Reinforced soil structures, in which reinforcement is embedded in soil mass, have a number of distinct advantages over conventional retaining structures because of their ductility, high tolerance to differential settlement without structural distress, rapid method of construction, cost effectiveness, and adaptation to different site conditions. For effective performance of reinforced earth structures, the design practices limited the soil to be well-graded, free draining coarse-grained materials as backfills. If it is difficult to find the materials commonly specified in design guidelines, locally available soils which may be fine grained or low permeable might be used as backfills, then the behavior of reinforced fine-grained or poorly draining soils needs to be evaluated.
A series of standard compaction and unconsolidated-undrained (UU) triaxial compression tests were conducted to investigate the behavior of geotextile-reinforced clay. Effects of inclusion of nonwoven geotextile to clay were investigated and evaluated by varying the magnitude of confining pressures, and number of geotextile layers. The response of single layer reinforced clay with provision of thin layers of sand around the reinforcement (sandwich technique) to undrained loading was also examined. In sandwich technique, confining pressure and thickness of sand layer were all varied to quantify its effect on clay-geotextile interaction. The investigations were conducted on clay specimens prepared at their maximum dry unit weight and optimum moisture content, as well on sand specimens prepared at relative density of 70%. The mobilized tensile strain of reinforcements was estimated from the residual tensile strain using a digital image-processing technique.
The compaction test results indicated the benefit of reinforcing clay with geotextile to increase the dry unit weight of soil, which were determined for pure compacted soil between geotextile layers. The UU test results showed that geotextile reinforcement improves the response of clay by enhancing the peak shear strength, and reducing the loss of post-peak shear strength. Failure patterns were gradually changed from apparent classical failures for unreinforced soil specimens to bulging failures between adjacent layers (barrel-shaped) for geotextile-reinforced specimens without sand around the reinformcent. As the number of geotextile layers increased, failure patterns for reinforced specimens was getting uniform. This demonstrates that the increase in number of geotextile layers, the more constraint to later deformation of clay specimen. As the number of geotextile layers increases, the strength ratio increases and with increasing confining pressure, the peak strength ratio also decreases. Effectiveness of reinforcing clay without providing sand around reinforcement was due to the increase in the cohesion of the reinforced specimen. In addition, the mobilized tensile strain of reinforcement increases as the number of geotextile layers, confining pressure and showed direct proportionality to strength difference between reinforced and unreinforced soil.
In sandwich technique, the investigations revealed that thin layers of sand provided around the reinformcent could enhance the strength and deformation behavior of reinforced clay particularly under high confining pressure and large thickness. The failure patterns were bulging of reinforced clay specimen and the thin sand layers almost appeared undeformed. This provides an evidence that sand layer around reinforcement is effective in improving the interfacial shearing resistance with geotextile and clay, by penetrating in to it and consequently, the failure is forced away from sand-geotextile as well as clay-sand interface to clay. The increase in shear strength was due to an increase in the internal friction angle. Similar to reinforced clay without sand, in sandwiched specimen the mobilized reinforcement tensile strain increases with the thickness of sand layers. In addition, the strength differences increases with thickness of sand layers. The negative strength differences observed at smaller thickness of sand layers were attributed to the thickness of shear band. This implies that the thickness of sand layers around the reinforcement should be large than the estimated thickness of shear band.

Reinforced soil structures, in which reinforcement is embedded in soil mass, have a number of distinct advantages over conventional retaining structures because of their ductility, high tolerance to differential settlement without structural distress, rapid method of construction, cost effectiveness, and adaptation to different site conditions. For effective performance of reinforced earth structures, the design practices limited the soil to be well-graded, free draining coarse-grained materials as backfills. If it is difficult to find the materials commonly specified in design guidelines, locally available soils which may be fine grained or low permeable might be used as backfills, then the behavior of reinforced fine-grained or poorly draining soils needs to be evaluated.
A series of standard compaction and unconsolidated-undrained (UU) triaxial compression tests were conducted to investigate the behavior of geotextile-reinforced clay. Effects of inclusion of nonwoven geotextile to clay were investigated and evaluated by varying the magnitude of confining pressures, and number of geotextile layers. The response of single layer reinforced clay with provision of thin layers of sand around the reinforcement (sandwich technique) to undrained loading was also examined. In sandwich technique, confining pressure and thickness of sand layer were all varied to quantify its effect on clay-geotextile interaction. The investigations were conducted on clay specimens prepared at their maximum dry unit weight and optimum moisture content, as well on sand specimens prepared at relative density of 70%. The mobilized tensile strain of reinforcements was estimated from the residual tensile strain using a digital image-processing technique.
The compaction test results indicated the benefit of reinforcing clay with geotextile to increase the dry unit weight of soil, which were determined for pure compacted soil between geotextile layers. The UU test results showed that geotextile reinforcement improves the response of clay by enhancing the peak shear strength, and reducing the loss of post-peak shear strength. Failure patterns were gradually changed from apparent classical failures for unreinforced soil specimens to bulging failures between adjacent layers (barrel-shaped) for geotextile-reinforced specimens without sand around the reinformcent. As the number of geotextile layers increased, failure patterns for reinforced specimens was getting uniform. This demonstrates that the increase in number of geotextile layers, the more constraint to later deformation of clay specimen. As the number of geotextile layers increases, the strength ratio increases and with increasing confining pressure, the peak strength ratio also decreases. Effectiveness of reinforcing clay without providing sand around reinforcement was due to the increase in the cohesion of the reinforced specimen. In addition, the mobilized tensile strain of reinforcement increases as the number of geotextile layers, confining pressure and showed direct proportionality to strength difference between reinforced and unreinforced soil.
In sandwich technique, the investigations revealed that thin layers of sand provided around the reinformcent could enhance the strength and deformation behavior of reinforced clay particularly under high confining pressure and large thickness. The failure patterns were bulging of reinforced clay specimen and the thin sand layers almost appeared undeformed. This provides an evidence that sand layer around reinforcement is effective in improving the interfacial shearing resistance with geotextile and clay, by penetrating in to it and consequently, the failure is forced away from sand-geotextile as well as clay-sand interface to clay. The increase in shear strength was due to an increase in the internal friction angle. Similar to reinforced clay without sand, in sandwiched specimen the mobilized reinforcement tensile strain increases with the thickness of sand layers. In addition, the strength differences increases with thickness of sand layers. The negative strength differences observed at smaller thickness of sand layers were attributed to the thickness of shear band. This implies that the thickness of sand layers around the reinforcement should be large than the estimated thickness of shear band.

