1. 1
Effect of gradation and sample size on compression index of
coarse grained clayey soils
Samira Nematzadeh1, Masoud Hajialilue Bonab2, Hamed Vafaei Molamahmood3
1- Master Student, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran
2- Professor of Geotechnical Engineering, University of Tabriz, Tabriz, Iran
3- Master of Geotechnical Engineering, Tabriz, Iran
Samira.nematzadeh70@gmail.com
Abstract
To identify the consolidation characteristics of fine-grained soils, the Oedometer test is usually used in
the laboratory. However, for coarse grained clayey soils, the Odeometer test could not be utilized because
of its small mold size. For these type of soils, a simple approach is to use the grade modification
techniques such as replacement and scalped methods that yields to an elimination of a large amount of the
soil, which causes an alteration in grain pattern. Another approach is to utilize Rowe consolidation cell,
which is not available in all soil laboratories. The Rowe consolidation cell provides the possibility of
controlling drainage pathways and recording pore pressure during the consolidation for specimens of
large diameter. In the present study, the consolidation test was performed on grade modified coarse
grained clayey soils by using Oedometer test, and the results were compared with the consolidation tests
carried out on base specimen soil by using Rowe consolidation cell apparatus. The results showed that the
value of compression index increases by performing modified gradation on samples. Besides, the
replacement technique indicates the closest results with the base soil specimen; therefore, it is more
reliable to determine the compression index of the coarse grained clayey soils by using this technique.
Keywords: Compression index, Coarse grained clayey soil, Gradation, Sample size, Rowe
consolidation cell.
1. INTRODUCTION
An appropriate estimation of compressibility of the soil layers due to excessive loading stresses is one of the
most important geotechnical issues. The compressibility of the soil is highly dependent to its characteristics
and the stress history of that. This behavior, which is different for each soil, is shown with a coefficient titled
compression index (Cc). It is mainly the gradient of the changes on void ratios (e) over alteration on effective
stress in logarithmic scale (compression curve). This coefficient is being determined by performing
consolidation tests and analysis of their results [1].
One of the main issues that is being faced in determining consolidation parameters of the coarse
grained clayey soils is that the conventional Oedometer test, which has a very small mold, is being used to in
most of the soil mechanic laboratories. Since the mold of this apparatus is very small, the grain size of the
coarse grained soils should be modified. As a result, a large part of the soil (coarse grained portion) is being
eliminated, and the results of the Oedometer test varies from the results of the in-situ base soil specimen.
The Rowe consolidation cell, which was developed by Rowe & Barden in 1966 at University of
Manchester [2], has been overcome the disadvantage of the conventional Oedometer apparatus. By
employing this apparatus, the coarse grained soils could be tested easily and the drainage pathways during
the consolidation could be controlled. Moreover, it is possible to apply back pressure and log the pore
pressure during consolidation with this apparatus [3].
The specimens of large diameter, first, was utilized by Rowe in 1968. Then, Barden & Lee
McGowan in 1974 performed their experiments on specimens with 150 mm of diameter. They concluded that
the usage of specimens with larger diameter yielded to more reliable results [2].
In the present study, the Rowe consolidation cell was employed to assess the consolidation behavior
and compression index of the coarse grained clayey soils. The soil samples were modified by using
techniques of replacement and scalped and tested by using Oedometer. Finally, the results of Rowe
consolidation cell and Oedometer test was compared and the effect of grade modification and mold size was
investigated and presented.

