining ceased, timber supports were removed and the shafts, chambers and tunnels simply allowed to collapse.

The high-speed line will pass on embankment. As it is designed to carry trains running at up to 330km/h, ground stability was of prime concern as the trains and track are very sensitive to movements.

'For these high speeds, even the smallest vertical or horizontal movement could pose a serious risk to the railway, ' explains Wolfgang Ast, a geotechnical consultant to the German rail authority, Eisenbahn-Bundesamt.

The area of concern is up to 1km long and 100m wide, affecting nearly 800m of track. While the positions of tunnels and shafts were known and stabilised with grout as part of preparatory works, voids are likely to exist, with risk of further collapse of between 0.5m and 1m, especially at tunnel crossing points, says Ast. 'The shafts are up to 4m in diameter and can appear in just one day - it is a broken field, ' he says.

Hermann Hubal from Eisenbahn-Bundesamt adds: 'We need to make sure that the cavities are filled but there may be voids at depth. Dynamic loading by highspeed trains can induce collapse and we need to prevent this.

'The track has to be built and controlled in such a way that, if collapse occurs, it can continue operating safely for four weeks while remediation is carried out, probably by grout injection.'

Any distortion of the sleeper framework has to be kept within 4.5mm over a 1.5m rail spacing.

Hubal says there was extensive discussion about whether to run the railway on bridges with foundations up to 70m deep, on concrete slabs, or to carry out large scale excavation, replacement and compaction.

Instead, a German team from geosynthetics firm Huesker Synthetic and instrument manufacturer Gl÷tzl came up with an innovative system comprising two layers of high strength geogrid to provide protection from movement for up to four weeks and a geofabric warning layer that can pinpoint the location of sinkholes.

The warning system consists of two layers of non-woven geofabric incorporating loops of electrical resistance wire, 250mm apart. The geofabric layers are laid at 90infinity to one another in 5m wide strips up to 40m long to form a 250mm by 250mm grid of wires. If these wires stretch (ie if a sinkhole forms beneath and the fabric deforms into the void) their resistance changes, alerting engineers to the collapse and pinpointing the location.

Field trials were carried out in 1997 to finalise design of the embankment, to confirm that the warning layer worked and that the geogrid was strong enough to maintain stability and track safety for four weeks under the dynamic loading caused by passing high-speed trains.

Construction at Gr÷bers began in May 2001. Local material, consisting of silty sands and slightly plastic clays, is excavated and mixed with a cement stabiliser in an on-site plant. A 400mm thick layer is then placed along the line of the railway, with some insitu mixing with underlying material to further improve ground strength.

This is followed by a gravel bedding layer. Extensometers, which act as a back-up to the warning layer, are placed horizontally with the two layers of geofabric laid on top. Nearly 700 sheets of geofabric are being used, laid by hand and overlapped to ensure full coverage over 90,000m 2.The resistance wires and extensometers are linked to monitoring controllers with measurement software and memory storage. Data is then sent via a network of fibre optic cables to a central controller, where engineers can check for any movements.

Next comes a crucial element of the system, the 300mm thick protective and load-inducing layer of sand and gravel. This ensures immediate response of the system to collapse, says Hubal.

Two layers of Fortrac 1200 geogrid are machine-laid crosswise on the load-inducing material in sheets overlapping by 11m longitudinally and 250mm with adjacent strips.

The grids comprise strips of Aramid with a strength of 1200kN/m joined by strips of 100kN/m material. The lower geogrid is laid with the Aramid strips running across the rail line, the upper with the strips running along it. Both sit within a 550mm thick gravel layer. A total of 213,000m 2of Fortrac is being used at Gr÷bers.

The 3.5m high embankment is then built above using cement stabilised material as before.

If collapse occurs a vault progressively forms in the stabilised material, with the static and dynamic loads taken by the geogrid. The load inducing layer will move downwards, forcing the warning layer down and stretching the wires, indicating collapse and pinpointing the sinkhole. If collapse is significant enough to snaps the wires, the system is not affected, says Hubal.

A colour-coded warning system ranging from green (no deformation) to red (deformation) alerts engineers to problems.

Construction of the loop line is due for completion by 2007.

Field trial Because Gr÷bers will be the first use of the warning layer system, a large-scale field trial was carried out between November 1997 and July 1998 at a test facility in Leipzig.

Modelling was used to design the system for the most unfavourable position of sinkholes. This was followed by a full-scale test, comprising a model embankment with a simulated sinkhole beneath - an excavation with water-filled rubber bags, drained to trigger collapse.

The embankment was heavily instrumented with inclinometers, pneumatic and electronic pressure cells, geophones and strain gauges placed directly on the geogrid.

Dynamic loading was then carried out to simulate train traffic. A 2.5m diameter steel plate on an oscillatory vibrator was vibrated at 27.5Hz with oscillation speeds of up to 30mm/s for 20 minutes - equivalent to four weeks of trains travelling at 300km/h.

Dynamic loads were up to 70t.

Maximum allowed differential settlement was set at 3mm between two rails 1.5m apart. Results showed that one rail settled by 1.5mm, while the other moved 3mm, well within the specified levels.

The structure even held when the sinkhole was enlarged to 4m by 8m in plan, a worst case scenario expected in about 5% of sinkholes. Maximum strain on the Fortrac geogrid was 0.8%, with static loads caused by collapse between 100kN and 200kN, occasionally up to 400kN.

The trial also showed that there was no settlement 4m from the edge of the sinkhole and so it was decided that a minimum of 7m of geogrid was needed either side of any potential collapse.

Tests to determine the compressive strength of the stabilised embankment and the tensile strength of the geogrid after loading were carried out with results used as the basis for designing the working embankment.

The route Work at Gr÷bers is being carried out under the TransEuropean Transport Networks programme (TEN-T). Line 8 between Berlin and Nürnberg will form part of the northsouth axis from the German capital to Verona in northern Italy. Deutsche Bahn is investing DM30bn (-13.5bn) of which the 500km Line 8 will cost -6bn, reducing travel time between Berlin and Munich from eight hours to four. The Gr÷bers node is part of a loop east from Erfurt to Leipzig airport, rejoining the main route at Bitterfeld.

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