Paper for SIBE 09 - Muhardi

8/8/2019 Paper for SIBE 09 - Muhardi

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76 / G: GEOTECHNICAL ENGINEERING

International Conference on Sustainable Infrastructure and Built Environment in Developing Countries

November, 2-3, 2009, Bandung, West Java, Indonesia

ISBN 978-979-98278-2-1

Physical Modelling of Fly Ash Embankment in Mini-Geotechnical Centrifuge

Marto, A.1, Makhtar, A.M.1, Kassim, K.A.1,

Yaacob, W.Z.2 and Muhardi3* 1

Faculty of Civil Engineering, Universiti Teknologi Malaysia,81310 UTM Skudai, Johor, Malaysia

2Faculty of Science and Technology, Universiti Kebangsaan Malaysia,

43600 Bangi, Selangor, Malaysia3Faculty of Engineering, University of Riau, Jalan Raya Pekanbaru Bangkinang Km. 12,5

Panam, Pekanbaru, Riau, Indonesia

*Corresponding Author: [email protected]

AbstractFly Ash has been used for many years in geotechnical engineering such as in embankment

construction. The relatively lightweight compacted of fly ash makes the material very suitable

as a replacement for backfill material in embankment construction, particularly on soft soil.

Physical modelling is usually performed to study the particular aspects of the behavior of prototypes. The physical models, constructed at smaller scales than the prototypes, could give

information of response more rapidly and the model details could be better controlled than

the full scale testing. Geotechnical centrifuge is an example of physical modeling where all

features of the prototype are reproduced at smaller scale. Large beam and drum centrifuges

are now regularly used to model geotechnical engineering structures. In this research, a

UKM mini-geotechnical centrifuge with 0.5m radius has been used and it has the advantage

that it is cheap to manage and modify, and allow large numbers of tests to be performed

rapidly. This paper discusses the components of the UKM mini-geotechnical centrifuge and

its application to model fly ash embankment, constructed on hard and very soft soil

foundations. Since there are no actuators or instrumentation present, tests are triggered by

increasing self-weight as the acceleration is increased and settlements and failure were

determined photographically. The settlement measurement method consists of a remotedigital video camera together with stroboscopic lighting. It is concluded that using fly ash as

fill material in embankment construction can affect the performance of the model whereby it

improves stability and reduces settlement of embankment and foundation layers, as observed

from mini-centrifuge model.

Keywords: fill materials, fly ash embankment, physical modelling, soft soils, UKM mini-

geotechnical centrifuge.

1. IntroductionPhysical modelling is usually performed to study the particular aspects of the behavior of

prototypes. Physical model is constructed at smaller scale than the prototype. By havingsmall scale models it is expected to obtain information of response more rapidly and morecontrol over a more details full scale testing could be carried out. Centrifuge model is an

example of physical modeling. It concerns the study of geotechnical events using small-scale

models subjected to acceleration fields of magnitude that is many times larger than the earth

gravity. With this technique, self weight stresses and gravity dependent processes are

correctly reproduced and observations from small-scale models can be related to the full-scaleprototype situation using well-established scaling laws (Muhardi et al., 2008).



G: GEOTECHNICAL ENGINEERING / 77

Whilst most centrifuge facilities are aware of the teaching potential of centrifuge

technology for demonstrating geotechnical problems, the cost of servicing this teaching tends

to be prohibitive other than on an occasional basis. Small beam provide an alternative

technique, which is more cost effective, but still requires specialist knowledge and technical

expertise. Much smaller centrifuges (less than 0.5m radius) have been built for teaching and

research purposes, but are described rarely in the literature. These very small centrifuge

facilities have the advantage that they are cheap to operate and modify, and enable large

numbers of tests to be conducted quickly (Newson and White, 2005).

Fly ash is one of solid waste material resulted from coal burning in the production of

electricity. Mahmud (2003) reported that in 2002 gas has been used as main resource fuel to

generate power plants in Malaysia. However coal has been projected as a possible resource

fuel in the forthcoming. It is projected that the total projected installed capacity on the coal

power plant will be 8,200 MW, requiring about 22.5 million tons of coal by the year 2010. As

a result of large utilization of coal, a large volume of coal ash as waste material will be

produced. In Malaysia, there is no report about the producing of coal ash annually. However,

according to Kim (2003), generally about 10% of the coal burned produces ash and up to 90%

of ash is fly ash. Fly ash has been used for many years in construction as a lightweight

replacement in cement, especially in high performance concrete. Although publishedscientific data on the performance of fly ash in geotechnical engineering is very limited, it is

well known that fly ash has also been used widely in ground stabilisation especially in road

construction, construction of embankments, reinforced soil retaining walls, and land

reclamation. The relatively lightweight compacted density of fly ash makes the material very

suitable as a fill material in embankment construction (Marto et al., 2009)

This paper discusses the behaviour of embankment that used fly ash (obtained from

Tanjung Bin, Johor coal-fired power plant) as backfill materials on hard and very soft soils

foundation, using the technique of centrifuge modelling. The embankment models were tested

in the centrifuge equipment with the gravitational force being increased until the model failed.

