Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanPERFORMANCE OF BURIED HIGH VOLTAGE
POWER CABLES DUE TO LIQUEFACTIONJohn M. EIDINGER1
1President, G&E Engineering Systems Inc., Olympic Valley, California
eidinger@geEngineeringSystems.com

ABSTRACT: Buried high voltage power cables failed in many locations in the February
22, 2011 M 6.3 Christchurch earthquake, due to liquefaction. The empirical evidence ofthese failures is presented. The original design of the cables is considered, and whatmade the cables so vulnerable. Nonlinear analyses of buried cables are presented to see
how they "theoretically" should perform due to liquefaction. Full scale tests areconducted for unreinforced and reinforced buried cable duct banks to examine theirrelative capability to absorb the effects of liquefaction. Recommendations are made forthe design of new buried cables in liquefaction zones.
Key Words: Christchurch earthquake, high voltage buried power cables, XLPE cables,
Oil-filled cables, liquefaction

INTRODUCTION
The recent sequence of earthquakes in Christchurch New Zealand resulted in functional damage tomore than 200 buried high voltage power cables due to liquefaction. A similar phenomena occurs tofiber optic and copper communication cables. In this paper, we examine the reasons for these failures,
as well as recent full-scale tests of buried power cable duct banks to try to determine cost effective
ways to seismically design buried cables through liquefaction zones.

OVERVIEW OF THE EARTHQUAKE IMPACT ON THE POWER SYSTEMThe February 22, 2011 M 6.3 earthquake was the second of four large earthquakes to impact the cityof Christchurch, New Zealand between September 2010 and December 2011. The February 2011
event had the worst impacts, resulting in 182 fatalities, many building failures, and widespreadliquefaction. A detailed review of the overall impacts of this earthquake sequence on lifelines isprovided in (Eidinger, 2011).
Three power companies serve the greater Christchurch community. Transpower provides regional
power transmission, with voltages up to 220 kV. Orion and Mainpower are the two distribution
systems for the area, taking power from Transpower's 220 kV grid, and delivering it to the retailcustomer. All three power companies sustained damage, including damage to office buildings (groundshaking and liquefaction); substation buildings (ground shaking, liquefaction and rock fall); substation
yard equipment (ground shaking). The worst overall damage, causing lengthy power outages and1858

costly repairs, was due to failure of buried power cables in the Orion and Mainpower distribution
systems, due to liquefaction.
This paper examines just one aspect of the overall power system damage, namely the failure ofburied 66 kV power cables in the Orion system. Figure 1 shows a map of the Orion power system,
highlighting the liquefaction zones in red (major liquefaction, settlements often 5 to 10 cm, withlateral spreads locally up to 1 meter) and yellow (moderate liquefaction, with settlements commonly 2
to 5 cm).

Figure 1. Location of Orion 33 kV to 66 kV Distribution System and Liquefaction Zones (Feb 222011)
In Figure 1, the red zone in the eastern part of the city suffered the most widespread concentration ofliquefaction, and the worst impact to buried power cables. The eastern part of the city is primarily
residential. From the Bromley 220 kV – 66 kV substation, there are 5 underground (UG) 66 kV
circuits that branch out to 66 kV substations: all these circuits had more than 1 faults; short term
emergency repair was to deploy emergency generators at low voltage substations; moderate term
repair was to re-energize substations with temporary above-ground cables; the long term permanent
strategy (2012 and beyond) is to install new end-to-end 66 kV buried cables, with some seismic designprovision. Exactly what will constitute the seismic design for the new buried cables remains to be
determined.
In Figure 1, the two yellow zones are drawn over the single Lancaster-to-Armagh 66 kV
XLPE-type circuit and the double Addington-to-Armagh 66 kV oil-type circuits. These failed at six
locations, were repaired with new splices at those six locations, and put back into service.
In Figure 1, the statistics for the February 22, 2011 earthquake are presented. If one counts bycircuit, including all of Orion's system, then 50% of 66 kV underground cables had one-or-more1859

