Compression behaviour of natural soils

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME
80
COMPRESSION BEHAVIOUR OF NATURAL SOILS
Nagendra Prasad.K1
, Manohara Reddy.R2
,
Chandra.B3
, Harsha Vardhan Reddy.M4
1
Professor, Dept. of Civil Engineering, SV University, Tirupati, India,
2
Research Scholar, Dept. of Civil Engineering, SV University, Tirupati, India.,
3
Post Graduate Student, Dept. of Civil Engineering, SV University, Tirupati, India.
4
Former Under Graduate student, Dept. of Civil Engineering, SV University, Tirupati, India.
ABSTRACT
Compressibility is an important characteristic feature of soils to evaluate magnitude of
deformation under a given loading. It is observed that compression behaviour of natural soils
is characterized initially by rigid response and rapid compression at greater stress levels.
Marked breaking point is noticed at the point of transition which is usually termed as yield
stress. Compression is relatively low if the applied stress level is within the yield point and
the stress levels are noticeable when the applied stress level is greater than the yield value.
Initial and final slopes of the compression curve represent the sample disturbance which is an
important parameter to estimate the quality of sample obtained in the field. Availability of
reliable engineering parameters for geotechnical design depends on careful testing. Testing
may be carried out in the laboratory or in the field, but in either case the most important
factor controlling the quality of the end result is likely to be the avoidance of soil disturbance.
The present paper deals with determining the compression index before yield and post yield
for evaluating settlements as also to quantify the degree of sample disturbance.
Keywords: Residual soil, compression index, consolidation, sample disturbance.
1. INTRODUCTION
Compressibility characteristics of natural soils are often the most important
parameters for settlement evaluation. The compression behaviour of a natural clay can be
classified into three regimes: the pre-yield regime characterised by small compressibility up
to the consolidation yield stress with soil structure restraining the deformation; the
transitional regime with gradual loss of soil structure when the effective stress is between the
INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 4, Issue 3, May - June (2013), pp. 80-91
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81
consolidation yield stress and the transitional stress; and the post-transitional regime
characterised by the same change law in compression behaviour as a reconstituted clay when
the effective stress is higher than the transitional stress. Pre and post yield slopes of the
compression curve represent the sample disturbance which is an important parameter to
estimate the quality of sample obtained in the field.
Sample disturbance is the most significant issue affecting the quality and reliability of
laboratory test data. All key design parameters such as compressibility, yield stress and un-
drained shear strength are adversely influenced by sample disturbance. A carefully planned
experimental investigation has been carried out on soil samples extracted from different
depths from various locations of Tirupati region. The properties of these soil samples
represent wide spectrum of soils normally encountered in this region. One-dimensional
compression test have been conducted apart from classification and identification tests. Based
on test results, a framework for analyzing the compression index at pre and post yield and
also degree of sample disturbance has been developed.
2. BACKGROUND INFORMATION
Researches were carried out to characterize the engineering properties of residual
soils (Hight & Leroueil, 2002), to investigate the effects of soil structure on the engineering
properties and analyse the compressibility behaviour (Nagaraj et al., 1998), and also to
evaluate the collapse behaviour of it (Rao & Revanasiddappa, 2006; Huat et al., 2008).
Sarma et al. (2008) observed that the consolidation properties of soils indicate an insight on
the compressibility behaviour of soils with associated expulsion of water. Abbasi et al. (2012)
brought out that the compressibility characteristics of fine-grained soils are often the most
important parameters for settlement evaluation. These characteristics are usually described
using two well-known coefficients: the compression index, Cc and the coefficient of
consolidation, Cv.
Hong et al. (2012) observed that the natural clays generally have a compression curve
lying above that of reconstituted clays owing to the effect of soil structure. It has been
recognised that the soil structure restrains the deformation of natural clays under effective
vertical stress up to the consolidation yield stress, consequently resulting in the low
compressibility of clays until the stress level exceeds the consolidation yield stress
(Butterfield, 1979; Burland, 1990). The difference of void ratio between natural clays and
reconstituted clays at the same stress level often increases with increasing consolidation stress
up to the consolidation yield stress, but decreases when the applied stress level is larger than
the consolidation yield stress. Nagendra Prasad et al (2007) brought out sample disturbance
index, using merely the slopes of compression paths, (representing mechanical response), in
the pre- and post-yield stress regimes under odeometric loading conditions. However, there
appears to be a need to examine the possibility of analysing the test results of residual soils to
understand the compression response and the possibility of evolving sample disturbance for
comprehensive understanding of the behaviour under compression and its application to solve
a practical problem.

