In order to ascertain the risk of collapse of a new road embankment over a historical railway tunnel, a number of intrusive and non intrusive investigations were undertaken. The non- intrusive investigations comprised: a visual inspection, a delamination survey, a ground probing radar survey from the ground surface and an electrical resistance tomography survey from within the tunnel itself. Intrusive investigations comprised of rotary coring of the surrounding rock and of t he tunnel lining. Numerical modeling of the proposed development including it's construction sequences were undertaken. A number of models were analysed including 2D boundary and finite element analysis and 3D stability analysis of the tunnel itself. The findings of the analysis were that the tunnel ‘bore’ itself was generally stable but the tunnel lining was in a state of degradation with approximately 11% failed to date. This led to the conclusion that a progressive failure mechanism may form, where the tunnel lining degrades further and collapses, leading to block failures throughout the surrounding rock, along the lines of the discontinuities, eventually progressing upwards to affect the embankment and road. The final remedial solution was to incorporate geotextiles within the embankment construction in order to mitigate the risk of catastrophic failure.

1.1 Proposals and site legacy

The regeneration of the former Gedling Colliery site in Nottingham, UK, will transform 35 hectares of the pit head area and associated spoil heaps of this former deep mining colliery site, into a mixed use development. The development will include a residential housing area of approximately 1200 homes, a commercial region for local businesses, primary school and other amenities.

(Figure in full paper)

Early in the planning stage, it was identified that the proffered line and orientation of the associated development access road would require construction of an embankment up to 12m in height crossing over a former railway tunnel.

The tunnel was constructed approximately 150 years ago and originally formed part of the Great Northern Railway, Erewash Valley Line, which served numerous collieries, ironworks and brickworks in both Nottinghamshire and Derbyshire, including the adjacent Gedling Colliery. Latterly the railway was dismantled and the tunnel line became disused in the 1960's. Figure 1.1 comprises an aerial photograph of the tunnel and figure 1.2 shows the approximate tunnel geometry.

In addition, the tunnel comprised a registered bat roost, which under UK law is protected from actions which may damage this habitat or otherwise disturb roosting bats.

(Figure in full paper)

1.2 Design challenge

The key design question was whether the embankment would remain stable throughout its 120 years design life, Or if loadings from the weight of the embankment and traffic, imposed onto the tunnel, located at a depth of 25m bellow existing ground level (mbegl), would cause the tunnel to become unstable and collapse. Considering the overall problem, the intention was to undertake numerical modelling to determine the ground stability. The first step required a ground investigation with two foci:

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