NHRP / Hazard themes / Geological Hazards / Landslide / Active Landscapes 2014-15 / Slopes Christchurch & Wellington

Seismic Response of Slopes in Christchurch and Wellington

Port Hills building damage

The 2010/11 Canterbury earthquakes triggered mass movements in the Port Hills including rockfalls, debris avalanches, landslides and cliff-top cracking. The most abundant mass movements with the highest risk to people and buildings were rockfalls and rock/debris avalanches. Over 100 residential homes were impacted leading to the evacuation of several hundred residents.

Our research looks at the effects that slope geometry, geology and earthquake source have on amplifying ground shaking leading to slope failure. This research was primarily focused on the effects of the 2010/11 Canterbury earthquakes on slopes, ranging from steep rock cliffs to shallow soil slopes in the Port Hills of Christchurch. The methods used and the lessons learned from Christchurch were then applied to some major slope types in Wellington to test the results and identify differences.

Results from the Port Hills cliffs

For the debris avalanches and rockfalls of the Port Hills, observations and monitoring showed that the higher and steeper cliffs produced more debris, as one might expect. The higher cliffs also suffered larger amounts of cliff-top cracking than smaller cliffs (below).

NHRP slope inclination

Relationships between slope height, inclination and extent of cliff-top cracking for cliffs in the Port Hills affected by the Canterbury earthquake sequence. Data: GNS Science.

Our data have shown that amplification of shaking did not increase linearly with increasing height, but instead reflected changes mainly in the cliff geology. Local topography, such as sharp breaks in slopes, can also amplify ground shaking. However, an important factor in amplifying ground shaking is contrasting geological materials, which in the Port Hills cliffs are quite variable.

Below is a schematic of the locations where we sampled the modelled accelerations (ground motions).  AMAX represents acceleration at the top of the cliff “crest”, AFF represents accelerations at the bottom “toe” of the cliff, and KMAX represents the average accelerations along a number of simulated slide surfaces (along which the displaced mass can move) within the rock mass forming the cliffs.

NHRP cliff failure schematic

Schematic of cliff failure modes in the Port Hills that occurred during the Canterbury earthquake sequence. Data: GNS Science.

For slope-stability assessments, characterising the shaking in terms of KMAX is more representative than just using the accelerations measured from a single point at the cliff top (AMAX) (see schematic above). Amplification ratios of peak ground acceleration (PGA) between the cliff crest and cliff toe were on average 2.0 (AMAX and AFF, respectively), and 1.5 (KMAX and AFF) for Port Hills slopes between 10 and 100 metres high and steeper than 60 degrees. That means the peak acceleration (‘ground motions’) experienced at the top of the slope could be two times greater than that at the bottom.

Thus, the well documented Port Hills case histories, involving site-specific assessments, can provide more certainty in seismic landslide assessments, in general. The results suggest that the ground motions used for assessing the stability of steep and tall cliffs in Christchurch (slopes greater than 60 degrees and more than 10 metres in height) should include an amplification factor to take into account the effects that slope shape and local geology have in amplifying shaking. However, for the design of structures located near the edge of steep slopes, the maximum acceleration at the slope crest (AMAX), may be a more useful parameter. 

Preliminary assessment of field and model data from slopes in Wellington, taking into account the differences in earthquake sources, geology and topography in Wellington compared to those in the Port Hills, suggest similar results. That is, tall and steep slopes, contrasting geological materials and sharp breaks in slope cause measurable increases in ground accelerations compared to those at the slope toe. The magnitude of amplification is currently difficult to quantify in Wellington because there is very limited subsurface information available from the Wellington hill slopes, and so more work will be needed in this area. Our findings in the Port Hills were used by regulatory authorities as one of the inputs to developing land zoning policy.  The lessons from Christchurch are influencing decision-making across the country.

Contact - Chris Massey, GNS Science
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Last updated 29 Oct 2015