List of Tables……………………………………………………………....xiii List of Figures………………..…………………………………………......xv Notations…..…………………………………………………………….....xxi
CHAPTER 1
INTRODUCTION……………...……………...…………………………....1
1.1 Background..…………………………………………………………..1
1.2 Objectives of Research…………………....…………………….…….4
1.3 Thesis Organization........……………..….……………………………5
CHAPTER 2
LITERATURE REVIEW……………………...…………………...…… ..9
2.1 Introduction….…………………………..……………………………9
2.2 Mechanisms of Reinforced Soil………………...……………….… ..9
2.2.1 Apparent Cohesion Concept….……………………………..11
2.2.2 The Enhanced Confining Pressure Concept………………..14
2.3 Behavior of Reinforced Specimen Observed in Triaxial
Compression Tests…………………………………………………..16
2.3.1 Reinforced Sand Specimen …………………………………16
2.3.2 Reinforced Clay Specimen …………………………………19
2.3.3 Reinforced Clay Specimen with Embedding Reinforcement in Thin layers of sand (Sandwich Technique)………………26
2.4 Behavior of Reinforced Specimen Observed in Compaction, Direct Shear and Pullout Tests……………………………………... 27
2.4.1 Reinforced Clay Specimen………………………………… 27
2.4.2 Reinforced Clay Specimen with Embedding Reinforcement in Thin Layers of Sand(Sandwich Technique)……………...29
2.5 Summary of Behaviors and Conclusions Drawn from Previous Studies …………………………... …………………………………30
CHAPTER 3
EXPERIMENTAL PROGRAM………………………...…………….. ...35
3.1 Introduction….………………………..…………………………35
3.2 Soil Properties……………………………………………….… .35
3.2.1 Index Properties….…………………………………….35
3.2.2 Shear Strength Parameters…………………………….36
3.2.2.1 Direct Shear Test ……………………………37
3.2.2.2 Triaxial Test…………………………………40
3.3 Geotextile Properties ……………………………………………46
3.3.1 Axisymmetric Tensile Strength Test…………………..47
3.4 Specimen Preparation……………………………………........... 53
3.5 Testing Program…………………………………………………56
3.6 Technique for Measurement of Reinforcement Strain…………..58
CHAPTER 4
COMPACTION OF GEOTEXTILE-REINFROCED CLAY………….61
4.1 Introduction….………………………..…………………………61
4.2 Approaches for Estimation of Compaction Parameters…………62
4.2.1 Composite Soil Approach …..….……………………...62
4.2.1.1 Dry Unit Weight and Water Content.………..63
4.2.1.2 Specific Gravity……………………………...64
4.2.2 Compacted Soil Only Approach …………………..….64
4.3 Compaction Curves ……………………………………………..67
4.3.1 Composite Soil Approach ……………..………………67
4.3.2 Compacted Soil Only Approach …...………………….69
4.4 Summary. ……………………………………............................ 71
CHAPTER 5
UNCONSOLIDATED-UNDRAINED TRIAXIAL
COMPRESSION TEST. …...…………………………………………. ... 73
5.1 Introduction….………………………..………………………73
5.2 Failure Pattern..……………………………………………....76
5.3 Stress-strain Behavior………………………………………..78
5.3.1 Clay-Geotextile….……………...........................83
5.3.2 Clay-Sand-Geotextile…………………………...85
5.4 Strength Improvement …………………………………. …..86
5.5 Failure Envelope ……………………………………………92
5.5.1 Clay-Geotextile…………………………………92
5.5.2 Clay-Sand-Geotextile…………………………...95
5.6 Mobilized Reinforcement Tensile Strain ………………..…..98
5.7 Summary……………………………………………………101


CHAPTER 6
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS.……..103
6.1 Summary……………………………………………………103
6.2 Conclusions ………………………………………………...104
6.3 Recommendations ………………………………………….107
REFERENCES …………………………………………………………109

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