1. International Journal of Civil Engineering and CIVIL ENGINEERING AND6308
INTERNATIONAL JOURNAL OF Technology (IJCIET), ISSN 0976 –
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME
TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 4, Issue 6, November – December, pp. 61-66
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IJCIET
©IAEME
CORRELATION BETWEEN CALIFORNIA BEARING RATIO AND SHEAR
STRENGTH ON ARTIFICIALLY PREPARED SOILS WITH VARYING
PLASTICITY INDEX
Dr. K.V.KRISHNA REDDY*
*Professor, Civil Engineering Department, MVSR Engineering College, Hyderabad, A.P.
ABSTRACT
The present study is aimed at evaluating a correlation between the soaked California Bearing
ratio (CBRSoaked) and Undrained Shear Strength (Su) for artificially prepared soils with varying
Plasticity Index. The naturally existing soil normally referred to as Red Moorum is considered for
testing. The soil fraction passing 425 microns is replaced by red powder, clay powder and sand
powder derived essentially from fines of red moorum, clay of medium plasticity and sandy soil in
various percentages to obtain samples with varying Plasticity Index. The non linear correlation
obtained was validated with experimental results.
Key Words: California bearing ratio, Undrained Shear Strength, Plasticity Index
1. INTRODUCTION
California bearing ratio (CBR) of the subgrade soils is an important factor in design of
flexible pavements. CBR is a measure of resistance of material at controlled moisture content and
density. In areas of moderate to heavy rainfall, 4 day soaking of the sample is considered to
determine the CBR value (CBRSoaked). The CBR of the soil is evaluated as a ratio of the force per
unit area required to penetrate a soil mass with a standard circular piston to that required for the
corresponding penetration of a standard crushed stone and is essentially an empirical value
influenced by the soaking time, soaking method, and interpolation of the CBR curves.
The un-drained shear strength of the soil is evaluated from cohesion(C) and angle of internal
friction (Phi) and is essentially an engineering property. The ‘C’ and ‘phi’ of the soil are dependent
on the type and quantity of fines and granular soil fraction in any soil, which in turn affect the
performance of the soil when used as a subgrade and subbase material. Most soils used for road
construction in India contain varying types and percentages of fines resulting in varied Plasticity
Index values. In the present study an attempt is made to correlate the soaked CBR value with the undrained shear strength of artificially prepared soils with varying plasticity Index.
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2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME
2. LITERATURE REVIEW
Danistan Joseph, et. Al. have reported extensive work done on correlation between CBR and
index properties of CH CL and SC soils and have concluded with correlations which can be used on
the specific soil groups. Patel, et. Al. developed correlations between Dynamic Cone Penetration
results with Plate Bearing Test, Unconfined Compression Strength and California Bearing Ratio
results for soils occurring in Gujarat. Soewignjo Agus Nugroho, et. Al. reported correlations between
Index Properties and California Bearing Ratio Test of Pekanbaru Soils.
Literature review suggests that there are a number of correlations developed between the
CBR and different soil parameters, however most of them are soil, gradation and location specific
and cannot be confidently used to evaluate the unknown parameter as the type of soil and gradation
varies. In this context a need is felt to obtain a correlation between undrained shear strength and
soaked CBR values for use on soils with varying fines.
3. RESEARCH METHODOLOGY
3.1 Soil considered
Naturally occurring red moorum soil is considered for testing. Though the percentage finer
than 75 microns is considered as fines, to have an advantage of varying plasticity, the percent passing
425 microns is considered to vary the fines. Hereafter the fraction passing 425 microns for the red
moorum is referred to as red powder. Two other soil powders passing 425 microns of medium plastic
clay soil and sand are considered and are here after referred to as black powder and sand powder
respectively. Table 1 indicates the Grain size distribution of the soils considered for testing.
In this study soil samples with varying plasticity index are prepared by replacing the 425 micron
passing fraction of the natural soil with varying percentages of red powder, clay powder and sand
powder and also the coarse grained and fine grained fractions have been changed to evaluate a
correlation.
