Civil Engineering Reference
In-Depth Information
movement of this wedge and reduce
δ
p
. In this latter case, however, the surcharge of
the footing would probably compensate for this effect on earth pressure, increasing
the resistance to failure.
The question of evaluating the parameters
δ
a
,δ
p
and
c
w
raises the problem that
these forces have not been measured in practice but are derived rather by inference.
Experience with load-bearing bored piles in granular soils suggests, from a good many
instances of load testing, that the ratio
δ/φ
is unlikely to be less in practice less than 0.7.
Current practice in virtually all soils is to take
φ
and this leads to satisfactory
δ
a
=
2
/
3
analysis.
The most vital part of the soil mass from the point of view of both wall stability
and deflection is the passive zone in front of the wall, and because errors on this side
are of much greater consequence than on the active side, it is current practice here to
take
φ
.
There is no ready means of inferring values of
c
w
in relation to
c
in clays, but by
analogy with adhesion factors
δ
p
=
1
/
2
α
, which are used in pile design in stiff clay, where
α
1
2
c
. Because
c
must be influenced by remoulding of the clay in the area immediately adjacent
to the wall, it is felt that
c
w
should be limited in stiff clay soils to a maximum of
about 15 kPa.
has values of the order of 0.5, so also the assumption is made that
c
w
=
6.2.2 Sensitivity analysis
Having determined the basic values of all the soil parameters to be used in a design,
it is often useful to consider potential errors and mis-selection in these values in the
specific design in question, in order to ensure that in a worst credible situation the
factor against rotational instability of a wall exceeds unity. If, however, this practice
is taken to extremes and combinations of several concurrent worst credible situations
are applied simultaneously, then the result will be uneconomic walls.
6.3 The basic design principles for bored pile
or diaphragm walls
Before the installation of any retaining wall commences, there exists a state of lateral
stress in the ground at that location. In normally consolidated soils, expressing the
ratio of lateral effective earth pressure to vertically applied effective earth pressure at
any point as
K
0
, then
K
0
=
φ
in cohesionless soils and is perhaps a little lower
1
−
sin
in clay soils.
In overconsolidated clays, which have been subject to high overburden pressures
in the past, the value of
K
0
is related to the normally consolidated
K
0
, and the over-
consolidation ratio (OCR), as indicated in Figure 6.3. The OCR is expressed as the
maximum vertical stress that the soil has experienced in the past divided by the present
vertical effective stress.
In practice the value of
K
0
in stiff clays may be as high as 3, but lower in the
near surface layers because these may have failed in a passive sense in the course of
overburden removal, (i.e.
K
0
cannot exceed
K
p
, where
K
p
is the coefficient of passive
earth pressure).
