AN INVENTORY OF UNREINFORCED LOAD-BEARING STONE MASONRY BUILDINGS IN NEW ZEALAND

SUMMARY Almost all unreinforced stone masonry (URSM) buildings in New Zealand were constructed between 1860 and 1910, typically in regions where natural stone was sourced from local quarries, fields and rivers. These buildings form an important part of the country’s architectural heritage, but the performance of URSM buildings during earthquake induced shaking can differ widely due to many aspects related to the constituent construction materials and type of masonry wall cross-section morphology. Consequently, as a step towards gaining greater knowledge of the New Zealand URSM building stock and its features, an exercise was undertaken to identify and document the country-wide URSM building inventory. The compiled building inventory database includes: (i) general building information, such as address, building owner/tenant and building use; (ii) architectural configuration, such as approximate floor area, number of storeys, connection with other buildings, plan and elevation regularity; and (iii) masonry type, such as stone and mortar types, wall texture and wall cross-section morphology. From this exercise it is estimated that there is in excess of 668 URSM buildings currently in existence throughout New Zealand. A large number of these vintage URSM buildings require detailed seismic assessment and the implementation of seismic strengthening interventions in order to conserve and enhance this component of New Zealand’s cultural and national identity. The entire stock of identified buildings is reported in the appended annex (688 total), including 20 URSM buildings that were demolished following the Canterbury earthquake sequence.


INTRODUCTION
Early European settlements and historic places are scattered throughout New Zealand, contributing to the nation's built heritage, being an inseparable part of the country's cultural and national identity that requires conservation and protection.A large proportion of existing unreinforced stone masonry (URSM) buildings in New Zealand were constructed in the second half of the 19 th century by early European settlers and as in many other countries, the construction of these buildings commonly followed traditional practices that did not take into consideration building stability and safety when subjected to earthquake induced lateral loads [1].This inherent vulnerability to earthquakes led to a large number of New Zealand's URSM buildings being lost during the strongest seismic events, including the 1855 Wairarapa earthquake (M 8.2) near Wellington, the 1929 Murchison earthquake (M 7.8) that affected Nelson, Westport and Greymouth, the 1931 Hawke's Bay earthquake (M 7.8) near Napier and more recently the 2010/2011 Canterbury earthquake sequence (M 7.1 and M 6.3) [2,3].
Following the 2010/2011 Canterbury earthquakes 18% of the stone masonry buildings in Christchurch were confirmed as being demolished due to safety reasons (Figure 1a) with the fate of an additional 13% being unknown at the time of the study.For the remaining 69% of URSM buildings in existence in Christchurch prior to the earthquakes (Figure 1b), only 21% were classified as having low damage (green placard) whereas 50% were classified as heavily damaged, corresponding to categories D4 and D5 in accordance with the EMS-98 scale [4,5,6].Most of the observed damage was attributed to the poor quality of constituent materials (particularly weak mortar) and/or the lack of interconnectivity between masonry leaves, see Figure 2 to Figure 6.Close examination of damaged URSM buildings allowed further investigation of historic construction methods and wall cross-section morphologies, being information that is typically only attained via destructive investigation methods.The performance of URSM buildings that had received seismic strengthening was also investigated following the Canterbury earthquakes [7].
Many of the vintage unreinforced stone masonry buildings in New Zealand are currently used for a variety of public functions, ranging from churches to schools and public offices.In addition to loss of the country's architectural heritage, the safety of building occupants and nearby pedestrians is of paramount concern due to the seismic vulnerability of URSM buildings.Detailed seismic assessment of these buildings, followed by appropriate improvement of their earthquake behaviour, will in many cases be necessary for both building safety and heritage conservation [7, 8 and 9].Currently, there is limited information available on how to assess New Zealand URSM buildings [10,11].In addition, there is a lack of readily available information for New Zealand professional structural engineers on best practice seismic improvement techniques for the URSM building type.In response to the aforementioned concerns and following the 2010/2011 Canterbury earthquakes, an exercise was undertaken to identify and document an inventory of New Zealand URSM buildings.The subsequent aim of the research project is to validate suitable and best practice methods for detailed seismic assessment and improvement of New Zealand URSM buildings.

