TRINITY CHURCH RESTORATION

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1 TRINITY CHURCH RESTORATION UNREINFORCED STONEMASONRY STABILISATION & STRENGTHENING PROGRAMME Richard Lloyd 11 December 2017

2 Introduction The former Trinity Congregational Church, located in the centre of Christchurch, has been part of the city s heritage architecture since its construction in The Christchurch earthquakes of 2010/11 caused serious but repairable damage to the building s stone facades. The interior of the building survived largely intact. The property is owned by the Christchurch Heritage Trust (CHT). The project includes restoration of the church and the equally significant Shand s Emporium, which has been relocated to the church site from Hereford Street. The restoration consists of four stages. 1. The strengthening and repair of the main church building. 2. The replication of the church tower (completely destroyed in Feb 2011). 3. The relocation and restoration of Shand s Emporium. 4. The construction of a new annex building to service the church and Shands. Stages 2 and 4 are primarily new builds using conventional construction systems. Stage 3 is a structural strengthening and repair of a timber framed building. The most complex stage is the restoration of the church itself. This report deals primarily with the strengthening and restoration of the church. It should be noted that the strengthening systems utilised have been tailored for the church, however they would be equally applicable on any URSM building constructed in a similar manner, such as the Christ Church Cathedral. Wall Structure The wall composition found in the Trinity Church is similar to that commonly found in New Zealand URSM buildings constructed in the mid-late 19 th century. The stonework can best be described as double-wythe, loosely-coursed stone with a rubble filled void and a degree of lateral interlocking. Figure 1: Typical stone wall cross-section 2

3 Structural Strengthening Solution The strengthening process consists of four stages. 1. Stabilisation of the remaining URSM walls 2. Repair and strengthening of the roof structure 3. Reconstruction of the masonry façades 4. Reconstruction of the rose windows Stabilisation of the URSM Walls The first stage involved injecting a lime-based grout into the rubble void between the inner and outer stone wythes. The process was carried out in courses, commencing at the base and working across and up the walls. Analysis Tests and inspections were conducted throughout the grouting process in order to gauge the penetration of the grout and the potential increase in performance of the strengthened wall. Grout Strength The strength of the grout was determined by producing grout cubes and core samples for compressive testing. Tests were carried out on grout in two states: Partially cured grout- cured in real-time conditions (inside the church). Estimated to be 50-60% cured at the time of testing. Fully cured grout- cured in a temperature and humidity controlled environment until fully cured. Figure 2 provides a graphical summary of the results achieved from the testing programme. The maximum rated strength of the fully cured grout is 18Mpa. 3

4 Grout Performance Cube-B3 9.5 Cube-B full cure partial cure Cube-B1 Cylinder-B3 Cylinder-B2 Cylinder-B1 Cube-A3 Cube-A2 Cube-A1 Cylinder-A Cylinder-A2 Cylinder-A Compressive Strength (Mpa) Figure 2: Summary of Test Data Grout Penetration In order to determine whether the grout fully penetrated the wall voids a number of 20mm holes were drilled at various points along the inner wall. The cores were extracted and inspected. While this proved useful it did not provide a complete picture of the grout coverage, as some samples were heavily damaged in the drilling process. A keyhole video camera was then inserted into the same holes. The footage taken was able to provide much better confirmation of the coverage and penetration of the grout. grout start stone penetration Figure 3: Core Inspection In addition to the video inspection, a limited amount of stone deconstruction was undertaken in areas where a complete rebuild of stonework was deemed necessary. This provided another opportunity to confirm the level of grout penetration. 4

5 Figure 3: Grout Penetration in Wall Cross-Sections In all areas inspected the grout appeared to have penetrated into at least 90% of the voids. Additional Stabilisation Systems The grout injection process may be augmented with other systems, if deemed necessary. The introduction of these systems incrementally improves the performance of the URSM wall. These additional systems may include: 1. Carbon fibre (or stainless steel) ties through the wall. 2. Structural plaster to the interior wall (integrated with the carbon fibre ties). 3. Deep repointing of the exterior stonework with a modern lime mortar. Roof to Wall Connections Seismic strengthening work was undertaken on the church in This consisted of improving structural connections in the roof, installing a new concrete floor, applying shotcrete to the internal walls and installing a concrete bond beam around the top of the URSM walls (tied into the shotcrete at the gables). While this work undoubtedly helped protect the building from complete collapse it can no longer be relied on to provide ongoing structural support. 5

6 Figure 4: Strengthening Work Circa 1975 In order to reinstate and improve the connection between the roof and walls, and to better transfer load through the walls, two options were considered. Option 1 Replace the existing bond beam with a new, larger bond beam and increase the roof/beam/wall connections. This option is a more conventional approach however it is likely to be costly and invasive. Option 2 Repair the existing bond beam by jacketing it with a combination of a high strength unidirectional steel fibre fabric and high performance cementitious mortar. In addition- grout deformed steel bars through the beam and into the consolidated wall below. This option has the least impact on the heritage fabric of the building but is the most difficult to quantify in terms of performance. Option 1 was chosen by the structural engineers as they felt it provided the most certainty. Reconstruction of the Masonry Façades The gable façades have been reinstated using two methodologies. Up to the top of the new bond beam a 150mm deep single wythe using the original stone was installed. Above the bond beam the stone was cut to a 40mm veneer and directly glued to a vented cavity system. This reduced the mass of the new stonework by approximately 90%. 6

7 Lightweight stonework Original stonework Reconstruction of the Rose Windows The reinstatement of the rose windows presents a number of structural and architectural challenges. The windows could not be reinstated in their original, unreinforced Oamaru stone sections without substantial structural support. The two options considered were: Option 1 Reusing and reinforcing the original pieces of Oamaru stone by dissecting them and installing an internal steel frame that would then connect to the structural steel installed in the gable. Option 2 Replicate the windows in a 40mm veneer using the same concept as the lightweight gable construction. Option 2 has been chosen as it achieves the best balance of authenticity with the minimum of additional structural requirements. Figure 5: Lightweight Rose Window Construction 7

8 Conclusion The aim of this restoration project was to bring the building up to at least 100% of NBS with as little impact on its heritage fabric as possible. In keeping with best practice in Europe and the United Kingdom the focus was on the use of strengthening systems that compliment and augment the techniques and materials used in the original construction of the building. Most of the materials used in the strengthening programme have been sourced from Mapei International. Mapei have been developing and using structural strengthening products for 80 years. Throughout the restoration the construction team worked closely with the technical team from Mapei. This included site visits from Mapei s chief engineer, Giulio Morandini. All the systems employed have been used in Europe for over 30 years and their performance has been well documented. Richard Lloyd Director 8