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1
8/2/2012
Priyantha Jayawickrama, Ph.D.
Associate Professor
CE 5331:
Design of MSE Walls
Texas Tech UniversityDepartment of Civil and Environmental Engineering
CE 5331-013: Design of Earth Retaining Structures
In this chapter…
Overview of design methods
Sizing for external stability
Sizing for internal stability
Design Details
Design Example
Limited to MSE walls having a near-vertical face and uniform length reinforcements
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CE 5331-013: Design of Earth Retaining Structures
Design Methods
Current practice….
Determine geometric and reinforcement requirements to prevent internal and external failure using limit equilibrium method of analysis
External Stability Evaluations treat the reinforced section as a composite homogeneous soil mass and evaluate the stability according to conventional failure modes for gravity type wall systems
CE 5331-013: Design of Earth Retaining Structures
Design Methods
Internal Stability Evaluations: Differences exist in
calculating the development of the internal lateral
stress and location of the most critical failure surface.
Internal stability is treated as a response of discrete
elements in a soil mass which suggests deformations
are controlled by reinforcements rather than the total
mass
But this is inconsistent, given the much greater volume
of soils
Therefore, deformation analyses are generally not
included in the current methods
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CE 5331-013: Design of Earth Retaining Structures
Design Methods
Working stress analyses
Limit Equilibrium Analyses
Deformation Evaluations
A complete design approach should consist of the following:
CE 5331-013: Design of Earth Retaining Structures
An analysis of working stresses consists of
Selection of reinforcement location and a check that stresses in the stabilized soil mass are compatible with the properties of the soil and inclusions
Evaluation of local stability at the level of each reinforcement and prediction of progressive failure
Design Methods
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CE 5331-013: Design of Earth Retaining Structures
Limit equilibrium analysis studies the overall stability of
the structure (External, Internal and Combined stability)
External stability involves the overall stability of the stabilized soil mass considered as a whole and is evaluated using slip surfaces outside the stabilized soil mass
Internal stability analysis evaluates potential slip surfaces within the reinforced soil mass
In some cases the slip surface is partly outside and partly inside the reinforced zone. Hence: Combined Analysis.
Design Methods
CE 5331-013: Design of Earth Retaining Structures
Deformation evaluations check the anticipated performance
of the structure with respect to horizontal and vertical
displacement
Horizontal deformation analyses are the most difficult and
least certain of the performed analyses
Approximate calculations are performed and/or it is
assumed that the usual FOS against external and internal
stability will ensure deformation within tolerable limits
Vertical deformation analyses are obtained from
conventional settlement computations, with particular
emphasis on differential settlement (both longitudinal and
transverse)
Design Methods
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CE 5331-013: Design of Earth Retaining Structures
Design Methods, Inextensible Reinforcements
Coherent gravity structure approach is adopted to determine external stability, similar to the analysis for any conventional or traditional gravity structure
For internal stability evaluations, a bi-linear critical slip surface is considered
The state of stress for external stability is assumed to be equivalent to a Coulomb state of stress with a wall friction angle δ equal to 0
For internal stability, a variable state of stress varying from a multiple of Ka to an active earth pressure state Ka are used for design
CE 5331-013: Design of Earth Retaining Structures
Design Methods, Extensible Reinforcements
For external stability, an earth pressure
distribution similar to that used for inextensible
reinforcements, is used
For internal stability, a Rankine failure surface
is considered, because the extensible
reinforcements can elongate more than the
soil before failure
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CE 5331-013: Design of Earth Retaining Structures
Sizing for External Stability
Four potential external failure mechanisms are usually considered in sizing MSE walls:
Sliding on the base
Overturning
Bearing Capacity
Deep Seated Stability (rotational slip surface or slip along a plane of weakness)
Due to the flexibility and satisfactory field performance of MSEW, in some cases, lower FOS values as compared to reinforced concrete cantilever or gravity walls are used.
CE 5331-013: Design of Earth Retaining Structures
External Stability Conditions
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CE 5331-013: Design of Earth Retaining Structures
External Stability Conditions
CE 5331-013: Design of Earth Retaining Structures
Sizing for External Stability
Flexibility of MSE walls should make
overturning failure highly unlikely. However,
overturning criteria (max. permissible eccentricity)
aid in controlling lateral deformation by limiting
tilting.
