Landslides

The term "landslide" 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, landslides are known also as landslips, slumps or slope failure.

Anatomy of a Landslide
Figure shows a graphic illustration of a landslide, with the commonly accepted terminology describing its features.

Types of Landslide

Rotational slide: This is a slide in which 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.

Translational slide: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting.

Block slide: is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass.

 

Rock Fall: Falls are abrupt movements of masses of geologic materials, such as rocks and 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.

Topple: Toppling failures are distinguished by the forward rotation of a unit or units about 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.

Debris flow: A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope. Debris flows include less than 50% fines. Debris flows are commonly caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes. Debris flows also commonly mobilize from other types of landslides that occur on steep slopes, are nearly saturated, and consist of a large proportion of silt- and sand-sized material.

 

Debris avalanche: This is a variety of very rapid to extremely rapid debris flow.

Earthflow: Earthflows have a characteristic "hourglass" shape. The slope material liquefies and runs out, forming a bowl or depression at the head. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. However, dry flows of granular material are also possible.

Mudflow:A mudflow is an earthflow consisting of material that is wet enough to flow rapidly and that contains at least 50 percent sand-, silt-, and clay-sized particles. In some instances, for example in many newspaper reports, mudflows and debris flows are commonly referred to as "mudslides."

Creep: Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. There are generally three types of creep: (1) seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature; (2) continuous, where shear stress continuously exceeds the strength of the material; and (3) progressive, where slopes are reaching the point of failure as other types of mass movements.

Lateral Spreads: Lateral spreads are distinctive because they usually occur on 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, cohesionless sediments (usually sands and silts) are transformed from a solid into a liquefied state. Failure is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced. When coherent material, either bedrock or soil, rests on materials that liquefy, the upper units may undergo fracturing and extension and may then subside, translate, rotate, disintegrate, or liquefy and flow.

 

Factors affecting landslide
(1)Closest distant to fault-rupture is the most important factor affecting landslides in the mountainous region.
(2 Closest distance to ridge-crest also plays an important role in the occurrence of landslides.
(3) Slope angle is the most important factor affecting landslides in each homogeneous zone.
(4) Slope aspect also shows some correlations to landslide ratio in each homogeneous zone.
(5) Goodness of correlation for each factor shows some differences among different homogeneous zones.

Effects of a landslide

Landslides kill people, destroy trees, crops and other plantations.This catastrophe could drastically affect everyone who lives near an elevated area. A single but relatively strong rainfall, thunderstorm, hurricane / tropical depression could make the soil erode, which will cause immense flooding, and will, definitely rage on the residents below.Earthquakes may occur because of landslides and tidal waves may also be effects.Landslides thus causes the property damage, injury, and death and adversely affect a variety of resources. For example, water supplies, fisheries, sewage disposal systems, forests, dams, and roadways can be affected for years after a slide event. The negative economic effects of landslides include the cost to repair structures, loss of  property value, disruption of transportation routes, medical costs in the event of injury, and indirect costs, such as lost timber and fish stocks. Water availability, quantity, and quality can be affected by landslides. Geotechnical studies and engineering projects to assess and stabilize potentially dangerous sites can be costly.

Causes of Landslides

1.Geological Causes

Weak or sensitive materials

Weathered materials

Sheared, jointed, or fissured materials

Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth)

Contrast in permeability and/or stiffness of materials

2.Morphological Causes

Tectonic or volcanic uplift

Glacial rebound

Fluvial, wave, or glacial erosion of slope toe or lateral margins

Subterranean erosion (solution, piping)

Deposition loading slope or its crest

Vegetation removal (by fire, drought)

Thawing

Freeze-and-thaw weathering

Shrink-and-swell weathering

3.Human Causes

Excavation of slope or its toe

Loading of slope or its crest

Drawdown (of reservoirs)

Deforestation

Irrigation

Mining

Artificial vibration

Water leakage from utilities

 Landslide Mitigation

Vulnerability to landslide hazards is a function of location, type of human activity, use, and frequency of landslide events. The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity. Local governments can reduce landslide effects through land-use policies and regulations. Individuals can reduce their exposure to hazards by educating themselves on the past hazard history of a site and by making inquiries to planning and engineering departments of local governments. They can also obtain the professional services of an engineering geologist, a geotechnical engineer, or a civil engineer, who can properly evaluate the hazard potential of a site, built or unbuilt.The hazard from landslides can be reduced by avoiding construction on steep slopes and existing landslides, or by stabilizing the slopes.Stability increases when ground water is prevented from rising in the landslide mass by
(1) covering the landslide with an impermeable membrane,

(2) directing surface water away from the landslide, 

(3) draining ground water away from the landslide, and
(4) minimizing surface irrigation. Slope stability is also increased when a retaining structure and/or the weight of a soil/rock berm are placed at the toe of the landslide or when mass is removed from the top of the slope.

About the Author: Kashif Ali Bhatti is a B.S in Geology from University of Sargodha, Pakistan