By the EERI Reconnaissance Team.
March 15, 2011. Earthquake Engineering Research Institute.
I spent today inspecting retaining walls and rockfalls in the suburbs of Lyttelton, Mount Pleasant, and Sumner. The focus of this blog post is on retaining wall performance.
Retaining walls in residential areas generally ranged from 1 to 4 meters high. Wall types observed included: gravity rock and masonry walls, gabion walls, masonry walls with ground anchors, concrete crib walls, timber crib walls, and timber pile walls. Example photos are shown in Figures 1, 2, 3, 4, 5, and 6.
Wall performance ranged from very good, to complete loss of the wall. Significant loss of ground above walls, such as causing road closure, was not observed. The worst performing walls were gravity-type walls with little or no lateral support systems. Rock and masonry walls were observed in multiple locations to be completely destroyed or had been removed at the time of inspection.
Gabion walls, although generally appearing to be in decent shape, frequently had ground cracking behind the walls.
Crib walls, although dependent on gravity, performed marginally better than rock and masonry walls. The tallest walls observed were concrete crib walls, which performed well. Cracking and movement of the cribs was observed, but significant damage was not present. Timber crib walls were observed to have good performance as well. It appeared that most high performing crib walls had more than one row of cribbing. A single concrete crib wall was observed to be destroyed, but this wall had only a single row of cribs.
Timber piled walls performed the best of all walls observed to day. No ground loss was observed behind any of the timber pile walls. In only a single instance was timber wall movement observed, but the wall was supporting landscaping and thus, posed no threat to traffic or structures.
Tomorrow I plan to undertake a comprehensive catalog of walls in the Mount Pleasant area.
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| Figure 1. Concrete crib wall with broken cribs. | Figure 2. Concrete crib wall with poor performance. | Figure 3. Ground distress above gabion wall. |
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| Figure 4. Masonry wall with ground anchors. | Figure 5. Timber crib wall with good performance. | Figure 6. Timper pile wall with good performance. |
