Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 180

Slides occur from shear failure along one or more surfaces that have reduced
resistance against movement. The mass of soil and/or rock that moves during a
slide may or may not experience deformation. The most common type of slide
occurs in clay soils and results in an approximately spoon-shaped slip surface.
These slides are called rotational slides and are generally characterised by back-
ward rotation of the sliding mass (Fig. 7. 1 ). They are also deep seated (the base of
theslide occurs at some depth). These slides are generally associated with little
deformation of the moving mass, and usually with the formation of only one
or a few discrete blocks or masses. It is also common for successive rotational
slides to occur at one location. This is a result of the scar left at the head of the
slope being near vertical and unsupported until successive slides have stabilised
theslope. Successive slides are defined as retrogressive slides and develop in a
headward direction with a common basal shear surface where the individual
planes of failure are combined (Bell, 1998).
Non-circular (non-rotational) slides occur in over-consolidated, weathered or
unweathered clays. They occur along quasi-planar slip surfaces. These slides are
called compound slides and are usually single events or progressive non-circular
slides (Crozier, 1986). Transitional slides tend to occur in inclined, stratified
deposits. Here, the slope movement occurs along a planar surface, usually a
bedding plane. These slides occur when the mass is dislodged due to the excee-
dence of the gravitational force over the frictional resistance along the slip
surface. Transitional slides are often slab slides where the slip surface is near
parallel to the ground surface. These slides may occur on more gently inclined
surfaces than rotational slides and also have the potential to be more extensive.
Failure usually occurs in clays, quick clays and varved clays, and results from
lateral spreading along bedding planes due to increased pore-water pressure. The
increase in pore-water pressure generally occurs in a permeable zone at shallow
depths that result in the mobilisation of the overlying layer. The mass generally
moves over planar surfaces and may divide into a number of semi-independent
slide units with complex movements that may include translational movements,
rotation and liquefaction (Bell, 1998).
Slides include both rock and debris slides. Rockslides usually occur on steep
slopes due to a gradual weakening of the bonds found within the rock mass.
They are usually translational slides controlled by discontinuity patterns within
theparent material. Again, freezing and thawing is one of the main factors
controlling this type of slide. Rockslides may travel significant distances. Debris
slides usually occur in the weathered parent material. Debris slides often occur
in the form of mud slides which are generated by an increase in the water
content along the contact between loose and/or saprolitic material and the firm