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Segment support

The Gerrards Cross tunnelisation was being carried out using a precast concrete arch system developed in France during the early 1990s.

The 20m span tunnel is composed of 2m wide, half span segments, designed and supplied by Reinforced Earth Company, a subsidiary of French contractor Freyssinet.

Segments were manufactured by Irish producer Macrete.

Segments are staggered, with each half span element bearing on to the two segments opposite.

Formwork was needed to support the first five elements, after which the system was self-stable.

An average of four segments were craned into position during night-time possessions.

Segments spring from cast insitu concrete piled foundations.

The capping beam is detailed with a 100mm deep trough into which the segments were seated. These footings were grouted up once the erection sequence was complete.

At the crown, a simple stainless steel-clad male-female 'hip' joint provides for up to 200mm of rotational movement as the arch deflects vertically. Movement arises as fill first squeezes the sides of the arch and then, as load is applied to the top of the arch, it returns to its original geometry.

Two longitudinal, cast-insitu beams, one either side of the crown joint, transfer stresses between units and provide resistance against longitudinal loading, but no resistance against lateral loading.

Fill consists primarily of grade 6N limestone quarry scalpings from the Mendips. This material was specified for the sides and top 1.5m covering the tunnel. The top layer of fill is graded ash derived from incineration of domestic waste in a London power station. Fill was to be placed and compacted in maximum layers of 500mm deep.

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