Many historic buildings have been uprooted and moved bodily to new locations over the years. It is always a tricky job but, in itself, is nothing new. But when the building in question is located just 17m from the edge of an 87m high cliff which could collapse at any moment, the job takes on an extra dimension.

British consulting engineer Abbey Pynford recently tackled just such a project. The building in question is the Belle Tout lighthouse, an early 19th Century structure which last shone its warning light to sailors in the early years of this century.

Located 87m above the shore at Beachy Head, East Sussex, on the south coast of England, the Belle Tout lighthouse has got nearer and nearer to the cliff edge over the years. Or rather the cliff edge has moved closer to the lighthouse.

When it was built in 1834, the 17m-high lighthouse was exactly 100ft (30.5m) from the cliff edge. This was deliberate: the light could not be seen by ships passing within a certain distance from the treacherous rocks near the shore, so if the mariners lost sight of the light, it meant that they were too close to the rocks.

But steady erosion of the soft chalk cliff brought the structure – by now a designated and protected historic building – teetering on the edge of the cliff.

Today, Belle Tout is a private dwelling, home to businessman Mark Roberts and his wife Louise. Abbey Pynford first investigated the structure in 1996, when the Roberts family realised that their home was at risk.

The cost of the operation was small by industry standards at just £250,000 ($400,000) but it was beyond the means of the Roberts family. While they attempted to raise the funds, time passed and the job remained undone, until a huge rock fall last November placed the lighthouse in imminent danger of collapse. At this point, Mark and Louise Roberts moved out, and the engineers moved in.

Preliminary design work started in 1997 and work on site started in December 1998. A new single storey basement structure, or reception box, had already been prepared on the former croquet lawn to the exact plan of the existing structure. All that remained to do was move the lighthouse onto these new foundations, 17m to the north.

To do this, the engineers cast a reinforced concrete ring beam under the structure’s load-bearing walls and installed four concrete slide track beams, tied into the reception box and extending out beneath the ring beam. These track beams became part of the jacking structure as well as providing the rails for the lighthouse to move on. To lift the 850t lighthouse, Abbey Pynford used 22 single action hydraulic jacks, each with a lifting capacity of 60t.

“All the jacks were linked into the same hydraulic circuit and we had a series of 10 extensiometers attached around the perimeter of the lighthouse to monitor any differential movements”, says Andy Parks, special operations manager with Abbey Pynford.

Monitoring was done with Abbey Pynford’s Equilift computer system, which used information fed back by the extensiometers to control the movement of the jacks. Once the lighthouse was jacked up, mechanical screw props were placed under the ring beam to support the structure while the lifting jacks were inverted and fixed to the ring beam.

Now fitted with grease skates on their underside, the jacks were extended again until the grease skates rested on the steel rails attached to the slide beams.

Much speculation had focused on whether the pushing force of the jacks would be too great for the fractured chalk at the cliff edge, initiating another rock fall, which this time would be disastrous.

“In fact, the force needed to move the lighthouse was actually quite small”, Parks says. “The skates are extremely efficient, and it needed only about 5% of the vertical load to get the lighthouse moving.” This equated to about 40t to overcome inertia, falling to 20t to continue the travel along the rails.

“We needed to control the jacks to slow the movement as well as to start it”, says Mr Parks.

“If there’d been a strong southerly blowing off the English Channel, we might not have needed any jacks at all, except then we wouldn’t have been able to stop it moving”.