GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE · z x s v x K a or, 32.43 = (γ s x z + q) x s v x K a or, 32.43= (18 x 10 + 39) x s v x 0.27 or, s v = 0.55 m Therefore, let us

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay


Module - 6LECTURE - 35

Geosynthetics for reinforced soil retaining walls

Recap of previous lecture…..


Gabion walls

General

Design of gravity gabion wall

Design of gabion wall with welded wire anchor mesh

as horizontal tie-backs (partly covered)

Internal stability:

Step 13: Calculate spacing and tensile force at each layer

The vertical pressure at any layer (σz) = γs x z + q

Tensile strength at any layer (Tcalculated) = σz x sv x Ka

z = depth of the layer from the top of the wall

Ka = co-efficient of active earth pressure = 0.27

Tultimate = 60 kN/m; Let, factor of safety = 1.85

Tallowable = (60/1.85) = 32.43 kN/m


Therefore, at the bottom, i.e. at z = 10 m, the maximumspacing can be calculated as,

Tallowable = σz x sv x Ka

or, 32.43 = (γs x z + q) x sv x Ka

or, 32.43 = (18 x 10 + 39) x sv x 0.27

or, sv = 0.55 m

Therefore, let us provide 0.5 m spacing at fewbottom layers and 1 m spacing at some top layers.

Calculate tensile strength (Tcalculated) at each layer


Step 14: Check the tensile strength at each layerTcalculated < Tallowable (= 32.43 kN/m) (OK)

z(m) Sv (m) σz (kN/m2) Tcalculated (kN/m) Tcalculated < 32.43 kN/m1 1 57 15.31 OK2 1 75 20.15 OK3 1 93 24.99 OK4 1 111 29.82 OK

4.5 0.5 120 16.12 OK5 0.5 129 17.33 OK

5.5 0.5 138 18.54 OK6 0.5 147 19.75 OK

6.5 0.5 156 20.96 OK7 0.5 165 22.17 OK

7.5 0.5 174 23.37 OK8 0.5 183 24.58 OK

8.5 0.5 192 25.79 OK9 0.5 201 27.00 OK

9.5 0.5 210 28.21 OK10 0.5 219 29.42 OK


Step 15: Calculation of minimum embedded length (Lem) Lem = 1.5 x Tcalculated/ (2 xσz x Ci x tanφ)

Step 16: Calculation of actual embedded length (Le)At any layer at a depth z, the actual length of embedmentpast the wedge,Le = L – tg – La x (Hg – z)/ Hg

La = Hg tan (45⁰ - /2) – Hg tan α = 10 x tan (45⁰ - 32/2) – 10 tan 6⁰ = 4.492 m

Step 17: Check for embedded lengthAt any layer, Le > Lem (OK)


z

(m)

Sv

(m)

σz

(kN/m2)

Tcalculated

(kN/m)

Tcalculated

< 32.43 kN/m

Le

(m)

Lem

(m)Lem< Le

1 1 57 15.31 OK 1.96 0.46 OK2 1 75 20.15 OK 2.41 0.46 OK3 1 93 24.99 OK 2.86 0.46 OK4 1 111 29.82 OK 3.30 0.46 OK

4.5 0.5 120 16.12 OK 3.53 0.23 OK5 0.5 129 17.33 OK 3.75 0.23 OK

5.5 0.5 138 18.54 OK 3.98 0.23 OK6 0.5 147 19.75 OK 4.20 0.23 OK

6.5 0.5 156 20.96 OK 4.43 0.23 OK7 0.5 165 22.17 OK 4.65 0.23 OK

7.5 0.5 174 23.37 OK 4.88 0.23 OK8 0.5 183 24.58 OK 5.10 0.23 OK

8.5 0.5 192 25.79 OK 5.33 0.23 OK9 0.5 201 27.00 OK 5.55 0.23 OK

9.5 0.5 210 28.21 OK 5.78 0.23 OK10 0.5 219 29.42 OK 6.00 0.23 OK

Detailed calculations:


