Civil Engineering Reference
In-Depth Information
When seismic effects have to be considered it is important to
o
= 0.5(20
z
- 10(
z
-2) + 10) kN/m
2
below the water table.
For cantilever retaining walls, the active earth pressure is
given by:
■
understand that the seismic effects are more akin to an inverted
triangle of forces, rather than a uniform load (i.e. not equivalent to
wind loading - there is a higher relative moment generated under
seismic loads than would be generated by taking the base shear
and assuming it acts at the centre of mass).
There is a need to specify and control the (internal) finishes for
a
= 0.33(20
z
+ 10) kN/m
2
above the water table, and
a
= 0.33(20
z
- 10(
z
-2) + 10) kN/m
2
below the water table.
■
assumptions related to friction to be valid.
Differential settlement of supports should be considered.
10.13.2 Allowable passive resistance provided by soil
Groundwater level in front of a retaining structure will depend
upon the degree of drawdown produced by local dewatering
array. Assume groundwater level to be at d
w
mBGL with no
seepage taking place. Excavation in front of the wall is assumed
to be up to 1 m depth.
For cantilever retaining walls, the passive resistance at depth
z
is given by:
■
■
Various codes can be used and these typically include geometrical
limits on silos and their openings (to ensure that the code design
approaches are 'valid').
10.13 Soil/earth loads
Generally the structures designed by structural engineers which
are loaded by soil or earth fall into the following categories:
p
= 3[20(
z
- 1)] kN/m
2
above the groundwater level, and
cantilever retaining walls;
■
p
= 3[20(
z
- 1) - 10(
z
-
d
w
)] kN/m
2
below the groundwater level.
Rules of thumb:
■
propped retaining walls (basement walls).
For the calculation of loads it is generally the active pressures
which are considered. For liquids, this loading is as per fluid
loads above and is simply a combination of the density of the
liquid and the depth from a zero pressure level as the pressure.
For granular soils the pressure is generally taken as being a
fraction of equivalent liquid pressure, with a coefficient related
to the internal angle of friction (angle of repose) and the soil/
wall friction. Typically the vertical friction 'load' on the wall
is neglected. The active pressure developed for a cohesive soil
differs. Generally unless specific guidance is provided related
to the soil properties simple assumptions are used (see rules of
thumb below); this is particularly the case where the designer
is not closely involved in the construction.
Active pressures are the 'loads' applied by a retained or con-
tained soil. The resistance of the soil mass to counterbalancing
loads (i.e. at the toe of an embedded retaining wall where the
wall is 'pushed' against the soil by the retained mass) is the
passive resistance. Passive pressures for granular and cohesive
soils are generally higher than active pressures and need to be
considered carefully before being used (i.e. they are potentially
non-conservative, particularly if the soil mass may be removed
or if for example with cohesive soils they become very dry
and a 'gap' develops between the wall and the soil, requiring
a large movement of the wall before the passive resistance can
develop).
Typical formulas are given below.
■
Where considering soil as a dead load generally a value of 20 kN/m
2
is suitable.
For design of retaining walls the minimum active pressure can
■
be found by treating the soil as liquid with a density of 5 kN/m
2
;
for retaining walls not higher than 5 m this will normally be the
controlling load case. For gravel soils Ka = 0.27 and if the average
bulk density = 20 kN/m
3
; Pa = 0.27 × 20 = 5.4 kN/m
3
. Therefore it
is conservative to use Ka = 0.27 for both clay and gravel soils.
A hydrostatic water pressure acting over 1/3 the height of the wall
■
should always be applied even if the wall is drained and there is no
groundwater shown by the site investigation. For soils with high
water tables a full hydrostatic head should be assumed.
For propped basement walls <7 m high where backfill has been
■
compacted behind the wall, a uniform pressure of 35 kN/m
2
should
be applied to the full height of the wall.
For propped retaining wall construction, i.e. a basement wall,
■
earth pressure at rest Ko should be taken as 0.7.
The usual assumption made in the absence of a detailed knowl-
■
edge of the actual groundwater conditions is that the water table is
located 1 m below the ground surface.
If the water table level is near the ground surface it is acceptable to
■
consider the factored (Ultimate Limit Strength) hydrostatic pres-
sure as being that which considers water at ground level, or other-
wise the 'maximum credible' value of the water pressure.
Some idea of the variation to loading based on different soils can
■
been reached by considering - Phi of 30 degrees for 'normally
consolidated loose sands', 35 degrees for medium dense well
graded sand, typical clay Phi = 25 degrees.
For t-section retaining walls, free to move at the base - design the
10.13.1 Static lateral loads from soil
The uniform surcharge load is assumed to be 10 kN/m
2
.
For walls laterally supported at top and bottom, and assum-
ing lateral strain in the soil is zero, the at-rest earth pressure at
depth
z
is given by:
■
wall stem using Ko, for overturning and sliding use Ka.
For embedded cantilever walls use Ka.
■
o
= 0.5(20
z
+ 10) kN/m
2
above the water table, and