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CE 5331-013: Design of Earth Retaining Structures
External Stability Computational Steps
CE 5331-013: Design of Earth Retaining Structures
Define Wall Geometry and Soil Properties
The following must be defined or established by
the designer
Wall height, batter
Soil surcharges, live load surcharges, dead load
surcharges
Seismic loads
Engineering properties (γ,c, ) of all the soils
(foundation soil, reinforced soil, retained fill)
Groundwater conditions
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CE 5331-013: Design of Earth Retaining Structures
Select Performance Criteria
External stability FOS
Global stability FOS
Maximum differential settlement
Maximum horizontal displacement
Seismic stability FOS
Design life
CE 5331-013: Design of Earth Retaining Structures
Preliminary Sizing
Add the required embedment, established under project
criteria (Section 2.7c) to the wall height in order to
determine the design heights for each section to be
investigated
A preliminary length of reinforcement is chosen should
be greater of 0.7H and 2.5m
Structures with sloping surcharge fills or other
concentrated loads generally require longer
reinforcements (0.8H to as much as 1.1H) for stability
H: Design height of the structure
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CE 5331-013: Design of Earth Retaining Structures
Earth Pressures for External Stability
MSE wall mass is assumed to act as a rigid body
For walls with vertical face (face batter less than
8º), earth pressures are assumed to develop on
a vertical pressure plane arising from the back
end of the reinforcements
CE 5331-013: Design of Earth Retaining Structures
Coeff. of Lateral Earth Pressure, Ka
• Vertical Walls (i.e. face batter <8 )
• Vertical Walls with a surchage slope,
• Walls with face batter, > 8
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CE 5331-013: Design of Earth Retaining Structures
CE 5331-013: Design of Earth Retaining Structures
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CE 5331-013: Design of Earth Retaining Structures
CE 5331-013: Design of Earth Retaining Structures
Vertical Pressure Computations
Weight of any wall facing is typically neglected in
calculating vertical pressure
Calculation steps for determining vertical bearing
stress are given in the next slide
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CE 5331-013: Design of Earth Retaining Structures
Vertical Pressure Computations
CE 5331-013: Design of Earth Retaining Structures
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CE 5331-013: Design of Earth Retaining Structures
Sliding Stability
The preliminary sizing should be checked w.r.t sliding at
the base layer
Resisting force is the lesser of the shear resistance
along the base of the wall or of a weak layer near the
base of the MSE wall
Sliding force is the horizontal component of the thrust on
the vertical place at the back of the wall
Soil passive resistance at the toe due to embedment is
ignored as the soil may be removed
5.1forces driving horizontal
forces resisting horizontald
Rsliding
PP
FS
CE 5331-013: Design of Earth Retaining Structures
Sliding Stability
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CE 5331-013: Design of Earth Retaining Structures
Sliding Stability
CE 5331-013: Design of Earth Retaining Structures
Sliding Stability
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CE 5331-013: Design of Earth Retaining Structures
Two modes of Bearing Capacity failures
exist
General shear failure
Local shear failure
Bearing Capacity Failure
CE 5331-013: Design of Earth Retaining Structures
Bearing Capacity Failure
General shear: Vertical stress at the base should
not exceed the allowable bearing capacity of the
foundation soil, determined considering a FOS of
2.5 w.r.t. Group I loading applied to ultimate
bearing capacity
FS
ultav
(FS <2 should be justified by geotechnical analysis)
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CE 5331-013: Design of Earth Retaining Structures
Bearing Capacity Failure
CE 5331-013: Design of Earth Retaining Structures
Bearing Capacity Failure
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CE 5331-013: Design of Earth Retaining Structures
Local Shear
To prevent large horizontal movements of the
structure on weak cohesive soils,
If adequate support conditions cannot be
achieved, ground improvement of foundation soil
is suggested
cH 3
CE 5331-013: Design of Earth Retaining Structures
Overall stability is determined using rotational or wedgeanalyses which can be performed by using a classical slope stability analysis method
The reinforced soil wall is considered as a rigid body and only failure surfaces completely outside a reinforced mass are considered
For simple structures (rectangular geometry, relatively uniform reinforcement spacing and a near vertical face) compound failure is normally not critical
For complex structures, compound failures must be considered
If FOS < 1.3, increase reinforcement length or improve foundation soil
Overall Stability
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CE 5331-013: Design of Earth Retaining Structures
During an earthquake, the retained fill exerts a
dynamic horizontal thrust PAE on the MSEW in
addition to the static thrust
The reinforced soil mass is subjected to a horizontal
inertia force PIR = M*Am
where M is the mass of the active portion of the
reinforced wall section assumed at a base width of
0.5H and
Am is the maximum horizontal acceleration in the
reinforced soil wall
Seismic Loading
CE 5331-013: Design of Earth Retaining Structures
Settlement Estimate
Conventional settlement analyses to ensure that
immediate, consolidation and secondary settlement of
the wall satisfy the performance requirements of the
project
Significant total settlements at the end of construction
indicate that the planned top of wall elevations need to
be adjusted
Significant differential settlements (greater than 1/100)
indicate the need of slip joints, which allow for
independent vertical movement of adjacent pre-cast
panels
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CE 5331-013: Design of Earth Retaining Structures
Settlement Estimate
Where the differential settlement cannot be taken care
of by these measures, consideration should be given
to ground improvement techniques like wick drains,
stone columns, dynamic compaction, use of
lightweight fill etc.