The failure mechanism of the embankment are also analysed and compared between the two

foundation types. The centrifuge modelling test carried out in this research took place at the

Geotechnical Centrifuge Laboratory, Universiti Kebangsaan Malaysia (UKM), using theUKM mini-geotechnical centrifuge system.

2. Methodology

2.1. UKM mini-geotechnical centrifuge systemBuilt in 2009, the centrifuge is a mini-sized beam-type centrifuge with a dead load

capacity of 6 kg, and is capable of applying 140g at an effective radius of 0.5 m (hence it is a

0.84g-tone machine). Since there are no actuators or instrumentation present on the package,

tests are triggered by increasing self-weight as the acceleration is increased and soil

displacements and failure are measured photographically. The displacement measurement

system consists of a remote digital video camera together with stroboscopic lighting.

Overview of UKM mini-geotechnical centrifuge system is shown in Figure 1. Soil packagesare placed in a strongbox that is fixed to the centrifuge arm. Unlike most large, fixed beam

centrifuges, the platform and box are combined and the box has a hinge at the top to allow for

swing-up during operation. This allows the box to have the largest possible size (giving a

larger sample) and enables different forms of box to be easily fixed into the apparatus. The

counterweight system is a box at the opposite end of the arm that could be moved manually

and fixed to the required position using pins. The video imaging and analysis system consists

of a VZOR digital video camera, mounted above the viewing hole of the centrifuge top plate.

To achieve the optimum picture quality and match the frequency of the strobe light, manual

control of exposure and focus is used. Image analysis was achieved using Particle Image

Velocimetry (PIV) software, which is a MatLab module that implements.

2.2. Model preparationTwo types of embankments were modelled that were by constructing it on two different

types of foundations; hard and very soft soil foundation. Foundation soils were constructed




from sand and kaolin. Dry sand was placed by raining technique to form a medium dense

sand with 16.4 kN/m3

unit weight. Kaolin was chosen as the base material representing very

soft soil to ensure a homogeneous soft soil foundation. The properties of sand and kaolin are

taken from the work of Khatib (2009). Kaolin samples were mixed at a moisture content of

62% (the liquid limit) and placed in the test box to the desired height. Then the centrifuge test

was carried out to control the consolidation of kaolin. Similar method of placement was

performed at Cambridge centrifuge center (Taylor, 1995). This method is based on the

measure of time (t ) at which 50 % of the final settlement has taken place. From consolidation

theory, it will be reached when cvt/h2

= 0.197 with cv is the coefficient of consolidation and h

is the height of the consolidating layer. With the height of kaolin, h = 0.05 m, radius of

centrifuge = 0.5 m, = 360 rpm, v = 6 m/s, g = v2 / r = 72g, cv = 10

-7m

2 /sec, then the test time,

t is obtained as 1.37 hr. Hence, after 1.37 hr of the consolidation of kaolin using the centrifuge

method, the undrained shear strength test was performed at the consolidated kaolin. The

average undrained shear strength of kaolin was found to be about 5 kPa, hence falls into a

very soft soil type (Su < 20 kPa).

Figure 1 Overview of UKM mini-geotechnical centrifuge system (cover plate removed)

Fly ash samples were mixed with water at an optimum moisture content of 19.75% and

the compaction was carried out by first laying the fly ash mix layer by layer with each layer

was compacted using a wood rod to get an homogeneous layer and to simulate the same

compactive effort such as in the standard compaction test. Three layers were laid up to the

desired embankment height and the specified slope inclination was achieved. To ensure a

uniform deposition of the embankment and at the required slope angle and density of fly ash

material, the pilot test was performed earlier using the same test box and wood rod. The unit

weight of fly ash was found to be 15 kN/m3. Once the sand or kaolin was placed to a height

slightly above the desired embankment height and slope angle, the embankment surface was

cut and levelled off to form a 70 cm height embankment with 30º slope inclination.

