Structural stability and mechanical strength of salt-affected soils

251 Pages·2016·13.6 MB·English

by  Abdolrahman Barzegar

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STRUCTURAL STABILITY AND MECHANICAL
STRENGTH OF SALT.AFFECTED SOILS
Thesis submitted by
ABDOLRAHMAN BARZEGAR
to
The University of Adelaide
Faculty of Agricultural and Natural Resource Sciences
for the degree of
Doctor of Philosophy
Department of Soil Science,
Waite Agricultural Research Institute'
The University of Adelaide, Australia
November 1995
TABLE OF CONTENTS
Page No.
ABSTRACT.. ...... vi
DECLARATION
"""'v111
ACKNOWLEDGMENTS .........
rx
LIST OF FIGURES .....x
LIST oF TABLES ""x'r
LIST OF PLATES .... xv
LIST OF PUBLICATIONS .....xvi
1 INTRODUCTION
CHAPTER ...........1
CHAPTER 2 REVIEW OF LITERATURE... ........4
2.T SALT.AFFECTED SOILS .4
STRUCTURE.. ........5
2.2 SOIL
2.2-I Introduction 5
2.z.z¿seedbed and Soil Structure. 7
2-2.3' Aggregate Hierarchy .. 8
2.2.4Formation of Soil Structure ... 11
2.2.4.1¡ Organic Irrtater t2
2.2.4.2:,ISotl Mellowing . .. L3
2-2.5 Soil Structural Deterioration 15
2.2.5.1 Factors Influencing Slaking ..15
2.2.5.2 Swelling 16
2.2.5.3 y Dispersion....... t7
2.3 SOrL STRENGTH. ...---.-.20
2.3.1 Introduction 20
2.3.2 Tensile Strength and Its Relation to Other Mechanical Properties . 22
2.3.3 Clay Content .... 24
2.3.4 Type and Size of Clay Particles......... 25
2.3.5 Exchangeable Cations...... 26
ii
2.3.6 Cementing Agents.. . . . 26
2-3.7 Water Content and Cycles of Wetting and Drying. 26
2.3.8 Matter and Soil Strength 28
2.3.9 Hardening 28
2.4 REQUTREMENTS FOR RESEARCH .... 30
CHAPTER 3 EFFECTS OF EXCHANGEABLE CATIONS AND
DISPERSIBLE CLAY ON SOIL STRENGTH. .....32
3.1 INTRODUCTION .... 32
.32
3.2 MATERIALS and METHODS
..32
3.2.1 Soil Samples
..32
3.2.2 Clay Mineralogy
3.2.3 Atærberg Limits .. JJ
3.2.4 COLE .. ... 33
3.2.5 Other Measurements aJõJ
3 -2.6 Experimental Procedure 34
3.2.7 Tensile Strength 36
3.2.8 Dispersible Clay . 36
3.2.8. pontaneously Dispersible ClaY ......... 36
3.2.8. cally Dispersible Clay. .........37
3.2.8.3 Calibration Curve for Dispersed Clay ........37
3.2.9 Preparation of Scanning Electron Micrographs.--..- ""'""' 37
Analysis. ""37
3.2.10 Statistical
3.3 RESULTS ....37
3.3.1 Exchangeable Cations and Tensile Strength 31
4l
3.3.2 Sodicity, Dispersible Clay and Tensile Strength.-- -...------.------.'
DISCUSSION. .......45
3.4
3.5 CONCLUSIONS..... 46
CHAPTER 4 PROPERTIES OF CLAY FRACTION AND SOIL
STRENGTH. .......48
4.I INTRODUCTION . ". 48
MATERIALS. .......