82
3. EXPERIMENTAL INVESTIGATION
3.1 Introduction
The study area lies to the extreme south of Andhra Pradesh state (India)
approximately between 12° 37' - 14° 80' north latitudes and 78° 30' - 79° 55' east longitudes.
Experimental investigations are carried out on tropical residual soils of Tirupati region.
3.2 Details of the Experimental Investigation
The present experimental investigation is carefully planned such that a framework for
analysis and assessment can be developed to understand the behavior of tropical residual
soils. The experimental program involves determination of the following aspects.
Basic properties of soils
Engineering properties of soil such as compressibility.
All the tests are conducted as per the relevant provisions stipulated in Bureau of Indian
Standards.
3.3 Soils Tested
The soils considered in the present investigation have been obtained from the
surroundings of Tirupati region. The details of locations of sampling are as follows:
1) Mullapudi (Village), Tiruchanur, Tirupati, Chittoor (District)
2) Yogimallavaram Residential Area, Tiruchanur, Tirupati, Chittoor (District)
3) Industrial Development Park, Gajula Mandyam, Renigunta, Chittoor (District)
4) Fire Station Building at Nagari, Chittoor (District)
5) Besides Thiruchanur by-pass road, Tiruchanur, Tirupati, Chittoor (District)
6) Renigunta Road, Near Hundai Show Room, Tirupati, Chittoor (District)
7 & 8) Nadavalur, Ramachandra Puram (Mandal), Tirupati, Chittoor (District).
3.4 Collection of Samples
Soil samples have been collected by exercising necessary care to see that the natural
constituents are represented and the same were transported to Geotechnical Engineering
laboratory. The samples were air dried and stored in air tight containers for use in rest of the
investigation.
3.5 Properties of Soils
3.5.1 Basic Properties of Soils
The properties of soils considered in the present investigation are presented in table 1.
It may be seen from the table that all the soils represent Clayey Sand (SC) excepting one
which is of Clay with Intermediate compressibility (CI). The grain size distribution curves for
soil samples are shown in figure 1. It may be noticed from the figure that the grain size
distribution curves are wide spread with fine fraction ranging from 30% to 70% and hence
the soils considered represent wide spectrum of soil samples normally found in this region.
The liquid limit values range from 47% to 85%. Further, the plastic index values range from
33% to 59 % covering wide spectrum of soils.

83
TABLE 1: SOIL PROPERTIES
Sl.
No:
Description
Values
Sample
1
Sample
2
Sample
3
Sample
4
Sample
5
Sample
6
Sample
7
Sampl
e 8
Depth at, m
2.0 2.8 2.1 3.0 2.4 2.3 1.7 2.5
1 Gravel (%) 1.4 1.4 7.90 10.15 9.1 7.6 0.3 1.3
2 Sand (%) 58.1 56.7 52.20 54.35 62.3 57.9 31.7 53
3 Silt+Clay (%) 40.5 41.9 39.85 35.50 28.6 34.5 68.0 45.7
4 0.425 mm Size (%) 67.0 58.8 61.65 42.50 38.71 42.9 90.7 67.6
5 Liquid Limit (%) 58.0 59.5 68.0 69.5 85 68.7 47.5 56.0
6 Plastic Limit (%) 15 20 14 24 26 23 14 16
7 Plasticity Index (%) 43.0 39.5 54.0 45.5 59 45.7 33.5 40.0
8 IS Classification SC SC SC SC SC SC CI SC
9 Free Swell Index (%) 40 90 35 110 320 25 30 50
10
Degree of Expansion Low Medium Low High
Very
High
Low Low
Mediu
m
11
Modified Liquid Limit,
WLm (%)
39.0 35.0 42.0 29.5 33 29.5 43 38
12
In-situ Density, γ
(kN/m3
)
18.67 20.15 17.48 20.07 20.87 20.48 19.17 17.63
13
Natural moisture
content, (%)
15.15 17.28 13.15 17.49 12.16 15.29 23.64 13.23
14 Dry density, γd (kN/m3
) 16.21 17.18 15.45 17.08 18.60 17.76 15.50 15.56
15 Initial void ratio, e0 0.646 0.553 0.727 0.562 0.435 0.503 0.722 0.715
16
Over burden pressure,σ0
(kN/m2
)
32.43 48.12 32.45 51.24 44.64 40.85 26.35 38.92
17
Shear Strength
Parameters
Angle of internal
friction, ф in degrees
7.48 9.55 25.08 12.99 27.92 7.02 4.19 7.38
Cohesion, C in kPa 42.24 73.16 31 44.95 26.5 46.56 58.66 49
18
Compression Index,
Cc (post yield)
0.117 0.170 0.162 0.149 0.219 0.083 0.113 0.231
Figure 1: Grain-size distribution curves for combination of all samples