CE 5331-013: Design of Earth Retaining Structures
Internal Failure of MSE Walls
Internal failure of a MSE wall can occur in two different ways Failure by elongation or breakage of
reinforcement: The tensile forces in the inclusions become so large that the inclusion elongate excessively or break
Failure by pullout: The tensile forces in the reinforcements become larger than the pullout resistance which increases shear stresses in the surrounding soil leading to large movements and possible collapse.
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CE 5331-013: Design of Earth Retaining Structures
Designing for Internal Failure
The process of sizing consists of determining
The maximum developed tension forces
Their location along the critical slip surface
Resistance provided by reinforcement for both pullout and tensile
CE 5331-013: Design of Earth Retaining Structures
Internal Design Process
The steps involved in internal design process:
Select a reinforcement type
Select the location of critical failure surface
Select a reinforcement spacing
Calculate the maximum tensile force at each reinforcement level (static, dynamic)
Calculate the maximum tensile force at the connection to the facing
Calculate the pullout capacity at each reinforcement level
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CE 5331-013: Design of Earth Retaining Structures
A – Critical Slip Surface
The most critical slip surface in a simple reinforced soil wall is assumed to coincide with the maximum tensile forces line
The shape and location of this line is assumed to be known from a large number of previous experiments and theoretical studies
The maximum tensile forces surface is assumed to be approximately bilinear in the case of inextensible reinforcement, approximately linear in the case of extensible reinforcement
Where the wall front batter is greater than 8 degrees the Coulomb earth pressure relationship may be used to identify the failure surface
CE 5331-013: Design of Earth Retaining Structures
Potential Failure Surface For internal Stability
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CE 5331-013: Design of Earth Retaining Structures
Potential Failure Surface For internal Stability
CE 5331-013: Design of Earth Retaining Structures
B- Calculation of Maximum Tensile Forces in the Reinforcement Layers
The resulting Kr/Ka for inextensible reinforcements ratio decreases from the top of the wall to a constant value below 6 m
The maximum tensile force is primarily related to the type of the reinforcement which is a function of the modulus, extensibility and density of reinforcement
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CE 5331-013: Design of Earth Retaining Structures
K/Ka Ratio
CE 5331-013: Design of Earth Retaining Structures
Maximum Tensile Forces (cont.)
The simplified coherent gravity method is used
The method is based on the same empirical data used to develop the coherent gravity method (AASHTO) and the structure stiffness method (FHWA)
Coeffcient of Lateral Earth Pressure is determined by applying a multiplier to Ka.
For vertical walls use the active earth pressure coefficient
)2
'45(tan 2
aK
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CE 5331-013: Design of Earth Retaining Structures
Maximum Tensile Forces (cont.)
For wall face batters equal to or greater than 80 use simplified form of Coulomb equation
2
3
2
sin
'sin1sin
)'(sinaK
CE 5331-013: Design of Earth Retaining Structures
Maximum Tensile Forces (cont.)
1. Calculate the horizontal stress, H
vrv
hvrH
qZ
where
K
2
Calculation steps of maximum tensile forces
v – Increment of vertical stress due to concentrated vertical
loads
h – Increment of horizontal stress due to horizontal
concentrated surcharge
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CE 5331-013: Design of Earth Retaining Structures
Distribution of stress from concentrated vertical load Pv
CE 5331-013: Design of Earth Retaining Structures
Distribution of stress from concentrated horizontal load
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CE 5331-013: Design of Earth Retaining Structures
Distribution of stress from concentrated horizontal load
CE 5331-013: Design of Earth Retaining Structures
Maximum Tensile Forces (cont.)
2. Calculate the maximum tension, Tmax
- For discrete reinforcements
- For discrete reinforcements and segmental concrete facing
vH ST .max
Rc is the coverage ratio b/Sh
At – area of 2 panel widths x the vertical spacing Sv
c
vH
R
ST
.max
tH AT .max
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CE 5331-013: Design of Earth Retaining Structures
Internal Stability with respect to breakage of the reinforcement
3. Calculate internal stability with respect to breakage of the reinforcement
The connection of the reinforcements with the facing, shall be designed for Tmax for all loading conditions
c
aR
TT max
Ta - The allowable tension force per unit width of the reinforcement
CE 5331-013: Design of Earth Retaining Structures
C - Internal Stability with Respect to Pullout
Stability with respect to pullout requires that the following criteria be satisfied
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CE 5331-013: Design of Earth Retaining Structures
C - Internal Stability with Respect to Pullout
CE 5331-013: Design of Earth Retaining Structures
Stability with Respect to Pullout (cont.)
The required embedment length in the resistance zone
The total length of reinforcement, L
- For MSE walls with extensible reinforcement
- For wall with inextensible reinforcement
Base up to H/2 Upper half of the wall
mRZCF
TL
cp
e 15.1
*
max
ea LLL
)2
'45(tan)( ZHLa
HLZHL aa 3.0)(6.0