Solution to last month’s coffee time challenge

The solution to our May edition challenge about tackling the problem of subsiding railway tracks started in Finland where the marshy landscape freezes solid in the winter but melts in summer.

Finnish roads are well known to subside, as do the buildings on occasion. As a result, they have come up with the idea of injecting polyurethane foam under floors to lift them up again and provide a stable base. This led on to injecting the stuff under road slabs, and doing the same for them. Now comes the idea of drilling a small hole, inserting a geotextile tube, and then inflating it with foam.

It is this idea, which is called by its inventors Uretek UK, the PowerPile. It offers a solution to stabilising railway lines running over soft ground. When we asked how it compared with the original Stephenson solution, David Hughes from the company says: "It is the same principle that we work on. We inject polymer resins and a catalyst. It then expands and compresses the ground. Network Rail is very interested."

For a rail solution, the tubes are pushed down through 40mm diameter holes, and can be expanded up to 350mm diameter between the lines and provide significant friction support in soft soils and peat.

For non-rail use, where the holes can be bigger, treatment depths can be 5m or more, and expanded diameter up to 1.5m. Compressive strength of the filled tube is said to typically be 4 to 6MPa. However, when incorporating safety factors this reduces to 1 to 2MPa.

Hughes added: "We have done a couple of smallish railway jobs, lifting and stabilising platforms, and stabilising a railway embankment in a horizontal direction in the last couple of months."

PowerPiles has also recently been used to successfully stabilise and underpin a historic building in the centre of York where three sides were shared walls with other buildings. Alternative strategies would have been noisier, dirtier, longer, more expensive and liable to threaten the stability of the adjoining structures.

Tom Shelley

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