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BAM Ritchies

Case Study: Hooley Cutting Slow Lines Stabilisation Works

Client: Network Rail

Designer: Arup

Contract Period: 2012 - 2013

Hooley cutting, site of the ‘Surrey Iron Railway’ (England’s first public railway) is a 170-year old 30m deep earth structure on the main London-Brighton line north of Redhill.

Each year sections of the gravel deposits slip which had caused train derailments on two occasions in the last twelve years.

During a possession over Christmas 2011, advance works were carried out to install a scaffold deck near track level and catch fence to protect the line during the main works. An internal JV of BAM Nuttall Rail and BAM Ritchies was then awarded the main stabilisation works contract in 2012. The project was split into three sections, each with different access constraints and engineering solutions.

The Houses section on the western cutting face required access to the top of the cutting utilising part of the rear gardens of 25 properties. The Dry Valley Gravels were stabilised through the use of 32mm and 38mm diameter soil nails, grouted in a 100mm diameter drill hole, up to 14m deep at nominal 1.5m centres. The highest line of nails was encapsulated in a 430m long one metre deep concrete crest beam which formed part of the permanent access and inspection walkway.

The concrete Beam and Grillage section on the eastern cutting face had no access from the top due to the close proximity of houses. The team installed soil nails capped with three concrete beams and 21 new concrete columns at 5m centres creating a buttressing effect.

Traditional scaffold decks for temporary access were unsuitable due to the instability of the temporary foundations on the unstable cutting face. A lightweight access system was developed that was supported by the new concrete columns below proving very adaptable to cope with the curved topography of the cutting face. This was only made possible by using sprayed concrete, which did not impose significant dead loads from pipelines and substantial formwork and lightweight formers.

The east side cutting face forms a spine like ridge between two parallel railway cuttings. Approximately 14,000m3 of the Dry Valley Gravel layer was removed using long-reach excavators to reduce the face slope angle so minimising the possibility of any future slips. The regarded cutting face was reinforced with 20km of soil nails installed via hydraulic drill masts mounted on long-reach excavators and netted with DELTAX high strength wire mesh.

In total some 6,000 soil nails were installed using long-reach excavators and slope rigs.