84
3.5.2. Compressibility
Compressibility represents volume change behavior of soils under loading; it is one of
the important engineering properties of soil representing the magnitude of settlement under
unit increase in pressure. As the field compression most often takes places under one-
dimensional compression, oedometer tests have been conducted on soil samples under
consideration. Necessary care has been exercised to retain basic constituents of the material
and the in-situ density. Samples have been saturated under a normal stress of 5 kPa to attain
nearly the state of saturations.
Figure 2: One-dimensional consolidation test curves for combination of all samples
The compression behavior of all eight soil samples is presented in figure 2. It may be
noticed that the compression behavior depicts initially stiff response up to a certain normal
stress value and shows greater degree of compression beyond this stress value. The same
compression behavior is noticed with respect to all the soil samples tested.
4. ANALYSIS OF TEST RESULTS
4.1 Introduction
The usual object of detailed experimental investigation will be to propose a basic
framework for analysis of the observed behavior so that assessment of behavior would be
based on mechanistic approach. A detailed analysis of test results is presented in the
following section.
4.2 Normal Compression Line (NCL) as State Boundary Surface
An attempt has been made to examine the compression behavior with respect to
Normal Compression Line (NCL) for which the equation given by Nagaraj et al. (1994) as
reproduced below has been adopted.
Eq. (1)
Where,
e = Void ratio at a given pressure of v
’
eL = Void ratio corresponding to liquid limit
'
log276.023.1 v
Le
e
σ−=

85
Most of the natural soils have particle sizes ranging from 4.75mm to 2. The liquid
limit is normally determined from the fraction passing through 425. Since, the soils
considered in the present investigation have fractions passing through 425ranging from
38% to 90%, modified liquid limit is considered as given by the following equation:
Eq. (2)
Where,
wl = Liquid limit of the soil passing 425
wlm = Modified liquid limit for the total soil
F = Fraction of passing through 425
Accordingly Equation (1) is modified as:
Eq. (3)
Where,
elm = Void ratio corresponding to modified liquid limit (Wlm)
Using equation (3) the Normal Compression Line (NCL) for all the soil samples is
shown in figures 3-10. It may be noticed that the compression behavior of natural soils is
located on left hand side of Normal Compression line (NCL) and hence the behavior of
natural soils is akin to the compression behavior of over consolidated soils. It is also seen that
the compression behavior of natural soils merge with Normal Compression Line, after a
particular stress value confirming that Normal Compression Line (NCL) forms the state
boundary surface.
Figure: 3 Comparision between normal
compression line and natural compressrion
line for sample 1
line for sample 2
'
log276.023.1 v
lme
e
σ−=

86
line for sample 3
line for sample 4
line for sample 5
line for sample 6

87
line for sample 7
line for sample 8
4.3 Compression Moduli and Yield Stress
Compression curve of natural soils is characterized by relatively rigid response at
initial stress levels and by greater compression at higher stress levels. The change of slope in
the compaction curve is characterized by the yield. The stress corresponding to yield point is
termed as yield stress. Accordingly, an attempt has been made to determine the compression
modulus before the yield (Cc1) and compression modulus after the yield stress (Cc2) as shown
in figures 11-18.
Figure: 11 Relation between specific
volume and effective stress for sample 1

88
Figure: 13 Relation between specific volume
and effective stress for sample 3

89
The compression index before yield stress (Cc1) and the compression index after yield
stress (Cc2) are determined for all the soil samples. Here, the sample disturbance is defined
for the purpose of the analysis, as given by following expression
100
'
2
1
x
Cc
Cc
edisturbancSample =
Cc2 represent compression index of NCL which corresponds to 100% disturbance and Cc1
represent pre-yield for rigid response of the soil.
If, Cc1 = Cc2
The soil sample is completely remolded; the sample disturbance is 100%. The sample
disturbance values from the table 2 indicate that the samples are disturbed from 7% to 34% in
the present investigation. The test results indicate that the value of Cc1 is of the order of 1/3 to
1/14 of Cc2 value depending on state of soil. It turns out that the ratio of compression moduli
for natural residual soils is significantly different from normally consolidated soils whose
ratio varies from 1/3 to 1/5 (as indicated in Atkinson et al.,1978).
Table 2: Compression index at pre and post yield, yield stress and over burden pressure
values
Description
Soil
Compression
Index, Cc1
(pre-yield)
Compression
Index, Cc2
(post-yield)
Cc1/Cc2
Sample1 0.033 0.117 0.283
Sample2 0.015 0.17 0.09
Sample3 0.042 0.162 0.259
Sample4 0.01 0.149 0.069
Sample5 0.027 0.219 0.127
Sample6 0.011 0.083 0.139
Sample7 0.038 0.113 0.34
Sample8 0.056 0.231 0.245

