Abstract:
Liquefaction is the phenomena when there is loss of strength in saturated
and cohesion-less soils because of increased pore water pressures and hence reduced
effective stresses due to dynamic loading. It is a phenomenon in which the strength
and stiffness of a soil is reduced by earthquake shaking or other rapid loading.
Liquefaction occurs in saturated, saturated soils are the soils in which the space
between individual particles is completely filled with water.
This water exerts a pressure on the soil particles that. The water
pressure is however relatively low before the occurrence of earthquake. But earthquake
shaking can cause the water pressure to increase to the point at which the soil
particles can readily move with respect to one another.
Although earthquakes often triggers this increase in water pressure,
but activities such as blasting can also cause an increase in water pressure. When
liquefaction occurs, the strength of the soil decreases and the ability of a soil
deposit to support the construction above it.
Soil liquefaction can also exert higher pressure on retaining walls,
which can cause them to slide or tilt. This movement can cause destruction of structures
on the ground surface and settlement of the retained soil.
It is required to recognize the conditions that exist in a soil deposit
before an earthquake in order to identify liquefaction. Soil is basically an assemblage
of many soil particles which stay in contact with many neighboring soil. The contact
forces produced by the weight of the overlying particles holds individual soil particle
in its place and provide strength.
Liquefaction is the process that leads to a soil suddenly losing strength,
most commonly as a result of ground shaking during a large earthquake. Not all soils
however, will liquefy in an earthquake.
The following are particular features of soils that potentially can
liquefy:
They are sands and silts and quite loose in the ground. Such soils
do not stick together the way clay soils do.
They are below the watertable, so all the space between the grai of
sand and silt are filled with water. Dry soils above the watertable won’t liquefy.
When an earthquake occurs the shaking is so rapid and violent that
the sand and silt grains try to compress the spaces filled with water,
but the water pushes back and pressure
builds up until the grains ‘float’ in the water. Once that happens the soil loses
its strength – it has liquefied. Soil that was once solid now behaves like a fluid
WHAT HAPPENS NEXT ?
Liquefied soil, like water, cannot support the weight of whatever is
lying above it – be it the surface layers of dry soil or the concrete floors of
buildings.
The liquefied soil under that weight is forced into any cracks and
crevasses it can find, including those in the dry soil above, or the cracks between
concrete slabs. It flows out onto the surface as boils, sand volcanoes and rivers
of silt. In some cases the liquefied soil flowing up a crack can erode and widen
the crack to a size big enough to accommodate a car.
AFTER THE EARTHQUAKE
After the earthquake shaking has ceased, and liquefaction effects have
diminished (which may take several hours).
The permanent effects include:
• Lowering of ground levels where liquefaction and soil ejection has
occurred. Ground lowering may be sufficient to make the surface close to or below
the watertable, creating ponds.
• Disruption of ground due to lateral spreading.
The liquefied soil that is not ejected onto the ground surface re-densifies
and regains strength, in some cases re-densified soil is stronger than before the
earthquake.
Careful engineering evaluation is required to determine whether ground
that has suffered liquefaction can be redeveloped.
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