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Wall Layout | Stability |
Design Procedures
Wall Layout
Carefully consider the location of retaining walls. The location of a wall can
affect the wall quantity significantly.
Embankment Side Slopes Consider a typical grade separation where
inadequate right of way requires retaining walls to be placed along the approach
embankment. In these situations, the walls can be placed at the edge of the
upper roadway with the top of wall coincident with the top of the embankment or
at some distance from the edge of pavement with the slope extending from the
edge of pavement to the top of wall. Placing the wall coincident with the edge
of pavement requires an expensive concrete rail on top of the wall and
eliminates any possibility for a future widening of the upper roadway; however,
it improves the long-term serviceability of the wall. Placing the wall a
distance from the edge of pavement requires the use of a guard fence or concrete
barrier at the edge of the pavement. It also allows future widening of the upper
roadway if the provisions are made in the design and detailing of the wall.
Widening Fill Sections Fill sections that are being widened present
special considerations. Typically some soil must be excavated to allow
construction of an MSE wall. Placing the face of wall as close as possible to
the toe of existing slope minimizes excavation and temporary shoring. Placing
the wall close to the existing top of embankment requires use of a cut-type wall
or a fill-type wall with extensive shoring.
Depressed Sections In depressed sections, consider additional width
for the lower roadway to allow for future lane additions. Once retaining walls
are in place, they cannot be moved to accommodate future width requirements.
Bridge Abutments Place retaining walls a reasonable distance in front
of bridge abutments to allow adequate clearance for wall construction. For most
retaining walls, the face of the wall should be at least 1.5 to 3 ft. in front
of the face of the abutment cap. For tied-back and MSE walls, this is especially
critical because the tiebacks and wall reinforcements may need to be skewed
around the abutment foundations. To improve the appearance of walls, control of
the top of wall profile with vertical curves rather than discreet elevations at
specific points results in a much smoother top of wall.
Structures behind Walls Consider the proximity of a retaining wall to
structures behind the wall. MSE walls are usually placed at least 1-3 ft. in
front of foundation to allow space for attachment of the reinforcements to the
facing panels and skewing of the reinforcements.
Stability
Unlike foundation failures, which can occur slowly over a period of years,
retaining walls can fail rapidly in stability with catastrophic results. The
failure of retaining walls can close a transportation facility just as quickly
as a bridge failure. As a result, thoroughly investigate retaining wall
stability. Stability analysis should be conducted for both short- and long-term
conditions.
Sliding and Overturning Sliding involves the lateral translation of a
wall due to inadequate resistance to movement at the base of the wall. Past
failures have involved marginal soil at the base of walls. Overturning does not
involve the soil under the wall but only the mass of the wall to resist the soil
driving forces behind the wall. Because the driving forces are applied to the
wall at roughly two-thirds the wall height above the base, the wall has a
tendency to overturn if the wall mass or geometry is inadequate. Consult the
governing standard for minimum factors of safety for these two modes of failure.
Eccentricity The combination of vertical and horizontal loads on a
wall combine to produce a resultant force at the base of a wall, which is not at
the middle of the footing. The distance between the middle of the footing and
the location of the resultant force is the eccentricity. The location of the
resultant force is limited to the middle third of the footing to ensure that the
rear part of the footing does not lift off the ground.
Bearing Pressure As a result of the weight of the wall mass and the
active driving forces behind a wall, pressure is exerted on the foundation soil
along the base of a wall. The pressure is greatest at the toe of the wall. If
the ultimate bearing capacity of the soil under the toe of the wall is exceeded,
the toe of the wall can plunge down into the foundation soil. The result is a
local distortion of the wall face. A safety factor of 2.0 in bearing capacity is
recommended.
Rotational Stability Rotational failures of walls encompass the
entire wall as well as a portion of the retained soil. This type of failure does
not depend on the wall design specifically but more on the strength of the
foundation and retained soil. Computer programs can evaluate rotational
stability. A safety factor of 1.3 or higher is usually considered adequate.
Settlement Settlement can be significant when walls are constructed
on soil softer than approximately 5/12 in. TCP. Settlement is mainly a problem
in the coastal areas of the state where soil softer than 2/12 in. occurs to
depths of 20 to 50 ft. If a bridge approach embankment is constructed over soil
subject to significant settlement, try either to allow as much settlement to
occur before completing the approach or to support the embankment with a
foundation improvement such as stone columns. Settlement can be accelerated by
installing vertical drains through the compressible subsoil. Construction of
embankments on very soft soil is also likely to result in rotational stability
failures during construction if no precautions are taken. When encountering
significant layers of soft soil, obtain samples for consolidation testing to
determine potential settlement. Note that data obtained from consolidation
testing is only approximate. Predictions of total settlement based on such data
are commonly higher than observed in the field, and the time predicted for such
settlement to occur can be incorrect by an order of magnitude. Temper any values
calculated for settlement with previous experience in the area. When significant
settlement is anticipated, the best solution may be to lengthen the bridge and,
thereby, reduce the height of the approach. This is often the most economical
and practical solution.
Design Procedures
The design of retaining walls requires a thorough knowledge of structural and
geotechnical engineering. This does not mean that one person has to design every
aspect of a retaining wall. Design loads and allowable pressures recommended by
a geotechnical engineer are often later used by a structural engineer to design
the wall. The following design procedures convey general methods and do not
address every design situation.
