Civil Engineering Reference
In-Depth Information
for
c
u
/σ
v
less than or equal to unity, and
τ
s
=
c
u
/σ
v
0
.
5
c
0
.
75
u
σ
0
.
25
v
(4.17)
nc
for
c
u
/σ
v
greater than unity.
The above expressions contain no artificial parameters other than the assump-
tion that
τ
s
equals
c
u
for normally consolidated clay. Since pile installation will
remould the soil, the strength ratio for normally consolidated clay should be that for
remoulded material. This may be estimated with reasonable accuracy from a knowl-
edge of the friction angle
φ
for the soil, using concepts of critical state soil mechanics
(Schofield and Wroth, 1968). To within a few per cent, the strength ratio for nor-
mally consolidated, remoulded clay with friction angle
φ
, may be estimated as
φ
/
100
φ
is given in degrees). The square root sign means that the variation will
be small; for most soils (
c
u
/σ
v
)
0
.
5
falls within the range 0.45 to 0.55. The value of
undrained shear strength,
c
u
, occurs to the power of 0.5 or 0.75 in equations (4.16)
and (4.17), thus reducing the sensitivity of the approach to accurate determination of
this property.
In applying the above expressions, design profiles of
c
u
and
(where
σ
v
with depth should
be determined, and then a profile of shaft friction with depth deduced. For most soils,
apart from at shallow depths, the value of
c
u
will be less than
σ
v
and equation (4.16)
will apply, giving values of shaft friction that are proportional to the geometric mean
of the shear strength and the effective overburden stress.
As an example of how the approach may be applied in practice, the capacity of a
driven pile in stiff overconsolidated clay will be determined. Soil conditions and pile
properties are taken from a group of six papers published in Ground Engineering in
November, 1979, and January andMarch, 1980, in particular Parry (1980), describing
the design of foundations for an offshore platform situated in the Heather Field in
the North Sea. The piles were 1.524 m in diameter, 63.5 mm wall thickness, driven
with a driving shoe of internal diameter 1.346 m to a final penetration of 45 m.
The piles were to carry maximum working loads of 29.5 MN in compression, with
a cyclic component of
2 MN. For a factor of safety of 1.5 (a typical figure for
piles where the amount of settlement is not critical), the required pile capacity should
be 44.3 MN.
Profiles of
±
21
.
σ
v
,
c
u
and
c
u
/σ
v
are shown in Figure 4.11. The site showed strong sandy
clay deposits, with shear strengths ranging up to 750 kPa in the upper 10 m, which
was very heavily overconsolidated. Parry (1980) has discussed the magnitude of these
strengths, and a probable profile of the overconsolidation ratio. He quotes values of
φ
for the soil of about 32
◦
for depths between 5 and 50 m. In deducing a profile
of shaft friction, the upper5mofsoil (3.3 pile diameters) has been ignored due
to the likelihood of loss of shaft friction due to lateral loading (and resulting post-
holing) of the pile. Below this depth, the relationships given above have been used
to obtain the profiles of
α
,
β
, and thus of the shaft friction,
τ
s
, which are shown in
Figure 4.11.
It is interesting to compare the deduced cumulative shaft capacity of the pile with that
estimated by Parry (1980), using a different effective stress approach (see Table 4.1).
At the design penetration of 45 m, the two approaches agree to within 5%. In addition
