Port Hills
Geotech update
Houses
Houses
Rockfall (and boulder roll)
Top of cliff
(source area)
Boulder fall
Top of cliff
(source area) (source cliff)
Rock falls are abrupt, downward movements of rock that
detach
Ash layerfrom steep slopes or cliffs. The falling material
usually strikes the lower slope at angles less than the
Ashof
layer
angle
fall, causing bouncing. The falling mass may break
on impact, may begin rolling on steeper slopes, and may
Ash layer
continue until Rock
the terrain
flattens. Rolling boulders may be
fall
Ash layerdowndebris
focussed
gullies or may fan out over a wider area if
Houses
Rock
fall
the slopes are not
dissected
by gullies.
debris
Houses
Occurrence and relative size/range
Common on steep or vertical slopes. The volume of
material in a fall can vary substantially, from individual
before
collapse
rocksCliff
orprofile
clumps
of soil
to massive block thousands of
Cliff profile
before collapse
cubic meters
in size.
Boulder fall
(source cliff)
Toe of cliff
(source area)
Toe of cliff
(source area)
Rockslope
Rockslope
Talus
Talus
Colluvial
loess
Colluvial
loess
Velocity of travel
Very rapid to extremely rapid, free-fall; bouncing and
rolling of detacthed rock, and boulders. The rolling
velocity depends on slope steepness and surface
Cracking (eg. vegetation).
characteristics
Cracking
Triggering mechanism
Undercutting of slope by natural processes such as
streams and rivers, weathering effects (such as the
freeze/thaw
cycle), earthquake shaking and human
Ash
layer
Ash layersuch as excavation during road building.
activities
Effects (direct/indirect)
Rock
Falling
befall
life-threatening. Falls can
Ash
layermaterial can
Rock fall
debris
Ash
layer
damage property beneath
debris the fall-line of large rocks.
Boulders can bounce or rollHouses
great distances and damage
Houses
Edge of
source
Edge of
source
Rock slope
(source
area)
Rock slope
(source area)
Edge of
Edge of
source
source
Fallen
boulders
Fallen
boulders
30°
30°
30°
30°
Boulders
on slope
Boulders
on slope
Most distant
Boulder
Most
distant
stopping
position
Boulder
stopping
position
Houses
Houses
Toe of
runout
Toe
of zone
runout zone
Houses
Houses
structures or kill people. Damage to roads can include
deaths in vehicles hit by rocks and blockage of key
lifeline roads.
Corrective
measures/mitigation
Cliff profile
after collapse
Cliff profile after collapse
JR 3580 September 11
At source:
Treatment at source to reduce the hazard may include
removal by scaling, blasting, containing loose rock
behind rock curtains or other slope covers, stabilisation
by use of rock bolts or other similar types of anchoring.
On slope:
Earthen bunds and/or catch fences or training walls can
be used to prevent rolling or bouncing to areas lower on
the slope or to direct boulders down gullies. Protective
covers over roadways may be an option in vary steep
terrain. Warning signs are recommended for awareness
in hazardous areas.
Schematic of rockfall mechanism and runout zone
Schematic of rockfall mechanism and runout zone
Port Hills
Geotech update
Boulder has rolled downslope and stopped on deck
Unstable rock block on bluff above houses. If it fell, this would be
expected to roll downslope.
Boulder that has rolled downslope and into a house
JR 3580 September 11
Boulder that has rolled downslope and through a fence