Civil Engineering Reference
In-Depth Information
stress changes within the arch, and also limiting conditions at the pile caps. Defining
δ
=
2
of the total area, the average pressure transmitted
directly to the soil is estimated from the higher of either
a
/
s
, so that the pile caps occupy
δ
(1
)
2(
K
p
−
1)
1
√
2
H
C
√
2
H
C
s
s
−
a
p
r
=
−
δ
−
+
γ
H
(4.102)
or
γ
H
p
r
=
2
K
p
K
p
+
(1
(4.103)
)
1
K
p
+
1
2
)
(1
−
K
p
)
−
δ
−
(1
−
δ
+
δ
−
δ
1
where
2(
K
p
−
1)
φ
1
+
sin
C
=
and
K
p
=
(4.104)
φ
2
K
p
−
3
1
−
sin
The first of the expressions for
p
r
governs for large
s
H
(low embankment height
relative to the pile spacing, while the second governs for low
s
/
H
(high embankments
or closely spaced piles). This method has been found to give good agreement with
measured data, such as the field studies reported by Reid and Buchanan (1983) where
over 80% of the surcharge loading was shed onto pile caps spaced such as to occupy
only 11% of the plan area. Reviews of this and other methods of analysis have been
given by Russell and Pierpoint (1997) and Love and Milligan (2003).
The main advantages offered by embankment piling are:
/
(a) short-term stability of the embankment will be ensured, avoiding the need for
staged construction;
(b) long-term settlement of the embankment close to the bridge will be reduced, giving
lower maintenance costs for the roadway; and
(c) lateral pressure on the piles for the bridge abutment will be significantly reduced.
4.6 Buckling of piles
There are two main areas where the axial stability of piles may be critical in design.
The first of these is during installation by driving, where instability can lead to the pile
deviating significantly from its proposed alignment. The other area is under working
conditions, where the axial load taken by a pile may be close to the critical buckling
load for the pile. This latter case is relatively rare, but can occur for end-bearing piles
driven through relatively soft alluvial deposits.
4.6.1 Instability during driving
Burgess (1976) has given a detailed analysis of the stability of piles driven or jacked into
soil of uniform shear strength (see also Omar, 1978; Ly, 1980; Burgess, 1980). Two
different forms of instability occur; one is a buckling phenomenon, and the other is
