So how good were those stone masons?

February 8, 2018

Well, that’s what we are working to find out.

Dr Dmytro Dizhur and assistant Dr Marta Giaretton were on site for three days in January investigating the makeup of our stone walls. Using a combination of techniques, the aim was to gain a lot more information on the properties of our unreinforced masonry walls. This is no simple matter as Taranaki Cathedral has walls of many different ages. The oldest wall erected in 1846 is to the left of the old entry facing Vivian Street. The eastern walls behind the organ were built in 1893. The stone masons who worked on each were a generation apart, so differences should be expected.

Dr Dizhur is a Research Fellow at QuakeCORE (Centre of Research Excellence) at Auckland University. His specialty is seismic assessment, investigation and retrofit of masonry buildings so when our Peer Review engineer Win Clarke recommended further site works be undertaken, Dr Dizhur was an obvious choice.

Dr Dmytro Dizhur and Dr Marta Giaretton about to start work at the eastern end of the church

Using a combination of very high tech equipment he and Marta worked from dawn to dusk off scaffolds and a cherry picker inside and outside the church recording huge amounts of information. Gaining access at the eastern end of the church was especially tricky. Careful measurement indicated it was possible to get the cherry picker in to place but theory and practice are sometimes not the same. Backing the trailer unit between trees and gravestones required millimetre precision and a team of helpers as well as John Pickering’s labrador barking orders!

Dr Marta Giaretton using the GPR unit to record information on the internal makeup of the stone walls.

Walls of different ages were selected to provide the fullest picture possible. A GPR unit (Ground Penetrating Radar) was used to indicate where voids in the walls might be. This was followed by the drilling of holes through mortar sections so that a tiny camera could be inserted in to the walls. Called a boroscope the camera recorded what it was seeing directly to a screen. We have not known if the huge corbel stones that stick out in to the church supporting the roof trusses extend to the outer wall or just how many stones cross over the inner and outer wall cavity providing extra strength. These camera shots will help fill in many gaps in our knowledge. Understanding the amount of space or voids in the walls improves our estimates on the amount and impact of grouting the cavity – a likely technique to be used in the strengthening.

We look forward to the results being with our Peer Review Engineer at the end of February.

And in case you are wondering – all the holes were refilled with mortar that will eventually fade to be indistinguishable.

Evening sets and Dr Dizhur drills the last of the exploratory holes on the western gable.

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Filed under Cathedral Remediation Work \ Our History \ Stones and Mortar

Add a comment2 Comments

Reply Richard Grevers (Author) | February 8th, 2018 at 1:35pm
I can vouch for the ability of heritage stonemasons to produce work indistinguishable from the original.
In November 2015, the first investigation of the make-up of the walls was performed by a Christchurch company who have worked extensively on the restoration of stone buildings there. In six walls of varying age, they removed one facing stone (a much harder task than anticipated due to the strength of the nineteenth-century mortar) and drilled a 40mm "core sample" through the wall behind.
They then mixed up mortar to match each different wall section to replace the facings stones. Walking around 48 hours later, if I hadn't known which stones had been removed, I'd have had no way of telling, the matches were so good.
Reply Jenny Goddard (Author) | February 8th, 2018 at 2:46pm
Agree - it takes some searching to find their patches!
The work of Dr Dizhur will build upon what the stonemasons found in 2015. The high tech equipment used allows large areas of wall to be investigated rather than just individual stones - this is key to understanding how the walls will perform as a whole in a quake. Additionally the samples removed in 2015 will be analysed by Dr Dizhur for compressive strength.

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