faults; 5.5% of 11 kV and 0.6% of 400 Volt. If one counts only those circuits located withinliquefaction zones, essentially all the 66 kV buried cables failed.
LOCATION OF CIRCUIT FAILURES
Figure 2 shows the alignment of 66 kV buried circuits in and around the central business district(CBD) of Christchurch, New Zealand. The red stars show the locations where the circuits failed. Theblue lines show the circuit alignments. Green areas are parks. The meandering Avon River ishighlighted by green parks along either bank. The six circuit failure locations are noted "1, 2, 3, 4, 5,6". The type of cable at each faulted location is listed as either Oil, or XLPE. Each blue line represents
a three-phase circuit. Oil-type cables have all three phases within pipe. XLPE-type cables have threeseparate cables.

Figure 2. Location of 66 kV Buried Circuit Faults, Central Business District, Christchurch (Feb 222011)
LOCATIONS 1, 2, 3 – OIL-TYPE CABLES
Figure 3 shows failure Location 1. Figure 4 shows the cross section of the trench design used atLocation 1, same as used at Locations 2 and 3. In the field, we observed the actual "230 mmcement-bound sand" and "80 mm red concrete cap" to have the consistency of a moderate strengthconcrete. Native soil in the top 3 meters, surrounding the cables, are typically moderate strength clays/ silts, able to withstand a vertical-cut trench wall. Figure 5 Shows the buckled cables, as crews dig
them up after the earthquake. Figure 6 shows the cable cross section, comprised of three aluminum
phases (300 mm^2), paper wrapped, with three oil-ducts; all housed within a corrugated aluminum
pipe and then with protective plastic outer sheath.

1860Figure 3. Location 1 – 66 kV Oil-filled Cable

Figure 4. Trench Design 66 kV Oil-type Cable

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Figure 5. Exposing the Damaged Oil-Type Cables, Location 1Figure 6. Oil-Type Cable Cross Section

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Figure 7 shows the cut-out section of a oil-type cable, from Location 3. The final deformed shapeshows that the cable tried to deform in sharp double curvature, to accommodate a shear across thecracked thermal concrete cross section. The high curvature lead to high strain in the aluminum pipe,tearing it. Post-earthquake inspection of the roadway surface at Location 3 showed street-levelknife-edge type vertical settlements on the order of 2 cm; no lateral spreads.

Figure 7. Oil-Type Cable (Location 3)
Post-earthquake inspection at Location 1 revealed that the cable alignment was parallel and about 6
meters from the edge of Lake Victoria. Liquefaction occurred under the lake, inducing somesettlements, and possibly a very slight lateral spread, as indicated by damage to the retaining wall at
the edge of the lake. One buried storm culverts goes under power cable, another over the power cable
one installed likely before, and one after, the cable installation. Surface level ground deformations in
the zone where the cable failed show sudden "knife edge" settlements on the order of 2 cm, graduallateral soil movements of perhaps 3 cm.
The damage pattern of the cable shows a classic "nonlinear beam on nonlinear soil" pattern,
expressed mostly of vertical settlement. The actual damage to the cable occurred due to high curvatureon the exterior aluminum pipe, leading to local wrinkling of the corrugations, pushing inward on theoil tubes / paper wrap. The aluminum pipe tore, releasing the oil and allowing air into the cable. The
actual aluminum cores did not appear worse-for-wear.
LOCATIONS 4, 5, 6 – XLPE-TYPE CABLES
The cables used at Locations 4, 5 and 6 are XLPE-type. Cores are 1600 mm^2 copper. Figures 8 and 9
show the typical trench installation and cross section of the cable.

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Figure 8. Trench Design 66 kV XLPE-type Cable

Figure 9. Cross Section, XLPE-type Cable (Top scale in inches, bottom in cm)
Figure 10 shows work crews exposing the fault cable at Location 6.

