Impacts of Liquefaction on the Potable Water System of Christchurch in the Canterbury Earthquakes

Transcription

1 9 th Int. Symposium on Water Supply Technology, Nov. 2012, Yokohama, Japan Impacts of Liquefaction on the Potable Water System of Christchurch in the Canterbury Earthquakes Misko Cubrinovski, University of Canterbury, Christchurch, New Zealand

2 Canterbury, South Island, New Zealand New Zealand population: 4.5 million Christchurch population:

3 Outline 1) Christchurch earthquakes 2) Liquefaction impacts in residential areas 3) Performance of the potable water system in the earthquakes 4) Summary remarks

4 Christchurch Earthquakes Christchurch

5 Fault-Normal Horizontal Acceleration Time Histories 22 February 2011 Earthquake (a max ) 0.19g 0.24g 0.24g 0.38g 0.27g 0.74g 0.70g 0.75g 0.13g 0.30g 0.50g 0.49g 0.42g 1.62g

6 Christchurch (NZ) versus Urayasu (Japan) records

7 Liquefaction Map February 2011 Earthquake

8 Severe Liquefaction in Suburbs (1) Over 400, 000 tons of silt ejecta removed from streets/properties Bexley: nearly 1 m subsidence Large areas covered by thick sand ejecta Large subsidence in some suburbs

9 Severe Liquefaction in Suburbs (2)

10 Liquefaction Maps from Three Earthquakes SEP 2010 FEB 2011 JUN 2011

11 Severe Liquefaction in Suburbs (3) 13 June 2011

12 Lateral Spreading

13 Christchurch Potable Water System CHRISTCHURCH 150 wells (at 50 locations) 8 main storage reservoirs; 37 service reservoirs 26 secondary pumping stations Gradually developed over the past 150 years

14 Potable Water Network Pipe network: 1600 km water mains and 2000 km submains Pipe materials: PVC, PE, GI, AC, Steel pipes Diamaters: Water mains: mm; Submains: 50mm and 63 mm Typical depth: 80 cm (less than 1.5 m) Backfill: Sandy gravel, P G 55%, F C 8-15%; compacted to 70%, 90% & 95% of dry density

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16 1650 repairs or 1 repair / km

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18 Water Mains Effects of Liquefaction 80% of the damage in liquefied areas

19 Water Mains Different Pipe Materials Pipes with relatively good performance Pipes with poor performance

20 Submains Different Pipe Materials Good performance Poor performance

21 Pipeline Repair Rates Steady state of repairs reached 50 days after the earthquake (2-3 repairs per day) Repair rate before the earthquakes was 0.5 repairs per day

22 Performance Objectives 1) Domestic service: provided to 99.5% of premises after 7 (14) days 2) Water quality: 90% of city receives water conforming to NZDWS after 1 month 3) Business continuity: 95% of industry/commercial sector able to resume normal operation after 1 month After Bruce Henderson (2011)

23 Liquefaction Resistance Index Map

24 Summary Remarks Widespread liquefaction causing extensive damage to buried pipe networks 80% of the damage in liquefied areas Relatively good performance of ductile materials and flexible systems Despite the large number of repairs/breaks, the potable water service was quickly restored It took 50 days to reach post-quake steady state of 2-3 repairs per day Rebuilding activities of $500 million NZD per year led by SCIRT

25 Acknowledgement UC postgraduate students: Merrick Taylor, Kelly Robinson, Anna Winkley, Duncan Henderson, Kun Ma, Simona Giorgini, Masoud Moghaddasi, Catherine Tatarniuk, Yusa Muhamed, Jawad Arefi, Patrick Kailey, Kelvin Loh,... Academic visitors / post-docs: Jenny Haskell, Yasuyo Hosono, Matthew Hughes Collaboration with: Tom O Rorke (Cornell University), US-GEER team, Jonathan Bray (UC, Berkeley), Russell Green (Virginia Tech), JGS team, Kiso-Jiban Consultants (Japan), McMillan Drillers (NZ), University of Auckland, SCIRT, Christchurch City Council Financial support: EQC (Earthquake Commission) University of Canterbury NHRP (Natural Hazards Research Platform) ECan (Environment Canterbury)

26 SEPTEMBER 26

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28 Failures of Submains 80% of the network consists of PE pipes

29 Pipeline Repairs

30 Soil Conditions (East-West Cross Section) Ground surface Water table CBD 20m Top of Riccarton Gravel 40m Thickness of recent alluvial soils Shallow water table (CBD and to the east)

31 CPT resistance in the red zone Clean sands and sands with non-plastic silts

32 Age of Soils Cubrinovski and McCahon (2011); data from Brown and Weeber (1992)

33 Hereford Street Soil Profile Sandy GRAVEL SAND SILTS, Silty SANDS, Peat Loose / medium-dense SAND Sandy GRAVEL Elder & McCahon (1990)

34 Lateral Spreading along Avon River 1.85m After 4 September 2010 Eq. 0.4 m 100 m

35 Road Bridges along Avon River

36 Spreading-induced damage to Bridges (1)

37 Spreading-induced damage to Bridges (2)

38 Spreading-induced damage to Bridges (3)

39 Spreading-induced damage to Bridges (4) ~ 1m

40 Observed Ground Response (Performance) Factor of safety estimated based on observed severity of liquefaction CSR values calculated based on recorded PGAs and earthquake magnitude

41 Water Mains: Correlation of damage and LRI zone Zone Estimated Ground Settlement (mm) Estimated Lateral Displacement (relative; transient) (mm) Equivalent ground strains & thickness of liquefied layer 0 > 500 > 400 ε v > 5%, γ > 4%, H = 5-10 m L WS Mains ε v = 5%, γ = 4%, H = 5-10 m L ε v = 3%, γ = 2%, H = 4-8 m L ε v = 1%, γ = 1%, H = 2-4 m L 4 < 20 < 20 γ < 0.5%, H = 0 m L % Damage