Barrett Byrd Associates

Rapid but safe lining of major rail tunnels in Kent is being attributed to a high degree of mechanisation, skilled miners and a cunning plan.

Closing a main line tunnel is not entertained lightly by Network Rail so when it happens – as currently with Strood and Higham tunnels on the north Kent line – it is for the best of reasons and the shortest possible time. Sections of both Strood and Higham needed refurbishment including lining to remove the risk of chalk falls. The question was, how best to do all the work in only a 12 month blockade?

The answer is being supplied by an integrated supply chain partnership which includes tunnel refurbishment specialist Amalgamated Construction (Amco). Amco along with partners Costain and Halcrow proposed a highly mechanised scheme for tunnel lining which has proved safe and immensely functional. One reason the scheme has gone so well is that congestion – potentially enormous, given that both tunnels have to be worked on simultaneously – has been written out of the equation.

“It was our cunning plan which helped win us the bid,” says Amco construction manager Andy Evans. The Strood and Higham bores began life in 1824 as a single canal tunnel later divided in two by a shaft excavated from the surface to provide a passing place for barges. Improvement of throughput was the canal company’s intention, along with a move into profitability, a vain hope as it happened. Hence the eventual conversion to rail use.

The shaft is 34m deep, roughly 60m square on plan and nicknamed the bombhole. “We reckoned it was the key to resource logistics, the movement of construction materials into each tunnel.” The base of the bombhole was prepared before the blockade began in January this year and much of the heavy stuff required for tunnel lining, from reinforcement steelwork to colliery arches, then delivered there by rail.

“It means we could work on both tunnels inwards from their entrance portals, drawing from a central stockyard between them,” Evans says. “Neither tunnel has disrupted operations in the other because of materials deliveries. We provided a concrete route into the shaft as well, so that foundation works could be supplied without impacting on lining activity.” At bid time, no other tenderer came up with a similar solution.

In fact, the partnership’s approach to logistics was only one factor in winning the circa £18M project. Others were Costain’s project management skills, Halcrow’s design/geological expertise (including that of the acclaimed ‘Dr Chalk’, geology specialist Colin Warren), and Amco’s abilities in mining and tunnelling.

“We have provided the miners, plant and tunnel lining knowledge,” Evans says. The canal tunnel was originally cut through chalk and partially lined in brick. This brickwork remains in good shape but in recent times there has been deterioration in the unsupported chalk sections. For safety’s sake, Network Rail’s predecessor Railtrack decided that the 918 unsupported metres in the 1400m long Higham tunnel and the 616 of the 2100m long Strood tunnel should be given a good, strong reinforced concrete lining. ‘Safety’ became the project’s watchword (see box).

The job had been designed to run like clockwork and in the event has done so, plus some, with seven weeks being made up on programme between May and September this year. “We’ve really stepped up productivity and optimised our time,” says Andy Evans. A major factor in terms of progress, quality and safety was the decision to use a self compacting, self levelling pumpable concrete mix. “Our lining operations as a whole went very well.”

Making as much use of the existing track as possible, rail mounted drilling, piling, colliery arch erecting, shuttering and ready mixed concrete delivery plant have effectively formed trains in both Strood and Higham tunnels. First came drilling and bolting where needed in the tunnel crown. Then came piling, raked outwards to clear the canal fill, to support capping beams either side of each tunnel.

Assembly, erection and waterproofing of colliery arches springing from these beams followed. Here, high definition lazer surveying equipment fulfilled an important role, playing with the geometry to create a profile sufficient to achieve the required railway envelope plus a minimum 250mm thickness of concrete lining, while minimising the removal of extraneous chalk.

With the arches up, the Cifa hydraulic slipform shutters were rolled into place. These were substantial beasts, each one capable of lining a 10m length of tunnel at a time, each tunnel measuring some 8.5m across at track bed level by 6m in height. Concrete brought to site by Hanson was pumped into a remixer for delivery by rail to the Cifa, which incorporated a fully automated distribution system.

A pour averaged 70m³ in volume; Amco achieved at least one pour every day except Sunday since concreting began in May; and with 86 pours in Higham and 57 in Strood, not a pour has been lost. The site has been working a 24 hour day each week day, with 12 hours on both Saturday and Sunday.

“We were striking shutters in the night shift, then advancing 10m, resetting and forming stop ends ready for an 8am start on the next pour,” says Evans. “Each Cifa operation involved only five or six men each shift. There were not many guys about – it emphasised the scope of the mechanisation.” Peak performance in one tunnel in one week was a staggering 88m of colliery arches and 60m of concrete lining – no mean feat.

With tunnel lining now just complete, attention has turned to installing new drainage in both tunnels as a prelude to cabling, track renewals and signalling works. The design of the drainage has been altered from its original concept to produce a scheme which can be installed more quickly, helping to recapture time. “The completion date of 17 January next year is non negotiable,” says Network Rail’s project manager John Kingsnorth. “The work has to be complete to our satisfaction by that time. The high productivity and quality of finish achieved on the lining has given us the best chance of achieving our objectives.”

SAFETY BOX

For the Strood and Higham railway tunnels, factors of safety used when formulating work methods for refurbishing the tunnel were based on risk levels three times those normally applied. Risk within the tunnels was driven down to minimal levels by a series of measures intended to ensure an injury free contract.

First of all the job has been run on conventional mining lines in terms of procedures and equipment. Stringent induction courses, an array of personal safety and lighting equipment plus a close tally kept on each person entering either tunnel has been the norm.

Beyond this, potential risk areas within unlined sections of the two bores were clearly identified and made virtual no go areas, except for the most necessary of works and under the most onerous of safety measures. The equipment for piling, erecting colliery arches and lining was all equipped with substantial canopies. No miner was allowed to put themselves in danger by working in an unprotected area.

Moving plant stops if tunnel workers about to be overtaken do not clearly indicate to the operator they have seen the plant and moved right out of its way. “We have created on the railway what we call a ‘high street environment’ with its own comprehensive set of rules,” says Amco construction manager Andy Evans.

VENTILATION BOX

There is natural ventilation through the Strood and Higham tunnels but Amco took the decision to force fresh air through the bores during refurbishment work and also to closely monitor air quality at all times.

Five gas monitoring systems are operating to ensure gas levels never approach HSE established maximums. Audible and illuminated warning mechanisms linked to the real time monitoring were installed to get people out quickly in the event of gas being detected. Graphs have been maintained of all gases monitored.

PRESS RELEASE

Completion of concrete lining to a tight programme marks a critical stage in Network Rail’s £35M project to refurbish the Strood and Higham tunnels on the North Kent line. Miners from Amalgamated Construction (working as an integrated supply chain partner to Costain) used specially developed tunnel lining plant to provide the bores’ 1534 unlined metres with high strength concrete support. The potential for chalk falls will be a thing of the past when the tunnels reopen on time next January, after work which embraces lining, drainage and track renewal.

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