Arrangement of reinforcement layersProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Mechanically stabilized reinforced soil gabion retaining wall (Photograph courtesy of Dr.. B. S. Asha)


Feasibility Study on Fly Ash as a Backfill Material


370

Experimental Program

Four parts

Direct Shear Test

1 D Consolidation Test

Permeability Test

pH value


Direct shear testExperiments were conducted for different moisturecontents: dry side of optimum: 12%, 16%, 20% wet side of optimum: 28%, 32%

MDD = 12.1 kN/m3

OMC = 24%

Dry unit weight versus moisture content curve

10.8

11

11.2

11.4

11.6

11.8

12

12.2

0 5 10 15 20 25 30 35

Dry

uni

t wei

ght

(kN

/m3

)

Moisture Content (%)

MDD = 12.1 kN/m3

OMC = 24%


0

5

10

15

20

25

10 15 20 25 30 35Ap

pare

nt C

ohes

ion

(kPa

)

Moisture content (%)

Variation of apparent cohesion of fly ash with different moisture contents

The apparent cohesion value is increased with increasingmoisture content and reached highest value 20.76 kPa atoptimum moisture content of fly ash. Thereafter the apparentcohesion values are decreased with increase in moisturecontent.cohesion minimum value of 2 kPa was observed for Koradi

fly ash when the compacted moisture content is 12% dry ofoptimum.


15

20

25

30

35

10 15 20 25 30 35

Angl

e of

inte

rnal

fric

tion

(°)


Variation of angle of Internal friction of fly ash different moisture contents

There is not much variation is seen in the value of angle ofinternal friction of fly ash. A difference of 5.4 was observedfrom dry side to wet side of optimum.Angle of internal friction has maximum value of 32.3 forKoradi fly ash when the compacted moisture content is 12%dry of optimum.


Permeability and Consolidation Tests

Combined experimental setup for 1D consolidation and falling head permeability test


0.4

0.6

0.8

1.0

1.2

1.4

1.6

10 15 20 25 30 35

k ave

x 10

-7 (m

/sec

)


10 kPa

20 kPa

30 kPa

50 kPa

100 kPa

200 kPa

400 kPa

800 kPa

Permeability and Consolidation Tests

Variation of average coefficient of permeability (kave) with different

moisture contents

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 10 100 1000

k ave

x 10

-7 (m

/sec

)

log σv

Dry(12%) Dry(16%)Dry(20%) OMC(24%)Wet(28%) Wet(32%)

Variation of kave with vertical stress for different moisture content values

The Koradi fly ash have coefficient permeability in therange of 1.42 x 10-7 m/sec to 6.31x 10-8 m/secVery fine sand; silts; mixture of sand silt and clay 10-5 to10-9 m/sec (Terzaghi et al.,1996)


0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1 10 100 1000

void

ratio

(e)

log σv

Dry(12%)

Dry(16%)

Dry(20%)

OMC(24%)

Wet(28%)

Wet(32%)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 200 400 600 800 1000

Cv x

10-4

(m2 /s

ec)

σv

Dry(12%) Dry(16%)Dry(20%) OMC(24%)Wet(28%) Wet(32%)

e-log v curve of fly ash for different moisture contents

Variation of coefficient of consolidation (Cv)with effective vertical stress (v)

The average value of compression index (Cc )0.078 when v 100 kPa and 0.092 when v 100 kPa The average value of recompression index (Cr) 0.014 Coefficient of consolidation (Cv) 0.5 x 10-6 to 1.8 x 10-4 m2/sec pH value 8.12 - lies in the range of pH values of reinforced backfills.


