Retaining walls
Retaining structures are considered as buildings and are therefore subject to the same Building Code performance requirements – to protect life, amenity and property.
Performance requirements
Generally, seismic forces should be considered in any design where the consequences of the structure failing are severe. Earth-retaining structures should be specifically designed to resist seismic forces in two main circumstances.
- Where the failure or excessive deformation of the retaining structure might contribute to loss of life within or safe egress from a building or loss of amenity for a building. This includes walls less than 3 m high.
- Where the retaining structure has an effective height greater than 3 m. This height includes the batter above or below the retaining structure within a horizontal distance of 1.5 times the retained height.
The New Zealand Geotechnical Society (NZGS) recommends the following performance requirements for most retaining structures.
Typical performance level for retaining structures. (Adapted from MBIE/NZGS, Earthquake resistant retaining wall design Figure 4.1 and Table 4.1)
| Wall type | IL | SLS | ULS |
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1 | No requirement | Wall movement should not be so excessive as to cause collapse of the building (eg less than 150 mm for normal timber framed construction to NZS 3604 |
| 2-3 | No significant movement | Wall movement should not be so excessive as to cause loss of structural integrity, or prevent means of safe egress (eg less than 50 mm for normal timber framed construction to NZS 3604) | |
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2-3 | No significant movement | Wall movement should not be so excessive as to cause loss of support, loss of structural integrity, or prevent means of safe egress (eg less than 100 mm for normal timber framed construction to NZS 3604) |
 |
2-3 | Minor movement (< 25 mm) | Wall movement should not be so excessive as to cause loss of structural integrity or prevent means of safe egress (eg less than 100 mm for normal timber framed construction to NZS 3604) |
 |
2-3 | Minimal visual impairment (< H/50) | There should be a low risk of collapse of the wall. Wall deformations should not impede egress from the building (noting that severe visual impairment of the wall may deter occupants from escaping the building) (eg less than 100 mm from vertical for typical cases) |
 |
1-3 | No requirement | There should be a low risk of collapse of the wall. Wall deformations should not be so excessive as to damage services or prevent use of driveway (eg less than 150 mm from vertical for typical cases) |
 |
1 | No requirement | There should be low risk of collapse of the wall |
In most cases, it is unnecessary to design and construct a retaining structure to withstand ultimate limit state (ULS) peak ground accelerations. The inertia and damping of the retained material tends to limit the amount of deformation the structure will experience during an earthquake. This means that most retaining walls are flexible enough to absorb high transient ground accelerations without damage.
Also, some permanent deformation is usually acceptable when exposed to a ULS event.
However, in all cases, retaining structures associated with facilities of importance level (IL) 4 or more require special engineering design.
Retaining wall design
The calculations required to design a retaining wall are beyond the scope of this resource.
However, the Building Code provides limited design direction via AS/NZS 1170.0:2002 Structural design actions – Part 0: General principles. This standard contains general procedures and criteria for the structural design of buildings, including retaining walls. It requires that seismic loads on retaining structures be determined in accordance with AS/NZS 1170.1:2002 Structural design actions – Part 1: Permanent, imposed and other actions.
The commentary of AS/NZS 1170.0:2002 also cites AS 4678-2002 Earth-retaining structures, which sets out requirements and recommendations relating to the design and construction of structures required to retain soil, rock and other materials.
Earthquake resistant retaining wall design, by MBIE and NZGS, also provides detailed and easy-to-follow guidance on the process to design most seismically resilient retaining structures.
Materials selection
During the 2010 and 2011 Canterbury earthquakes, many retaining walls that used natural, river-rounded drainage gravel as the backfill material exhibited significant damage.
When exposed to strong shaking, the rounded material tends to settle to bottom of the volume retained by the wall. The accumulation of mass at the base lifts the wall out of plane and prevents it from returning to its original position.
For this reason, NZGS recommends designers avoid rounded metal as a backfill material. Crushed aggregates that are well compacted during installation should be used instead.