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Figure 10. Exposing the Damaged XLPE-Type Cables, Location 6Figure 11 shows the tell-tale reverse-curvature bend in the XLPE cable from Location 4, with ablow-out hole caused when the cable faulted. Each of the three cables had similar reverse curvature at
this location. Figure 12 shows this cable after the outer HDPE coating is removed. The high reverse
curvature has caused the XLPE insulation around the copper core the wrinkle outwards, tearing theouter copper sheath.
It appears that the damage to the cables at these 6 locations was due to the effects of liquefaction.
Why do we say this? The following observations are made:
Ground shaking. The level of ground shaking varied throughout Christchurch in this earthquake.
In the areas with buried 66 kV cables, the motions were typically 40 to 70 cm / sec. Assuming a wave
propagation speed of perhaps 3.5 km/sec, and the cable moves with the ground, we get a ground strainof 0.0002. This should produce a maximum stress in the aluminum (or copper) on the order of 10% ofyield. While locally the ground velocities might have been twice as high owing to normal variations ofmotions over distance, it is not credible to posit that the motions would have been high enough tocause yielding in the metal.
At none of the six locations was the ground deformation "huge", and there were no major groundcracks. Figure 13 shows Location 5; the remaining locations are not all that much different withrespect to damage to the streets / sidewalks.

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Figure 11. Failed XLPE-Type Cable, Location 4

Figure 12. Failed XLPE-Type Cable, Location 41866

Figure 13 shows the buckling of the XLPE-type cables at Location 5.

Figure 13. Failed XLPE-Type Cable, Location 5
ESTIMATED PERMANENT GROUND DEFORMATIONS
We field surveyed each of the 6 locations. With the exception of Location 1 we observed nolateral spreads. At all 4 of the 6 locations, we can observed up to 2 cm of "knife edge" settlements
nearby, as expressed by cracked sidewalk curbs or other cultural features. Nearby single family
residential structures showed slight to moderate damage (most were "green tagged".) One older
unreinforced masonry structure had partial collapse near Location 2, but that is not surprising given
the high levels of shaking (PGA about 0.5g).
At all 6 locations, there are is clear evidence, within 50 m of the damaged cables, of liquefaction,
as expressed by sand boils.
In discussions with Orion, it was observed that multiple people who looked at the damage
locations could not discern any particular reason why the cables broke at those specific locations.
Figure 14 shows the street in the vicinity of Location 5: no major ground deformations are observed.
Ongoing work to superimpose Lidar-based movements (differential between locations measured
after the September 10 2010 event and the February 22 2011 event) remains to be done.
FAILURE MECHANISM
A series of nonlinear structural analyses of buried cable were performed to try to duplicate the fieldobservations in Christchurch. Based on these analyses, and in combination with the field observations,the causative movements / forces to cause the cable failures are thought to be as follows. A
combination of traveling waves (peak velocities of perhaps 40 to 70 cm/sec) and local differential1867

settlements (locally 2 to 5 cm) create high bending moment about the horizontal axis of theunreinforced cemented sand / concrete thermal backfill. Before the concrete cracks, the cables are fine.

Figure 14. Street at Location 5
With increasing bending moment, the induced tensile stress in the concrete finally finds a "weak spot"
and the concrete cracks, with the depth of the crack about mid-way through the cross section. Highcompressive forces occur in the cables as the cable pipes try to behave "like reinforcing". These highcompressive forces may cause spalling of the sides of the cemented-bound sand. Within a minute (or afew minutes) after the start of ground shaking, liquefaction-induced pore pressures begin to dissipate,
and the ground settles into its final position. By that time, the earthquake bending-moment-caused
crack in the thermal concrete then allows the differential settlement to accumulate along the crack,
resulting in a shearing action of the now-divided two pieces of unreinforced concrete. The crack in the
top "soil cap" allows for "sloshing" of the top ground layer, and when the two parts "come backtogether", high compressive forces occur in the thermal concrete, forcing the cables to buckle
sideways (towards the Lake) and downwards / upwards. The corrugated cables, being anchored within
the concrete either side of the crack, cannot slip; the XLPE cables might slip a bit into the concrete.
The 2 cm to 5 cm of resulting "knife edge" shearing offset of the thermal concrete.
The net effect was that the shearing action across the cracked concrete was enough to locally crush
the power cable, or to put the cable into reverse bending over a short enough crack-width zone so as tolocally wrinkle a lead sheath within the cable; the buckled aluminum or lead sheath crushed theinternal insulation around the copper / aluminum core, leading to a short (fault).