Table 1. Grain size distribution of soils considered for testing
Red soil
Black Powder
Sand powder
Size
% Retained
% Retained
% Retained
38.8
4.75mm
21
2mm
35.6
425 µ
2.8
17.5
83.0
75 µ
1.8
82.5
17
Pan
3.2 Sample preparation with varying Fines
As explained in the research methodology two sets of soil samples are prepared each with
subsets to get samples of varied plasticity index. The first set with all its subsets has the red moorum
soil retained on 425 microns at 70% and the soil fraction passing through 425 microns is varied (70%
coarse and 30% fines, fines being varied). The second set has 9 subsets, with the portion of red
moorum retained on 425 microns kept at 30% and the portion passing 425 microns is varied (30%
coarse and 70% fines, fines being varied). Table 2 shows the varying fines content for both set I and
set II samples.
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3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME
Table 2. Table showing Soil sample preparation by varying fines
Sand Powder
Red Powder
Black Powder
40%
60%
30%
70%
10%
90%
100%
75%
25%
50%
50%
25%
75%
100%
4.0 TESTING, RESULTS AND ANALYSIS
4.1 Testing
The Atterberg Limits of all the soil subsets are computed. Proctor standard compaction test
was conducted on all the soil subsets to determine the optimum moisture content and maximum dry
density. California Bearing ratio (CBR) test is conducted at the corresponding optimum moisture
content and maximum dry density for all the soil samples under 4 day soaked condition. Cohesion
(C) and angle of internal friction (Phi) were determined by direct shear test. The allowable
compressive strength in the subgrade was taken as 1.5kg/sqcm (Yoder) to evaluate the undrained
shear strength.
4.2 Results
Figure1 shows the CBR curves plotted between load Vs penetration for the set I soil samples
with 70% coarse and 30% fines, fines being varied as per Table 2. Figure 2 shows the CBR curves
plotted for the set II soil samples with 30% coarse and 70% fines, fines being varied as indicated in
Table 2. Figure 3 depicts the plot between Normal stress and Shear stress form the direct shear test
for set 1 soil samples and Figure 4 shows the results of direct shear test for set II soil samples. The
test results obtained and derived from the graphs for all the tests conducted are depicted in Tables 3
and 4 for set I and set II soil samples respectively.
load in KG
40% Sand P owder and 60% Red P owder
30% Sand P owder and 70% Red P owder
600
10% Sand P owder and 90% Red P owder
100% Red Powder
550
50% Red P owder and 50% Black P owder
25% Red P owder and 75% Black P owder
500
100% Black P owder
450
400
350
300
250
200
150
100
50
Penetration in mm
1
2
3
4
5
6
Figure 1. CBR curves for Set I soil samples with 70% coarse and 30% Fines
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4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME
Load in KG
40% Sand Powder and 60% Red Powder
10% Sand Powder and 70% Red Powder
100% Red Powder
75% Red Powder and 25% Black Powder
300
25% Red Powder and 75% Black Powder
250
200
150
100
50
Penetration in mm
1
2
3
4
5
Figure 2. CBR curves for Set II soil samples with 30% coarse and 70% Fines
Shear Stress (kg/sqcm)
40% Sand Powder and 60% Red Powder
30% Sand Powder and 70% Red Powder
0.45
10% Sand Powder and 90% Red Powder
100% Red Powder
50% Red Powder and 50% Black Powder
0.4
25% Red Powder and 75% Black Powder
100% Black Powder
0.35
0.3
0.25
0.2
0.15
0.1
0.05
Normnal Stress (kg/sqcm)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Figure 3. Mohr Envelopes for Set I soil samples with 70% coarse and 30% Fines
Shear stress (kg/sqcm)
40% Sand powder & 60% Red powder
10% Sand powder & 90% Red powder
100% Red powder
75% Red powder & 25% Black powder
f(x)=0.60645161*x+0.090043011; R²=0.9955
0.4
0.3
0.2
0.1
Normal Stress (kg/sqcm)
0.1
0.2
0.3
0.4
Figure 4. Mohr Envelopes for Set II soil samples with 30% coarse and 70% Fines
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5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME
Subset
1
2
3
4
5
6
7
Subset
1
2
3
4
5
Table 3. Results of Testing done on subsets of Set I soil samples
Red
Black
PI
C
Ph
Su
CBRSoaked
Sand
powder powder powder
%
Kg/sqcm
Deg
Kg/sqcm
%
40%
60%
8.8
0.03
40
1.289
24.6
30%
70%
9.6
0.034
40
1.293
23.7
10%
90%
13.2
0.044
39
1.259
22.0
100%
16.7
0.05
39
1.265
22.5
50%
50%
23.0
0.09
36
1.180
19.9
25%
75%
23.5
0.10
35
1.150
19.6
100%
28.8
0.12
33
1.094
14.4
Table 4. Results of Testing done on subsets of Set II soil samples
Sand
Red
Black
PI
C
Ph
Su
CBRSoaked
powder powder powder
%
Kg/sqcm
Deg
Kg/sqcm
%
40%
60%
8.8
0.10
28
0.898
11.2
10%
90%
13.2
0.13
26.5
0.878
9.0
100%
16.7
0.15
26
0.882
9.6
75%
25%
22.3
0.18
24
0.848
8.2
25%
75%
23.5
0.232
20
0.778
5.8
4.3 Analysis
Relation between CBR soaked and Su is obtained by plotting a graph as depicted in Figure5. A
polynomial correlation is found to better correlate the two parameters. The correlation obtained is
validated by the actual tests conducted on different soil samples. The validation details are presented
in Table 5.
