Engineering Geology

Chapter 9
destroy the structure of the clay and initiate a new process of consolidation that drags the
piles in a downward direction, indeed they may settle on account of their contact with the
remoulded mass of clay even if they are not loaded. Sensitive clays are affected in this way,
whereas insensitive clays are not. Small displacement piles include some piles that may be
used in soft alluvial ground of considerable depth. They also may be used to withstand uplift
forces. They are not suitable in stiff clays or gravels. Non-displacement piles are formed by
boring and the hole may be lined with casing that is or is not left in place. When working near
existing structures that are founded on loose sands or silts, particularly if these are saturated,
it is essential to avoid the use of methods that cause dangerous vibrations that may give rise
to a quick condition.
For practical purposes, the ultimate bearing capacity may be taken as that load which causes
the head of the pile to settle 10% of the pile diameter. The ratio between the settlement of
a pile foundation and that of a single pile acted upon by the design load can have almost any
value. This is due to the fact that the settlement of an individual pile depends only on the
nature of the soil in direct contact with the pile, whereas the settlement of a pile foundation
also depends on the number of piles and on the compressibility of the soil located between
the level of the tips of the piles and the surface of the bedrock.
Bearing Capacity
Foundation design is concerned primarily with ensuring that movements of a foundation
are kept within limits that can be tolerated by the proposed structure without adversely affecting
its functional requirements. Hence, the design of a foundation structure requires an
understanding of the local geological and groundwater conditions and, more particularly, an
appreciation of the various types of ground movement that can occur.
In order to avoid shear failure or substantial shear deformation of the ground, the foundation
pressures used in design should have an adequate factor of safety when compared with
the ultimate bearing capacity of the foundation. The ultimate bearing capacity is the value
of the loading intensity that causes the ground to fail in shear. If this is to be avoided, then
a factor of safety must be applied to the ultimate bearing capacity, the value obtained being
the maximum safe bearing capacity. But even this value may still mean that there is a risk
of excessive or differential settlement. Thus, the allowable bearing capacity is the value that
is used in design, also taking into account all possibilities of ground movement, and so its
value is normally less than that of the safe bearing capacity. The value of ultimate bearing
capacity depends on the type of foundation structure as well as on the soil properties.
For example, the dimensions, shape and depth at which a footing is placed all influence
the bearing capacity. More specifically, the width of a foundation is important in sands;
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