USE OF NATURAL STONE IN NEW ZEALAND
During the pre-European period, New Zealand native Maori used cobbles and stones to construct paths and property boundary walls.With the arrival of European settlers, local natural stone was used mainly for prestigious construction projects, with the local geology and extent of access to local natural stone supplies largely determining the number of URSM buildings and the type of stone used in construction in a given region.For example, in the Otago and Canterbury regions, early settlers made use of natural stones from the nearby fields and/or rivers to build prestigious private houses and farms [12, 13, 14 and 15].
Working as a stonemason was exhausting and a dirty profession, injuries were common, and employment numbers were low.Therefore, unreinforced stone masonry buildings soon became prohibitively expensive and after approximately 1910, building stones were used mainly for facings, basement retaining walls, pillars and façades instead of for the entire load bearing structure.With increasing urbanization, timber was favoured as a building material due to its availability and ease of construction.Based on the 1901 census, only 4% of residential dwellings were constructed using clay brick or natural stone, whereas 90% were constructed using timber [16].Currently, natural stone from local quarries is typically used for paving, fireplaces and chimneys, property boundary walls and a variety of ornamental works [16].

INVENTORY COLLECTION PROCEDURE
In order to identify and document the full inventory of URSM buildings in New Zealand, information was collected and documented beginning from the records available within the Register of the New Zealand Historic Places Trust (NZHPT) [17].Subsequently, visits were conducted to those districts with an expected high concentration of URSM buildings.The available literature [12, 13, 14 and 15] and observations from Google Street View technology that is featured in Google Maps were also used for preliminary data collection.61% of the collected URSM buildings are currently registered with the NZHPT [17], see Figure 7. Difficulties encountered in identifying the non-registered URSM buildings suggests that it is likely that more buildings are scattered throughout the country which have not yet been identified in the present inventory.Nevertheless, the assembled URSM building inventory is considered to be appropriately accurate and reliable.
The inventory database of URSM buildings is subdivided into geographical regions and the information is clustered into three main data groups including: (i) general data; (ii) architectural configuration; and (iii) masonry information.The use of natural stone was particularly common for the construction of public facilities such as churches, schools and universities, government properties (courthouses, chambers, courts, boundary markers, fortresses, lighthouses, prisons) and civic facilities (museums, libraries, stations, post offices, theatres, banks).Many of the earliest settlements were also constructed using natural stone, especially in the Otago countryside.Clusters of buildings and estates were built following a traditional code of practice and were used as rural houses, farms, manufacturing facilities and settlements.
Natural stone was also used for the construction of hotels, restaurants, warehouses and shops where townships were expanding.As shown in Figure 9a, approximately 62% of the existing URSM buildings were constructed for private use (settlements, residential and commercial use), whereas 38% of the buildings were constructed as public facilities.Based on observations from past earthquake damage, it was identified that alterations to a building's original structure and configuration are likely to result in modification of the seismic performance of the building [18,19].New openings that altered the structural system of the façades, transformation of the internal spaces that involved removal of load-bearing walls, replacement of roof, floors or other part of the structure, and the addition of new storeys are some examples of typical alterations that have a potentially adverse effect on the seismic performance of a URSM building.Such alterations are likely to be related to a change in the original intended use of the building.Hence, both original and current building use were recorded in the database of URSM buildings (see Figure 9).Figure 9b shows that the number of URSM buildings which are still used as originally intended is high (52%), compared with 24% of URSM buildings that have had a changed use.The presence of ruins or vacant buildings was estimated to be approximately 11% (NZHPT, [17]).

