OBSERVATIONS OF THE PERFORMANCE OF EARTH BUILDINGS FOLLOWING THE FEBRUARY 2011 CHRISTCHURCH EARTHQUAKE

A reconnaissance survey of earth walled buildings in the Christchurch area was carried out following the February 2011 Christchurch Earthquake. Twenty six earth buildings were inspected during the survey including historic earth buildings and recent reinforced earth buildings. Some of these buildings had previously been inspected following the September 2010 Darfield Earthquake. The February 2011 Earthquake caused comparable patterns of damage to earth buildings as the September 2010 Darfield earthquake except for unreinforced pressed brick buildings which performed particularly badly. Reinforced earth buildings constructed since the 1990’s performed well during the February 2011 earthquake provided the overall wall bracing was adequate and detailing of the reinforcement and connections were generally in accordance with the NZ Earth Building Standards. Some older unreinforced rammed earth buildings constructed between 1950 and 1980, all of which had reinforced concrete foundations and bond beams, performed relatively well with only minor cracking. Unreinforced cob and adobe buildings in the area of strong shaking suffered significant damage and will require reconstruction or repair of the walls and strengthening of the upper floor or ceiling diaphragms. The performance of six houses are discussed as case studies that cover the range of buildings observed.


INTRODUCTION
Two significant damaging earthquakes occurred near Christchurch.After the first event in September 2010, the Earth Building Association of New Zealand (EBANZ), organised a reconnaissance survey of earth buildings in October 2010 as reported in the NZSEE Bulletin.(Morris et al. 2010).
After the 22 February 2011 earthquake this second survey was organised by EBANZ and undertaken in March 2011.More details of damage examples from September were also outlined at a conference (Morris et al. 2011).Earth wall terminology used for this investigation can be summarised as Adobe: sun dried bricks, Rammed earth: cement stabilised soil heavily compacted between shutters, Pressed earth brick: cement stabilized bricks compressed in a mechanical press, Cob: soft soil laid in layers and later trimmed, Sod: soil blocks cut from the ground and placed directly into the wall.
Over half the world's population live in earth houses, lessons learnt about performance and effective seismic resisting systems are of significance in New Zealand and worldwide.

Seismological Context
On 22nd February 2011 a shallow magnitude M6.3 earthquake occurred at a depth of 5 km near Lyttleton and approximately 5 km south east of Christchurch causing severe local peak ground accelerations of up to 1.4g horizontal and 2.2g vertical.Modified Mercali intensities (MMI) of up to MMVIII were recorded in the Christchurch area.Further damaging earthquake aftershocks have occurred which have had smaller peak ground accelerations than the February event.
Figure 1 shows the locations of the houses, the approximate location of the fault (no surface trace) and the earthquake epicentre.

EBANZ RECONNAISSANCE SURVEY
The EBANZ reconnaissance was carried out in March 2011 by three engineers, two experienced in the design and construction supervision of earth buildings, and an engineering academic: Earthquake (Webster and Tolles, 2000) and further modified by the Morris, Walker and Drupsteen for use in the survey of earthbuildings following the Darfield 2010 Earthquake.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.Details are provided in the earlier paper (Morris et al. 2010).

DAMAGE TO EARTH BUILDINGS INSPECTED
Table 2 provides a summary of the earthquake damage to the earth buildings inspected during the reconnaissance survey after the February 2011 Christchurch Earthquake.Houses 8 to 14 were also inspected in the earlier survey.
All identified earth buildings with access available were inspected.It is estimated that they represent over 60 percent of all known earth walled houses, cottages and garages within the area of significant shaking, greater than MM V, in the Christchurch area.The exact number of earth buildings in the Christchurch area is unknown.Some similar typical types of seismic damage experienced in unreinforced earth houses are shown in Figure 2.

Unreinforced cob and adobe buildings
Historic unreinforced cob and adobe buildings in the area of strong shaking suffered significant damage and will require reconstruction or repair of the walls and strengthening of the upper floor or ceiling diaphragms.One historic adobe house constructed in 1854 with 500 mm thick walls on the ground floor and earth walls with timber framing on the upper floor appeared worse initially due to the cracking of incompatible stiff cement plaster.However cracking within the actual adobe wall, where visible, appeared to be relatively minor and repairable.

