Indian Geotechnical Conference (IGC-2010)igs/ldh/files/igc 2015 pune... · constructed with coal ash as a structural fill material. Coir fiber, derived from coconut husk, is a natural

50

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IG

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INDIAN GEOTECHNICAL CONFERENCE

17th

– 19th

DECEMBER 2015, Pune, Maharashtra, India

Venue: College of Engineering (Estd. 1854), Pune, India

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SEEPAGE AND PIPING ANALYSIS OF POND ASH REINFORCED WITH COIR

FIBERS AND COIR GEOTEXTILES

J. V. Reeba1, Y. E. Sheela

2

ABSTRACT

Ash is a by-product obtained from the combustion of pulverised coal and is primarily generated

from the coal based thermal power plants. In India, about 250 million tonnes of ash is being generated

annually. But its utilization is only about 38 million tons per year, mainly in the areas of cement as well

as concrete manufacture and building products, and to some extent in earth fills. Conventionally the

unutilized volumes of ash (both bottom ash and fly ash) are mixed with large amounts of water and then

disposed onto the ash ponds, which have already occupied 65,000 acres of valuable land in India and

many million acres of land all over the world. This disposal methods poses a great threat to the

environment.There are two types of ash produced by thermal power plants, viz. (1) fly ash, (2) bottom

ash. Pond ash is the combination of fly ash and bottom ash mixed with water to form a slurry and is

pumped in the ash ponds. In ash ponds, water is removed and the ash settles as residue. This residual

deposit is called pond ash. It is a lightweight and self-draining material compared to natural soil contains

particles of fine sand to silt sizes. There exists a tremendous potential of utilization of pond ash in

geotechnical constructions in order to preserve the valuable top soil. Today, pond ash is being used as a

structural fill material in highway and railway embankments, for ash pond bunds, levees, filling low lying

areas etc.

‘Piping’ refers to the development of channels which develop at the downstream side of the

structure where the flow lines converge and a high seepage pressure occurs. Piping is a form of seepage

erosion and involves the development of subsurface channels in which soil particles are transported

through the porous medium. Seepage induced failures in the form of piping can weaken and affect the

1J. V. Reeba, PG student, College of Engineering Trivandrum, India, [email protected]

2Y. E. Sheela, Associate professor, College of Engineering Trivandrum, [email protected]

J. V. Reeba, & Y. E. Sheela

performance of embankments, drainage projects, river levees, contour bunds, temporary canal diversion

works, temporary check dams, and soil structures, constructed with coal ash as a structural fill material.

When the seepage velocity exceeds the critical velocity, piping occurs and the soil in the constructed

areas flows out and the structures are weakened. Therefore, an effective counter measure against the

piping is needed and the coir fiber mixed pond ash is useful in this application. Coir fiber, derived from

coconut husk, is a natural material available abundantly in South India and other coastal areas. It is

stronger than other natural materials such as jute or cotton. Coir geotextiles have been used in various

slope stabilization and erosion control projects.

This study was done to examine the effect of coir fibres on the piping behaviour of pond ash. It

was conducted on a specially developed experimental set up and measured the discharge, hydraulic head,

seepage velocity and piping resistance of pond ash reinforced with coir fibers for different fiber contents

and fiber lengths. Since fibers are distributed throughout the soil mass, they impart strength isotropy and

reduce the possibility of formation of weak zones. Reinforcing pond ash with coir fibers reduced the

seepage velocity and contribute to improved piping resistance in the range of 20- 50%. Numerical

analysis of the embankment model constructed with pond ash using MIDAS software was also studied.

The paper describes details of experimental work carried out, numerical analysis using finite element package and

comparison of results and discussion.

Keywords: Piping, Seepage, Pond ash, Coir fibers, MIDAS GTS-NX

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Seepage and Piping analysis of Pond ash reinforced with Coir fibers and

Coir geotextiles

J. V. Reeba, PG student, College of Engineering Trivandrum, [email protected]

Y. E. Sheela, Associate professor, College of Engineering Trivandrum, [email protected].

ABSTRACT: Pond ash is used as a structural fill material in embankments and drainage projects. Seepage

induced failures in the form of piping can weaken and affect the performance of embankments

constructed with coal ash as a structural fill material. Coir fiber, derived from coconut husk, is a natural

material used as a reinforcing material to improve piping resistance. This study was done to examine the

effect of coir fibres and coir geotextiles on the piping behavior of pond ash. One dimensional piping tests

were carried out for different fiber contents and fiber lengths. Seepage velocity, critical hydraulic gradient

and piping resistance was found out for all the cases. Numerical analysis of the embankment model

constructed with pond ash using MIDAS software was also studied. The paper describes literature review on

the subject, details of experimental work carried out, numerical analysis using finite element package, comparison

of results and discussion.