Architectural configurations
Typically unreinforced stone and clay brick masonry buildings represent a box-type structural system composed of vertical (walls) and horizontal (floors and roof) structural elements.Gravity and seismic loads are transferred from the floors, acting as horizontal shear/flexural members, to the bearing walls, acting as vertical compression members that transfer loads to the foundations.Hence, the structural arrangement and the architectural shape are significant factors in the global seismic performance of the building [1].
Plan and elevation symmetry and regularity, approximate dimensions (foot-print area) and number of storeys were identified and documented for URSM buildings during the data collection stage.A building was considered regular in plan when the foot-print area was comparable to a simple geometric shape.Buildings with a 'T', 'L', or 'U' shaped footprint and/or buildings with multiple projections were considered as irregular in plan.A building was considered regular in elevation when the floor height was similar in all its parts without projections, and there was an absence of ornamental structures (such as pinnacles, spires, flying buttresses, dome lanterns, and other similar features An important factor that influences the global seismic performance of URSM buildings is the interaction with neighbouring buildings, also known as pounding effects [1].It has been shown in previous earthquakes that isolated buildings belonging to the same age and type of construction frequently exhibit different failure mechanisms when compared to row buildings [18,19].In order to evaluate the possibility of pounding, a distinction between isolated and connected buildings was identified and documented in the inventory.Figure 13 shows that there are more isolated URSM buildings (64%) than connected buildings (28%), with the latter category including both row and clustered buildings.Small townships in the Otago countryside were typically built using URSM buildings and were arranged in a row or cluster with minimal use of timber construction.Igneous sub-types used in construction (see Figure 14) were basalt and scoria (or tuff) extracted in Auckland, Bay of Islands, and Banks Peninsula (see Figure 15), trachyandesite from Leith Valley (see Figure 16), trachyte from Banks Peninsula, New Plymouth andesite, Port Chalmers breccia (see Figure 17) and Waikato ignimbrite [13,14].The properties (colour, density, porosity, compressive strength, etc.) of each type of natural stone vary depending on the quarry where the stones were extracted and consequently result in varying rates of deterioration [20,21].Sedimentary rocks, such as sandstone (Charteris Bay and central Canterbury) and greywacke (Nelson and Southland), form from sediments (e.g.mud, sand and gravel) that accumulated and compacted in layers in the ocean, lakes and river valleys [13,14].A particular type of sedimentary rock is limestone (sourced from Oamaru, Pleasant Valley and Hawke's Bay) which typically forms via accumulation, compaction and cementation of deposits of marine shells (calcium carbonate).Limestone is softer and easier to cut in comparison to other stone types and therefore was widely used in construction (22% of the URSM building inventory) and for building ornamentation, such as around openings, corners, pillars, stringcourses and spires (see Figure 16).Selective examples of URSM buildings constructed in north-east Otago using limestone are shown in Figure 18 to Figure 20.

Figure 20: Totara Estate, near Oamaru, built by the owner with local limestone (1867-1881).
Metamorphic rocks, such as schist, arise from the transformation of existing rock types (igneous or sedimentary) that have been subjected to heat and pressure, causing profound physical and/or chemical change [13,14].A wide variety of schist was used in the construction of URSM buildings in central Otago (36% of the URSM stock); see Figure 21 to Figure 23 for typical examples.

Geographical distribution
The majority (92%) of the identified URSM buildings are concentrated in the South Island, with 65% of the buildings being located in Otago and 25% of the buildings being located in the Canterbury region (data from prior to the 2010/11 Canterbury earthquakes, where 20 stone masonry buildings were demolished due to the heavy damage suffered), see Figure 24.Mapping of the recorded data showed that the use of construction stone types was directly related to the local geology (see Figure 25).
The geographical distribution of URSM buildings was compared with the seismic hazard map of the country [11], and as shown in Figure 25b it can be observed that a large number (approximately 35%) of unreinforced stone masonry buildings are located in the highest earthquake hazard zones of the country, i.e.Z hazard factor between 0.

