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Excavation shoring
Excavation shoring is a system that braces an excavation and
retains earth, water, and adjacent structures. Excavation shoring
design can be complicated. The excavation shoring designer has
to account for many unknowns and factors that influence the behavior
of the excavation shoring. Typically, there are two excavation
shoring 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 the available excavation shoring
systems are:
Performing detailed calculations for both excavation 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.
Excavation 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 excavation shoring 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 excavation shoring 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 excavation 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 excavation shoring: 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 excavation 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 excavation shoring 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- Excavation shoring design
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