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TEST PROGRAM AND DESIGN IMPLICATIONS
A test program for full scale duct bank specimens is now being conducted at Berkeley, California,
sponsored by Pacific Gas and Electric. The test specimens include four 150 mm diameter (6-inch)
ducts (either PVC or HDPE) supported on spacer grids every 1.5 m (5 feet) within unreinforced (orreinforced) concrete, about 0.8 m x 1 m in cross section. As of writing this paper, complete results
are not yet available, but initial tests on two specimens show the following.
Unreinforced duct: as soon as imposed bending moment on the concrete duct bank exceeds thetensile cracking strength, a large crack forms in the concrete. As the imposed displacement isincreased to force increased curvature on the 150 mm ducts within (but preventing any shearing), theducts maintain their circular cross section sufficiently so as to prevent crushing of the power cablewithin, even through rotations of 5 degrees.
Reinforced ducts. The internal reinforcement includes steel to resist bending and shear due toliquefaction; with extra shear reinforcement to assure that bending controls. Tests show thesteel-reinforced duct bank easily absorbs the cracking via ductile performance of the reinforcing bars;
there is no significant shear deformation on the internal ducts. Internal PVC ducts do crack at theirjoints; this damage would not likely damage an existing XLPE cable within, but might preclude
pulling a new XLPE cable, until such time that the cracked PVC is identified and repaired.
Based on the empirical evidence, coupled with nonlinear analyses and tests, the following
preliminary design recommendations are made for 66 kV (or higher) buried cables in liquefaction
zones:
If at all possible, avoid the liquefaction zone; or use overhead circuits in liquefaction zones.
Direct burial in moderately-strong thermal backfill concrete can result in cable failures. Failure
rates observed are about 2 faults per cable per km, given settlements on the order of 2 to 5 cm. Buriedcables in zones that had higher settlements (5 to 10 cm+ with some lateral spreads), had a high enoughnumber of failures as to warrant outright replacement rather than repair.
Where buried cables are required in liquefaction zones, a reinforced concrete thermal backfill cancontrol curvatures and prevent sharp knife-edge offsets. Reinforcement (either steel, or possibly withfiber reinforcement) should be used for such direct burial situations. Where ducts are placed within thethermal concrete, the cemented PVC joints can be expected to crack under high bending action.
CONCLUSIONS
Unreinforced thermal concrete should not be used either for direct burial or duct-bank-type burial of
66 kV (or higher) cables in zones prone to liquefaction.
ACKNOWLEDGMENTS
Many people contributed to the findings in this paper. John O'Donnell and Shane Watson of Orion
Energy graciously provided many of the details and repair photos. Sonia Giovinazzi, University ofCanterbury helped with maps. Barry Davidson helped with field reconnaissance. Donald Duggan
performed numerical analyses of the buried cables. Pacific Gas and Electric sponsored the tests.
REFERENCES
Eidinger, J., editor, “Christchurch earthquake sequence of 2010-2011.” Technical Council on Lifeline
Earthquake Engineering, draft: url http://www.geEngineeringSystems.com, Monograph 40, ASCE,
2012 (in press).
Giovinazzi, S., Wilson, T., David C., Bristow D, Gallagher M., Schofield, A, Villemure, M, Eidinger J,Tang, A., Lifeline performance and management following the 22 February 2011 Christchurch
earthquake, New Zealand: Highlights of Resilience, New Zealand Society for Earthquake
Engineering, Christchurch, April 13-15, 2012 (in press).

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