Civil Engineering Reference
In-Depth Information
4.3.2.2 Pressuremeter analogy
Solutions based on rigid-plastic response of the soil may overestimate the true capacity
where the plastic flow is confined. An alternative approach uses the analogy of the
deformation field around a pressuremeter, such as noted by Swain (1976) from tests
on deeply-embedded strip anchors. The geometry of the strip anchor is similar to that
of a pile loaded laterally, and the experimental results indicated a pattern of outward
radial movement of the soil in front of the strip anchor, and inward radial movement
behind the anchor.
For this pattern of deformation, it is reasonable to assume that the pressure exerted
by soil in front of a laterally-loaded pile will approach the limiting pressure measured
in a pressuremeter test. Behind the pile, the lowest normal stress attainable will be
a suction of the order of 100 kPa. If a gap forms, then this stress will increase to
zero (in a dry hole) or to the ambient head of water if free water is available (such as
for offshore piles). In addition, at the sides of the pile, some friction may be present
between the pile and the soil. Typically, this friction may be estimated as
c
u
d
, per unit
length of pile.
The limit pressure,
p
l
, obtained from a pressuremeter test may be written
c
u
ln(
G
1
p
l
=
σ
h
+
/
c
u
)
+
(4.66)
where
σ
h
is the (total) in-situ horizontal stress in the soil, and
G
is the shear modulus of
the soil. In practice, for typical values of
G
/
c
u
, this expression may be approximated
(Marsland and Randolph, 1977) by
p
l
∼
σ
h
+
6
c
u
. Thus the net force per unit length
acting on the pile lies between the limits
σ
h
+
(
7
c
u
)
d
≤
P
u
≤
(
σ
h
+
p
a
+
7
c
u
)
d
(4.67)
where the lower limit corresponds to the case of the ambient head of water act-
ing behind the pile, and the upper limit is where a suction develops behind the pile
(
p
a
denoting atmospheric pressure).
For normally or lightly overconsolidated clay,
σ
h
/
c
u
will be about 2, and the
lower limit in equation (4.67) corresponds to the result for a smooth pile obtained
from plasticity theory. For short-term loading, some suction will probably develop
behind the pile, and the plasticity solution will give the lowest estimate of limit-
ing force per unit length acting on the pile. For stiff, overconsolidated clay, the
ratio
σ
h
/
c
u
will be considerably lower and may be as low as 0.5 at shallow depths.
Even if suctions develop behind the pile, the limiting force per unit length may fall
below 9
c
u
d
.
4.3.2.3 General profile of limiting pressure
The discussion provides a theoretical background for choosing particular profiles of
limiting pressure for laterally loaded piles. In many cases, however, the suggested
values for
P
u
lead to a profile which increases from 2
c
u
d
at the ground surface
up to a value of about 9
c
u
d
at a depth of 3 pile diameters, much as was origi-
nally suggested by Broms (1964). (Broms conservatively took
P
u
equal to zero at
the ground surface, to allow for any gap which might be present around the head
