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Shoring design - Excavation shoring design
Shoring is a term used to describe a system that functions to
retain earth, water, and adjacent structures when an excavation
is required. Shoring design can be a very complicated matter.
The designer has to content with many unknowns and factors that
influence the behavior of the excavation shoring. Typically, there
are two systems in excavation shorings that must be designed:
A) the Earth Retention System that contains the earth i.e. the
support wall (sheet pile, diaphragm wall, etc.), and b) the Support
System (i.e. the internal or external bracing such as rakers,
struts, or tiebacks) that supports the earth retention system.
Performing detailed calculations for both systems can be a very
time consuming process, especially when parameters have to be
changed. In addition, many current software programs do not offer
an integrated platform of structural and geotechnical analyses
required to design shoring excavations. As a result, the designer
is forced to use numerous software programs to analyze the excavation
and the structural system seperately. With the exception of finite
element analyses, there are very few theoretical solutions for
calculating lateral soil pressures from complex surface profiles.
Furthermore, the designer has to save under different filenames
different stages for the same excavation. As a result, the whole
process can become unescessarily complicated and time consuming.
DeepXcav addresses most of these issues and provides an integrated
structural and geotechnical platform for designing deep excavations.
Shoring can be designed with both traditional and non-linear
methods of analyses. While it is realized that traditional methods
of analysis have obvious limitations in predicting real behavior
accurately, they are important for framing the problem and providing
a back-check for more rigorous finite element methods.
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 shoring system. Two calculations
are typically performed:
b1) Passive resistance of shoring systems: This calculation
considers the available earth resistance in the horizontal direction
below the excavation.
Factor of safety sliding = Resisting horizontal forces / driving
horizontal forces
For temporary conditions a safety factor of atleast 1.2 is required.
b2) Moment - rotational stability: This type of calculation
considers the stability of the shoring for rotational failure
of the wall.
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 for shoring design Another item
of concern is the overall global stability of the excavation.
In some cases, while the other 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 a shoring system: Once a stability
checks are satisfactory then one can design the actual individual
shoring components. For concrete retaining walls this involves
the proper sizing of longitudinal and shear reinforcement if required.
DeepXcav
2010- Shoring Design
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