1. INTRODUCTION

Geotechnical constructions like embankments,

retaining structures etc requires huge amount of

earth materials. Rapid industrialization and non

availability of conventional earth materials have

forced the engineers to go in search of alternate

options and in particular the waste materials, which

poses a threat to environment. There exists

tremendous potential for the utilization of Pond ash

in engineering structures.

In India, about 200 million tons of ash is

generated annually from the thermal power plants

and only a small portion is being utilized in an

effective manner. Conventionally the unutilized

volumes of ash (both bottom and fly ash) are mixed

with large amounts of water and disposed on to the

ash ponds. This disposal method poses a great

threat to the environment. Pond ash contains fine

sand to silt sized particles. It is a light weight and

self draining material compared to natural soil.

As the water flows through the soil there is

transfer of energy to the soil skeleton. This causes

seepage forces to act on the specimen. When the

flow of water is upwards and the hydraulic gradient

is high enough, the resultant force is zero and the

hydraulic gradient is called by the name critical

hydraulic gradient(ic). In this case the contact force

between soil particles will be zero and have no

strength leading to erosion of soils known by the

name piping. It involves the development of

channels in which particles are transported through

the porous medium which develops at the

downstream side of the structure where the flow

lines converge and high seepage pressure occurs.

In the context of sustainable watershed

management, natural fibers have application in

irrigation and drainage projects, soil structures etc

for controlling seepage. Coir fiber obtained from

coconut is a natural material available in plenty in

India and other coastal areas. Due to its high lignin

content it is durable than any other natural fibers

like jute, cotton etc.

In this study, it is intended to examine the effect

of coir fibers and coir geotextile in the piping

behaviour of Pond ash samples and to find the

variation of seepage velocity and piping resistance

for different fiber lengths and fiber dosages. The


optimum fiber length and fiber content is to be find

out and also the optimum depth of placement of

geotextile to reduce the piping effect.

1.1 Literature review

The fiber inclusions have significant effect on the

failure deviator stress and the shear strength

parameters of soil. The length of the fibers

influences the behavior of the specimens [3]. In the

triaxial shear tests, the fiber inclusions had a

significant effect on the stress-strain behavior of

the specimens. In Unconfined compression tests,

the raw fly ash specimens attained a distinct axial

failure stress at an axial strain of about 1.5-2.5%

following which they collapsed, when reinforced

with polyester fibers. But, the fiber-reinforced

specimens have a highly ductile behavior. The

internal erosion indicated by the loss of fine

particles causes changes in the void ratio and a

significant increase in hydraulic conductivity [5].

There occurs drastic changes in strength for

hydraulic gradients above 0.5. In most of the

drainage projects and embankments constructed

with fill materials, failure occurs by piping. When

seepage velocity exceeds the critical velocity,

piping occurs and the soil in the constructed areas

flows out and structures are weakened [1]. The

inclusion of polyethylene and polyester fibers in

fly ash reduced the seepage velocity, increased the

piping resistance and increased the critical

hydraulic gradient hence, delaying the occurrence

of piping [2].Coir fibers are very effective in

controlling seepage and improving the piping

resistance of soils [6]. It can be used for the

construction of river levees and drainage projects.

The fibers contributed to increased piping

resistance.

2. METHODOLOGY

Coir fibers of length 10mm, 25mm, 50mm and

75mm were made and mixed with Pond ash at

different fiber contents 0.5%, 0.75%, 1%, and 1.5%

of dry unit weight of Pond ash. The experiments

were conducted for reinforced and unreinforced

samples. The experiments include Unconfined

compressive strength test (U.C.C), compaction and

one dimensional piping tests. The strength

characteristics of reinforced and unreinforced pond

ash samples were obtained from U.C.C tests. The

optimum moisture content and maximum dry

density was obtained from compaction tests. The

same percentage of pond ash and coir fibers are

subjected to one dimensional piping test in a

specially developed experimental set up. Different

parameters like seepage velocity, critical hydraulic

gradient and piping resistance was obtained from

the discharge collected during the tests. The piping

test was also repeated by placing woven and non-

woven geotextile instead of coir fibers at various

depths.