SURFACE TEXTURE AND WALL CROSS-SECTION
One significant defect for multi-leaf stone masonry walls is the absence of link elements oriented in the wall's transverse direction, such as when a wall is constructed of small pebbles, contains a rubble infill, or when two well-ordered external leafs are not interconnected [1,8].Observations from previous earthquake damage investigations indicate that a regular surface texture on the wall face does not necessarily correspond with a regular cross-sectional morphology [4, 5, 18 and 19].Therefore, an analysis of the mechanical behaviour of existing multi-leaf masonry walls must incorporate a detailed investigation of the wall's cross-section arrangement [8,9].The texture classification method presented by Lowndes [22] was used in this study to identify the recurring stone masonry typologies in New Zealand (see Figure 26).Ashlar and rubble stonework are the two main surface texture categories.Ashlar texture refers to natural stones that are cut on four sides with the adjoining sides oriented at right angles to each other, having straight and horizontal mortar bed joints, and with the vertical joints kept plumb [22].Several ashlar stonework examples were identified in New Zealand, including coursed ashlar (3%, Figure 27a), blocked-course ashlar (13%, Figure 27b), random-coursed ashlar (2%, Figure 27c) and broken ashlar (6%, Figure 27d).As shown in Figure 26, rubble stonework was used in construction more frequently than was ashlar stonework.The rubble type of surface texture refers to natural stones in which the adjoining sides are not required to be oriented at right angles.Such stonework is generally laid with mortar joints dressed with strong cement based pointing mortar.It was identified that structural mortar is likely to be weak or locally absent.Natural rubble stones are set irregularly in the wall and the interstices are filled with spalls and mortar [22].Typical rubble stonework that was identified included rubble walls (36%, Figure 28 a-b), field-stone walls (7%, Figure 28c) and coursed rubble (16%, Figure 28d).In central Otago, schist is a particular case of texture surface where the typical foliation makes it difficult to cut ashlar stone.Schist was commonly used as rubble stonework but a refined ashlar stonework finish with regular mortar joints was sometimes superficially simulated (see Figure 29).Several examples of a simulated ashlar surface texture were identified in the Fruitlands area (central Otago), Alexandra and Clyde.A visual inspection of the building is usually insufficient to identify the cross-section morphology of the wall, such as the number of leaves, individual leaf thickness and constituent materials.Sonic and radar tests, core drilling and boroscopy are useful methods that could be adopted for onsite investigations of the masonry cross-section morphology [8,9].
It is noted that during the inventory documentation phase no non-destructive tests (NDT), minor destructive tests (MDT) or destructive tests (DT) were undertaken on any of the documented URSM buildings.
A combined arrangement of natural stone facing and clay brick backing (see Figure 30 d-e) was observed to be a common construction technique used in city centres and it is highly likely that a similar technique was used for the construction of iconic URSM buildings in several centres including Christchurch, Dunedin and Auckland.Combining two constituent construction methods resulted in simplified and accelerated construction, thereby reducing the construction cost of large and complex buildings when compared to the use of stone masonry construction only.Some examples of combined construction are the Arts Centre in Christchurch and the University of Otago in Dunedin, where the grandeur was obtained by constructing the building façade using natural stone but the less visible walls were constructed using only clay brick masonry (Figure 31).Another example of mixed construction is the Cathedral of the Blessed Sacrament in Christchurch where the walls were built using two leaves of ashlar Oamaru limestone and a cast in-situ nofines concrete core (Figure 30f).

CONCLUSIONS
The 2010/11 Canterbury earthquakes have once again highlighted the typically inadequate seismic performance of URSM buildings [4,5], with approximately 20% of the affected URSM buildings demolished due to the heavy damage suffered.Due to the high seismic hazard across New Zealand [2,11], it follows that assessment and mitigation of the risk associated with unreinforced stone masonry buildings is needed in order to guarantee personal safety and to protect this component of the country's built heritage.In response to a perceived lack of knowledge about the traditional construction techniques used in New Zealand's URSM buildings and suitable methods for analysis and appropriate interventions for improving the seismic response of this building type, a detailed inventory of unreinforced stone masonry buildings throughout New Zealand was completed, with a total of approximately 688 URSM buildings being identified, including 20 buildings that were demolished after the recent Canterbury earthquake sequence.It is likely that more URSM buildings (such as huts, sheds, stables) are scattered throughout isolated parts of the country.The accuracy of the collected data was improved by circulating the URSM building inventory to the NZHPT District Offices and obtaining feedback.Data processing led to the following observations on the URSM built heritage in existence in New Zealand:  A large number of URSM buildings are concentrated in high seismic hazard zones (i.e.Z hazard factor between 0.3 to 0.6) and the stone type used in construction is usually related to the geology of the region.
 URSM buildings were constructed chiefly between 1860 and 1890 and now approximately 66% of the entire inventory is registered with the NZHPT.
 A representative type of URSM building is isolated and regular in shape (plan and elevation) with a maximum of two storeys and a foot-print area less than 200 m 2 .Connections between load-bearing walls or between walls and floors or roof are commonly insufficient.Timber floors and roofs are typically present in URSM buildings.
 Rubble stonework built using local schist or basalt and ashlar stonework of limestone were mainly used in the construction of URSM buildings.Several cases appear to be well constructed using regularly arranged stones and strong mortar, but in contrast an in-depth assessment shows the widespread use of irregular rubble core and surface pointing of mortar joints.Structural mortar is likely to be weak or locally absent.
 Multi-leaf masonry walls are common in New Zealand URSM buildings but the wall cross-section is often composed using different materials.It was identified that several public URSM buildings in the city centres were built using stone facing with a clay brick back leaf or a cast in-situ no-fines concrete core.Full cross-sections constructed of natural stone were mainly used in rural buildings.Both forms of construction usually lack connection between leaves that results in increased earthquake vulnerability.
Due to the aforementioned problems, which involve the constituent materials of each leaf in the wall cross-section and the local absence of structural mortar, a thorough investigation is recommended for a proper seismic assessment of the building and associated design of suitable seismic strengthening [8,9].Further development of the project presented here will include characterization of constituent materials (stone and mortar) used in URSM construction based upon laboratory testing of samples extracted on-site.Knowledge of the original materials will support the identification and validation of suitable retrofit interventions and will be helpful for the selection of compatible materials.
The inventory presented here will support the identification of priority buildings and provide basic information including address, owner contact details, and current state of the building.The overarching goal of the project is to support the conservation and protection of the URSM built heritage of New Zealand and the safety of the people working in and around these buildings.