Unreinforced rammed earth buildings
Nine cement-stabilised unreinforced rammed earth houses constructed between 1950 and 1980 were inspected.Each house had reinforced concrete foundations and reinforced concrete bond beams and well constructed rammed earth walls between 150 and 250 mm thick.These walls do not comply with the thickness or reinforcement requirements of the current NZ earth building standards but performed relatively well, most with only minor cracking.

Unreinforced pressed earth brick buildings
Two houses had a light timber post and beam structure with infill pressed earth (Cinva) brick walls and experienced strong shaking (estimated MM VI).The walls comprised double skin 100 mm thick pressed bricks laid on their edge with a 50 mm cavity with metal ties across the cavity.Major failures of the walls occurred for both these houses with significant collapse of the outer skin and some drop outs of bricks from the inner skin.The timber structure in both cases remained intact and the houses did not collapse.However the overall wall bracing in both houses was compromised by the collapse of these walls and both houses will require substantial repairs and strengthening.
In two other pressed brick houses which experienced very strong MM VIII shaking, the internal non load bearing walls comprised 100 mm thick pressed bricks laid on their edge without any form of reinforcement or additional support or concrete bond beam.Most of the longer 100 mm thick walls in both these houses suffered complete or partial collapse.Shorter walls with support from timber posts each end of the wall generally remained intact.Collapse of the thin internal walls in both these houses posed a serious hazard to the inhabitants.This unreinforced wall construction does not comply with the New Zealand Earth Building Standards.

Reinforced adobe buildings
One reinforced adobe house constructed near Diamond Harbour in 1995 was inspected.This house has adobe walls on the ground floor and a timber second storey and experienced moderately strong shaking with estimated Modified Mercalli intensity of MM VII.The earth walls have both vertical and horizontal reinforcement similar to the details in the NZ earth building standards.There was some minor cracking particularly adjacent to openings but no significant structural damage.There were no other known reinforced adobe houses in the area of strong shaking.

Reinforced rammed earth buildings
One reinforced rammed earth house constructed on the Banks Peninsula in 1997 was inspected.This house has rammed earth walls on the ground floor and a timber second storey and experienced moderate shaking with estimated Modified Mercalli intensity of MM V.Only very minor cracking at some locations was evident, otherwise the rammed earth walls performed well.There were no other known reinforced rammed earth houses in the area of strong shaking

Reinforced pressed earth brick buildings
Two pressed brick houses with external double skin pressed brick walls and a 50 mm thick reinforced concrete core and total wall thickness of 250 mm experienced very strong shaking (estimated MM VIII).The external walls in one of the houses appeared to suffer no damage while the external walls in the other suffered limited damage.
A large house on the Banks Peninsula with pressed brick walls constructed in 2000 experienced strong shaking (estimated MMI VI).The earth walls are reinforced both vertically and horizontally and are generally in accordance with details in the NZ Earth Building Standards.The house has a timber second storey with pressed brick veneer walls.The house performed well with only very minor cracking mainly near openings and with some cracking of some of the veneer bricks but no significant structural damage.

Unreinforced adobe building-Cracroft House 20
Cracroft House is an historic 1851 adobe building in Hoon Hay that has been subject to a number of alterations.This has included a conventional brick rear section and a 1990 timber structure that is largely seismically independent at the rear.
Some of the adobe structure has been repaired with burnt brick and plastered with a cement plaster.The cracking in the plaster indicated significant deformation while the underlying adobe has suffered significant fracture it appeared to be to a lesser extent than the plaster.Figure 3 illustrates the overall house and crack damage, in particular on the front wall.Figures 4 and 5 show detail of the front wall damage.Where the walls were visible inside the structure, significant movement was evident as shown in figure 6.The upper level was light construction with mud plaster on lath for both walls and ceilings.This suffered significant damage as shown in Figure 7.    House 23 had only suffered moderate shaking of about MM V. Some internal walls were less than 150 mm thickness and additional restraint should be considered.