Earth Pressure Distribution Determine the pressure applied by soil on
a retaining structure by different methods depending upon the wall type. The
soil behind walls, which are free to deflect or move in response to the applied
loads, is considered to achieve the active state. For this condition, calculate
the earth pressure based on Rankines or Coulombs methods. The pressure
distribution is triangular in shape with the maximum pressure occurring at the
bottom of the wall. This is the case for spread footing, MSE, drilled shaft, and
sheet pile walls. Usually soil pressure is assumed to increase downward at a
rate of 40 psf per ft. of depth.
Structures such as tied-back walls or braced excavation shoring are more or
less fixed and, therefore, unable to achieve the active state. For this
condition, use an earth pressure distribution as proposed by Terzaghi and Peck.
The pressure distribution is in the shape of a trapezoid.
Internal Analysis Internal analysis refers to the design of the wall
structure to resist the stresses induced by the earth pressure applied to the
wall. This aspect of design comprises mostly structural engineering. The various
elements of the wall must be designed to carry the stresses generated so that an
adequate factor of safety is attained.
- Mechanically Stabilized Earth (MSE) Walls. The internal design of
MSE walls involves checking the earth reinforcements for allowable stresses
and anchorage into the mass of select fill behind the face. Make allowances
for metal section loss on the reinforcements when computing tensile
stresses. Alter the reinforcement density and size to attain proper stresses
and anchorage. The overall dimension of the reinforced mass is governed by
external stability.
- Tied-back Walls. The internal design of tied-back walls involves
the analysis of a continuous beam (soldier pile) to determine the support
reactions (tied-back loads) for an applied load diagram (earth pressures).
Correct the tied-back loads determined by the continuous beam analysis to
account for the anchor inclination. Select a soldier pile that will
adequately resist the maximum bending moments from the continuous beam
analysis. Then design the wall facing that spans between the soldier piling.
Analyze this as a simple beam to support the maximum soil pressure. Then
design the facing-soldier pile connection. The typical soil loading is
trapezoidal with a maximum intensity of 36H psf (where H is the wall height
in feet). Walls supporting rock are designed for a 25H psf trapezoidal
pressure distribution. Design pressures higher than 36H may be justified if
walls are constructed in expansive soil.
- Drilled Shaft and Sheet Pile Walls. The design of these walls
involves the analysis of a continuous beam on nonlinear supports. The
nonlinear supports model the soil in which the beam is embedded. This
approach accounts for the bending stiffness of the shaft or pile foundation
unlike other methods, which consider the foundation to be infinitely stiff.
Use the computer program COM624 or LPILE to conduct the analysis. Use the program to
determine the foundation response to the applied load for a range of
embedment depths. Determine a foundation length by examining the
embedment-deflection relationship for a suitable deflection either at the
ground line or the top of wall.
External Analysis The external analysis of walls examines whether
walls stay where built. A number of failures of walls and embankments prove that
external stability is just as important as internal design. External stability
is routinely evaluated for fill-type walls. Cut-type walls are not routinely
checked for external stability due to the different approaches to their design.
However, if exceptionally soft soil is present, check the various aspects of
external stability for cut-type walls. As always, sound engineering judgment
should prevail.
- Sliding and Overturning Sliding of a retaining wall occurs when
the active driving forces from the soil behind the wall exceed the
frictional or cohesive forces along the base of the wall and the passive
resisting force in front of the wall. Whether to include passive forces in
front of a wall depends on whether that soil will be present during
construction or at some future date. For most calculations, the subsoil is
assumed to be cohesionless with an angle of friction of 30 deg. The resistance
to sliding is the weight of the wall and soil comprising the wall times the
tangent of 30 deg. (0.58), a valid assumption unless soil borings indicate
it is not conservative. When a questionable soil is present, use triaxial
testing to determine the cohesion and angle of friction, which you can then
use to determine sliding resistance. Overturning occurs when the active
driving forces exceed the gravitation resisting forces of the wall mass. The
mass of the wall is considered the reinforced volume for an MSE wall or the
weight of the concrete and soil above the heel for a spread footing wall.
The safety factor is determined by adding moments about the toe of the wall.
- Eccentricity The eccentricity is the sum of the moments of the
forces acting at the base of the wall divided by the sum of the vertical
forces. The moments are normally calculated at the rear of the base of the
wall.
- Bearing Pressure Bearing capacity failures under walls involve the
displacement of soil from under the wall. Use bearing capacity equations to
determine the ultimate capacity of the foundation soil. These equations
require cohesion and friction values determined by triaxial testing. If this
data is not available, use Texas cone penetration data to obtain allowable
bearing pressures from the drilled shaft and spread footing design chart.
The classical bearing capacity equation for the ultimate soil pressure is:
where Nc, Nq, Ng are
theoretical factors based on the geometry of the failing mass of soil beneath
a footing, c is the soil cohesion, and g is the density of the soil. A safety
factor of two is typically required for bearing capacity. The following figure
gives these factors.
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Rotational Stability Rotational stability of walls
is a special case of slope stability. The limits of the wall affect where a
potential failure surface can develop. The failure surface for a rotational
failure can be either circular or noncircular depending on the
stratification of the foundation soil. For walls on uniform soft clay, the
failure surfaces tend to be circular. If the soft zone is fairly thin, the
failure surface tends to be noncircular following the soft zone. TxDOT uses
both the GSTABL 7 and UTEXAS computer programs to analyze for stability.
While the subsoil can be tested in advance to obtain strength data for
analysis, the future embankment material properties are unknown. An accurate
answer is difficult to obtain because normally about half of the failure
surface passes through the embankment behind a fill wall. Local experience
may provide some insight into the strength of the proposed fill. While
computer programs are used to evaluate wall stability, an approximate hand
check of the results may be conducted by the method of slices.
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