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Proven design for extra strength

Copying the design of structures that have survived Christchurch’s earthquakes is helping to speed up reconstruction work. GE reports.

christchurch

While images of earthquake-hit Christchurch in New Zealand have focused on scenes of destruction, some structures have survived the increase in seismicity in the region unscathed.

Replicating the designs of these structures is helping engineers to ensure new infrastructure can withstand any future earthquake and ongoing
aftershocks.

Repairs to damage on Fitzgerald Avenue had be fast-tracked to allow this key arterial route into the city to take full traffic load, and engineers looked to proven designs for the solution.

“Fitzgerald Avenue suffered extensive damage during the February 2011 disaster,” says Opus International geotechnical engineer Brabha Pathmanathan. “For a length of 200m, the underlying ground had spread laterally towards the adjacent Avon River, creating a longitudinal rent up to 2m deep along the northbound carriageway.

“The road normally carries around 38,000 vehicles a day, so repairing this route became a priority in restoring Christchurch’s infrastructure.”

The high seismic intensities created by the Christchurch earthquakes, together with more than 10,000 aftershocks, resulted in extensive damage to the areas around the rivers which pass through Christchurch, due to inward movement of the banks.

This included Fitzgerald Avenue, which borders the Avon River and which is constructed over weak river silts.

“Repairing this route became a priority in restoring Christchurch’s infrastructure”

The extreme forces involved in the main quake are illustrated by strong motion data from the Christchurch Resthaven site, just over 1km west of Fitzgerald Avenue, where peak ground accelerations of 0.52g vertically and 0.71g horizontally were recorded.

Strong motion of this magnitude affects a wide range of structures such as roads and buildings.

To find a solution, Opus’s engineers studied the performance of ground improvement techniques and structures which had survived the 2010 Darfield event 35km from the city centre and those closer to Christchurch.

Local Tensar dealer Maccaferri NZ had noticed one interesting survivor - a riverside section of Carlton Mill Road located 2.6km upstream that was built using Tensar geogrid 20 years ago and which showed no visible signs of earthquake damage.

At Carlton Mill Road, the 20-year-old construction comprises an earth embankment and precast concrete panel retaining wall with the soil mass reinforced by Tensar uniaxial geogrids fastened to the full height facing panels.

Tensar believed that the reinforcement by geosynthetics held the key to the structure’s survival and proposed a similar reinforced embankment for Fitzgerald Avenue.

At Fitzgerald Avenue, any reinforced soil solution had to be able to resist lateral spread and unacceptable deformations from future seismic action.

Tensar

The final design incorporated stone columns driven into the silt foundation, over a geogrid reinforced retaining wall. Short lengths of Tensar RE uniaxial geogrid were cast into the precast concrete facing panels at 500mm intervals.

Each geogrid tail was fastened with a simple bodkin joint to 7.5m of geogrid laid back into the embankment, over which was compacted well graded mineral aggregate (GAP65) fill to form the soil mass.

The finished structure has a near vertical, structurally stable retaining wall up to 3.2m high, which enabled the full width of the road to be reinstated, and a cycle/pedestrian path to be added on the river side.

The restored road was fully reopened to traffic in May this year after just eight months of construction work. Tensar believes this took less time than alternative designs would have.

And if the resilience of Carlton Mill Road has been replicated at Fitzgerald Avenue, then the durability has almost certainly been improved too.

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