Design of RE Slope

8/13/2019 Design of RE Slope

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T 1 SEMINARON

Shahane Hrishikesh A. (121321011)

Shaikh Mohammed Asif .Y S (121321012)

Ingawale Harshvardhan D.(121121008)

Presented by-

DESIGN OF REINFOR ED SLOPES

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INTRODUCTION

The design approach principally assumes that the slope is to

be constructed on a stable foundation.

The reinforcement of slopes may be undertaken for a number

of applications including:

reinforcement of fill in new construction,

reinforcement of failed slopes,

reinforcement of existing ground in cut slopes

reinforcement of existing cut or fill slopes which are

marginally stable,

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Reinforcement of existing ground The techniques described in this clause do not involve

disturbance of the ground, apart from insertion of the

reinforcement.

They may be used in walls or slopes which have failed buthave not collapsed completely to prevent future failure of awall or slope or to enable an existing slope to be cut back to

a steeper angle.

These techniques are also employed in new construction.Four techniques may be identified:

1. soil nailing;2. reticulated root or micro piles;

3. soil dowelling;

4. ground anchors



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CLAUSES

The density of reinforcement in a reinforced soil structure

with a steep face will generally result in a stiff reinforced

structure and hence the soil pressures acting on the

reinforced block should be taken into account.

As the angle of the face declines from the vertical the

influence of the retained soil reduces and the proportion of

the stability provided by the reinforcement decreases.



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ASSUMPTIONS

The angle of the slope will have some influence on the method of

analysis to be employed, but most importantly will determine the

type of facing to be employed and the method of construction to

be used.

A distinction is therefore made between steep slopes (slope

angles greater than 45 to the horizontal), and shallow slopes

(slope angles less than or equal to 45to the horizontal).

provide some form of facing for steep slopes to enable anchorage

of the reinforcement in the active zone and to provide erosion

protection.



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For shallow slopes it is usually possible to establish

vegetation for long term erosion protection.

Some suitable fills have adequate stability at 45providing

resistance against deep-seated slips thus obviating the need

for structural facings.

Therefore it is possible to place, compact and trim fill to a

face slope of 45without permanent or temporary support of

the face.



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AREAS OF APPLICATION

The methods of constructing reinforced soil slopes can be

divided into two main categories:

a) the reinforcement of fill materials, new or excavated and

replaced, by reinforcement which is placed horizontally

within the compacted layers of fill.

b) the reinforcement of existing ground by reinforcementwhich is inserted from the slope face at appropriate angles to

suit the design.



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FACINGS

It is usually difficult to establish permanent vegetation to

cover the exposed face of steep slopes.

A permanent facing will therefore usually be necessary to

prevent erosion and ensure face stability.

Facings connected to the reinforcements may also be

necessary to ensure load transfer in the active zone.



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REINFORCED SLOPE ANALYSIS

Reinforced soil structures are designed to conformed to two limitstates-

a) Ultimate limit state- Relevant potential collapsemechanisms are identified.

b) Serviceability limit states- Relevant working conditions areidentified and the structure checked to ensure that it willretain the characteristics necessary for it to fulfill its function

throughout its life.

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The limit states which should be considered are as follows-

a) External stability:

-Bearing and tilt failure;

- Forward sliding;

- Slip failure around the reinforced soil block;

- settlement of the slope foundation.b) Internal stability:

- Tensile failure of the individual reinforcement,

- Bond failure of the individual reinforcement,



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External stability:1.Bearing and tilt failure-

The typical bearing pressure by a reinforced soil structure on thefoundation strata is shown in fig-a



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For design, a bearing pressure qr based upon a Meyerhof

distribution may be assumed,



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The imposed bearing pressure qr should be compared with theultimate bearing capacity of the foundation soil as follows:



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2) SLIDING ALONG THE BASE-

The stability against the forward sliding of the structure at the

interface between the reinforced fill and the subsoil should beconsidered.



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3) settlement of the slope foundation-a) I nternal settlement of reinforced soil f i l l -

The amount of settlement within the reinforced volume will depend

mainly upon the nature of the fill, its compaction, and the verticalpressure within fill.



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b) Differential settlement -



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Where these stability assessments indicate that one or moremodes of potential collapse exits, several option may be available

-Reduce the slope angle;-Increase the width of the reinforced zone;-Use better quality fill;-enhance the foundation with ground treatment;

employ a counterweight such as a berm;-Use lightweight fill;-Incorporate reinforcement at formation level;-Introduce drainage to reduced pore water pressures.



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Internal stability:

Internal stability of the reinforced slope depends upon the ability of

reinforcement to resist the loads imposed upon them.

A large number of method are available to choose form, these includes,

- two-part wedge analyses

- Circular or non-circular analyses

- Log-spiral failure analyses

- Coherent gravity method



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two-part wedge analyses -The two-part wedge analysis assumes a bilineal failure surface.

This is a logical extension of the Coulomb wedge approach for

vertical walls.



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1. The gross disturbing force for an unsurcharged slope is give by,



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2. The recommended vertical reinforcement spacing Svj to

prevent reinforcement rupture can be determine,



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3. The bond failure is not attained the reinforcement bond length Lej

is given by,



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CONTAIN:

Need of reinforcement to earth slope.

