The National School for Boys – Structural Assessment
Services:
Met Consultancy Group provided an engineering survey & appraisal at The National School for Boys where distress and cracking visible in the external walls.
The National School for Boys, located next to Knaresborough Castle was built in 1814 and is a Grade II listed building. It is partially built over the Castle moat causing differential settlement to occur to certain parts of the building. The settlement is estimated to be as much as 100-150mm in places.
Met Consultancy Group (Met) were asked to inspect the Old School Building and comment on the distress and cracking visible in the external walls. The investigation recommended that underpinning the building walls would be required to transfer the loading onto suitable firm strata. Following the digging of trial pits for structural and archaeological investigation, a scheme was designed by Met. This involved the existing building walls being supported on in-situ reinforced concrete needles bearing onto small diameter (150mm) steel driven piles. Small diameter piles were chosen over larger diameter augered piles to reduce the detrimental effect on the archaeology present.
Monitoring during construction work
As well as a visual inspection, the building needed to be monitored for any movement especially in areas already suffering from stress. It was possible that damage might occur as a result of the preparation works, the piling itself or the subsequent reinstatement.
To maintain an effective watch on the building, monitoring points were installed at key locations around the perimeter. The monitoring points consisted of threaded sockets resin anchor drilled into the wall. Customised steel levelling balls are screwed into the sockets and a precise levelling exercise by Met staff undertaken at regular intervals and at strategic times. Temporary Bench Marks’s (TBMS) were installed an adequate distance away from the building so results were not distorted. The expected accuracy of the levelling exercise was ±0.3mm. In addition to this, our survey staff installed small crack monitoring markers either side of some of the largest cracks to see whether they increased or decreased in size. This was monitored using a simple micro meter.
This method meant the settlement of the building could be monitored over time and any changes assessed. Monitoring continued for a short period after works were completed to make sure that any movement had been halted.
An example of a similar monitoring point with a steel levelling ball and levelling staff is shown below. The monitoring points remain so that longer term measurements can be taken if required.