2. 2
2. MATERIALS AND METHODS
In order to fulfill predetermined goals of the research, the consolidation tests were conducted on two type of
coarse grained clayey soil. Then, the base specimens were grade modified by using replacement and scalped
methods and they were tested by Oedometer apparatus.
2.1. UTILIZED MATERIALS
In this research, the soil samples were prepared manually by mixing the coarse grained with kaolinite and
bentonite soils. The coarse grained soil was obtained from the vicinity of the station 2 of Tabriz subway. The
kaolinite and the bentonite was also purchased from Iran china Clay Industries Company, Iran, and Silica
Sand MFG Company, Iran, respectively. Figure 1 shows the grain size distribution curve of the used mixture.
Figure 1. Used soil materials in preparation of samples.
2.2. GRADE MODIFICATION
In this research, the aforementioned soil types were combined to form two type of coarse grained clayey
samples. In addition, the created soil samples were grade modified by using replacement or scalped
techniques, therefore, 4 type of grade modified samples were created along with the initial samples. The
information of the prepared specimens along with the associated consolidation test have presented in Table 1.
Since the maximum particle size of the sample in consolidation test should be less than the 1/6th of
the compaction mold height [4], the maximum particle size was determined as 8.33 and 3.28 mm for
hydraulic consolidation and Oedometer tests respectively. However, by comparing these values with the
available sieves, the sieve number 3/8 inch (9.5 mm) and 8 inch (2.36 mm) were chosen.
The grade modification could be approached by using replacement or scalped techniques [5]. In
replacement method, the base soil is being sieved from sieve number 8, and the amount of the additional soil,
which is over this sieve, is being replaced by a sandy soil, which has the particle size between sieve numbers
of 200 to 8. In scalped method, however, the amount of surplus soil over the sieve number 8 is being
removed from the sample to prepare the new sample.
2.3. EXPERIMENTAL SETUP
Prior to performing any sort of consolidation test, the general characteristics of each sample was determined
by using performing several tests. The values of liquid limit, plastic limit and plasticity index were calculated
by using the testing method of ASTM D4318 and the value of specific gravity was obtained by using ASTM
D854. Additionally, the grain size analysis and the soil classification were carried out based on ASTM D422
and ASTM D2487 respectively. Ultimately, the values of optimum water content and maximum dry density
was acquired by performing compaction test on the samples considering ASTM D698. Table 2 presents the
information regarding general characteristics of the tested samples.

3. 3
Table 1. General characteristics of the prepared samples.
Testing
Apparatus
CharacteristicsSamples
Rowe CellBase soil type (15% Bentonite + 30% Kaolinite + 55% sand and gravel)A
OedometerThe grade modified sample of soil type A by using scalped gradation methodA-I
OedometerThe grade modified sample of soil type A by using replacement gradation methodA-II
Rowe CellBase soil type (25% Bentonite + 20% Kaolinite + 55% sand and gravel)B
OedometerThe grade modified sample of soil type B by using scalped gradation methodB-I
OedometerThe grade modified sample of soil type B by using replacement gradation methodB-II
Table 2. General properties of tested samples
Properties
Samples
A A-I A-II B B-I B-II
Maximum particle size (mm) 9.5 2.36 2.36 9.5 2.36 2.36
Gs 2.75 2.66 2.7 2.72 2.64 2.67
LL (%) 48 48 36 78 78 64
PL (%) 27 27 23 46 46 41
PI (%) 21 21 13 32 32 23
Passing percent through sieve #
200(%)
35 57.4 35 39 63.94 39
Maximum dry density (gr/cm3) 1.99 1.82 1.91 1.96 1.77 1.89
Optimum moisture content (%) 10.75 13.6 11.7 11.9 14.87 10.97
Soil type (USCS) GC CL SC GC CH SC
Furthermore, the Oedometer test was conducted based on ASTM D2435. The height of the sample
mold was 1.97 cm and the inner diameter of it was 6.29 cm. The specimens were tested by using the
Oedometer apparatus by applying vertical stresses of 5,25, 50, 100, 200, 400 and 800 kPa followed by
unloading up to 400 and 5 kPa.
The Rowe consolidation test was performed based on BS 1377-6 and Head (1985) by using the ELE
apparatus (Fig. 2). The shape of the sample mold was cylindrical, and the height and diameter of it was 5 and
15.1 cm respectively. The samples were molded by using their optimum moisture content and the associated
maximum dry density for each sample. Since the vertical double drainage condition provides the closest
results to the Oedometer test, all samples were tested in in this condition with equal strain. The samples were
saturated by incrementing back pressure, and the consolidation stage was achieved by opening the drainage
outlet [6]. Additionally, the vertical load was applied to the specimens by using the hydraulic load system in
which water pressure acts on a flexible diaphragm, and the values of them was set to be 5,25, 50, 100, 200,
400 and 800 kPa followed by unloading up to 400 and 5 kPa.
Figure 2. General arrangement of ancillary equipment used with the Rowe consolidation cell: (a) water de-airing
system, (b) pressure generator, and (c) data logger.

