OBSERVATIONS OF THE PERFORMANCE OF EARTH BUILDINGS FOLLOWING THE SEPTEMBER 2010 DARFIELD EARTHQUAKE

Reinforced earth houses constructed since the 1990s performed well provided the overall wall bracing was adequate and detailing of the reinforcement and connections was generally in accordance with the NZ Earth Building Standards. Several unreinforced earth buildings constructed before 1930 (or reconstructed historic buildings) suffered significant structural damage and will require reconstruction or substantial repair. Unreinforced rammed earth buildings, and reinforced cinva ram brick buildings, constructed between 1930 and 1990 with reinforced concrete foundations and bond beams and adequate overall wall bracing generally performed moderately well given the level of shaking they experienced.


INTRODUCTION
On 4 th September 2010 a magnitude 7.1 earthquake occurred in Canterbury, approximately 40 km west of Christchurch, at a depth of approximately 10 km near the town of Darfield.Modified Mercalli intensities were estimated from data supplied by GNS Science.The accelerations were analysed by Gledhill et al. [1] and are plotted in another paper in this issue of the Bulletin, these estimates are noted for comparison with the observed damage.Some historic and older earth buildings were damaged during the earthquake and some more recent earthbuldings suffered some minor cracking damage.
Earth buildings use heavy low strength masonry or low tensile strength monolithic walls panels, buildings in this survey were constructed using a range of earth wall techniques.These are: Adobe -sun dried brick; Rammed earth -stabilised soil heavily compacted between shutters; Cinva brick -bricks of cement stabilised soil hand pressed with a Cinva Ram mechanical press; Cob -soft soil mixture laid into a wall in layers; Sod -soil blocks cut from the ground and placed directly into the wall; and Poured eartha cement stabilised soil mixture poured or vibrated into formwork layer by layer.

EBANZ RECONNAISSANCE SURVEY
The Earth Building Association of New Zealand (EBANZ) organised a reconnaissance survey of earth buildings in the Canterbury area following the Darfield 2010 Earthquake.This survey was carried out on 14-16th October 2010 by an engineer with research interest in earth buildings and two engineers experienced in the design and construction supervision of earth buildings.Fifteen earthbuildings were visited during the survey including several historic (or reconstructed historic) and older unreinforced earthbuildings and several more recent reinforced earth buildings and two timber framed buildings with external pressed earth brick veneer walls.A summary of the earthbuildings inspected, including their location, main construction details, and damage state are shown in Table 1.
The buildings were assessed with respect to design criteria and details within the NZ Earth Building Standards and the performance and damage criteria of the Modified EERI Methodology.This modified methodology was developed by Fred Webster and E. Leroy Tolles following the 1994 Northridge, California Earthquake and further modified by the authors for use with this reconnaissance survey [2,3].

NEW ZEALAND EARTH BUILDING STANDARDS
As described by Walker and Morris [5,6], three performance based standards for earth walled buildings were published in 1998.NZS 4297 Engineering Design of Earth Buildings [7] specifies design criteria, methodologies and performance aspects and is intended for use by structural engineers.NZS 4298 [8] defines the material and workmanship requirements of earth walls to comply with the requirements of the NZ Building Code.NZS 4299 [9] provides the methods and details for the design and construction of earth walled buildings not requiring specific engineering design.NZS 4299 is the earth wall construction equivalent of NZS 3604 Timber Framed Building.The New Zealand earth building standards are the most comprehensive in the world and have received attention from overseas and are cited by ASTM E2392 [10].

EARTH BUILDING DAMAGE STATES
The modified EERI damage state definitions are shown in Table 2 below.The damage states A through to E are shown in the first column, the EERI standard definition is shown in the second column and the definition developed by Webster and Tolles [2,3] with respect to earth walled buildings is shown in the third column.The damage states A to E were subdivided further on a scale of 1 to 3 for this reconnaissance survey with 3 being more serious than 1.

EARTH BUILDING DAMAGE TYPES
The concept of Standardised Damage Types, as defined by Webster and Tolles for earth buildings following the Northridge Earthquake, was used for the assessment of damage in this reconnaissance survey.The more important types of earthquake damage to earth buildings are as follows: Out

DAMAGE TO SOME EARTHBUILDINGS
Table 3 provides a summary of the earthquake damage to the fifteen earth buildings in Canterbury inspected during the reconnaissance survey.Ground accelerations are estimated (for approximate comparison only) by interpolating from the graph by Gledhill et al. [1] The methodology is outlined later with buildings plotted with Modified Mercalli Intensities.
The 15 earth buildings are a small proportion of the earth buildings within Canterbury located by EBANZ members and by owners volunteering.A comprehensive survey by Allen [11] in 1991 located 46 earth houses in Canterbury this will have increased but the exact number is not known.No damage or evidence of new cracking.