4.2 METHODS AND 48
4.2.1 Soil Samples 48
4-2.2 Separation of Clay Fraction .... 49
4.2.3 Clay Mineralogy 49
iii
4.2.4 Sample Preparation.....-.......
4.2.5 Clay Particle Size..
4.2.6 Dispersible Clay .
4.2.7 Transmission Electron Microscopy .--..
5l
4.2.8 Specific Surt'ace Area.
4.2.9 Statistical Analyses... """"'- 52
4.3 RESULTS .-.. - -. 52
4.3.1 Soil Strength and Clay Content 52
4.3.2 Soil Strength and Clay Type.. 54
4.3.3 Number of Contact Points and Soil Strength 60
4.3.4 Dispersible CIay and Soil Strength 63
4.3.5 Principal Component Analyses of Data. 63
4.4 DISCUSSION. 66
4.s CONCLUSIONS..... .--..--
67
CHAPTER 5 EFFECTS OF SODICITY AND SALINITY ON
STRUCTURAL STABILITY AND SOIL STRENGTH ....... 68
TNTRODUCTTON .........
5.1 ' 68
METHODS..... ...
5.2 MATERIALS AND 69
5.2.1 Soils. .69
.69
5.2.2 Aggregate Preparation
5.2.3 Aggregate Stability
7l
5.2.4 Dispersible Clay .
5.2.5 Tensile Strength 7T
5.2.6 Bulk Density of the Aggregates 7l
5.2.7 Other Measurements 72
5.2.8 Experimental Design . 73
5.2.9 Statistical Analyses.. . 13
5.3 RESULTS 73
5.3.1 Aggregate Stability ... 73
5.3.2 Eft'ect of Salinity and Sodicity on Aggregate Stability . . - - -. -. - 74
5.3.3 Spontaneously Dispersible Clay as Related to Mean Weight
Diameter..... ..76
5.3.4 Soil Strength .... ..79
5.3.5 Aggregate Stability and Soil Suength 84
. .......
5.4 DTSCUSSION 87
tv
5.s coNcLUSroNS..... .....-.89
MELLOWING ........90
CHAPTER 6 SOIL
6.1 Mellowing of compacted soits and soil Aggregation as
Influenced by Ctay Type and Rate of Wetting .....90
6.1.1 INTRODUCTION . .... 90
6.1.2 METHODS AND MATERIALS .. 91
6.1.2.1 Soil Samples. 91
6.I.2.2 Sample Preparation 92
6.1.2.3 Aggregaæ Søbility. 92
6.I.2.4 Soil Compaction.... 93
6.I.2.5 Wetting and Drying Cycles 93
6.1.3. RESULTS 94
6.1.3.1 Aggregation and Clay Type. 94
6.I.3.2 Strength of Soil at-ter Wetting and Drying Cycles. 96
6.1.3.3 Mellowing Ratio....... 96
6.I.4 DISCUSSION. 101
6. 1.5 r CONCLUSIONS 102
in
6.2 Mellowing and Aggregate Hierarchy salt-Affected
Soils 103
6.2.1 INTRODUCTION ..... 103
6.2.2METHODS AND MATERIALS ... 104
6.2.2.1 Soil Samples. ... .. 104
6.2.2.2 Sample Preparation . .... 105
6.2.2.3 Aggregate Stability Measurements ...... .. ..... 105
6.2.2.4 Wetting and Drying Cycles and Strength ..... 106
6.2 3 RESULTS. .... .. 106
6.2.3.1 Soil Aggregation ..... 106
Mellowing.........