Property FHWA –NHI-00-024 [Berg et al., 2009]

Ram Rathan Lal and Mandal (2012)

Cohesion Cohesion less 2 kPaAngle of internal friction () 34 32.3Coefficient of uniformity (Cu)

4 11.12

Plasticity Index 6 NPpH 3 pH 9 (a)

5 pH 10 (b)8.12

Organic content (%) 1 0.1035(a) Geosynthetic reinforcement (b) Steel reinforcement

Low specific gravity 2.15 Less lateral pressures on retaining walls


The results of the permeability and consolidation testsshowed that the coefficients of permeability andconsolidation characteristics of the compacted fly ashwere comparable to those of non-plastic silts and veryfine sands.

Coefficient of permeability: 1.42 x 10-7 m/sec

Angle of internal friction: 32.2

Apparent cohesion: 2 kPa

Moisture content: 12% dry side of optimum

Major Findings


Compression Index Cc 0.078 when v 100 kPa and0.092 when v 100 kPa fly ash can be used as backfill material in reinforced retaining walls as thedeformations due to loadings will be less when it wasproperly compacted.

The pH = 8.12 value is fall within the recommendedlimits of electrochemical properties for reinforced fillswith steel and geosynthetic reinforcements.


Model Tests on Geocell Walls Subjected to Strip Loading


Model No. Structure type Facing angle () Reinforcement type

1 GTW 90 CRW2 GTW 80 CRW3 GTW 70 CRW4 FTW 90 CRW5 FTW 80 CRW6 FTW 70 CRW7 FTW 90 CRW8 FTW 80 CRW9 FTW 70 CRW10 FTW 90 CRW+ Top layers11 FTW 90 CRW + Bottom layers12 FTW 90 CRW +Total layers

GTW: Gravity type wall; FTW: Facing type wallCRW : Cellular Reinforced Wall

Experimental programme on geocell walls


Geometry of Models


Fly ash used as backfill material; Locally available wasteplastic water bottles were used to prepare geocells. Thediameter and height of unit geocell was 70 mm

Photograph of geocells made up of waste plastic water bottles


CityTotal plastic

waste generated (tons/day)

Plastic waste collected for

recycling (tons/day)

Uncollected plastic waste

(tons/day)

Kolkata 425.35 255.21 170.14Chennai 429.30 257.58 171.72Mumbai 421.01 252.60 168.40Delhi 688.84 413.30 275.536

Plastic waste quantification in four metro cities ofIndia (CPCB,2012)


Results and DiscussionLateral Deformation of wall

Effect of facing angle(δh/H) values increased withincreasing in surcharge pressureuntil failure occurred.

The failure surcharge pressures (qf) are increased withdecreasing the facing angles. Similar results wereobserved for facing type of walls.

(δh/H) values increased withincreasing (yh/H) values indicatingthat the maximum lateraldeformation (δhmax) of the walloccurred at the crest portion.


For bottom layer reinforced backfill geocell wall, the(δh/H) values increased with increasing (yh/H) valuesindicating that the maximum lateral deformation of the walloccurred at the crest portion.

Whereas, for top layer reinforced backfill geocell wall themaximum lateral deformation occurred near about midheight of the wall.


Settlement of the Backfill

The relationship between backfill settlement and surchargepressure showed non linear for all the facing angles.Backfill settlements are increased with increasing facingangles.


Model number Facing angle () (qf/H) (hmax/H)% (max/b)%1 90 12.51 0.525 6.3902 80 14.11 0.557 6.6963 70 14.68 0.575 7.1964 90 13.92 0.535 6.7345 80 15.43 0.547 7.2196 70 16.65 0.563 7.8967 90 13.17 0.513 6.3258 80 14.39 0.568 5.5299 70 15.52 0.598 7.902

10 90 16.27 0.315 7.99111 90 15.33 0.353 7.95212 90 18.34 0.308 9.312

Gravity type of wall Facing type of wallFacing type of wall (backfill reinforced)

Results and Discussion

The lateral deformation was reduced by 32 to 40% in caseof reinforced backfill compare to unreinforced backfill wall


Major Findings

Normalized lateral deformation of wall (δh/H) increasedwith increasing surcharge pressure (q) until failureoccurred.