Figure 1 :
Figure 1: Condition of URSM building stock in Christchurch after the 2010/2011 Canterbury earthquakesdata updated June 2013.

Figure 2 :
Figure 2: Canterbury Provincial Council Buildings, Bellamy's (1864).3 leaf basalt (from Halswell) wall.Delamination and local collapse of the outer leaf due to the absence of bond stones.

Figure 3 :
Figure 3: Canterbury Provincial Council Buildings, Bellamy's (1864).Halswell basalt facing with clay brick backing wall.Out-of-plane partial collapse of a gable due to the absence of bond between leaves.

Figure 4 :
Figure 4: Hakatere Station (1862) greywacke rubble walls with mudstone gables.Lack of bond between walls with activation of out-of-plane mechanism.

Figure 5
Figure 5: Cathedral Church of Christ (1864), Christchurch.3 leaf basalt (from Banks Peninsula) wall and limestone ornamentation.In-plane failure shown by diagonal cracking (highlighted for clarity) and twisting of the buttress.

Figure 6 :
Figure 6: Cathedral of the Blessed Sacrament (1899), Christchurch.Oamaru limestone facing with cast in-situ no-fines concrete core walls.Collapse of the towers due to inadequate lateral resistance of the masonry walls.
Figure 8:Construction decade of URSM buildings in New Zealand.

Figure 13 :Figure 14 :
Figure 13: Presence of isolated and connected (including in a row and clustered) URSM buildings.

Figure 16 :
Figure 16: University of Otago in Dunedin.Built between 1878 and 1923 with Leith Valley trachyandesite and Oamaru limestone as ornamentation.

Figure 17 :
Figure 17: Carey's Bay Hotel in Port Chalmers, built in 1874 using local breccia.

Figure 19 :
Figure 19: Harbour Street in Oamaru.The buildings were constructed between 1875 and 1885 using Oamaru limestone.

Figure 24 :Figure 25 :
Figure 24:Estimated provincial populations of stone masonry buildings in New Zealand.
Figure 26: Prevalent stonework used on the texture surface of the URSM buildings in New Zealand.
Figure 27: Examples of ashlar stonework in New Zealand: limestone wall in (a) Oamaru and (b) Clark's Mill, Maheno; (c) basalt wall of the Dunedin Railway Station; (d) scoria wall of the Arts Centre, Christchurch.

Figure 28 :
Figure 28: Examples of rubble stonework in New Zealand: basalt wall in (a) the church of the Holy Innocents, Mt.Peel; (b) the Bluestone Store, Auckland; (c) Kinder House, Parnell; and (d) schist wall, St. James church in Roxburgh.

Figure 30 :
Figure 30: Typical unreinforced stone masonry crosssection in New Zealand.3 leaves with rubble fill in (a) the Trinity Church and (b) the Provincial Chambers towers, Christchurch; c) 3 leaves with headers in St Paul's Church, Auckland; stone front façade with clay brick back leaf in (d) Bellamy's and (e) St Luke's Church, Christchurch; f) stone facing with cast in-situ no-fines concrete core in the Cathedral of the Blessed Sacrament, Christchurch.The use of natural stone for construction of the full crosssection of masonry walls was common in construction of settlements and estates spread throughout the Otago countryside, with a few examples present also in Christchurch and Auckland (Figure 30 a-c).Recurring cross-section arrangements include single-leaf and multi-leaf stone masonry walls with two external load-bearing leaves and a rubble infill.The absence of bond between the leaves (headers or ties) and the high percentage of voids are primary contributing causes of earthquake vulnerability, for both in-plane and out-of-plane loading directions[8,9].

Figure 11: Number of storeys for existing URSM buildings. The category "other" reports all buildings with high open space and large gable walls, typically being churches and halls.
] Figure 12: Approximate foot-print area in m 2 of existing URSM buildings.