B Slight
Minor damage to non-structural elements.Building may be temporarily closed but could probably be reopened after minor cleanup in less than 1 week.Only incidental hazard.
Pre-existing cracks have opened slightly.New hairline cracks may have begun to develop at the corners of doors and windows or at the intersection of perpendicular walls.

C Moderate
Primarily non-structural damage; there also could be minor but non-threatening structural damage.Building probably closed 2 to 12 weeks.
Cracking damage throughout the building.Cracks at the expected locations, and slippage between framing and walls.Offsets at cracks are small.None of the wall sections are unstable.

D Extensive
Extensive structural and non-structural damage.Long-term closure could be expected due either to amount of repair work or uncertainty on feasibility of repair.Localized, life threatening situations would be common.
Extensive crack damage throughout the building.Crack offsets are large in many areas.Cracked wall sections are unstable; vertical support for the floor and roof framing is hazardous.

E Complete
Complete collapse or damage that is not economically repairable.Life-threatening situations in every building of this category.
Very extensive damage.Collapse or partial collapse of much of the structure.Repair of the building requires reconstruction of many of the walls.Hororata Cottage (Figure 1) has cob walls and Penfolds Cottage Ferrymead (Figure 3) is sod and repaired in the 1950s with cob.Both were constructed around 1860 without reinforcment.
Coton's Cottage (Figure 2) is part of a museum complex.It was initially constructed with cob walls and substantially reconstructed with unreinforced rammed earth walls in 1977.Buildings at Camp Bay (Figure 6) and Beckenham (Figure 7) were constructed between 1950 and 1980 and have unreinforced rammed earth walls, reinforced concrete foundations and reinforced concrete bond beams that connect to lintels at the tops of walls.Both these buildings experienced strong shaking but performed quite well with slight cracking damage to the rammed earth walls particularly adjacent to window and door openings.The Sefton house has single storey rammed earth walls with vertical steel reinforcing.The vertical reinforcing was located at each end of the rammed earth wall panels similar to the details for partially reinforced earth walls in NZS 4299 [9].There was some slight cracking adjacent to some openings.One short internal wall had some minor horizontal cracking which the owner advised occurred during construction.

Double Skin Cinva Brick House.
Figure 9 shows a house constructed in 1978 with double skin pressed earth brick load bearing external walls with a reinforced concrete core and single skin pressed brick internal walls and reinforced concrete foundations and bond beam and a timber framed upper storey.This building experienced strong shaking but generally performed well with slight diagonal cracking along mortar joints particularly near door and window openings.There was moderate cracking to two exterior unreinforced cantilever pressed brick walls and some separation of the fireplace structure from the main house walls.

Adobe Houses.
The Charing Cross house (Figure 10) was located approximately 700 metres from the Greendale Fault with very strong shaking with Modified Mercalli intensity estimated at MM8.

Figure 9: Governors Bay Double Skin Cinva Brick House (Building 8). Subject to strong shaking but with minor damage.
The damage to paving outside and tiling in the bathroom indicated significant ground movement.This single storey house was constructed in 1997 with 2700 mm high and 280 mm thick adobe walls with both vertical and horizontal steel reinforcing, reinforced concrete foundations, timber top plate and timber ceiling diaphragms.An adobe house near Leeston (Figure 11) experienced strong shaking with ground accelerations of approximately 0.3 g and estimated MMI of VII.There was evidence of ground liquefaction at the site.This single storey house was constructed in 1999 with mainly 2,250 mm high and 275 mm thick adobe walls with both vertical and horizontal steel reinforcing, reinforced concrete foundations, timber top plate and timber ceiling diaphragms.The building performed well generally.There was some slight cracking adjacent to doorways and openings and some minor gaps at ends of lintels and more significant cracking at a very high 4.1 m high adobe wall.There was some slight horizontal cracking at the top course of adobe bricks at some locations.This was considered to be due to insufficient connection of adjacent timber topplates and insufficient anchorage between the top course of adobe bricks and the underside of the timber top-plate.The adobe house (Figure 12) near Staveley experienced moderate to strong shaking with an estimated MMI of greater than VI.This house was constructed in 2008 with adobe walls on the ground floor and a timber framed second storey.The adobe walls were reinforced.The detailing of the reinforcing and overall wall bracing was not in accordance with NZS 4299.There was horizontal cracking in the mortar joints of the adobe walls at one end of the building where the reconnaissance team considered the wall bracing and reinforcement fixing is likely to be insufficient.