What is analysis of reinforced slopes?

General method of analysis of reinforced slope.

Slices method for circular slip analysis.

Assumptions made in wedge method of analysis.

What is design of reinforced slopes?

Step by step procedure of design of reinforced slopes.

Example on design of reinforced slope.

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DETAILED ANALYSIS METHOD

One approach to the design is to determine the required

strength of reinforcement by means detailed limit

equilibrium analysis such as a bishop method.

When a failure surface intersect the reinforcement layer, an

additional resisting moment is added to the over all moment

of equilibruim.

Tensile force is assumed to be horizontal.as shown in fig.3

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The factor of safety for unreinforced slopes is

Fs(u)=c*L*R/(W*x)=MR/MD

Where, c=cohesion,

L,R,W,X =as shown in fig.3

Mr=Resisting moment

Md=Deriving moment

Reinforcement is directly contribute to resisting moment

So the factor of safety for reinforced slope is given by,

Fs(r) = (MR+MG) /MD =Mr +(T hor *D) /MD

Where, MG=resisting moment due to reinforcement

T hor , D = as defined in fig.3

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Orientation of tensile reinforcement is effect on resisting moment and

ultimately on factor of safety of reinforced slopes.

Thus the maximum resisting moment due to reinforcement is ,

MG= T (inclined) *R

Calculation of resisting moment for multi layered reinforcement is

given,

MG= intigration of Ti *Yi from limit 1 to n .

Where, Ti= tensile force of thet particular reinforce layer

Yi= height of that particular layer of reinforcement

from centre of failure surface.

n= no of layers of reinforcement31


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Slice method for circular slip analysis

Suitable for slopes of varying geometry and multiple soil strata

in this method in reinforced slope it is assumed that the inter slice forces are

ignored because of complexity of the reinforcement.

It is also assumed that the reinforcement layer is horizontal and only

considered when they intersect with assumed failure surface on a particular

slice.

The MR of combined effect of soil and reinforcement should not be less

than the MD due to weight of soil.

the moment should be calculated about the centre of rotation of the

disturbed mass.

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For equilibrium :

MD < M RS + M RR

where,

MD is disturbing moment due to weight of soil and

surcharge;

M RS is restoring moment due to shear strength of soil;

M RR is restoring moment due to presence of the reinforcement

in the slope.

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SIMPLE WEDGE METHOD OF ANALYSIS

This method is based on assumptions as following;

1. Extensible reinforcement elements are used.

2. Slopes are constructed within uniform, cohesion less soil ; `,c=0 .

3. No pore pressure within slopes,

4. No seismic loading,

5. Competent , level foundation,

6. Flat slope face and horizontal slope crest,

7. Uniform surcharge load at top of slope,

8. Horizontal reinforcement layer with coefficient of interaction (ci) equal to 0.9

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DESIGN STEPS FOR R E SLPOES BY


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DESIGN STEPS FOR R E SLPOES BYSIMPLE WEDGE METHOD

STEP 1.calculation of modified height of slopes (H`) to take into accountany uniform surcharge loading at the top of slope.

The modified height of slopes (H`)= H+(q/)

STEP 2 . Calculation of factored friction angle (`f)

(`f) = tan^ - 1 ( t a n `/F.s)

w h e r e , ` = soil friction angle

`f = factored soil friction angle

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STEP 3.

Calculation of force coefficient k from standard graph given by

BIS and here shown in next slide.

Calculation of maximum tensile force requirement (T max ).

T max is calculated from formula given as,

T max =0.5 * k * *H` ^2

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STEP 4:

Determination of length of top (L T) and bottom (B T)reinforcement of the reinforced section.

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STEP5. Next steps is to select appropriate primary geogrid

and calculate the no of layers required.

The term primary geogrid layers is refer to the geogrid required

to satisfy internl , external , global stability requirment.

The spacing of primary geogrid layers at the bottom of slope

should not be less than 8 to 12 inches .this is coorespond to

typical earth fill thickness.

The spacing of primary geogrid should not be more than 4 feet.

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If calculation yields geogrid spacing less than the practical

limit then, the stronger primary geogrid can be chosen.

And if yield geogrid spacing more than 4 feet then, lighter

geogrid can be used.

To determine the appropriate geogrid , calculate the long term

design strength (LTDS) of the material as follows,

LTDS = T ult / (RFCR * RFD * RFID)

Where, T ult= ultimate tensile strength of reinforcement as per

ISTM D6637

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RFCR = reduction factor due to creep

RFID = reduction factor due to installation

damage

RFD = reductio factor due to durability

Then no of reinforced lsyerd id cslulated by ,

N=T max / LTDS AND SV= H/N

Where, N= no of geogrid layers

Tmax=the total geogrid force

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Note that the T max for low section of slope is equal to the total

geogrid force requirment for the entire height of slope.

For higher slope section , T max can be distribute over several

zones.

For example for three zone section t max can be distribute as;

T Bottom=(1/2)T max

T middle =(1/3) Tmax

T top =(1/6) T max

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In other words the section is divided into three zones and the

requirment and spacing of geogrid is different.

So it result in most efficient and cost effective design

Design of RE Slope

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