mse walls design for internal & external stability [recovered]

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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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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


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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Design Methods

Working stress analyses

Limit Equilibrium Analyses

Deformation Evaluations

A complete design approach should consist of the following:


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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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)

Design Methods

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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 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


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

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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

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


Sliding Stability

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Sliding Stability


Sliding Stability

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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

FS

qq

ultav

(FS <2 should be justified by geotechnical analysis)

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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


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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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


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 failure surface


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 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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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

)2

'45(tan 2

aK

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

2

3

2

sin

'sin1sin

)'(sinaK



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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Distribution of stress from concentrated vertical load Pv


Distribution of stress from concentrated horizontal load

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Distribution of stress from concentrated horizontal load



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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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


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

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

mse walls design for internal & external stability [recovered]

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