Land instabilities

Mechanism of destruction

Landslides destroy structures, roads, pipes and cables either by the ground moving out from 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. Rockfalls 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, tilling 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. Liquifaction 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 (piling, ground anchors, retaining walls). Shallower angles for embankments and cuttings, terracing slopes and forestation can prevent loss of surface material to depth of root penetration. Debris flows can be directed into specially constructed channels if they are expected. Rockfall protection barriers (trenches, slit dams, vegetation barriers) can protect settlements.

Onset and warning

Most landslides occur gradually at rates of a few centimeters an hour. Sudden failures can occur without warning. Rockfalls are sudden but noisy. Debris flows sudden, but precursory trickles of material may give a few minutes of warning if population is prepared.

Elements most at risk

Settlements built on steep slopes and softer soils or along cliff tops. Settlements built at the base of steep slopes, on alluvial outwash fans or at the mouth of streams emerging from mountain valleys. Roads and other communication lines through mountain areas. Masonry buildings. Buildings with weak foundations. Large structures without monolithic foundations. Buried utilities, brittle pipes.

Main mitigation strategies

Location planning to avoid hazardous areas being used for settlements or as sites for important structures. In come cases relocation may be considered. Reduce hazards where possible. Engineering of structures to withstand or accommodate potential ground movement. Piled foundations to protect against Liquefaction. Monolithic foundations to avoid differential settlements. Flexible buried utilities. Relocation of existing settlements or infrastructure may be considered.

Community participation

Recognizing land instability potential and identifying active landslides. Avoidance of siting houses in hazardous locations. Construction of strong foundations for structures. Compaction of ground locally. Slope stabilization through terracing and forestry. Rockfall barriers (trees and earth banking).