Innovation In Structural Investigation

HAPPISBURGH LIGHTHOUSE INVESTIGATED BY GBG USING NON-DESTRUCTIVE TESTING


Cambridgeshire-based structural investigation specialist, GBG, has been commissioned by the University of Lincoln to undertake a structural assessment of the grade II listed Happisburgh Lighthouse in East Anglia using a range of non-destructive testing (ndt) techniques. This work forms part of the European Commission-funded PHAROS programme looking at the conservation of European historic lighthouses.

Happisburgh is the oldest working lighthouse in East Anglia. Saved from closure by the local community and now maintained by the Happisburgh Lighthouse Trust, it is the only independently run lighthouse in the country. Built in 1790, originally as one of a pair, the tower is 85’ tall and the lantern is 134’ above sea level. Much of the inside of the lighthouse is a void. A locker room below the lantern is accessed via a 96 cantilever stone step spiral staircase.

Dr. Belinda Colston of the University of Lincoln is project manager for the PHAROS programme, working with Dr. David Watt of Hutton+Rostron Environmental Investigations Ltd. David is a chartered building surveyor and historic buildings consultant, and was keen to use a non-destructive testing approach to provide a better understanding of the original construction of the lighthouse and any later interventions.

He commented: “I commissioned GBG to undertake the survey because I had worked with them in the past. I know that unlike some organisations who offer ndt surveys, GBG really do focus on interpretation and providing meaningful information, rather than just a mass of data.”

Specifically, Dr. Watt wanted the survey to provide information on how far the stone spiral treads and the supporting steel went into the walls, and the thickness of the render.

The non-destructive survey undertaken by a team from GBG used impulse radar, long wavelength thermography, dynamic impedance and ultrasound.

These techniques, combined with GBG’s experience in investigating historical structures, identified an original brickwork core with both internal and external layers of reinforced concrete render, probably dating from the 1920s or ‘30s. The internal concrete render was c250mm thick with steel reinforcement at a depth of c120 mm, whilst the external concrete render was c150 mm thick. The stone spiral steps were embedded some 140 mm into the brick work, whilst the steelwork was embedded to a depth between 400 and 450 mm. Overall the brickwork of the lighthouse was found to be generally in a sound condition, although there were some areas where the mortar between bonds had deteriorated. Cracks in the external render appeared to be allowing water to enter the structure, causing the brickwork core to deteriorate and the embedded steel reinforcement – which were hitherto unknown - to corrode. This has caused the render to delaminate, evidence of which was found during the survey.

Based on the findings of the one day ndt survey, GBG has recommended the extraction of two cores from the inside and outside of the lighthouse in order to confirm the thickness of the render and to assess the condition of the bond between the render and the brickwork core. Chemical analysis of both the concrete and brickwork would provide information on salt levels and their source.

The report and recommendations from GBG are being considered, together with an assessment of the funding required to carry out further investigative work.

Notes for editors
Specialising in innovative techniques for structural investigation, GBG is headquartered in Cambridgeshire with a Structural Engineering Division in Watford, and overseas offices in Australia and New York.

For further information, please contact:
George Ballard () or David Wilson ()


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