Slope Failure - Land slides

Slope failures

Slope failure describes a wide variety of processes that result in the downward and outward movement of slope - forming materials including rock, soil, artificial fill or a combination of these processes. the various types of landslides can be differentiated by the kinds of material involved and the mode of movement.

Types of slope failures / movements

1) Slides
2) Falls
3) Topples
4) Flows
5) Lateral spreads

(1). Slides 

There are two major types,

- Rotational slides

The surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide.

- Transitional slides

The landslide mass moves along a roughly planar surface with little rotation or backward tilting.  There is a sub category of transitional slides called Block slide, it is moving mass consist of a single unit or a few closely related unit that move down slope as a relatively coherent mass.

 

 

(2). Falls

Abrupt movement of masses of geological materials, such as rocks boulders, that become detached from steep slopes or cliffs.

Separation occurs along discontinuities such as fractures, joints and bedding planes and movement occurs by free fall, bouncing and rolling. falls are strongly influenced by gravity, mechanical weathering and the presence of interstitial water.
  
(3). Topples


 
Toppling failures are distinguished by the forward rotation of a unit or units above some pivotal point, below or low in the unit, under the actions of gravity and forces exerted by adjacent units or by fluids in cracks.

(4). Flows

 There are different types of flows,

 - Debris flow

Involve the mass movement of loose soil, rock, organic matter, air and water mobilize as a slurry that flows down slope.

- Debris avalanche

Rapid to extreme rapid debris flow

- Earth flow

The slope material liquefies and runs out, forming a bowl or depression at the bead (hourglass shape). usually occurs in fine grained materials or clay bearing rocks on moderate slopes and under saturated conditions

- Mud flow

A mud flow is an earth flow consisting if material that is wet enough to flow rapidly and that contains at least 50% sand, silt and clay size particles.

- Creep

Creep is the imperceptibly slow, steady, downward movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure.   

(5). Lateral Spread

Usually occur an very gentle slopes or flat terrain. the dominant mode of movement is lateral extension accompanied by shear or tensile fractures. the failure is caused by liquefaction, the process whereby saturated, loose, cohesion less sediments (usually sand and silts) are transformed from a solid into a liquefied state.