MSE Walls Design for Internal & External Stability [Recovered]

8/2/2012

Texas Tech Tech Universi University ty Department of Civil and Environmental Engineering

CE 5331: Design of MSE Walls

Priyantha Jayawickrama, Ph.D. Associate Professor

In this chapter…     

Overview of design methods Sizing for external stability stability

Sizing for internal stability Design Details Design Example

Limited to MSE walls having a near-vertical face and uniform length reinforcements

CE 5331-013: Design of Earth Retaining Structures

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Design Methods 

Current practice …. 

Determine geometric and reinforcement requirements to prevent internal and external failure using limit limi t 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


Design Methods 

Internal Stability Evaluations: Evaluations : Differences exist in calculating the development of the internal lateral stress and location of the most critical cri tical 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 volume of soils



Therefore, deformation deformation analyses are generally not included in the current methods


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Design Methods A complete design approach should consist of the following:   

Working stress analyses Limit Equilibrium Analyses Deformation Evaluations


Design Methods 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


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Design Methods 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 

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)


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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 stabil ity 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 varyi ng from a multiple of K a to an active earth pressure state Ka are used for design


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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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.


External Stability Conditions


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External Stability Conditions


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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External Stability Computational Steps


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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Select Performance Criteria 

External stability FOS



Global stability FOS



Maximum differential settlement



Maximum horizontal displacement



Seismic stability FOS



Design life


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 CE 5331-013: Design of Earth Retaining Structures

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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


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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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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Vertical Pressure Computations



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Sliding Stability 

The preliminary sizing should be checked w.r.t sliding at the base layer FS sliding

horizontal resisting forces horizontal driving forces

P R P d

1.5



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


Sliding Stability


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Sliding Stability


Sliding Stability


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Bearing Capacity Failure 

Two modes of Bearing Capacity failures exist 

General shear failure



Local shear failure


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 v

qa

qult FS

(FS <2 should be justified by geotechnical analysis) CE 5331-013: Design of Earth Retaining Structures

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Bearing Capacity Failure


Bearing Capacity Failure


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Local Shear 

To prevent large horizontal movements of the structure on weak cohesive soils,

H



3c

If adequate support conditions cannot be achieved, ground improvement of foundation soil is suggested


Overall Stability 

Overall stability is determined using rotational or wedge analyses 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


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Seismic Loading 

During an earthquake, the retained fill exerts a dynamic horizontal thrust P AE 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


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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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.


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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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


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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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 surface CE 5331-013: Design of Earthfailure Retaining Structures 

Potential Failure Surface For internal Stability


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Potential Failure Surface For internal Stability


B- Calculation of Maximum Tensile Forces in the Reinforcement Layers 

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



The resulting K /K r a for inextensible reinforcements ratio decreases from the top of the wall to a constant value below 6 m


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K/Ka Ratio


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

K a

tan 2 ( 45

' ) 2


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For wall face batters equal to or greater than 80 use simplified form of Coulomb equation sin 2 (

K a sin

3

1

') sin ' sin

2



Calculation steps of maximum tensile forces 1. Calculate the horizontal stress, H H

K r

v

h

where v

Z

r

2

q

v

v–

Increment of vertical stress due to concentrated vertical loads

h –

Increment of horizontal stress due to horizontal concentrated surcharge


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Distribution of stress from concentrated vertical load P v


Distribution of stress from concentrated horizontal load


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Distribution of stress from concentrated horizontal load


Maximum Tensile Forces (cont.) 2. Calculate the maximum tension, Tmax T max

-

.

S v

For discrete reinforcements

T max -

H

. S v Rc

H

For discrete reinforcements and segmental concrete facing

T max

H

. At

Rc is the coverage ratio b/Sh At – area of 2 panel widths x the vertical spacing Sv CE 5331-013: Design of Earth Retaining Structures

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Internal Stability with respect to breakage of the reinforcement 3. Calculate internal stability with respect to breakage of the reinforcement

T a

T max Rc

The connection of the reinforcements with the facing, shall be designed for T max for all loading conditions Ta - The allowable tension force per unit width of the reinforcement


C - Internal Stability with Respect to Pullout 

Stability with respect to pullout requires that the following criteria be satisfied


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C - Internal Stability with Respect to Pullout


Stability with Respect to Pullout (cont.) 

The required embedment length in the resistance zone 1.5 T max Le 1m CF * Z p Rc



The total length of reinforcement, L L

La

Le

- For MSE walls with extensible reinforcement ' ) La ( H Z ) tan (45 2 - For wall with inextensible reinforcement Base up to H/2

L

0.6 ( H Z )

CE 5331-013: a Design of Earth Retaining Structures

Upper half of the wall

La

0.3 H

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MSE Walls Design for Internal &amp; External Stability [Recovered]

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