FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS
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Two types of multiple rotational
failures are recognized:
·Deep-seated
retrogressive failures: These types of failures are common in sea cliffs
where continuous removal of the toe is experienced.
·Shallow multiple
failures: Common in stiff clay soil
as a result of reduction in shear strength due to weathering.
Translational slides
These are shallow features having a straight slide planes
with curvature towards the crown. Involve the movement of many rock or soil
units along a plane. If few
units are involved, the movement is called translational block slide. Often
took place along pre-existing failure plane.
In soil, normally starts with sliding and end –up with flow at the
bottom due to increase in water content (debris slides to debris flows). The
shear planes long which the movement takes place are roughly parallel to the
ground surface. The depth to the slip
surface is generally less than one – twenty of the distance from the toe to the
rear scarp. In rocks, movements occur along a well-defined plane usually along
pre-existing foliation, bedding or joint surfaces. Generally these types of
failures tend to occur during heavy rainfall due to increase in pore – water
pressure.
Most rotational and translational slides occur rapidly. However, some
earth slumps and slumps in weak rocks can occur slowly over many days or even
years. When movement is slow and rock mass is rafted on soft clay beds, the
movement is commonly called rock glides
iv)
Lateral spreading
The slow-to-rapid lateral extensional
movements of soil masses on almost level ground are known as lateral spreads. In fine-grained soils, such as quick clays,
lateral spreading occurs if the soils are remolded or disturbed by human
activities or natural processes (earthquakes).
Loose, granular soils commonly produce lateral spreads through the
process of liquefaction (Fig.3.4). The
liquefaction is the transformation of a granular material from a solid state
into a liquefied state as a consequence of increasing the water pressure in the
spaces between the grains of sand.
Fig. 3.4.
Lateral spread across the road
v) Flows
Involve mainly unconsolidated materials. Flows are divided into wet
and dry flows
Wet flows: Range from debris flows in coarse granular materials
to mudflow in predominantly clays (Fig. 3.5).
A critical component of any flow in soils is its water content. Usually
show a sharp boundary (slip surface) between the moving mass and the ground
across which is moving. Some wet flows occur slowly while others occur rapidly.
In flowing mass velocity decrease with depth and laterally.
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