2.3. Centrifuge Test MethodologyOnce the model was prepared, the centrifuge test was started by applying the acceleration

that was slowly increased until failure occured. During testing, once a certain speed was

reached the tests were maintained before increasing the speed again. In order to monitor the

deformation of the slopes, the initial shape of the embankment and foundation layer were

drawn on the transparent perspex, at the side surface of the model. After failure was achieved,

centrifuge machine was stopped and the undrained shear strength test was performed using a

mini vane shear at the fly ash embankment. The undrained shear strength was found to be

about 6 kN/m2.




3. Result and Discussion

3.1 Obtaining theoretical valuesBased on the Taylor‘s stability chart (Craig, 2004), the stability coefficient number, N s

(. .

u

s

c N

F γ H ) is found to be 0.13 if the factor of safety, F is taken as 1 (critical condition),

while the slope angle, = 300

and depth parameter, DH is 1.

Taking the factor of safety, F of 1 with the undrained shear strength of embankment, cu of

6 kN/m2

and the unit weight of embankment, of 15 kN/m3, the height of prototype to cause

failure could be obtained as follows:

prototype. .

u

s

c H

F γ N =

6

1 15 0.13 x x= 3.1 m .......................................(1)

This height represents the real height of the prototype embankment being modelled. In

order to find the scaling factor, N where the model theoretically failed, the following equation

is used:

N =prototype

mod el

H

H =

3.1m

0.07m= 44.3 ......................................................(2)

3.2 Performance of Embankment based on SettlementIn order to examine the performance of embankment, graphs of the settlement of the top

of embankment models due to increase gravity under consideration have been produced. The

settlement was obtained from image analysis using PIV software. Figure 2 shows the

settlement of the top of the embankment models at various gravitational accelerations for

medium dense sand and very soft soil foundation, respectively. It can be seen from Figure 2

that the settlement on very soft soil increased rapidly until about 50 x g. The settlement is

continuous at a constant rate until the test stopped at about 76.5 x g. However, at about 45.4 xg, large settlement started to occur, whereby the settlement rate seemed to increase beyond

that. This demonstrated that slipping and tension cracking is occurring within the

embankment. Hence, 45.4 x g could be the ultimate limit state (ULS) condition for very soft

soil, which failure occurred. Beyond that, there were excessive lateral and horizontal

movement occurred in front of the embankment and eventually the final settlement was found

to be about 28 mm. On the other hand, the settlement of embankment on medium dense sand

increased slowly and there seemed to be no clear change in the rate of settlement. The final

settlement was about 5 mm when centrifuge test was stopped at about 76.5 x g. At this point,

it was observed that there was no excessive lateral and horizontal movement in front of the

embankment and the embankment was relatively stable. Hence, the model might have not

reached its ultimate limit state (ULS) condition yet. However for comparison purposes, the

ULS is taken at 76.5 x g for medium dense sand foundation. The comparison between model

and prototype for two types of foundation are shown in Table 1.




Figure 2 Gravity vs. Settlement of the top of fly ash embankment model

Table 1 Comparison between model and prototype based on settlement

Very soft soil foundation Medium dense sand foundation

Model Prototype Model Prototype

Settlement at ultimate

limit state (ULS)17 mm 45.4 x 17 = 771.8 mm 5 mm 76.5 x 5 = 382.5 mm

Embankment height 0.07 m 45.4 x 0.07 = 3.18 m 0.07 m 76.5 x 0.07 = 5.36 m

Comparing between theoretical values with centrifuge test, the prototype of embankment

height matched reasonably well for embankment on very soft soils (3.1 m for theoretical and

3.18 m for centrifuge test). However, for embankment on medium dense sand, the centrifuge

test gave much higher value compared to theoretical calculation. It means that the factor of

safety for fly ash embankment model on medium dense sand is higher than 1. This is

supported by the condition of the model that was observed to be relatively stable and there

was also no clear sign of failure.

In ordinary design of foundations, the serviceability limit state (SLS) is usually expressed

in terms of allowable settlements. A simple conservative way of defining SLS for the

embankment models is to assume the settlement at 10% of ULS as shown in Table 2. As can

be seen from Table 2, the allowable settlement of 38.28 mm can be accepted for embankment

on medium dense sand. However, for the embankment on very soft soil, the settlement of

77.18 mm could not satisfy the allowable settlement for any kind of structure supported on the

surface of the embankment. From the observation through centrifuge tests, excessive

settlements occurred at the top and base of embankment model, constructed on a very soft soil

foundation. In general, a settlement of less than 50 mm can be accepted of which the structurecould continue being functional.

