The heritage listed St Paul’s Church in Tai Tapu, Christchurch, was significantly damaged during the earthquakes that hit New Zealand’s South Island in 2010 and 2011.
Constructed in the early 1930s, St Paul’s Church is a category one heritage listed building with foundations of locally quarried rubble stone.
Although the building suffered little damage during the earthquakes, St Paul’s Church was rendered unusable as a result of subsequent soil liquefaction and ground movement. Liquefaction occurs when the ground loses strength in response to applied stress, such as an earthquake, causing otherwise solid soil to temporarily behave as a viscous liquid.
This resulted in ground settlement of up to 300mm, causing the church’s heavy, unreinforced masonry to subside. The church tower also settled out of level and rotated away from the main building.
Higgs Construction contracted Mainmark to help remediate the ground underneath the fragile church structure and tower on behalf of the client, Church Property Trustees (CPT). CPT oversees church assets in New Zealand and manages the repair and rebuild program of 234 earthquake-damaged buildings.
Mainmark recommended JOG Computer-Controlled Grouting (JOG) as the most suitable solution to treat the liquefied ground conditions, stabilise the rubble foundation and relevel the building, without causing further damage to the brittle masonry façade.
The success of the overall restoration project was acknowledged by Selwyn District Council, with the St Paul’s project winning the Art, Culture & Heritage category at the Selwyn Awards 2018. The awards are presented every two years to honour outstanding work or inspirational achievements by individuals or organisations.
The overall objective was to carefully and accurately rotate the tower back to its correct orientation, and the tower and church building needed to be gradually lifted whilst the foundations were re-supported. First, Mainmark was required to treat the liquefied ground beneath the church to stabilise and strengthen the rubble foundation.
As with any remediation on a heritage property, it was imperative that the trustees of St Paul’s Church felt confident that the solution would not add any additional stress to the original foundations or impact the structural integrity of the building.
Mainmark’s proprietary JOG was identified as a suitable solution because it is an extremely accurate method for raising sunken foundations and can be precisely controlled to provide very small amounts of lift at a time.
Sequenced injections of cementitious grout were applied at the lowest point of the foundation, allowing the church tower to be carefully and gradually rotated back into alignment with the main building. Once the church building and tower were re-aligned, uniform lifting of the building could commence.
JOG cementitious grout was delivered to targeted areas of the church footings through 60 small injection points that were 25-40mm in diameter. The injections were controlled using a central computer, allowing the structure to be raised evenly and gradually with pinpoint accuracy, avoiding undue impact on the heritage building.
A slower setting grout was injected into the rubble beneath the church in order to bind the loose foundation, first strengthening it and then enabling all parts of the building to be lifted in unison.
Using a total of 55.3m³ of grout, Mainmark successfully re-levelled the church building, achieving a maximum of 130mm lift. As the JOG process is non-invasive and required no excavation around the church, minimal stress was placed on the very fragile structure during the remediation process.
Higgs Construction Senior Project Manager Chris Yearsley said: “Mainmark undertook the JOG relevelling of the old stone church in Tai Tapu. We found them to be thoroughly professional in their communication, operation, and health and safety on site. We would recommend them to any contractor where their technology and experience is appropriate.”
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