6.2.3.2 ----107
6.2.3.2.1 Sodium- and Calcium-Treated Soils....------......' 107
6.2.3.2.2'Equilibrated Soils. . ... 110
6.2.3.3 Rggreglte Hierarchy ...111
6.2.4 DISCUSSION. 113
6.2.5 CONCLUSIONS 113
CHAPTER 7 FORMATION OF WATER STABLE
AGGREGATES IN SODIC SOILS.... rt4
7.1 organic Matter, sodicity and clay Type: Influence on
Soil Aggregation. ""' II4
r14
7.I.1 INTRODUCTION
,7.I.}MWHODS AND MATERIALS ... tL4
1.2.1 Experiment I SoiI Mixtures. . . . . .... .. 115
I.2-2 Experiment 2 Natural Soil Aggregates-....--.-.-. ........ 1 16
.......tr7
7.1 RESULTS
1.3.1 I Soil Mixtures....... ....__.rr7
7.r.3 riment 2 Natural Soil Aggregates'...--- ----------------I2I
7.r.4 I SSION..-.. ""'r24
7.r.5 ONCLUSIONS. ...126
7.2 Decom tion of Added Organic Matter as Influenced
by Sodicity and CIay TYP"..... tzl
7.2.1 INTRODUCTION ....
7.2.2 MATERIALS AND METHODS ..
7t2 2.I Analytical Methods....
1.2 3k',RESULTS.. r28
7\I2 3.1 ....... 128
7.2.3.2 ay Type .. ..... 133
(
7.2.3. Sodicity .......133
...... .r37
7.2.4 D SSION.
7.2.5 CONCLUSIONS .......139
CHAPTER 8 GENERAL DISCUSSION AND CONCLUSIONS. I+O
TNTRODUCTTON ...
8.1 140
8.2 MECHANICAL STRENGTH.. ..... .. 140
8.3 STRUCTURAL STABILITY . . . ... . .. 143
8.4 HIERARCHICAL ORDERS OF AMELIORATION t44
REFERENCES .. r47
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ABSTRACT
This thesis ourlines the factors at'tecting both soil strength and st¡uctural stability and
their interrelationship in salt-affected soils. Salt-affected soils comprise about 33Vo oÏthe
porenrial arable lands which are mainly found in arid and semi-arid regions of the world.
The dominant clay minerals in these legions are usually montmorilloniæ and illite.
The literature on soil strength, soil structure and dispersible clay, with emphasis on
sodic and saline soils, is reviewed. The objectives of this study were to investigate:
(i) the intluence of clay particles on soil densification and mellowing,
(ii) the mellowing of compacted soils and soil aggregates as influenced by
solution composition,
(iii) the disaggregation of soils subjected to diftèrent sodicities and salinities and
its relationship to soil strength and dispersible clay, and
(iv) the etï'ect of organic matter and clay type on aggregation of salt-at1ècted
soiIs.
Soil strength was intluenced by the type of exchangeable cations, natule of clay, clay
size, clay content and amounts of either spontaneously or mechanically dispersible clay-
Soil densitjcation increased with clay type; smectites > illites > kaolinites. The results
also indicated that not only clay type but also the size distribution of the clay fiaction
atTected soil strength. The importance of the number of contact points in enhancing soil
srrength was contirmed experimentally. Highly significant relationships were obtained
between either CEC of clay particles or surt'ace area of spontaneously dispersible clay and
tensile strength of sodic soils.
Soil strength and aggregation of compacted soils as tunctions of clay type, rate of
wetting and solution compositions were investigated. The results showed that soils with
high capacity to shrink and swell exhibited higher soil strength before the soils were
subjected to a number of wetting and drying cycles. However, soil aggregation and soil
mellowing increased with increasing shrink-swell potential. Mellowing of both
compacted soils and soil aggregates was aft-ected by solution composition and shlink-
swell porential of soils. The higher the soil shrink-swell potential and electrical
conductivity of solutions the lower was the soil mellowing afier cycles of wetting and
drying. It is suggested, therefore, that the use of natural processes, e.g. cycles of wetting
and drying, to alleviate of soil compaction is not applicable under sodic conditions. The
conceptual theory of aggregate hierarchy was also examined in these experiments- The
hierarchical orders of soil structure in salt-soils was shown to be related to solution
composition and shrink-swell potential of soils. The results indicated no evidence of
aggregate hierarchy in sodic soils.
