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).
