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TABLE 1Damage severity categories (Riley Consultants 2010, 2011)CategoryNo DamageMinor DamageModerate DamageMajor DamageSevere DamageDescriptionNo observed damageCracks up to 5 mm wide and/or 300 mm deep. Negligible settlement of crest.Cracks up to 1 m deep. Some settlement of crest.Cracks greater than 1 m deep. Evidence of deep seated movement and/or settlement.Severe damage or collapse. Gross lateral spread and/or settlement, cracks showing deformation of500 mm or more.dinal cracks to connect undetected transverse cracks or flawsthat only penetrate partway through opposite sides of the levee.Such a tortuous seepage path could potentially enlarge rapidlydue to internal erosion and piping at high river levels.Transverse cracks in the levees were less commonlyobserved than longitudinal cracks and were often associatedwith sharp bends along the length of the levees and/or slumpingof the embankment (Figure 5B). Because these cracks provide adirect seepage path from one side of the levee to the other, theycan severely impact the functionality of the levees. Even transversecracks of minor width could potentially enlarge rapidlydue to internal erosion and piping at high river levels and leadto the failure of that section of the levee.Settlement of levee sections resulted from both post-liquefactionconsolidation in the foundation soils and bearingcapacity failures due to the reduced strength of the liquefiedfoundation soil. In addition to the degradation in levee functionalitydue to cracking associated with the settlement (similarto that discussed above), settlement also reduces the amountof freeboard at high river levels. The significance of this loss isdependent on the magnitude of the settlement, but in generalit is not thought to be a significant issue with the levee system.Another liquefaction-related mode of degradation to thelevees’ capacity is where liquefaction and/or lateral spreadingformed on both sides of the levee. In these cases a potentialflow path is formed down through the vertical feeder dike onthe river side of the levee, laterally through the source stratumunder the levee, and up through the vertical feeder dike on theother side of the levee. Extensive liquefaction was observed onboth sides of the levee along an approximately 0.5-km stretchof the Waimakariri River on Coutts Island Road (Figure 6).From interviews with local land owners and review of maps ofthe area from 1865, this area was part of an old river channel(Wotherspoon et al. 2011). Additionally, borings performedby the authors using a hand auger showed a deep sand depositalong this 0.5-km stretch of levee and buried sticks and logs onboth ends, consistent with an old river channel and river channelbanks.SEVERITY OF DAMAGE AND FOUNDATION SOILSTo examine the relationship between the severity of theinduced damage to the levees and the liquefaction response ofthe foundation soil, a stretch of levees along the Kaiapoi Riverwas examined in more depth. As shown in Figure 4, theselevees sustained damage ranging from No Damage to SevereDamage (Table 1). Following the Darfield earthquake, theNew Zealand Earthquake Commission (EQC) contracted alocal firm to perform a series of cone penetration tests (CPTs),among other in-situ tests, throughout north and south Kaiapoi(Tonkin and Taylor 2010). The locations of the CPT soundingsperformed on, or adjacent to, the levees along the KaiapoiRiver are shown in Figure 7.Representative CPT soundings from the north and southbanks of the Kaiapoi River are presented in Figures 8A and 9A.From interpreting 27 such CPT logs, as well as available boreholedata (Tonkin and Taylor 2011), the soil profile along thenorth bank of the Kaiapoi River east of the Williams StreetBridge is characterized by approximately 4 m of very loose toloose sand overlying approximately 4 m of loose to mediumdense gravelly sand. Below approximately 8 m, the sand andgravel layers tend to be significantly denser than the overlyinglayers. The depth to the ground water table varies, but isapproximately 1.5 m deep. Samples of the liquefiable soilstaken adjacent to the levees on the north bank had fines contentsaround 15%, with the fines being non-plastic. On thesouth bank of the Kaiapoi River east of the Williams StreetBridge, the soil profile is characterized by approximately 6 m ofvery loose to loose silty sand/sand overlying an approximately2-m-thick layer of loose to medium dense sand/gravelly sand.Below approximately 8 m the sand and gravel layers tend to besignificantly denser than the overlying layers. The ground watertable is approximately 2 m deep.Using the 27 Kaiapoi levee CPT soundings and twoadditional soundings performed adjacent to the levees alongthe southern bank of the Waimakariri River in Kainga andBrooklands, the authors analyzed liquefaction of the foundationsoils following the procedures outlined in Youd et al.(2001). The strong motion seismograph station KPOC, locatedin north Kaiapoi (Figure 7), recorded motions from both earthquakes.The geometric mean of the horizontal peak groundaccelerations (PGAs) of the motions recorded during theDarfield and Christchurch earthquakes were 0.32 g and 0.20 g,respectively. The distance from the strong motion station to theCPT sounding locations ranges from approximately 0.7 to 3.7km, with the majority of the soundings being located less than1 km from the station. Because of this close proximity, 0.32 gand 0.20 g PGAs were used to compute the cyclic stress ratios(CSRs) imposed on the soil at all the sounding locations duringthe Darfield and Christchurch earthquakes, respectively.Seismological Research Letters Volume 82, Number 6 November/December 2011 943