| Type |
Form of Initial Failure
Surface |
Subsequent
Deformation |
| Fall (rock or stiff
soil) |
|
|
| Detachment from:
pre-existing discontinuities or tension
failure surfaces |
a) Planar surface
b) Wedge (two or more intersecting joints)
c) Stepped surface
d) Vertical surface |
Free fall, may break up,
bounce, slide or flow down slopes. May involve fluidisation,
liquefaction, cohesionless grain flow, heat generation or other
secondary effects on disintegration when failed rock hits the ground
surface. |
| Topple (rock or stiff
soil) |
|
|
| Detachment from:
pre-existing discontinuities or tension
failure surfaces |
a) Single
b) Multiple |
As above |
| Slide |
|
|
| Rotational movement (failure surfaces
essentially circular; occurs in soils) |
a) Single
b) Multiple
c) Successive |
Toe area may deform in a
complex way. The ground may bulge, the slide may creep or even flow,
possibly over existing failures. Failure might be retrogressive or
progressive. |
| Non-rotational compound movement
(non-circular failure surface; may be listric or bi-planar; found in
soils and rocks) |
a) Single
b) Progressive
c) Multi-stored |
Graben often develops at the
head of the landslide. It may include a toe failure of a different
type. |
| Translational movement (Often associated
with discontinuity controlled failures in bedded or foliated rocks) |
a) Planar
b) Stepped
c) Wedge
d) Non-rotational |
May develop complex run-out
forms after disintegrating (see falls and flows). |
| Spread (soils and weak
rock) |
|
|
| Lateral spreading of ductile or soft
material that deforms |
a) Soft layer beneath a hard rock
b) Weak interstratified layer
c) Collapsing structure |
Can develop sudden spreading
failures in quick clays when the slope opens up in blocks and
fissures followed by liquefaction. Might be a slow movement
associated with denudational unloading. Can be represented by
cambering and valley bulging. |
| Flow (usually
associated with soils but rock flows do occur) |
|
|
| Debris movement by flow |
a) Unconfined
b) Channelized |
Flow involves complex run-out
mechanisms. it may be catastrophic in effect and it may move in
sheets or lobes.
The form of movement is a function of the
rheological properties of the material. |
| Creep movement |
Failure surface rarely clearly defined |
Creep may be superficial
gravity movement, seasonal movements or it might represent
pre-failure and progressive movements prior to a larger scale
failure. |
| Rock flow (sometimes referred to as
sagging or Sackung). Usually associated with mountain terrain or
areas of rapid and deep incision. |
a) Single-sided
b) Double-sided
c) Stepped
(Failure surface may be rotational,
compound, listric, biplanar or intermittent) |
May be slow gravity creep or
the early stages of larger scale movements that only show as bulging
in the topography without a clearly defined toe deformation. Where
controlled by discontinuities it may involve toppling. |
| Complex |
|
|
| a) Movements involving two or more of the
above mechanisms (referred to as compound when two types of movement
occur currently) |
Dependant on the form of failure described
above |
As described for the various
categories above. |
| b) Rock or debris avalanche |
Often initiated as fall/slide of rock
and/or debris |
Complex long run-out
mechanisms, including fluidisation and cohesionless grain flow. |
