Geotechnical Properties of Soil-Ball Milled Soil
Mixtures
M.R. Taha1
Abstract. Laboratory experiments were conducted to study the fundamental
geotechnical properties of mixtures of natural soils and its product after ball milling operation. The product after ball milling process is termed nano-soil herein.
SEM analysis showed that much more nano size particles were obtained after the
milling process. Testing and comparison of the properties of original kaolinite,
montmorillonite and UKM soil with regard to its liquid limit, plastic limit, plasticity index, and specific surface and after addition of its nano-soil were also conducted. Laboratory tests results showed that the values of liquid limit and plastic
limits were higher after nano-soil addition. However, its plasticity index reduces
which is advantageous in many geotechnical constructions. Compressive strength
of original soil-cement-1% nano-soil mixture showed almost double its value
without nano-soil. It demonstrated that a small amount of these crushed particles
or nano-soil can provide significant improvement in the geotechnical properties of
soil. Thus, nanoparticles are potentially suitable for improving the properties of
soil/clay for various applications.
1 Introduction
As the demand for land for development purposes increases, soil improvement
techniques are sought after to convert land in which construction are impossible,
such as soft clays and peat into one where structures can be built. One of the most
common techniques is soil stabilization with additives or admixtures. In this technique the geotechnical properties of soil is enhanced by mixing it with another
material. The additives that have been used in the past include cement, lime, calcium chloride, fly ash, bitumen, etc. In addition, especially for soft clays and peaty
deposits which are composed of fine particles with high moisture content, granular
materials such as sand and industry by-products have also been used. Mixing these
M.R. Taha
Universiti Kebangsaan, Malaysia
e-mail: [email protected]
http://pkukmweb.ukm.my/~jkas/webjkas/cv/ProfRaihan.html
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M.R. Taha
additives in the ground served the following purposes [1]: strength increase, deformability/settlement reduction, volumetric stability (control of shrinking and
swelling), reduction of permeability, reducing erodibility, increasing durability,
and control of variability.
The use of additives is advantageous because one of the fundamental requirement for civil engineering projects is that it must be inexpensive. At times, land to
be improved covers a huge area and also involving large distances (for example
highways). Thus, the need to keep the project economical calls for low cost materials. In order to meet this requirement, industrial by-products or waste have also
been extensively studied. However, these raised another important question, i.e.
toxicity of the materials. Leaching of toxic chemicals to the environment posed
health related issues and is therefore another basic requirement for candidate materials for soil improvement.
In this study, experiments were conducted to evaluate the behavior of soil upon
mixing with nano-soil. The nano-soil is actually a product of milling of a natural
soil in which a greater portion of its particles was pulverized into nano sizes
(1-100 nm). The use of such materials almost entirely eliminates the toxicity questions, as usually raised for nanomaterials applications, since the material o be used
is originally a natural soil. Much study has been conducted and documented on the
use of nanoparticles or nanotechnology in conceivably all fields of knowledge except geotechnical engineering. Although it is possible to speculate the general
properties of soil when nanoparticles are added but real data are lacking due to inexistence of such research. This study could also pave the way to more extensive
research on nanotechnology and nanoparticles in geotechnical engineering.
2 Materials and Methods
The soils studied and involved in this study include the following:
i.
ii.
Metasediment (a sedimentary residual soil) obtained from a site within the
campus of Universiti Kebangsaan Malaysia (UKM) in which this type of soil
dominates its surficial geology. This soil is termed as UKM soil.
Commercial kaolinite and montmorillonite. These are clay minerals quite
commonly used in geotechnical engineering to represent in-situ clays.
In order to obtain the nano-soil, the soils were pulverized using a ball mill. The
soil specimens were loaded in small amounts in a ball mill of type “Planetary
Mono Mill Pulverisette 6” produced by Fritsch, Germany. The 20 balls used for
grinding have a diameter of 5 mm each and is made of sintered corundum.
The time taken for milling each batch is between 10-13 hours. In between the cyclonic milling operation, de-aired water was added to the bowl to prevent excessive
heating. The original soil and the soil material from the milling process were then
analyzed with an electron microscope at the UKM Electron Microscope Unit.
Geotechnical Properties of Soil-Ball Milled Soil Mixtures
379
Almost all basic geotechnical properties were mainly conducted in accordance
to BS 1377:1990 [2] in which the liquid limit was tested using the cone penetration test. The soil-nano soil mixtures were set at 98 % original soil and 2 % nanosoil by weight due to the very limited amount of the nano-soil obtained. The
specific surface determination was done using the ethylene glycol monoethyl ether
(EGME) method [3].
The compressive strength test was conducted by compacting the soil mixtures
in a mortar cube with dimensions of 50 x 50 mm. The control mixtures consist of
94 % UKM soil and 6 % Portland cement. In order to evaluate the effects of nanosoil on the strength of the soil-cement mixtures, 1 and 2 % nano-soil was added.