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The concept of number of contact points intluencing soil strength and the importance
of clay and organic matter were further investigated at both micro- and macroaggregate
scale. An inverse relationship between soil structural stability expressed as mean weight
diameter (MWD) and amount of spontaneously dispersible clay was obtained- The
sensitivity of soil structure to sodicity decreased as the amounts of organic matter content
increased. The amounts of dispersible clay from dry aggregates were higher than from
aggregates wetted under 2kPa suction. Dispersive breakdown of aggregates of sodic
soils occurred irrespective of the rate of wetting. Soil strength was inversely related to
structural stability of soils; the slope of the relationship being dependent on the amounts
of organic matter present. Analyses of the data showed the most important factor
atTècting soil strength was suspended clay particles after wet sieving, tbllowed by MWD.
Furthermore, the results of this experiment showed that soil strength was aff-ected not
only by sodicity bur also by salinity. The relationship between tensile strength and
sodium adsorption ratio was nearly linear and parallel for soil water electrical
conductivities of 0.1 and 4 dS m-1. A discriminant line to separate hardened and
mellowed soils in relation to sodicity and salinity was also obtained.
The process of soil aggregation as intfuenced by sodicity, clay type and organic
matter was studied in further experiments. Addition of organic mâtter caused
sponraneously dispersible clay to decrease with time of incubation. Mechanically
dispersible clay increased after 7 days of incubation and then decreased atier 67 days of
incubation compared to the soils without organic matter addition. The higher the shrink-
swell potential of soils the greater the sensitivity of soils to sodicity. Organic matter had a
positive influence on soil aggregation irrespective of clay type or sodicity in the range of
ESp 2.6 to 36. Analysis by 13ç NMR indicated that microbial products, polysaccharide
mucilages, peptides and aliphatic compounds were responsible for improvement in
aggregate stability of sodic soils.
In the tìnal chapter, the results are integrated into a comprehensive hypothesis and a
strategy fbr management of salt-aff-ected soils using an hierarchical order of ameliorative
treatments is discussed. Future research needs are also hightighted.
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DECLARATION
I declare that this thesis contains no material which has been accepted tbr the award
of any other degree or diploma in any university. To the best of author's knowledge and
belief, this thesis contains no material previously published or written by another person,
except where due reference is made in the æxt of the thesis-
I give consent to this copy of my thesis, when deposited in the University Library,
being available for loan and photocopying.
IGNBD: DATE s
S
A. Rahman Barzegar
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ACKNOWLEDGMENTS
I am much indebted to my supervisors, Professor J.M. Oades, Drs. P. Rengasamy
and R.S. Murray. They have provided helpful and invaluable advice, support and
direction throughout my candidature. Special appreciation goes to Dr. G.J. Churchman
for his guidance during the first stage of my project.
I also would like to thank other members of the staff of the Department of Soil
Science particularly Drs. D.G. Lewis, A.M. Alston, Professor S.E. Smith, Drs- D.J-
Chittleborough, C.D. Grant, Mr. P.N. Nelson , Dr. C. Mao for their commenls and to
Mr. D. Iandiorio, Mr. C.M. Rivers, Mr. J.F. Davey, Mrs. H. Taylor, Mrs. J. Ditchfield,
Mrs. A.G. Waters and Dr. P. Clarke for providing technical assistance. My thanks also
exrend to Ms L. Giles and Ms R. Middelberg (Biometry Section of the Department of
Plant Science) for help in the statistical analyses of the data-
I thank members of CSIRO Division of Soils, Adelaide, Dr. W.W. Emerson for his
invaluable comments, Mr. S.G. McClure for SEM, Mr D. Wiseman for clay particle size
analysis and Mr. G. Riley for X-ray diffraction.
The work reported here was futly funded by the Iranian Ministry of Culture and
Higher Educaúon and The University of Ahwaz,Iran.
Finally and most importantly, completion of this work would not have been
possible without the constant understanding, support and encouragement of my parents,
my wife, Mehin, and children,Zahra, Maryam, Reza and Bahareh.