Clifton Hill and Moa Bone Point Rockfall Protection

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Published 01 December 2016
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Clifton Hill and Moa Bone Point Rockfall Protection

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During the Christchurch Earthquakes in 2011, the cliffs at Clifton Hill and Moa Bone Point experienced significant ground shaking and as a result a large amount of material was lost from cliff collapse and mass movement. The sites at Clifton Hill and Moa Bone Point were part of the Sumner to Lyttelton Corridor works, which includes mitigating the risk to traffic along Main Road below Clifton Hill and Moa Bone Point.

Abseil Access was contracted by Fulton Hogan to carry out a significant portion of the cliff stabilisation works, designed by Golder Associates in Christchurch. The cliff stabilisation works carried out at each of the sites included the following,

 Clifton Hill

• Removing vegetation to gain access to the site and cliff face
• Scaling of loose rocks and protrusions that might affect the integrity of the draped mesh
• Supply and installation of approximately 3400 m2 of draped Geofabrics PVC coated double twist mesh
• Supply and installation of face anchors at locations determined by the Engineer

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Moa Bone Point

• Removing vegetation to gain access to the site and cliff face
• Heavy rock scaling of isolated columns of rock using multiple airbags, in an area approximately 1200 m2
• Moderate rock scaling of the talus slope to stabilize large blocks using airbags and crow bars, in an area approximately 1600 m2
• Light rock scaling of the slope directly above the Moa Bone cave, in an area approximately 450 m2
• Supply and installation of two 3 m 100 mm diameter rock bolts to support a potentially unstable block.

Due to the high volume of traffic along Main Road (up to 20,000 combined traffic count), the traffic management plan only allowed for short duration, 4 to 5 min, road closures for any work that could an increase risk to traffic users. In particular at the Moa Bone Point site where columns of rock up to 24 m3 were being scaled from the face using airbags.

Under traffic management control, a helicopter was used to lift the 45 rolls of rock fall netting to the edge of the cliff from the carpark. This allowed the netting to be positioned directly on to the top cable reducing the need for manual handling. With each roll weighing approximately 170 kg moving them by hand would have taken much longer and an increased risk of injury from manual handling.

The rolls of netting were then rolled down the cliff face and stitched together, secured at the base of the cliff with a 19 mm diameter galvanized wire toe cable. The work was completed within the allocated timeframe and allowed the removal of the shipping containers providing protection to traffic along Main Road after 5 years.

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Issue 92
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ISSN 0111-6851