Unreinforced pressed brick building -Little River House 17
Little River was over 20 km from the epicentre where moderate shaking of approximately MM VI experienced.This house was timber post and beam construction with unreinforced pressed earth brick double skin infill panels on the lower floor.It suffered major damage and the overall structure lost most of its lateral strength expected from the infill panels (see Figure 10).This reinforced adobe house used detailing and reinforcement that is similar to the New Zealand Standards ( see Figure 15).This adobe house experienced MM VII shaking.The most significant damage was to the concrete chimney (Figure 16) which had been reinforced only part height.There was considerable minor cracking that spalled small plaster fragments, cracked window corners (Figure 16 right), and hairline cracks in the adobe brickwork as visible in Figure 17.

Rammed earth building -Little River House 19
At Little River, House 19 had 2.4 m high and 400 mm thick rammed external earth walls and 300 mm internal walls (Figure 20) that formed part of the lower storey.A 400 mm architectural feature wall stands alone as shown in figures 21 and 22.The walls had a strong timber well fastened bond beam.The house was greater than 22 km from the February epicentre with a reported intensity of MM V and although the walls supported a light timber frame upper level the only damage was hairline cracks that extended less than 500 mm.There was no damage to the foundations or to the upper level, the west side of house had 100 by 40 mm studs at 400 mm crs with 140 mm thick pressed brick veneer.

MAIN CONCLUSIONS FROM THE SURVEY
The February 2011 Earthquake caused comparable patterns of damage to the September 2010 Darfield Earthquake except for unreinforced pressed earth brick buildings which performed particularly badly.
Reinforced earth buildings constructed since the 1990s and inspected during the survey generally performed well during the February 2011 Christchurch Earthquake provided the overall wall bracing was adequate and the detailing of reinforcement and connections were in accordance with the NZ Earth Building Standards.
Some limited minor 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 and in accordance with the requirements of the NZ Earth Building Standards.The requirement of the standards for continuous vertical reinforcement from foundation to top plate provides integrity that has been evident.
Cracking is likely to be greater and more widespread in older unreinforced earth buildings with greater structural significance.
Some older unreinforced cob and adobe earthbuildings constructed before 1900 suffered significant structural damage during the February 2011 Christchurch Earthquake and will require reconstruction or substantial repair work.
Unreinforced rammed earth buildings constructed between 1950 and 1980 with reinforced concrete foundations and bond beams and lintels performed well.Most were subjected to moderate shaking during the February 2011 Christchurch Earthquake with generally only minor cracking.
Unreinforced pressed brick buildings including those with double skin and a cavity performed badly and the walls will require dismantling and reconstruction.

REVIEW OF THE PERFORMANCE OF EARTH BUILDINGS IN RELATION TO THE NZ STANDARDS AND RECOMMENDATIONS
Although none of the damaged pressed brick walls complied with the New Zealand earth buildings Standards, modification to the pressed earth brick section of the Standards will be required.Double skin cavity construction is not covered by the Standards but should be specifically excluded.
Unreinforced earth walls of any existing NZ houses in high seismic zones thinner than 200 mm and without any lateral support from timber framing are also not covered by the NZ Standards and should also be specifically excluded.
These unreinforced thin earthwalls and double skin earth masonry walls with a cavity should be dismantled or strengthened by providing additional lateral support to the walls.
Second storeys of earth in all high seismic risk zones should be discouraged.A review of the provision for second storeys in NZS 4299 is recommended.Some apparent poor designs indicate the need to check the competency requirements for designers of second storeys under NZS 4297.
A number of earth walls had weakened earth wall material due to weather damage that may have contributed to failures.
Weather protection of earth walls in accordance with the provisions of NZS 4299 remains very important and some of these weather protection details should be made mandatory for earth walled houses specifically designed by a structural engineer.

Figure 1 :
Figure 1: Locations of houses relative to the February epicentre.Approximate fault location transcribed from GNS Science.

Figure 3
Figure 3 also shows the very symmetrical plan configuration of the original adobe.It provides a good basis for restoration and provision of a floor diaphragm if this is economic.

Figure 4 :
Figure 4: Cracroft House front corner with plaster cracks.The small plaster drop out revealed burnt brick repairs.

