Sarjeant Gallery: Checks on historic building's movement proves solid design

A new concrete retaining wall installed behind the Sarjeant Gallery prevents subsidence during construction of the extension wing. Photo / Supplied

Systems are in place and regular checks show no significant movement of Whanganui's 100-year-old Sarjeant Gallery, now in the full swing of redevelopment.

The Sarjeant Gallery redevelopment project involves earthquake-strengthening of the existing heritage building and construction of a new wing – Pataka o Sir Te Atawhai Archie John Taiaroa, named in honour of the much loved Whanganui kaumātua.

During excavation for the construction of the new wing there was risk of subsidence due to the removal of earth close to the existing 100-year-old structure.

The construction team is satisfied it has found a good design method to ensure the stability of the building, and the design team is very responsive to the needs and issues that can arise within a 100-year-old building restoration project.

Steel buttresses support the stone exterior walls of the Sarjeant Gallery. Photo / Supplied

After a number of earthquakes shook the Whanganui and Central North Island region earlier this year, the structural engineers Clendon Burns & Park reviewed data from in and around the building and found no detectable movements.

To prevent any movement, the construction team completed ground retention involving the installation of a sloping concrete retaining structure held in place with anchors driven deep into the ground.

Two A-frame structures have also been installed to support the old stone walls during strengthening work inside the gallery.

The construction team has introduced several simple surveying or measuring methods to detect any movement the building may have. These include using pencil markings, dazzle paint and fragile glass plates across existing cracks inside and outside the building.

Registered surveyors Harrison & O'Sullivan take measurements against reference points on the gallery and neighbouring buildings on a weekly basis.

While measurements are accurate, recording movement down to a millimetre, it's accepted the building can be subject to a normal variation of +/- 2mm because of thermal effects – heating and cooling – and activities in close proximity.

Protocols are in place if the recorded movement is between 2-5mm, 5-10mm and more than 10mm but so far there's been none in excess of the normal variation.