Land Instabilities

LAND INSTABILITIES________________________________

Mechanism of destruction:-

Landslides destroy structures roads pipes and cable either by4he ground moving out form beneath them or by burying them Gradual ground movement causes tilted, unusable buildings. Cracks in the ground split foundations and rupture buried utilities. Sudden slope failures can take the ground out from under settlements and throw them down hillsides. Rock falls cause destruction from fragmentation of exposed rock faces into boulders that roll down and collide into structures and settlements. Debris flows in softer soils, slurry material, man-made spoil heaps and soils with high water content flow like a liquid, filling valleys, burying settlements, blocking rivers (possibly causing floods) and blocking roads. Liquefaction of soils on flat land under strong vibrations in earthquakes is the sudden loss of the strength of soils to support structures that stand on it. Soils effectively turn temporarily to liquid allowing structures to sink or fall over.

Parameters of severity:-

Volume of material dislodged (m3), area buried or affected, velocity (cm/day), boulder sizes.

Causes:-

Gravitational forces imposed on sloping soils exceed the shear strength of soils that hold them in position: High water content makes soil heavier, increasing the load, and decreasing shear strength. With these conditions heavy rainfalls or flooding make landslides more likely to happen. The angle of slope at which soils are stable is a physical property of the soil Steep cuttings through some types of soils makes them unstable. Triggering of the collapse of unstable soils can be caused by almost any minor event: storms, minor ground tremors or man-made actions. Liquefaction is caused by earthquake vibrations through loose soils, usually with high water content.

Hazard assessment and mapping techniques:-

Identification of previous landslides or ground failures by geotechnical survey Identification of probability of triggering events such as earthquakes Mapping of soil types (surface geology) and slope angles (topographic contouring) Mapping of water tables hydrology and drainage. Identification of artificial land fill, man-made mounds, garbage pits, slag heaps. Investigation into the probability of triggering events, especially earthquakes

Potential for reducing hazard:-

Landslide risk for a slope reduced by shallower slope angles (excavating top layer to cut back slope)., increasing drainage (both deep drainage and surface run-off) and engineering works (pilling, ground anchors, retaining walls). Shallower angles for embankments and cuttings, terracing slopes and forestation

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