90
5. CONCLUDING REMARKS
Based on detailed experimental investigation and analysis of test results the following
concluding remarks may be made.
• The compression behavior depicts initially stiff response up to a normal stress value
and shows greater degree of compression beyond this stress value. The same
compression behavior is noticed with respect to all the soil samples tested.
• The compression behavior of natural soils is located on left hand side of Normal
Compression line (NCL) and hence the behavior of natural soils is akin to the
compression behavior of over-consolidated soils.
• The compression behavior of natural soils merge with Normal Compression Line
after a particular stress values confirming that Normal Compression Line (NCL)
forms the state boundary surface.
• The compression curves of natural soils are characterized by relatively rigid response
at initial stress levels and by greater compression at higher stress levels.
• sample disturbance is defined as the ratio of compression indices at pre and post
yields, given by following expression
100
'
2
1
x
Cc
Cc
edisturbancSample =
• Sample disturbance in the present investigation ranges from 7% to 34%.
• The test results indicate that the value of Cc1 is of the order of 1/3 to 1/14 of Cc2 value
depending on state of soil. It turns out that the ratio of compression moduli for natural
residual soils is significantly different from normally consolidated soils whose ratio
varies from 1/3 to 1/5.
REFERENCES
[1] Abbasi, N., Javadi,A.A. & Bahramloo,R. (2012) “Prediction of Compression
Behaviour of Normally Consolidated Fine-Grained Soils”, World Applied Science
Journals 18 (1): 06-14, 2012. ISSN 1818-4952.
[2] Atkinson J.H and Bransby P.L (1978), “The Mechanics of Soils-An Introduction to
Critical State Soil Mechanics”, McGraw-Hill Book Company (UK) Limited.
[3] Burland, J. B. (1990). On the compressibility and shear strength of natural clays.
Géotechnique 40, No. 3, 329-378.
[4] Butterfield, R. (1979). A natural compression law for soils. Géotechnique 29, No. 4,
469-480.
[5] Hight, D. W. and Leroueil, S. (2002) "Behaviour and properties of natural soils and
soft rocks", Characterisation and Engineering Properties of Natural Soils, A.A.
Balkema, Publishers, United States, pp. 29-254.
[6] Hong, Z.S., Zeng, L.L., Cui,Y.J., Cai,Y.Q.& Lin,C. (2012) “Compression Behaviour
of Natural and Reconstituted Clays”, Author manuscript, published in "Géotechnique
62, 4 (2012) 291-301".
[7] Huat, B. B. K., Aziz, A. A., Ali, F. H. and Azmi, N. A. (2008) "Effect of wetting on
the collapsibility and shear strength of tropical residual soils", Electronic Journal of
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[8] Nagaraj, T. S., Prasad, K. N., Reddy, V. M. C. and Reddy, N. G. (1998) "Analysis of
residual tropical cemented soil behaviour", the geotechnics of hard soils - soft rocks.
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Italy, A.A.Balkema, 2, pp. 715-723.
[9] Nagaraj,T.S. & Srinivasa Murthy B.R., Vatsala, A. (1994), Analysis and Prediction of
Soil Behavior, Wiley Eastern, New Delhi, p.294.
[10] Nagendra Prasad,K., Triveni,S., Schnaj,T. & Nagaraj,T.S.(2007) ‘Sample
Disturbances in Soft and Sensitive Clays: Analysis and Assessment’, Marine
Georesources and Geotechnology, 25:181–197, 2007
[11] Rao, S. M. and Revanasiddappa, K. (2006) "Influence of cyclic wetting drying on
collapse behaviour of compacted residual soil", Geotechnical and Geological
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[13] Sarma, M.D. & D. Sarma,D. (2008) “Prediction of Consolidation Properties of
Partially Saturated Clays” The 12th
International Conference of International
Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6
October, 2008 Goa, India
[14] Ch. Sudha Rani and K Mallikarjuna Rao, “Compositional and Environmental Factors
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6308, ISSN Online: 0976 – 6316.
[15] Nagendra Prasad.K, Sivaramulu Naidu.D, Harsha Vardhan Reddy. M and Chandra.B,
“Framework for Assessment of Shear Strength Parameters of Residual Tropical
Soils”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4,
Issue 2, 2013, pp. 189 - 207, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
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