Figure 5. Correlation between CBR soaked and Su
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6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME
Table 5. Validation of the correlation with test results
SET
Sand
powder
Red
powder
Black
powder
PI
C
Kg/
sqcm
Phi
Deg
Su
Kg/sqc
m
CBRSoaked
Actual
CBRSoaked
Correlation
%
variation
I
25
75
-
10.4
0.04
39.5
1.277
22.9
23.25
1.5%
II
-
50
50
23
0.23
24
0.897
9.7
9.82
1.2%
5.0 ACKNOWLEDGEMENT
At the outset the author would thank the Head, CED, MVSR Engineering College and SE
(R&B) for their valuable guidance and encouragement during experimentation.
6.0 CONCLUSION
1. The soil samples with varying plasticity Indices studied, has resulted in a polynomial
relationship between the soaked CBR and undrained shear strength.
2. The correlation obtained is CBR Soaked = 11.08Qu2 + 11.27 Qu - 9.201 with R- Squared value
of 0.983. The same is validated with test results and the variation was fond to be within 1.5%.
7.0 REFERENCES
[1]
Danistan Joseph, et Al. (2010), “Correlation Between California bearing Ratio and soil
parameters” , Proceedings of CIGMAT-2010, Conference and Exhibition.
[2] Dr. K.V.Krishna Reddy, “Stabilization of Medium Plastic Clays using Industrial Wastes”,
International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 3, 2013,
pp. 38 - 44, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
[3] Dr. K.V.Krishna Reddy, “Benefit Analysis of Subgrade and Surface Improvements in Flexible
Pavements”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4,
Issue 2, 2013, pp. 385 - 392, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
[4] Dr. K.V.Krishna Reddy, “Influence of Subgrade Condition on Rutting in Flexible Pavementsan Experimental Investigation”, International Journal of Civil Engineering & Technology
(IJCIET), Volume 4, Issue 3, 2013, pp. 30 - 37, ISSN Print: 0976 – 6308, ISSN Online:
0976 – 6316.
[5] Dr. K.V.Krishna Reddy and Mr.K.P.Reddy, “Maturity Period and Curing as Important Quality
Control Parameters for Lime Stabilized Clay Subgrades”, International Journal of Civil
Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp. 393 - 401, ISSN Print:
0976 – 6308, ISSN Online: 0976 – 6316.
[6] SoewignjoAgus NUGROHO, et. Al. (2012), “Correlation Between Index properties And
California bearing Ratio Test Of Pekanbaru Soils With And Without Soaked” Canadian
Journal on Environmental, Construction and Civil Engineering, Vol. 3, No. 1, pp. 7-17.
[7] Yoder, E.J. and Witczak, M.W. (1975) Principles of Pavement Design, 2nd edition, John
Wiley & Son, Inc, New York, USA.
[8] Dr. K.V.Krishna Reddy, “Rutting Resistance of Filler Modified Bituminous Concrete
Surfaces”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4,
Issue 2, 2013, pp. 250 - 257, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
[9] Patel, S.R. and Desai, M.D. 2010. CBR Predicted by Index Properties for Alluvial Soils of
South Gujarat, Dec. 16-18, 79-82, Proceedings of Indian Geotechnical Conference–2010,
India.
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