Civil Engineering Reference
In-Depth Information
φ
and
c
, the other parameters that
have to be considered apart from the physical geometry of the system are
Having established the basic soils parameters
•
active side wall friction angle
δ
a
;
•
passive side wall friction angle
δ
p
; and
• wall adhesion
c
w
.
6.2.1 Wall friction and adhesion
Piles are frequently required to carry load by shaft friction and the existence of such
friction is unquestionable in all manner of soils. In the case of a load-bearing pile it
is the lateral stress that is locked in between the soil and the pile that gives rise to a
force acting through friction on the pile surface in the axial direction. Bored pile and
diaphragm-retaining walls have surfaces of similar roughness and are acted upon by
lateral earth forces in the active and passive senses. Hence, provided there is differential
vertical motion between the wall and the soil, frictional forces must be mobilized which
affect the stability of the wall, and these forces should be taken into account in wall
design.
Corresponding to the shear strength parameter
φ
, the forces acting on a wall are
related to the parameters
δ
a
for the active side,
δ
p
for the passive side, and correspond-
ing to
c
,
c
w
as wall adhesion. To neglect these factors is certain to lead to uneconomic
designs.
In order to justify the normal reduction of active, and the increase of passive, earth
pressure due to these effects, it is first necessary to ensure that the movements of the
soil around the wall in any potential failure mechanism are directionally such as to
give benefit.
The soil on the active or retained side of the wall has to move down relative to the
wall as shown in Figure 6.2 in order to mobilize friction in the beneficial direction
and, on the passive side, the displaced soil has to move upward. These conditions
arise naturally with most walls, but if the wall itself is going to suffer settlement
under sufficient vertically applied load, then
δ
a
will diminish; similarly if a heavy strip
footing is placed in front of the wall on top of the passive zone it may prevent upward
Figure 6.2
Directions of soil movement at failure.