2.1 Material characterization

The materials used for this study are pond ash,

coir fibers, woven and non-woven geotextiles. The

material properties evaluated as per IS standards

are reported in table 1 and table 2.

2.2 Experimental programme

The apparatus consists of a transparent 150mm

diameter cylinder in which a moist compacted

sample was placed as shown in figure 1. It was

subjected to an upward flow of water, the flow

being increased in small steps until piping occurs.

Height of the sample was maintained as 70mm for

all the tests. Basically two chambers in the

cylinder, in which sample was placed. Through the

bottom chamber, the water was allowed to flow

vertically in to the sample. A thin non-woven

geotextile fabric was kept at the bottom to contain

the pond ash particles and to distribute water flow

uniformly.

Table 1 Properties of Pond ash

Property Value

Specific gravity 2.38

Particle size distribution

Sand (%)

Silt (%)

Clay (%)

70

30

0

Plasticity index Non-plastic

Effective particle size, D10(mm) 0.38

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Uniformity coefficient, Cu 2.36

Coefficient of curvature, Cc 1.5

Maximum dry unit weight(kN/

m3)

13

Optimum moisture content (%) 26.5

Cohesion (kPa) 0

Angle of internal friction

(degrees) 29

Table 2 Properties of coir fibers used

Property Value

Specific gravity 1.12

Tensile strength (kPa) 1x105

Tensile modulus (kPa) 2x106

Length (mm) 10,25,50,75

Table 3 Properties of coir geotextiles used

Property Value

Woven geotextile

Thickness (mm) 5.55

Mass per unit area

(g/m2)

350

Tensile strength

(kN/m)

Apparent opening size

(mm)

2.2

1.12

Non-woven geotextile

Thickness (mm) 6.95

Mass per unit area 1550

(g/m2)

Tensile strength

(kN/m)

Apparent opening size

(mm)

35

0.16

Fig. 1 One dimensional piping test set up-

schematic diagram

Discharge under various head conditions was

monitored by collecting water flow through the

sample into a jar attached to the mould. Each time

the head was increased in steps by 20mm. The

experiments were conducted for different fiber

contents (0.5%, 0.75%, 1% and 1.5%) of dry unit

weight of pond ash and fiber lengths (10, 25, 50

and 75mm). The coir fiber mixed pond ash was

filled in the cylindrical mould in approximately 3

equal layers and each layer was statically

compacted. All the specimens were prepared at

optimum moisture content and maximum dry unit

weight obtained from the compaction curve of

pond ash. After preparing the natural and

reinforced soil samples, it was saturated for one

hour. The piping tests were carried out by

increasing the head of water in the reservoir at

increments of 20mm while the level of water above

the sample was constant. The duration of each

increment was nearly 10 min and during this time

the discharge water from the sample was collected

and measured. The level of water in the reservoir


was increased until the piping occurred in the

sample.

Piping failure was always observed by the

formation of small bubbles and local boiling.

Seepage velocity increases with the increase in

hydraulic gradient.

Seepage velocity was calculated using the

equation htnA

QLVs

………..(1) where Q =

Discharge in m3/s, L = Length of specimen, n =

porosity of sample, A = Cross sectional area of

specimen, ∆h = differential head varies from 0 to a

value at which piping occurs.

Critical hydraulic gradient ic, is defined as the

ratio of differential head at which pond ash

particles starts lifting due to the upward flow of

water ∆hc , to the length of specimen,L

L

hi

cc

..............(2)

As the water flows through the soil, a force is

applied to the soil particles which is referred to as

seepage force. Therefore the seepage force acts in

the direction of flow of water. The piping

resistance of soil acts in the opposite direction of

seepage force. When these two forces are equal, the

particles of soil start to lift due to the upward flow

of water. The seepage force at critical gradient can

be calculated by the relationship given below. At

the onset of piping, seepage force is equal to piping

resistance, P.

P = Ὑw ic V …………..(3)

where P is the seepage force at critical gradient ic

, Ὑw is the unit weight of water and V is the

volume of sample.

3. RESULTS AND DISCUSSIONS

The results of Unconfined compressive strength,

Compaction and One dimensional Piping tests are

detailed below.

3.1 U.C.C test Results

The variation of unconfined compressive

strength with varying fiber content and length is

shown in Fig 2. As the fiber content increases, in

all the fiber lengths used, the strength increases.

The stress is maximum for 50mm fiber length.