Figure 5 :
Figure 5: Cracroft House front wall plaster cracks and minor lateral spreading damage to the concrete porch.

Figure 6 :Figure 3 :
Figure 6: Cracroft House interior plaster with significant damage, underlying adobe wall showing less damage.

Figure 7 :
Figure 7: Cracroft House damage to upper level walls of plaster and lath.

Figure 8 :
Figure 8: Unreinforced Rammed Earth Kennedy's Bush Road (Building 23) showing overall layout.New conventional addition to left front.

Figure 10 :
Figure 10: Unreinforced Rammed Earth Kennedy's Bush Road (House 23) showing damage at the bond beam interface over a lintel.(Photo Drupsteen)

Figure 10 :
Figure 10: Little River House 17 post and beam construction supporting upper storey, detail view showing ties pulled out of mortar.

Figure 11 :
Figure 11: Little River House 17 showing major damage to outer skin of pressed earth brick and significant damage to the inner skin.

Figure 12 :Figure 13 :
Figure 12: Little River House 17 showing overall house, some minor in-plane cracking and out-of plane failure of the inner and outer skin.(G North)

Figure 15 :
Figure 15: Diamond Harbour reinforced adobe house overall view.

Figure
Figure 16: Diamond Harbour reinforced adobe house showing detachment of inadequately reinforced concrete chimney and crack damage at south window.

Figure 14 :
Figure 14: Diamond Harbour adobe house plan showing location of damage observed and vertical reinforcement.

Figure 17 :
Figure 17: Diamond Harbour house showing cracks on south wall adjacent to entry.(G North)The main item of structural interest was the stub wall supporting a beam which fractured at about half height as shown in figures 18 and 19.

Figure 18 :
Figure 18: Diamond Harbour house showing beam landing on the stub wall.

Figure 19 :
Figure 19: Diamond Harbour house showing the stub wall mid height horizontal cracks due to out-of-plane flexure.

Figure 22 :
Figure 22: Little River rammed earth House 19 internal feature wall and stairway.(Photo G North)

Figure 24 :
Figure 24: Little River pressed earth house 18 overview of north facing walls.

Figure 25 :
Figure 25: Little River pressed earth house 18 side view showing garages with insufficient adjacent bracing walls.(G North) Figures 26 and 27 show the types of cracking adjacent to openings at the locations highlighted in Figure 23.

FigureFigure 23 :
Figure 26: Little River pressed earth house 18 showing cracking of the pressed brick veneer.

Figure 27 :
Figure 27: Little River pressed earth house 18 showing cracking of veneer.

Table 1 .
The buildings were assessed with respect to design criteria and details from the NZ Earth Building Standards and the performance and damage criteria of the Modified EERI Methodology.This modified methodology was developed by Webster and Tolles following the 1994 Northridge, California

Table 1 : Summary of Earth Buildings inspected after the February 2011 Christchurch Earthquake. No. * Location and Type ** Earth Wall Material Construction date MM
**** Damage State A -None, B -Slight, C -Moderate with cracking damage throughout the building D -Extensive crack damage throughout E-Very extensive damage with collapse or partial collapse of structure The digit indicates where in the scale eg B1 low within the B category, B2 mid B category, B3 borderline to C Note: Damage scales further subdivided 3 more serious than 1,

EARTH BUILDING DAMAGE TYPES The
(Webster and Tolles, 2000) as defined by Webster and Tolles(Webster and Tolles, 2000)for earthquake damage to earth buildings are as follows:Out of Plane Flexural Damage,

Table 2 : Summary of Damage to Earth Buildings inspected after the Christchurch 22 nd February 2011 Earthquake. Out-of-Plane Gable Ends Horizontal Upper wall Mid Height Flexure Diagonal Crack Damage Wall Intersection Separation Corner Cracks Crack damage at Openings Foundation Movement Horiz Movement at Foundation Moisture Damage Vertical Cracks Estimated MM Intensity New Damage State* Pre-Existing Damage State Subsoil Class** No. Location and Type 8a
Governors Bay House No.1 External Walls (250 thick)