Port Hills
Geotech update
Cliff collapse
Cliff collapse as a result of the earthquakes has affected sea
cliffs at Whitewash Head and other headlands around Banks
Peninsula, and old (abandoned) sea cliffs at Redcliffs,
Peacocks Gallop and along Wakefield Avenue in Sumner.
Houses
Smaller failures have also been recorded on old cliffs
within McCormacks Bay and in some former quarries.
Historically these cliff areas have been relatively stable,
with only isolated local failures involving relatively small
volumes of material. The 4 September 2010 earthquake
had very little impact on these cliffs, but on 22 February
and 13 June 2011 widespread cliff collapse resulted from
the extremely high levels of earthquake shaking generated
by active faults directly below the Port Hills.
Ash layer
Ash layer
Rock fall
debris
Houses
The effects of this type of extreme earthquake shaking on
cliffs are listed below.
Collapse mechanism
A cliff collapse begins with the detachment of soil and/or
rock from the cliff, most often along a pre-existing fracture
or weakness. The material subsequently descends, mainly
by falling, and may break up in flight or on impact. The
collapse material will normally accumulate as a debris
wedge or cone at the base of the cliff but falling rock
impacting on the debris may travel significant distances as
‘fly rock’.
Cliff profile before collapse
Cracking
Cracking of the cliff top behind the collapse zone is common.
Occurrence and relative size/range
In the Port Hills, cliff collapse involves modern or old sea
cliffs and/or abandoned quarries. The volume of material
involved can vary substantially, from individual rocks to
massive blocks.
Velocity of travel
Rocks falling from a cliff move very to extremely rapidly
with a combination of free-fall, bouncing and rolling of
detached soil, rock, and boulders.
JR 3580 September 11
Triggering mechanism
The main causes of cliff collapse are undercutting of slope
by natural processes such as sea or river erosion, and
earth­quake shaking.
Ash layer
Ash layer
Rock fall
debris
Houses
Cliff profile after collapse
Port Hills
Geotech update
Effects (direct/indirect)
Material falling as a result of cliff collapse can be lifethreatening. Falling debris can damage property beneath
the fall zone, and boulders that bounce or roll can damage
structures or kill people in houses, on roads or in open spaces.
Collapse of the ground behind the cliff top can also be life
threatening. Cracking can damage buildings and indicate
that further more complete collapse is likely.
Corrective measures/mitigation
Large cliffs are very difficult to stabilise even at great cost.
Rock bolts or other similar types of anchoring can be used
to stabilize cliffs, and scaling of loose or unstable rock can
lessen the hazard.
Rock curtains or other slope covers, protective covers over
roadways, removal of rocks or realignment of key lifelines
can be used as other mitigation methods.
Cliff collapse close to houses, Whitewash Head, after 13 June 2011
In addition to such measures, warning signs are commonly
used to advise against stopping, parking or walking under
hazardous cliffs.
The options for cliff tops are most commonly limited
to avoidance of at risk areas (e.g. with setback
requirements). Cliff edge stabilisation may be attempted
but is very expensive and must be considered in relation
to the stability of the whole cliff not just an individual site.
Predictability
Indicators of imminent cliff collapse include cracking of
the cliff top and sometimes the presence of overhanging
rock or fractured rock in the cliff face. Very strong earthquake
shaking that may trigger collapse cannot be predicted reliably.
Cliff collapse and debris wedge at Peacocks Gallop, after 13 June 2011
JR 3580 September 11
Cracking in cliff and cliff top between houses after 13 June 2011