(δh/H) increased with increasing normalized height of wall(yh/H) indicating that the maximum lateral deformation ofwall occurred at the crest portion. (/H) decreased withdecreasing the facing angle () of the wall.

For top and total layers reinforced backfill models themaximum lateral deformation occurred at about midheight of the wall.


The failure surcharge pressures (qf) are increased withdecreasing the facing angles.

Reinforcement in the backfill reduced the lateraldeformation of the geocell wall. The reduction innormalised maximum lateral deformation (hmax/H)% ofthe geocell wall was about 30% to 40% with respect tounreinforced backfill.

Relationship between normalized surcharge pressures(q/H) and normalized settlement of the backfill (/b) wasobserved to be non linear for all the facing angles.

Settlement of the backfill was increased with decreasingfacing of wall.



Behaviour of Cellular Reinforced Fly Ash Wall under Strip Loading

Strip load is applied on the top of the backfill through a

loading plate having dimensions,

thickness (t) 10 mm;

width (b) 100 mm and

length (l) 350 mm

Position : 150 mm from the facing panel.

C5010 = Cellular reinforcement of diameter 50 mm andheight 10 mm


Models and objectives

Cellular reinforcement

Dimensions Aspect ratio (h/d) Vertical spacing (Sr) Length (Lr)

Model-1. Unreinforced - - - -Back fill reinforced with cellular reinforcement mattress

Model-2.different length of cellular reinforcement(Lr) values and with a constant value ofvertical spacing of cellularreinforcement (Sr)

C5010 0.2 0.22H 0.7H, 0.6H, 0.5H, 0.4H and 0.3H

C5020 0.4 0.22H 0.7H, 0.6H, 0.5H, 0.4H and 0.3H

Model-3.different vertical spacing of cellularreinforcement (Sr) and with differentlength of cellular reinforcement (Lr).

C5010 0.20.32H

and 0.42H

0.7H, 0.6H, 0.5H, 0.4H and 0.3H

Model-4.varying the height (h) and diameter (d)of cellular reinforcement.

C5010 0.2 0.22H 0.7HC5020 0.4 0.22H 0.7HC7015 0.21 0.22H 0.7HC7030 0.43 0.22H 0.7H

Back fill reinforced with cellular reinforcement stripsModel-5.reinforcement coverage ratio (Rc) = 0.5,0.6 and effect of single anchored (SA)and double anchored (DA) systems onthe behaviour of cellular reinforced wall.

C5010 0.2 0.22H 0.7H, 0.6H, 0.5H, 0.4H and 0.3H

C5020 0.4 0.22H 0.7H, 0.6H, 0.5H, 0.4H and 0.3H

H = height of the facing panel; 500 mm ; C5010 Cellular reinforcement of diameter 50 mm and height 10 mm

Experimental Program


Line sketch indicating different vertical spacing of the cellular reinforcement


Cellular reinforced fly ash wall - model test setup photograph


Line sketch Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Cellular ReinforcementThe cellular reinforcement used in the model experiments

was made up of locally available waste plastic water bottles.

Cellular reinforcement mattresses (a) C5010 (b) C5020 (c) C7015 and (d) C7030Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Placement of cellular reinforcement mattress


Placement of cellular reinforcement strips

Rc = 0.5 Rc = 0.6


Failure surcharge pressures and Lateral Displacement of Facing Panel

Lr/HSr/H = 0.22 Sr/H = 0.32 Sr/H = 0.42

(qf /H) (max/H)% (qf /H) (max/H)% (qf /H) (max/H)%

0.7 10.74 1.596 8.68 1.492 7.74 1.532

0.6 10.26 1.584 7.84 1.50 6.94 0.855

0.5 9.16 1.135 7.10 0.81 6.52 0.885

0.4 7.05 0.725 5.47 0.562 5.21 0.642

0.3 5.47 0.694 4.52 0.839 4.31 0.9432

Effect of length and spacing of cellular reinforcement C5010

(Cellular reinforcement in the form of mattress)Unreinforced backfill (qf /H) = 2.84

Results and Discussion


Increasing Lr/H ratio corresponded to higher failure

surcharge pressures.

