Civil Engineering Reference
In-Depth Information
10.4.2 Dynamic load tests
These tests are less popular than static load tests and monitor the response of a pile subjected to hammer
blows applied at the pile head. The measured response parameters are analysed to give predictions of
the soil resistance that would be mobilised by the pile under static load conditions, based on stress wave
theory. The analysis can also provide prediction of the load/settlement performance of the pile.
10.4.3 Ground tests
A ground investigation will invariably be carried out for any foundation project. Results from soil samples
can be used to give an indication of the shear strength parameters of the soils acting along the shaft of
the pile and at the pile base. These parameters can be used in the design process as will be seen in Sec-
10.5 Determination of the bearing capacity of a pile
A pile is supported in the soil by the resistance of the toe to further penetration plus the frictional or
adhesive forces along its embedded length.
Ultimate bearing capacity
=
Ultimate base resistance
+
Ultimate skin friction:
Q
Q Q
=
+
u
b
s
10.5.1 Cohesive soils
Q
b
for piles in cohesive soils is based on Meyerhof's equation (1951):
Q
N c
A
= × ×
b
c
b
b
Where
N
c
=
bearing capacity factor, widely accepted as equal to 9.0
c
b
=
undisturbed undrained shear strength of the soil at base of pile.
Q
s
is given by the equation:
Q
= × ×
α
c A
s
u
s
Where
α
=
adhesion factor
c
u
=
average undisturbed undrained shear strength of soil adjoining pile
A
s
=
surface area of embedded length of pile.
Hence
Q
=
c N A
+
α
c A
u
b c
b
u
s
The adhesion factor
α
Most of the bearing capacity of a pile in cohesive soil is derived from its shaft resistance, and the problem
of determining the ultimate load resolves into determining a value for
α
. For soft clays
α
can be equal to
or greater than 1.0 as, after driving, soft clays tend to increase in strength. In overconsolidated clays
α
