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Mini piles find footing for rail power pylons

DIFFICULT ACCESS, overhead power cables and changes in foundation design meant that foundation contractor Roger Bullivant had to use its own specially designed mini piling rigs on a railway embankment stabilisation contract in southern England.

Some of the overhead power line pylon foundations alongside the main London, Tilbury and Southend railway line at Laindon, Essex, have been slipping towards the edge of railway embankments. Track owner Railtrack regularly has to lift the pylons and bases back into position using a rail-mounted crane during night-time possessions.

To reduce maintenance, the company is now replacing more than 200 overhead line foundations and masts in a major £3.44M refurbishment project. Awarded to Balfour Beatty Rail Projects last October, the contract involves installing new pylons founded on tubular steel piles, driven in by rail-mounted vibratory piling equipment.

Some sections have particularly poor access and ground conditions, and so BBRP awarded Bullivant a £250,000 subcontract to install 48 concrete pylon bases supported on clusters of raking mini piles. Bullivant, working on the edge of the 8m high embankment, had to work under live overhead power lines just 2m from the track while trains rushed by at up to 100km/h.

Bullivant's original intention was to install 150mm diameter cast insitu bored piles and precast concrete pile caps. But the small rotary auger rigs that were earmarked for the contract were found to be too light to overcome obstructions and so it was decided that pile diameter should be increased to 200mm and slightly heavier and more powerful 1.5t Bullivant RB 700 series crawler-mounted mini piling rigs used.

The pile caps, which were to have been placed at night by track-mounted crane, were instead cast insitu because of restrictions on night working.

Four rigs installed a total of 288 piles. Clusters of four piles support each of the 36 standard pylon bases, while six piles are needed for the 12 larger mid-point catenary anchor bases. All piles were drilled on a 15 rake to between 8m and 12m depth through ash, gravel, compacted fill and London Clay and fan out to accommodate lateral loading. They are designed to accommodate up to 120kN compression and 100kN tension.

Piles were constructed from a grout mix with a strength of 40N/mm2 at 28 days. The grout was tremied in and a single T20 reinforcing bar inserted centrally for the full depth. After piling, the 1.6m2 reinforced concrete pile caps were cast.

Bullivant finished the seven-week contract in early December 1997. BBRP is now erecting the new masts and is due to finish in mid-April.

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