In early 2023 we were asked to conduct a partial topographical and measured building survey of Great Chalvedon Hall, a Grade 2 listed building experiencing subsidence on its western side.
With a plan to return to the site every three months to monitor any movement, the data we gathered would determine what impact the subsidence may be having on the walls and chimney.
Methodology
Establishing a control network
The first challenge we faced was how best to create an adequate control network in a semi-wooded area with few permanent features on three sides and potentially untrustworthy ground on which to place survey markers.
The solution was to establish a number of permanent survey stations in the car park, away from the area of subsidence, alongside a loop of temporary stations that enabled our team to traverse the site during each visit.
Using our Trimble Total Station (±1mm in 300m) and a closed loop traverse approach, 14 trees located around the perimeter of the site were marked for resectioning on subsequent visits, ensuring the data was tied to reliable reference points regardless of any ground movement around the building.
Monitoring target permanence
The second challenge we faced was the placement of retro reflective targets and other features that could be used for monitoring purposes. As an active and popular pub, there was a risk that curious patrons could remove the targets or other markers if left within their reach.
Having consulted with a structural engineer, our team set out 19 ground nail markers where there was a limited risk of them being removed around the area of the subsidence to monitor any ground movement. We also set a further 61 targets on the walls (30 on the western face), including existing details such as screws and downpipe brackets that were unlikely to move over time, and retro reflective targets placed above head height using ladders.
On our second visit, after clearing some unwanted foliage, and with the exception of a single ground nail that had been lost and two targets that had been damaged, we were able to resurvey the monitoring targets as planned.
Measured building survey
We surveyed the building using our ZEB Horizon handheld 3D laser scanner and a 4m carbon fibre rod that allowed us to reach above the eave line and scan the roof in greater detail.
The mobile laser scanner was extremely useful in measuring the small details, including the lean of each of the walls (these are seldom straight in a listed building!).
Following the survey, the point cloud data we had collated enabled us to produce three floor plans and four elevations of the building. At a later date, we also provided two additional sections to aid engineers in assessing the movement of the building.
Photographic survey
As we do with the majority of our surveys, we photographed the site extensively during our visit for quality assurance purposes and as a deliverable for the client, allowing a comparison on each return visit during the monitoring process.
