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Shoring systems
Shoring system is a term used to describe a system that functions
to retain earth, water, and adjacent structures, when an excavation
has to be performed. Design of a shoring system system can complicated.
The designer has to account for many factors that influence the
behavior of the shoring system. Typically, there are two shoring
system systems 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.
For vertical walls some of available shoring systems are:
Performing detailed calculations for both shoring 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 systems 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 shoring system 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 Shoring system 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 system 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 a shoring system: 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 system 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 shoring system design: 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 system design
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