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Retaining wall design
Retaining wall design can be a tedious simple task to carry out.
A retaining wall design has to account for a number of factors,
foremost being the stability of the wall itself. Last, the design
has to account for the specific retaining wall type that is used.
In simple terms, different retaining wall types might require
some additional design checks. Typically a retaining wall design
has to consider the following:
a) Earth - Water pressures in retaining wall design
Before all, a designer has to appropriately select the type of
lateral earth pressures that are expected to act on the wall.
For most retaining walls active or at-rest
earth pressures are appropriate. Passive soil resistance
should be used with caution. The possibility of including water
pressures has to be considered if sufficient drainage is not provided.
In the USA, depending on the design approach, some design codes
(LRFD) apply safety factors that multiply each pressure by a safety
factor. In Europe, a strength design approach is applied where
soil strength is divided by safety factors and loads are multiplied
according to their nature (temporary and permanent). Each method
has its benefits and its shortcomings.
b) External Stability checks in retaining wall design
External stability checks refer to calculations that represent
the overall stability of the retaining wall as if the retaining
wall structure acts as a whole single body. Two calculations are
typically performed:
b1) Sliding stability of retaining walls: This calculation
considers the retaining wall stability in the horizontal direction.
The horizontal components of forces are calculated and separated
into driving and resisting forces. Soil and retaining wall weights
are calculated and then the horizontal shear resistance at the
base can be computed as Base Shear Resistance= Sum of Vertical
forces x tan (soil friction angle) + Base Length x Soil-Wall Adhesion.
Then the overall sliding stability if given by:
Factor of safety sliding = Resisting horizontal forces / driving
horizontal forces
Under normal conditions a safety factor of atleast 1.5 is required.
b2) Overturning stability of retaining walls: This type
of calculations considers the stability of the wall againgst toppling
(i.e. turning over). This calculation is performed by calculating
the moment each force component is generating about a given point
in the wall. The toe of the wall is usually taken as the point
of rotation. Moments are then subdivided into resisting and driving
moments and the overturning safety factor is calculated as:
Factor of safety overturning = Resisting moments / driving moments
Under normal conditions a safety factor of atleast 1.5 is required.
c) Bearing Stability in retaining wall design: In all
cases a retaining wall has to be founded in some kind of base
material (be that rock or soil). When a retaining wall is based
on soil the bearing stability tends to be more critical. The first
task in this check is to properly compute bearing stresses on
the toe and heel of the wall. The reason why bearining stresses
have to be computed on both sides is because the overturning causes
increased stresses in the toe and reduced stresses on the heel
base. The bearing stresses have to be examined againgst the permissible
bearing stresses and a minimum safety factor of 3.0 is typically
specified. Using such a high safety factor typically ensures that
wall settlements are kept within acceptable levels. Otherwise
detailed settlement alculations are required if settlement control
is critical.
d) Global stability in retaining wall design: Another
item of concern is the overall global stability of a retaining
wall. In some cases, while the overturning and sliding resistance
as well as the bearing checks yield acceptable factors the wall
might be succeptible to an overall rotational type failure that
extends well below the retaining wall itself. Such a failure mode
is most commonly accounted in hillsides where weaker soil zones
exist or when a soft geomaterial is found below the wall base.
e) Structural checks in retaining wall design: Once a
stability checks are satisfactory then one can design the actual
retaining wall structure itself. For concrete retaining walls
this involves the proper sizing of longitudinal and shear reinforcement
if required. Limited wall bending is generated in most gravity
walls that solely rely on their own weight for stability. Hence,
in many cases the provided reinforcement is the minimum required
for thermal and shrinkage effects.
DEEP2008
- Gravity module performs retaining wall design computations efficiently,
in an easy to use manner.
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