Civil Engineering Reference
In-Depth Information
used to circumvent the problem. One technique, which may be relatively costly, is
to shield the upper part of the piles using cylindrical caissons. An alternative solu-
tion, particularly suitable for embankment approaches to bridge abutments, is the
use of piles beneath part of the embankment. This technique, known variously as
'embankment piling' or 'bridge approach support piling' (Reid and Buchanan, 1983),
is shown schematically in Figure 4.58. Widely-spaced piles (up to 10 diameters apart)
are used, with small pile caps that typically occupy as little as 10% of the overall area.
Arching action is relied on in order to shed most of the embankment load on to the
piles, but Reid and Buchanan recommend the use of a latticed fabric membrane over
the tops of the pile caps, in order to prevent cavities from forming. The membrane
will carry a proportion of the embankment loading, and will thus aid the degree of
arching between the pile caps.
The performance of the pile support may be expressed in terms of an 'efficacy',
E
,
which gives the proportion of the total embankment load carried by the pile caps.
Figure 4.59 shows a detail of reinforcing membranes spanning pile caps, of side,
a
,
and centre-line spacing,
s
.
For granular fill of height,
H
, and friction angle,
φ
, overlying the pile caps, Hewlett
and Randolph (1988) proposed a method of analysis based on the formation of arches
spanning between the pile caps. Two alternative estimates of the average vertical
pressure,
p
r
, acting on the ground between the pile caps were arrived at, considering the
Figure 4.58
Typical section through embankment piling.
transverse
reinforcing
strips
side a
longitudinal
reinforcing
sheet
s
s
Figure 4.59
Detail showing reinforcing membranes overlying pile caps.
