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Suburban renewal

Individual pile design, tight tolerances and restricted work periods on a Stent Foundations' contract in Kent exemplify the approaches needed for geotechnical railway projects.

Contractor Stent Foundations recently finished piling for the signal posts and overhead gantries as part of network manager Railtrack's Dartford Area Resignalling Scheme in Kent.

The project involves renewal of signals on the suburban rail lines in north Kent from Woolwich, Eltham and Mottingham in the west to Strood in the east.

Stent, working for contractor Balfour Beatty Rail Projects and main contractor Westinghouse Signals, installed some 300 driven steel H section piles for the contract over 16 weekend possessions. A combination of careful planning and specially designed equipment was needed to meet the tight programme.

Ground conditions varied considerably over the length of the project and included clay, granular material and chalk. This meant pile design had to be tailored for each structure, with site investigations at each location using static cone penetration tests. Chosen instead of boreholes because of their speed and practicality, these were carried out by site investigation contractor Fugro, usually in advance of piling.

Foundations were designed for three types of signal structure: single post, cantilever post and gantry. Single posts have a single pile, a group of three piles was installed for each cantilever post and a pair of piles placed for each portal leg of the gantry. Piles are designed to resist axial loads, horizontal loads and overturning moments.

Steel foundations were chosen because of the short programme times, as their use removed the need for time-consuming concrete pours.

Piling began in October last year and was carried out over continuous periods of either 28 or 52 hours during the weekend possessions.

During the week piles were cut to the required lengths and joint plates drilled and welded, ready for transportation to site in the order they were to be installed.

Two 'piling trains' were used, comprising wagons housing the hammers, vibrators and power packs, along with the pile sections, a canteen/store carriage and a 12t capacity rail crane which also propelled the train.

Using equipment specially designed by Balfour Beatty Rail Plant, the crane lifted the piles into a pile gate which was extended out from the wagon to above the pile location.

While the pile was held in place, the vibrator wa s l i f ted on to i t and u s ed to v ibra te i t to de s ign toe level. At locations where the vibrator proved too slow, a hydraulic piling hammer had to be used.

Pile length varied from 5m to 26m depending on ground conditions. The maximum pile length that could be safely handled by the equipment was 10m, so some piles had to be jointed using specially designed bolted joint plates. Once installed, piles were trimmed and any excess steel loaded back onto the train.

A 'top hat'was then bolted to the pile head.

This connection for the signal post was specially designed by BBRP to cater for the tolerances required for pile installation.

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