Structural GPR for Void Detection

28th August, 2018

Wellington Monument is a striking landmark on the edge of the Blackdown Hills. The monument stands as a tribute to the Duke of Wellington and his victory at the battle of Waterloo and is the tallest three-sided obelisk in the world.

The construction history of the monument is complicated, with budget issues and lightning strikes the construction process was stop start and the final monument design changed on several occasions leading to varying degrees of build quality over the multiple construction phases.

The National Trust took over management of the monument in 1934 and have been undertaking substantial restoration every 10-15 years since. They are looking for a more permanent solution and are fund raising with the aim of completing a full restoration. This will secure the monument’s long term future and enable it to be fully reopened to the public. The National Trust are currently commissioning and coordinating survey work to help better understand why the monument is deteriorating in order to determine a long-term solution and restoration plan.

As part of these survey works, LandScope were commissioned to undertake a structural ground penetrating radar (GPR) survey on the curtain wall at the base of the monument. The aim of  LandScope’s structural survey was to detect any areas of potential voiding behind the outer facing stones. It is understood that the National Trust have been trialling techniques (including injection void filling) at ground level which, if successful, will be applied to the entire monument structure. LandScope’s GPR survey will be compared to legacy GPR data to determine how successful this void filling process has been.

To complete the structural GPR survey LandScope deployed a SIR3000 GPR unit with a 900MHz antenna. This antenna frequency was selected to meet the required depth penetration whilst still providing a high enough resolution to identify potential voiding or delamination type features. The GPR data was collected on a 10cm transect spacing over the North West side walls of the obelisk.

The survey grids were marked on the wall using chalk and tape to ensure our survey works did not cause any damage to the structure. The grids were then positioned by the use of a laser scanner. The laser scanner produced a full 3D point cloud of the structure, which had the additional benefit of mapping any potential bulging in the wall as a result of the injection filling process.

On completion of the survey works the data was taken back to LandScope’s head office for post processing and interpretation. Post processing of GPR data is a fundamental step. It allows advanced filtering and innovative visualisation of the data in a 3D environment. The final survey results and interpretation were submitted in AutoCAD and PDF formats.

The National Trust – Wellington Monument