Fig. 2 Variation of U.C.C strength with fiber

content

3.2 Compaction characteristics

Results for compaction tests of Pond ash for

different fiber content and fiber length are shown in

Fig 3 and Fig 4.The results shows that optimum

moisture content increases as the fiber length and

fiber content increases. This is due to the surface

texture of coir fibers. At the same time, maximum

dry density decreases when fiber content increases.

This is due to the low density of coir fibers.

Fig. 3 Variation of optimum moisture content with

fiber content and fiber length

0

10

20

30

40

50

60

0 0.5 1 1.5 2

Str

ess

(Kg

/cm

2)

FIBER CONTENT (%)

l=50mm

l=25mm

l=10mm

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Fig.4 Variation of maximum dry density with fiber

content and fiber length

3.3 Piping test results

From piping tests, the variation of seepage

velocity with hydraulic gradient for fiber lengths

10, 25, 50 and 75mm and fiber contents0.5%,

0.75%, 1% and 1.5% are shown in figures 5, 6, 7,

and 8. Seepage velocity catastrophically increased

once piping was started. It is clear that critical

hydraulic gradient increases as fiber content and

fiber length increases. However for the case of

75mm length fibers, it was found that critical

hydraulic gradient and piping resistance decreases.

Fig. 5 Variation of seepage velocity with

hydraulic gradient (fiber content = 0.5%)




hydraulic gradient (fiber content = 1%)




Table 4 Piping test results

Fig. 9 Variation of critical hydraulic gradient

with fiber content and fiber length

Fig. 10 Variation of piping resistance with fiber

content and fiber length

From Fig 10, it can be noted that, as the fiber

content increases, the piping resistance increases.

But with increase in fiber content piping resistance

increases upto 0.5%, and after that it remained

almost constant. Maximum piping resistance for

50mm length is 27.69N and for 10mm it is only

15.63N. The piping resistance of soil is maximum

for 50mm fiber length and for the case of 75mm,

piping resistance decreases. Thus it can be noted

that coir fiber mixed pond ash with a fiber content

of f =0.5% and fiber length, l = 50mm exhibits a

distinct increase in critical hydraulic gradient and

piping resistance.

3.4 Effect on critical hydraulic gradient with the

placement of geotextile

The piping tests were repeated by placing

geotextiles at different depths. The critical

hydraulic gradient is more when non-woven

geotextile is placed than a woven geotextile. In

both the cases, critical gradient is maximum, when

geotextile is placed at a depth of 1/3h from the

bottom. The variations in both cases are shown in

Fig 11 and Fig 12.

Fiber

Content

(%)

Fiber

Length

(mm)

Critical

Hydraul

ic

Gradien

t (ic)

Piping

Resistan

ce (N)

Permeabili

ty

(m/s)

0 0 0.79 15.63 5.9 x 10-5

0.5

10 0.94 18.59 4.3 x 10-5

25 1.0 19.78 3.9 x 10-5

50 1.4 27.69 3.5 x 10-5

75 1.1 21.76 4.1 x 10-5

0.75

10 0.82 16.22 5.8 x 10-5

25 0.95 18.79 4.4 x 10-5

50 1.1 21.76 3.2 x 10-5

75 1.05 20.02 3.6 x 10-5

1

10 0.9 17.80 5.8 x 10-5

25 1.05 20.77 4.3 x 10-5

50 1.25 24.73 4.1 x 10-5

75 1 22.7 4.2 x 10-5

1.5

10 0.92 17.91 4.5 x 10-5

25 1.11 21.21 4.1 x 10-5

50 1.21 23.44 3.8 x 10-5

75 1.05 21.47 3.9 x 10-5

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Fig. 11 Effect on critical hydraulic gradient when

non-woven geotextile is placed

Fig. 12 Effect on critical hydraulic gradient when

woven geotextile is placed

3.5 Numerical Analysis using MIDAS GTS-NX

The finite element analysis was carried out using a

new package – MIDAS GTS NX, which is purely

used to solve geotechnical engineering problems.

Steady state seepage analysis of Pond ash

embankment model was carried out. The

embankment model constructed with the help of

MIDAS software is shown in Fig 13 and the hybrid

mesh generated for the analysis is shown in Fig 14.

Pond ash embankment models were constructed for

0.5%, 0.75%, 1%, and 1.5% fiber contents and for

the case of unreinforced case. Variation of seepage

velocity of pond ash embankment model is shown

in figures 15, 16, 17, 18 and 19. The input

parameters for software analysis are tabulated in

Table 5. The numerical results from the analysis

are tabulated in Table 6.