Table 2 Settlement at Ultimate Limit State (ULS) and Serviceability Limit State (SLS)

Very soft soil foundation Medium dense sand

foundation

Model Prototype Model Prototype

Settlement at ULS 17 mm 771.8 mm 5 mm 382.5 mm

Settlement at SLS 1.7 mm 77.18 mm 0.5 mm 38.25 mm

Figure 3 shows the failure mechanism in the fly ash embankment on a very soft soil

foundation and a medium dense sand foundation, respectively. The failure mechanisms

observed from the transparent perspex at the side of the embankment show a similar pattern




with the observation made through VZOR digital video camera, mounted above the viewing

hole of the centrifuge top plate.

(a) Very soft soils foundation (b) Medium dense sand foundation

Figure 3 Failure mechanisms of model on different foundation types after 76.5 x g

acceleration

4. Conclusion

Two centrifuge fly ash embankment model tests were performed each on very soft soil

foundation and medium dense sand foundation. The acceleration at failure and the failure

mechanisms were obtained photographically using PIV and these results were compared to

the theoretical predictions. Comparing between the theoretical values with centrifuge test, the

prototype of embankment height matched reasonably well for embankment on very soft soil.

However, for embankment on medium dense sand, the centrifuge test gave much higher value

compared to theoretical calculation. This shows that the factor of safety is higher than 1 for

embankment on medium dense sand. The allowable settlement calculated for the prototype

can be accepted for embankment on medium dense sand. However, the embankment on very

soft soils could not satisfy the allowable settlement for any kind of structure supported on thesurface of the embankment.

Observed variations suggest degrees of conservatism in the analytical methods, problems

with the boundary conditions of the tests, inaccuracy with the estimate of undrained shear

strength or problems with the visual identification of failure. The other difficulty was to

define the constitutive failure. The failure was governed by Serviceability Limit State rather

than the Ultimate Limit State. This approach appears to be very suitable to be used in research

and with suitable caveats would give researchers an excellent introduction to allowable

settlements and failure mechanism. The centrifuge tests, using the mini-centrifuge system,

proved to be very quick since the test can be conducted, starting from the soil preparation

until test to failure, in less than 2 hours. The modelling using mini-centrifuge system is also

simple to perform making them suitable for geotechnical engineering research and

demonstration works.

5. Acknowledgement

The authors would like to thank the Ministry of Higher Education (MOHE), Malaysia for

funding this research by Fundamental Research Grant Scheme (FRGS) Vot No.78220, and the

Universiti Kebangsaan Malaysia for providing the centrifuge testing facilities.

6. References

Craig, R.F. (2004), Soil Mechanics, E & FN Spon, London, United Kingdom.

Khatib, A. (2009). Bearing Capacity of Granular Soil Overlying Soft Clay Reinforced with

Bamboo-Geotextile Composite at the Interface. PhD Thesis, Universiti Teknologi

Malaysia, Skudai, Johor Bahru, Malaysia (unpublished).

Kim, B. (2003). Properties of Coal Ash Mixtures and Their Use in Highway Embankments.PhD Thesis, Purdue University, Indiana, USA (unpublished).




Mahmud, H.O. (2003). Coal - Fired Plant in Malaysia. The 15th

JAPAC International

Symposium 19 September 2003, Tokyo.

Marto, A., Mahir, A. M., Lee, F. W., Yap, S. L. and Muhardi (2009). Morphology,

Mineralogy and Physical Characteristics of Tanjung Bin Coal Ash. Proceedings of 4th

International Conference on Recent Advanced in Materials, Minerals and Environment

(RAMM) & 2nd

Asian Symposium on Material & Processing (ASMP), 1-3 June 2009,

Pulau Penang, Malaysia.

Muhardi, Marto, A., Kassim, K. A. and Craig, W. (2008). Geotechnical Centrifuge Physical

Model of Reinforced Clay Slope. International Graduate Conference on Engineering

(IGCES) 23-24 December 2008, Skudai, Johor, Malaysia.

Newson, T. and White, D. (2005). Modelling Geotechnical Problems in Soft Clays using the

Mini-Centrifuge. K.Y. Lo Symposium, 7-8 July 2005, The University of Western

Ontario, Canada.

Taylor, R.N. (1995), Geotechnical Centrifuge Technology, Blackie Academic, London,

United Kingdom.

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Paper for SIBE 09 - Muhardi