The specimens were compacted at the same energy and upon releasing it from the
mold, the specimens were cured for 7 day by covering it with a damp cloth and
spraying it daily with water. Finally, it was tested in a universal testing machine.
3 Results and Discussion
The SEM images of the original sample of kaolinite and the corresponding samples which were milled for a period of 10 to 13 hrs is shown in Fig. 1 (magnified
20,000 times). It generally shows that the milled samples have greater portion
Fig. 1 Kaolinite before (top ) and after (bottom) milling process
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M.R. Taha
nano size particles in which the 40-80 nm particles are clearly evident. This is actually the target of these exploratory experiments.
The Atterberg limits (ie. plastic and liquid limits) result which shows the
change in plasticity characteristics of the original soils and its mixtures mixtures
(98 % original and 2 % milled soil) are shown in Fig. 2. It generally shows that the
plastic limit (PL) and liquid limit (LL) increases upon mixing with the finer materials. However, since the increase in the LL is less that of PL, the plasticity index,
PI (PI=LL-PL) reduces for all samples. This is important in geotechnical engineering as compaction of high plasticity soils will generally result in high shrinkage
upon drying [4]. Eventually the soil will have high hydraulic conductivity which is
a disadvantage for structures such as landfill liners and caps. Thus, the addition of
finer particles such as nano-soil, even at low doses, could enhance the properties
of soils.
Fig. 2 Atterberg limits of soils and its mixtures (98% original soil dan 2 % nano-soil)
Specific surface of the soils evaluated using the EGME method is given in Table 1. It shows that the specific surface increases which explained the changes in
the plasticity characteristics as shown by the Atterberg limits. With the increase of
specific surface, the water required to cover the particles increases and hence increasing plastic and liquid limits.
The results from strength tests of UKM soil-cement-UKM nano-soil is shown
in Fig. 3. The addition of cement to soil is a common soil improvement technique.
As previously mentioned, the control mixtures consisted of 94 % UKM soil and 6
% Portland cement. In order to evaluate the effects of nano-soil on the strength of
Geotechnical Properties of Soil-Ball Milled Soil Mixtures
381
Table 1 Specific surface area of the soils
Kaolinite
Montmorillonite
2
Original
2
(m /g)
After
Original
milling
25.3
UKM soil
2
(m /g)
39.8
(m /g)
After
Original
milling
730.1
792.7
After
milling
2.4
3.9
the mixtures, 1 % and 2 % nano-soil was added. The results show significant improvement in the strength of the nano-soil mixtures over the control test albeit the
amount of nano soil added was minimal or rather quite low. The strength of the
soil-cement mixture almost doubled when 1 % UKM nano-soil was added. In addition, the mixture which is usually termed as cement-modified soil in geotechnical engineering will have lower tendency of volume change and PI in addition to
increase in load bearing capacity [5].
This study has demonstrated that even a small addition of nanoparticles will
show marked enhancement in soil behavior. It is also possible to engineer
nanoparticles to improve behavior of soils to suit design and practice requirements. This research is the first effort to introduce nanoparticles a soil improvement material. More intensive research is needed before the materials can be
utilized for its intended use in the field.
Fig. 3 Compressive strength of soil-cement-UKM nano-soil mixtures
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M.R. Taha
4 Conclusions
Nano-soil was used as a possible soil improvement material. It was obtained from
ball milling operations of a parent soil. From SEM analysis, it is evident that after
between 10 to 13 hours of cyclonic milling operations, there were more nano particles compared to the original soils. The plastic and liquid limits of soil mixtures
consisting of 98 % original soil and 2% nano-soil increases compared to the values of 100 % original soil. However, the plasticity index reduces which is advantageous in geotechnical construction such as landfill liners and caps. Mixing 96 %
UKM soil, 4 % cement and 2 % UKM nano-soil results in almost doubling the
compressive strength compared with samples without nano soil. Thus, nano-soil or
more generally nanoparticles is an excellent candidate material for soil improvement. However, research need to be intensified beyond this initial study before
real field applications can be realized.
References
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York (1990)
2. Taha, M.R., Lim, S.Y., Chik, Z.: Ciri-ciri asas beberapa tanah yang dikisar (Basic properties of milled soils). Jurnal Kejuruteraan (Engineering Journal) (2009) (accepted and
in-print)
3. Cerato, A.B., Lutenegger, A.J.: Determination of surface area of fine-grained soils by
the Ethylene Glycol Monoethyl Ether (EGME) method. Geotech. Test J. 25(3), 314–320
(2002)
4. Mitchell, J.K., Soga, K.: Fundamentals of soil behavior, 3rd edn. John Wiley & Sons,
New York (2005)
5. Winterkorn, H.F., Pamukcu, S.: Soil stabilization and grouting. In: Fang, H.Y. (ed.)
Foundation Engineering Handbook, 2nd edn. Van Nostrand Reinhold, New York (1991)