Fig. 13 Embankment model

Table 5 Input parameters in MIDAS

Input parameters Value

Elastic modulus Pond ash (kPa)

Coir fiber (kPa)

24000

2 x 106

Poisson’s Ratio 0.3

Cohesion 0

Angle of internal

friction

290

Maximum dry

density (kN/m3)

13


Fig. 14 Hybrid mesh generated for finite element

analysis.

Table 6 Numerical Results

Fiber

length

(mm)

Fiber

content (%)

Flow

velocity

(m/s)

0 0 4.20 x 10-6

10

0.5 5.31 x 10-7

0.75 7.34 x 10-7

1 4.61 x 10-6

1.5 4.57 x 10-6

25

0.5 6.72 x 10-7

0.75 6.31 x 10-7

1 5.21 x 10-6

1.5 4.70 x 10-6

50

0.5 3.22 x 10-7

0.75 6.89 x 10-7

1 4.74 x 10-6

1.5 4.70 x 10-6

75

0.5 7.73 x 10-7

0.75 7.72 x 10-7

1 4.35 x 10-6

1.5 4.32 x 10-6

Fig. 15 Variation of flow velocity for the

unreinforced Pond ash embankment

Fig.16 Variation of flow velocity for the Pond

ash embankment with coir fiber content of

0.5% for 50mm fiber length

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Fig. 17 Variation of flow velocity for the

Pond ash embankment with coir fiber content

of 0.75% for 50mm fiber length

Fig. 18 Variation of flow velocity for the Pond

ash embankment with coir fiber content of 1%

for 50mm fiber length

Fig. 19 Variation of flow velocity for the Pond

ash embankment with coir fiber content of

1.5% for 50mm fiber length

From the embankment model analysis , the seepage

velocity is the minimum for the case of fiber

content = 0.5% and fiber length = 50mm.

4. CONCLUSIONS

The following conclusions are made out from

this study:

The unconfined compressive strength

increases as the fiber content increases

and is maximum for the case of 50mm

length.

Reinforcing Pond ash with coir fibers

reduced the seepage velocity and

increased the piping resistance in the

range of 20-50%.

As the fiber length is increased, the

critical hydraulic gradient is also

increased and hence the piping resistance.

Piping resistance increased until 0.5%

fiber content and then reached a constant

value.

Permeability of reinforced pond ash

sample decreases in comparison with

unreinforced ones and the amount of

decrease is a function of fiber content and

fiber length.

The critical hydraulic gradient is found to

be more when non-woven geotextile is

used.

The optimum depth of placement of

geotextile is 1/3h from the bottom of

sample.

Critical hydraulic gradient is the

maximum for the case of placement of

non-woven geotextile than reinforcing

with coir fibers.

The optimum fiber content was found to

be 0.5% and the optimum fiber length

was 50 mm.

From the embankment model analysis

also, the seepage velocity is the minimum


for the case of fiber content = 0.5% and

for fiber length = 50mm

REFERENCES

1. Arghya Das, Jayashree and Viswanadham,

B.V.S. (2009) Effect of randomly

distributed geofibers on the piping

behaviour of embankments constructed

with fly ash as a fill material, Geotextiles

and Geomembranes 27,341–349.

2. Estabragh, A.R, Soltannajad, K., and

Javadi, A.A. (2014) Improving piping

resistance using randomly distributed

fibers, Geotextiles and Geomembranes, 42,

15-24.

3. Kaniraj, S.V. and Gayathri, V., (2003)

Geotechnical behavior of fly ash mixed

with randomly oriented fiber inclusions,

Geotextiles and Geomembranes, 21

4. Kumar, R.,kanaujia, V.K. and Chandra, D.,

(1999) “Engineering behaviour of fibre-

reinforced Pond ash and silty sand,

Geosynthetics international, 6, 6.

5. Lin Ke and Akihiro Takahashi. (2012).

Strength reduction of cohesionless soil due

to internal erosion induced by one-

dimensional upward seepage flow, Soils

and Foundations, 52(4):698–711.

6. Sivakumar Babu, G.L. and Vasudevan,

A.K. (2008).Seepage Velocity and Piping

Resistance of Coir Fiber Mixed Soils,

Journal of irrigation and drainage

engineering, 10.1061/ASCE, 0733-9437,

134:4(485).

Documents

Indian Geotechnical Conference (IGC-2010)igs/ldh/files/igc 2015 pune... · constructed with coal ash as a structural fill material. Coir fiber, derived from coconut husk, is a natural