Increasing Sr/H ratio corresponded to lesser failure

surcharge pressures.

Increasing the height of the cellular reinforcement (h)

corresponded to higher failure surcharge pressures,

indicating better performance of wall in terms of

carrying load before failure.


Lr/H

Cellular Reinforcement C5010 Cellular Reinforcement C5020

Rc = 0.5 Rc = 0.6 Rc = 0.5 Rc = 0.6

qf/H (max/H)% qf/H (max/H)% qf/H (max/H)% qf/H(max/H)

%

0.7 7.73 0.7905 8.89 0.870 8.74 0.816 10.47 0.841

0.6 7.10 0.796 7.68 0.790 7.84 0806 9.26 0.831

0.5 6.79 0.422 7.26 0.426 7.16 0.814 8.26 0.815

0.4 5.73 0.442 6.42 0.440 6.31 0.448 7.05 0.449

0.3 4.15 0.4629 5.47 0.483 5.05 0.452 5.47 0.441

Effect of length and coverage ratio of cellular reinforcement

(Cellular reinforcement in the form of strips)

C50 10 = Cellular reinforcement of diameter 50 mm and height 10 mmProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay


The variation between (y/H) and (δ/H) was found to be

almost linear for all surcharge pressures.

The (δ/H) values increased with increasing in surcharge

pressure until failure occurred.

The (δ/H) values increased with increasing (y/H) values

indicating that the δmax occurred at the crest portion of the

facing panel.

For a particular value of surcharge pressure (q), increasing

the height of the cellular reinforcement (h) showed less

facing panel displacements.Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Effect of (Lr/H) ratio on the (max/H) for different (Sr/H) and Rc values

Mattress

Strips


Backfill settlement

Relationships between normalized surcharge pressures

(q/H) and normalized settlement of backfill (/b) were

plotted.

The key variables are

= settlement of backfill, and

b = width of the loading plate = 100 mmProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Relationship between (q/H) and (/b) with different (Lr/H) Values

Effect of length of cellular reinforcement C5010:(Cellular reinforcement in the form of mattress)


The most important factors which governed thebehaviour of cellular reinforced fly ash wall are found tobe failure surcharge pressures, horizontal displacementof the facing panel and backfill settlement.

These factors are significantly influenced by theparameters like: length of cellular reinforcement toheight of facing panel ratio (Lr/H), vertical spacing ofcellular reinforcement to height of facing panel ratio(Sr/H), dimension of the cellular reinforcement (diameterand height) and reinforcement coverage ratio.

Conclusions


Increasing height, coverage ratio of cellular reinforcementcorresponded to a higher value of failure surchargepressure (qf) indicating better performance of wall interms of carrying load before failure. Also failuresurcharge pressures are increased with increasing (Lr/H)ratios and decreasing (Sr/H) ratios.

The model testing findings suggest that it may bepossible to use (Lr/H) ratio less than typical value of 0.7when the backfill is reinforced with cellular reinforcementin reinforced walls. However, more studies must becarried out before it can be used in actual practice.


The normalized displacements of facing panel (δ/H)increased with increasing normalized height of facingpanel (y/H) values indicating that the maximumhorizontal displacement of the facing panel occurred atthe crest portion of the facing panel. The variationbetween (y/H) and corresponding (δ/H) was found to belinear for all surcharge pressures.

The relationship between backfill settlement andsurcharge pressure is found to be non linear for all themodel walls. The settlement values at failure conditionincreased with increasing (Lr/H) ratios and dimensionsof the cellular reinforcement.


Applications of GeowebProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


Documents

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE · z x s v x K a or, 32.43 = (γ s x z + q) x s v x K a or, 32.43= (18 x 10 + 39) x s v x 0.27 or, s v = 0.55 m Therefore, let us