4. 4
3. RESULTS AND DISCUSSION
Figure 3 plots the variation of void ratio (e) versus effective stress for soil type of B, B-I and B-II. It could be
understood that the soil type of B-I, which is the grade modified version of soil type B by using scalped
method, has the most alteration in void ratio in both loading and unloading phases. For example, the value of
void ratio for this soil type has been decreased from 0.97 to 0.58in loading phase, while, for the soil type of
B-II this value has been decreased from 0.75 to 0.50, and for the soil type of B this value has been declined
from 0.65 to 0.45. The same procedure could be mentioned for the unloading phase. The reason for this trend
could be attributed to the elimination of coarse grained materials form samples. By eliminating the coarse
grained part, the portion of fine grained soil increases. Moreover, another result could be ascribed from this
figure; the alteration trend of B-II sample is very close to the base sample of B in both loading and unloading
phases. The reason for this could be the replacement of coarse grained part of soil with sandy soil, which has
a larger grain size in comparison to the B-I sample.
Fig. 3. e-log p diagram for soil type of B, B-I and B-II.
In order to determine compression index, the slope of e-log p on line NCL (Normal Consolidation
Line) curve is being utilized based on Eq. (1).
2 1
2
1
( )
log
( )
c
e e
C
p
p
−
= (1)
Where Cc is compression index, e is void ratio, p is effective stress and the subscripts 1 and 2 denote
two arbitrarily selected points on the NCL.
The results for compression index has been provided in Table 3. By emphasizing on the results
presented in this table, it could be deduced that the values of compression index for base soil specimens are
less than corresponding values for grade modified samples. As an instance, the value of compression index
for soil type B in loading stage of 25 KPa is 0.0148, which is less than the corresponding values for B-I
(0.06) and B-II (0.0599) samples. The reason for this phenomenon could be ascribed to the amount of coarse
grained materials in the samples. In grain modified samples the amount of coarse grained materials is less
than the base soil specimen, which yields to the larger values in comparison to the base soil specimen.
Fig. 4. Plots the compression index values presented in Table 3. This figure shows the general trend of
compression index alteration in tested samples. The results show that compression index increases with an
increase in effective stress for all samples. Besides, grade modified samples by using replacement method fits
better with the base soil specimens. Additionally, it is obvious that the associate values of compression index
for soil type of B is more than the values of soil type A. The reason could be the higher amount of Bentonite
(more fine grained soil) in soil type of B.

5. 5
Table 3. Compression Index for tested samples.
Compression Index (Cc)
Loading
Stage
Sample ASample B
AA-IIA-IBB-IIB-I
0.0170.030.03760.01480.05990.0625
0.018630.0890.09850.03470.15180.121150
0.0330.11840.14930.09710.17960.2455100
0.050.15570.19250.09730.20910.3549200
0.06990.16270.21970.19380.21810.3647400
0.10050.16370.22850.20680.19690.3777800
Fig. 4. Plot of compression index versus effective stress.
4. CONCLUSION
In this research the following conclusion were obtained based on the presented results and discussions:
- By grade modifying the soil samples and decreasing the sample size, the value of compression index
increases. Generally, an increase in compression index in replacement method is less than scalped method.
- the replacement techniques showed a better result in comparison to the scalped technique; therefore, it
could be mentioned as more reliable method for determining compression index by using grade modification
method for coarse grained clayey soils with Odeometer in comparison to other methods.
- With an increase in bentonite content in tested samples, the values of compression index increases.
11. REFERENCES
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Soils”. Journals of Agricultural Engineering Research, 2(24).
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Horizontal Permeability for Fine Grained Soils by Using Rowe Consolidation Cell”. 5th national congress
on civil engineering, Mashhad, Iran. (In Persian).
3. Rowe, P. W. and Barden, L. (1966), “A New Consolidation Cell”, Geotechniqu, Vol.16, No.2, pp.162-170.

6. 6
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