Civil Engineering Reference
In-Depth Information
8.2.4 Deep Foundations
Deep foundations are used when the upper soil stratum is too soft, weak, or compressible
to support the static and earthquake-induced foundation loads. Deep foundations are also
used when there is a possibility of undermining of the foundation. For example, bridge
piers are often founded on deep foundations to prevent a loss of support due to flood con-
ditions which could cause river bottom scour. In addition, deep foundations are used when
the expected settlement is excessive (Chap. 7), to prevent ground surface damage of the
structure (Sec. 7.3), or to prevent a bearing capacity failure caused by the liquefaction of an
underlying soil deposit.
Types of Deep Foundations.
The most common types of deep foundations are piles and
piers that support individual footings or mat foundations. Piles are defined as relatively
long, slender, columnlike members often made of steel, concrete, or wood that are either
driven into place or cast in place in predrilled holes. Common types of piles are as follows:
●
Batter pile:
This pile is driven in at an angle inclined to the vertical to provide high
resistance to lateral loads. If the soil should liquefy during an earthquake, then the lateral
resistance of the batter pile may be significantly reduced.
●
End-bearing pile:
The support capacity of this pile is derived principally from the resis-
tance of the foundation material on which the pile tip rests. End-bearing piles are often
used when a soft upper layer is underlain by a dense or hard stratum. If the upper soft
layer should settle or liquefy during an earthquake, the pile could be subjected to down-
drag forces, and the pile must be designed to resist these soil-induced forces.
●
Friction pile:
The support capacity of this pile is derived principally from the resistance
of the soil friction and/or adhesion mobilized along the side of the pile. Friction piles are
often used in soft clays where the end-bearing resistance is small because of punching
shear at the pile tip. If the soil is susceptible to liquefaction during an earthquake, then
both the frictional resistance and the lateral resistance of the pile may be lost during the
earthquake.
●
Combined end-bearing and friction pile:
This pile derives its support capacity from
combined end-bearing resistance developed at the pile tip and frictional and/or adhesion
resistance on the pile perimeter.
A
pier
is defined as a deep foundation system, similar to a cast-in-place pile, that con-
sists of a columnlike reinforced concrete member. Piers are often of large enough diameter
to enable down-hole inspection. Piers are also commonly referred to as drilled shafts, bored
piles, or drilled caissons.
There are many other methods available for forming deep foundation elements.
Examples include earth stabilization columns, such as (NAVFAC DM-7.2, 1982):
●
Mixed-in-place piles:
A mixed-in-place soil-cement or soil-lime pile.
●
Vibroflotation-replacement stone columns:
Vibroflotation or another method is used to
make a cylindrical, vertical hole which is filled with compacted open-graded gravel or
crushed rock. The stone columns also have the additional capability of reducing the
potential for soil liquefaction by allowing the earthquake-induced pore water pressures
to rapidly dissipate as water flows into the highly permeable open-graded gravel or
crushed rock.
●
Grouted stone columns:
These are similar to the above but include filling voids with
bentonite-cement or water-sand-bentonite cement mixtures.
●
Concrete Vibroflotation columns:
These are similar to stone columns, but concrete is
used instead of gravel.