Poured Earth House.
A house under construction (Figure 13) near Sandy Knolls experienced very strong shaking with ground accelerations of approximately 0.8 g and estimated MMI of VIII.This house with cement stabilised poured earth walls and steel vertical reinforcing and reinforced concrete foundations and bond beams was under construction at the time of the earthquake.
All the earthwalls and roof framing were completed but the roofing was not on and the timber ceiling diaphragms were not yet installed.The building was designed by the owner in accordance with NZS 4299.The building performed well with no damage evident.

Cinva Brick Veneer House and Garage.
A timber framed house (Figure 14) and timber framed garage (Figure 15) were built with 140 mm thick pressed earth brick veneer walls which underwent strong shaking.The veneer walls in both buildings were fixed to the timber frame with brick ties bedded within the mortar.The mortar for the house was sand-cement.The mortar for the garage was soil-cement.
There was no positive connection with nails or screws through the tie into the top of the bricks.
The veneer walls in the house generally performed well but had some slight cracking under most of the window openings.
The support timber frame for the pressed brick fireplace had separated 15 mm from the main timber framed wall for the house.This was considered to be due to inadequate connection of the fireplace to the wall.
There was a major failure of the support system for the veneer walls on the south side of the garage.This is considered to be due to the pullout of the ties from the mortar.This will be checked when the walls of the garage are demolished and reconstructed.

LOCATION OF BUILDINGS AND INTENSITY ESTIMATES Table 4: Detail of Building Locations
Buildings in this survey were located across the region of high intensity shaking, the longitude and latitude of the sites is listed in table 4.
The building locations are plotted in Figure 16 with approximate Modified Mercalli intensity and instrument PGA's.The MM contours are based on more than 2000 reports recorded by GNS Science in the area of interest.There were multiple values reported at most places, an intensity value between the average and the maximum was determined for each point shown.The simplified contour plot was made using general purpose software from these intensities.The plot provides a general context with individual buildings also compared with nearby reported values.
The accelerations by Gledhill et al [1] are also noted on the figure, more detail is available in that paper.The general trend Designated Location Longitude and Latitude of accelerations was also informed by GNS Science on-line information [12] and is consistent with the intensities.Interpolated accelerations were made between recorded sites using hand drawn contours consistent with the trend.Accelerations are site dependant but these approximate indicative values will assist when comparison is made with Webster [2] and other such reports.

OBSERVATIONS AND DISCUSSION
Reinforced earth buildings constructed since the 1990s and inspected during the reconnaissance survey generally performed well during the Darfield Earthquake provided the overall wall bracing was adequate and the detailing of reinforcement and connections was in accordance with the NZ Earth Building Standards.
Some limited cracking can be expected in most earth buildings during major earthquake events, particularly adjacent to windows and door openings.This cracking is generally of no structural significance if the buildings are provided with vertical and horizontal reinforcing and the overall wall bracing provided in the building is adequate.
Cracking in a large event would be more widespread in older unreinforced earth buildings with possibly greater structural significance.For example the bond beam would need to be properly fixed along the wall length in the Hororata house to avoid in-plane cracking that would result in falling material.
Some older unreinforced earth buildings constructed before 1930 suffered significant structural damage during the Darfield Earthquake and will require reconstruction or substantial repair work.
Unreinforced rammed earth buildings and reinforced cinva brick (cement stabilised pressed brick) buildings constructed between 1930 and 1990 with reinforced concrete foundations and reinforced concrete bond beams and lintels and adequate overall wall bracing performed quite well during the Darfield Earthquake with generally only some minor cracking.

CONCLUSIONS
The NZ earth building standards deal well with key types of failure observed but need to be applied with a good understanding of the earth material characteristics.
Adequate anchoring of the top plate and full height continuous reinforcement are critical in all forms of new earth construction.Minimum bracing wall lengths must be used as specified with good intersection details.Earth brick veneers with soil cement mortar require positive connection to the ties.

Figure 1 :
Hororata Cob Cottage, (Building 2) unreinforced walls with horizontal cracking and wall intersection cracks.a) North wall of cottage showing major chimney damage after initial event .b) South wall, east corner failure.(Photos John Le Harivel Historic Places Trust) c) Internal view showing complex damage of south wall, east corner .d) South wall collapse, west corner.

Figure 10 :
Figure 10: Charing Cross Adobe House (Building 4) 1999.Located between the epicentre and major fault rupture.Reinforcing similar to that specified in the NZ Earth Building Standards.Minor cracking mostly due to differential ground movement.

No. Location and Type Wall Material Construction Date Foundation Bond Beam Wall Height / Thickness Wall Reinforcing Sub- Soil Type Damage State*
* Further detail in table 2,Estimated Site Subsoil Class based on AS/NZS 1170.5[4]

Table 2 . Standardised Damage States used for the EBANZ Darfield Earthquake Reconnaissance Survey Damage State EERI Description Commentary on damage to historic and earth buildings
A NoneNo damage, but contents could be shifted.Only incidental hazard.