what we learned from christchurch taking the risk out ... - PlaceMakers

FEATURESBRANZEarthquake performanceBRANZ research shows that houses built to the current Building Code and standards generally stood upwell to the Canterbury earthquakes. However, more complex, specifically designed houses often had issuesThe earthquakes that have struckCanterbury since 4 September2010 have provided anopportunity for engineers to comparethe actual performance of a widerange of structures against theoreticalexpectations.This has been particularly true forBRANZ structural engineers, whohave been closely involved from theinitial event, first in the emergencyresponses to the main earthquakes of4 September 2010, 22 February 2011and 13 June 2011, and then in therecovery operations centred on theregion’s housing stock.HOUSE AND GROUNDPERFORMANCEFollowing the June 2011 earthquake,BRANZ conducted a comprehensiverandom survey of over 300 housesin Christchurch to gain a betterappreciation of the performance ofthe range of house types, includingthe effects of ground conditions.Houses on the flat had either beenaffected by ground failure resultingfrom the earthquakes (liquefaction)or by earthquake shaking. Whilepredominantly affected by shaking,some hillside houses were also affectedby ground instability such as rock fall,rock roll, and slope and cliff failure.To varying degrees, liquefactionaffected a large proportion of houseson the flat. Of the properties surveyedon the flat, 30% experienced liquefactionof the site, with half of these havingliquefaction in all three major earthquakeevents before the survey.CHRISTCHURCH FOUNDATION STYLESThe survey sample confirmed there havebeen two predominant foundation stylesin Christchurch over the history of thecity. Houses built in the early and middle20th century generally have floors onpiles with perimeter concrete foundationwalls, while most houses built since 1980have concrete slab-on-ground foundations.When the ground liquefied, neitherfoundation style was sufficiently robust,leaving many houses uninhabitable.Many of the perimeter concrete foundationwalls supported heavy brick or blockcladdings and heavy concrete or clay tileroofs. When the liquefaction was severe,these walls settled into the ground at agreater rate than the more lightly loadedpiles, resulting in a badly distorted house.The slab-on-ground floors were also hithard by the soil liquefaction. Many ofthe slabs were unreinforced (acceptablein NZS 3604 construction before theearthquakes), and those that were hadbrittle reinforcing mesh fractured as theslab distorted. The then DBH issued anamendment to its citation of NZS 3604,requiring all new floors in New Zealandbe reinforced with ductile reinforcing.NEED FOR STRONGER FOUNDATIONSThe Engineering Advisory Group(EAG) has developed a range ofmore robust foundation systems fornew houses on properties wherethere is a likelihood of liquefaction infuture earthquakes. Some foundationsare expected to accommodate theassociated ground distortions withminimal distortion of the house –stiff rafts – while others have beendesigned so re-levelling is a relativelyeasy process – suspended timber floors.Other solutions involve treating theground before the house is built tocontain the liquefying soils and providea stiff soil platform for the foundation.Repairs would still be required in manyinstances, but these would be minor andthe disruption to occupants is expectedto be low.Now, Christchurch probably has thebest database of subsoil informationof any territorial authority (TA) in thecountry, allowing a more informedselection of appropriate foundationsystem for the conditions.Other TAs with known areas ofliquefiable soils, particularly where thereis also potentially high seismicity, shouldconsider what house foundations orground treatments are most appropriateto ensure resilient performance.14www.branz.co.nzThis house was effectively destroyed because of damage to foundations resulting from liquefaction