BOOKS:
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SOIL MECHANICS by T WILLIAM LAMBE & ROBERT V.
WHITMAN, Wiley ASTERN
FOUNDATION ANALYSIS & DESIGN by JOSEPH E. BOWLES,
Mc Graw Hills
GEOTECHNICAL ENGINEERING by SHASHI K GULHATI &
MANOJ DATTA, TATA Mc Graw Hill
SOIL MECHANICS AND FOUNDATION ENGINEERING by B C
PUNMIA, A K JAIN & A K JAIN, Laxmi Publications Pvt. Ltd.
 Provide
a solid massive foundation for heavy
loads and high horizontal thrusts.
 Drilled
piers are structural members of
relatively large-diameter massive struts
constructed and placed in a pre-excavated
hole.
 They are referred to variously by civil
engineers as bored piles, large-diameter
piles, foundation piers, sub-piers, and drilled
caissons.
 The shafts can be enlarged at the base,
resulting in belled or under-reamed piers
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In areas where pile penetration is difficult, piers can be
provided.
Vibration and heave of soil are not caused as in installation
of a driven pile. This is a decisive factor when the
adjacent structure is on spread footings or short piles.
Equipment used in the construction of drilled piers
produces less noise and, hence, is quite suitable for areas
near hospitals and similar institutions.
There is a possibility of inspection and physical testing of
the soil or rock conditions at the bottom of the pier.
In the construction of piers, there is no displacement of
volume of soil, and the problems of shifting and lifting are
eliminated.
Drilled piers generally require light construction
equipment.
They can resist high lateral stresses.
A
drilled pier derives its supporting power from
both skin friction and bottom bearing as in a
pile.
 Generally, the skin friction developing along
the shaft is less compared to the end bearing
capacity.
 The surface area of a drilled pier is less
compared to the one available in a pile group.
Thus, in many instances, the drilled piers are
designed as a compression member subjected
to a load on top and an equal reaction at the
bottom, neglecting the skin friction.
The ultimate load-bearing capacity of drilled
piers can be computed as for piles as
Qu = Qf + Qp
For Cohesive soils:
Qf = a cu As
(Qp)n = 9 cu Ap
a = 0.35 to 0.4
As = surface area
cu = cohesion
Ap = area of base
In Non-Cohesive Soils
(Qp)a = (g Db Bk) Ap
(Qp)n = 𝐴𝑝 𝑞 ′ (𝑁𝑞 − 1)
(Qp)ns = lower of (Qp)a OR (Qp)n /F
Qf = (K s´v tand) (p Db L)
(Qf)s = Qf /F
Net Allowable Load on Pier:
Qns= (Qp)ns + (Qf)s
Type of soil
Skin friction
(kN/m2)
Silt and soft clay
Very stiff clay
Loose sand
Dense sand
Dense gravel
7–29
48–192
12–34
34–67
48–96
 are
structural boxes or chambers that are
sunk in place through the ground or water by
systematically excavating below the bottom
of the unit, which thereby descends to the
final depth.
 These have a large cross-sectional area and
hence provide high bearing capacity, which is
much larger than what may be offered by a
cluster of piles.
 are
generally used for major foundation
works because of the high construction cost.
 In general, a caisson foundation is
recommended and found to be advantageous
when
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(i) large-size boulders are encountered and
(ii) a massive sub-structure is required to
withstand large lateral stresses.
Open Caissons
 These are concrete or masonry shafts which
remain open both at the top and at the
bottom during construction.
 The caisson is sunk into place, as the soil is
removed from the inside, till the well sinks
to the required depth. Then, a bottom
concrete seal is made by depositing
concrete. The well is pumped dry, after
maturity of the bottom concrete seal, and
filled with concrete or sand.
Box Caissons
 These are structures with a closed bottom. They
are constructed on land and transported and
sunk in to the prepared foundations below the
water level.
Pneumatic Caissons
 These caissons have their top closed, and
compressed air is used to stop the entry of water
into the working chamber. Thus, excavation and
concreting are done in a dry condition. The
caisson is sunk as the excavation proceeds, and
after reaching the required depth, the working
chamber is filled with concrete.