An innovative interlocking piled solution has been devised to strengthen the base of The Needles lighthouse off the Isle of Wight. Adrian Greeman reports.
It’s been a while since the bunk beds were used in the Needles lighthouse just off the Isle of Wight.
They have remained empty for nearly 15 years since the tower, like most of the UK’s marine warning stations, was automated.
But the lighthouse keepers’ curved wall galley, shower room and sleeping compartments were briefly in use again this summer by a small work group from Bam Nuttall, which spent the summer strengthening the rock base foundations for the cylindrical tower.
Staying at the site was the best way to tackle the job of repairing the lighthouse foundations.
Though the picturesque jagged rock pillars, known as The Needles, on the far western point of the island, remain connected physically to the shore, the lighthouse at the end is accessible only by boat and repeated trips to and fro would have been required.
There was a need in any case for the six workers and a site manager on the project to be in place at the oddest of times.
The base of the lighthouse is on the dangerous chalk rocks which are its raison d’être and these become flooded at high tide.
“On top of that the chalk dips at a 60 degree angle and the northern side is only reachable during the very lowest point of the spring tides,” explains Ron Blakeley, principal civil engineer for Trinity House, which owns and maintains the lighthouses in England and Wales.
“To work there meant using the low half of the 12-hour tidal cycle,” elaborates Marek Weroniecki who supervised the work by the six-man team from the contractor. “This gave us a six hours on, six hours off shift pattern, night and day, and always a little later each day.” The crew worked in 11-day stretches, therefore, with three days off.
The work was needed to strengthen the flat platform which was originally formed by cutting away the most seaward of The Needles when the lighthouse was built in 1859, replacing an older one on the cliffs above.
The wall base was cut into the chalk and backfilled with lime mortar. But the surrounding spit has gradually been eroded by the sea.
“It has stood up very well considering, because the chalk is surprisingly hard, but it loses a little material every year,” says Blakeley.
Cornish granite used to build the straight-sided tower - the only non-tapering lighthouse in Britain - was also used for protective “sets” around the base, which have loosened.
Trinity House did a value assessment and decided that a one-off strengthening project would be more cost effective than seasonal maintenance.
Just how was put out for tender, with the proposal from Nuttall proving most suitable: to build a protective concrete wall around the tower base with piles which would be infilled with granite “plums” and then grouted.
Nuttall’s plan for the protective wall used slotted, interlocking precast concrete piles, each connected into the previous one by sliding a tongue down a groove in the side “like traditional carpenter’s dovetailing” says Blakeley.
The piles were toed into a 700mm wide circlular trench in the chalk around the tower dug some 750mm deep with hydraulic hammers.
The concrete piles, weighing up to 0.75t each, were lifted into position with a purpose-built gantry crane on a wheeled carriage.
This ran on two H-section rails fixed by drilled connections into the base of the tower, one at ground level and one about 1.5m up.
“It is not a very elaborate and on dry land you could do it in less than a month, maybe even two weeks,”
It was loaded by a second fixed crane on the little jetty, which was used for lifting the piles and other materials from a work barge. The “mobile” then ran them round to the right position.
“It also allowed a kind of jig to be fitted to help position the trench at a radius 2.5m out from the lighthouse,” says Blakeley.
The pile wall was low, just a metre or so high, but sufficient to take the backfill “which is less like plums than granite rugby balls” adds Blakeley.
Once grouted in, stainless steel rods were used to connect the pile tops to drilled anchor points in the tower walls, and these supported stainless steel mesh reinforcement for a 300mm thick concrete cap on the whole structure.
“It is not a very elaborate and on dry land you could do it in less than a month, maybe even two weeks,” concedes Weroniecki.
But it took longer on the headland where, even though the project was timed for the summer period from the end of July to November, there was still some severe weather to cope with.
Not only that, but the inaccessibility meant much more careful pre-planning than usual was necessary to ensure essential items were on site and ready for use when needed.
It all went smoothly, says Weroniecki and with one of the work crew doubling up as a chef in the lighthouse galley, some decent meals were had. But he says he still would not have fancied one of the three-month stints served by the old lighthouse keepers.