Despite several advantages, field applications of fiber reinforced soil have been quite limited and still lag behind the traditional planer reinforcement method. The contribution of fibers to the improvement of soil strength in different states of formation has been examined and a comparison has been made on the behavior of clay soil reinforced with glass fibers and sandy soil.
LIST OF TABLES
155 Table 6.11 Adaptation statistics for larger principal stress at failure 158 Table 6.12 ANOVA table for principal stress at failure 159 Table 6.13 Summary of t-statistics for principal principal stress at failure 159. 203 Table 7.10 Summary of (EAC)f values of all reinforced sand samples 204 Table 7.11 Goodness of fit statics for principal principal stress at failure 206 Table 7.12 ANOVA table for larger principal stress at failure 207 Table 7.13 Summary of t-statistics for principal principal stress at failure 207.
INTRODUCTION
- INTRODUCTION
- SOIL REINFORCEMENT
- FIBRE-REINFORCED SOIL
- ADVANTAGES OF FIBRE-REINFORCED SOIL
- OBJECTIVE AND SCOPE OF THE STUDY
- ORGANIZATION OF THE THESIS
To conduct unconfined compression tests on glass fiber reinforced clayey soil and to evaluate the effects of fiber content, fiber length, formation moisture content, and change in dry unit weight on the unconfined strength and behavior of deformation. To conduct California bearing ratio tests on glass fiber reinforced clayey soil and to study the effects of fiber content, fiber length, formation moisture content and variation of soaking period on bearing capacity behavior.
LITERATURE REVIEW
INTRODUCTION
SHEAR STRENGTH BEHAVIOUR OF FIBRE-REINFORCED SOILS .1 Direct Shear Tests on Sandy Soils
- Direct Shear Tests on Clayey Soils
- Triaxial Tests on Sandy Soils
- Triaxial Tests on Clayey Soils
It was found that the shear strength of the soils increased with fiber content and fiber length in terms of cohesion and friction angle. The incorporation of fibers increased the peak shear strength of the sand and reduced the loss of post-peak strength.
UNCONFINED COMPRESSIVE STRENGTH BEHAVIOUR OF FIBRE- REINFORCED SOILS
At constant fiber content and moisture content, an increase in the dry unit weight of the reinforced specimens resulted in a significant increase in the UCS. At the same fiber content and dry unit weight, an increase in the moisture content of the reinforced samples resulted in a decrease in the UCS.
BEARING CAPACITY OF FIBRE-REINFORCED SOIL .1 Based on California Bearing Ratio Tests
- Based on Laboratory or Field Model Tests
The addition of rubber fibers increased the CBR value of the mixture, but the addition of shredded rubber decreased it. The maximum increase in CBR and modulus of the substrate was with 4% strip content having an aspect ratio of 3. Adding 4% strip content with an aspect ratio of 3 to fly ash increased the CBR value of fly ash by approximately 8.0 times , while for the same reinforcement condition, the increase in CBR value for stone dust and recycled product waste was 7.0 times and 2.4 times their unreinforced values, respectively.
MODELS OF FIBRE-REINFORCED SOILS .1 Force Equilibrium Models
- Models Based on Statistical Analysis
- Energy-Based Homogenization Model
- Discrete Framework Model
- Other Models and Numerical Studies
At pullout failure, the distributed stress induced by the fiber is a function of volumetric fiber content, interface shear strength, and fiber aspect ratio. Michalowski and Cermak (2003) developed a model for predicting the failure stress of fiber-reinforced sand based on drained triaxial compression tests.
APPLICATIONS OF FIBRE-REINFORCED SOIL
Two examples were presented to illustrate the use of the kinematic approach for anisotropic friction materials. In the laboratory, a large direct shear and a standard triaxial apparatus were used to evaluate the shear strengths of the reinforced materials. The fibers in the soil reduced the earth pressure and displacements of the wall and increased its stability.
This method made it possible to excavate the existing clay soils in the embankments, mix them with fibers and recompact them into the fill without importing selected fill or disposing of the existing fill (i.e. the mortar).
SUMMARY OF LITERATURE REVIEW
Fiber-reinforced soil models based on the approaches of force equilibrium and superposition of the effects of soil and fibers can be used to estimate the engineering properties of all reinforced soil types, while the model based on energy dissipation is useful for reinforced cohesionless soils. Because the discrete approach requires an independent investigation of the soil and fibers, but not of the fiber-reinforced soil, the results obtained are more realistic. It has been noted that studies on fiber reinforced soils have mainly been conducted on samples compacted at a certain moisture content and dry unit weight or relative density.
In this study, the strength and deformation behavior of glass fiber reinforced clayey and sandy soil samples under varying as-cast conditions are investigated.
MATERIALS AND METHODS
- INTRODUCTION
- MATERIALS USED .1 Clayey Soil
- Sandy Soil
- Glass Fibres
- LABORATORY TESTS CONDUCTED .1 Compaction Tests
- Relative Density Tests
- Unconfined Compression Tests
- California Bearing Ratio Tests
- PREPARATION OF TEST SPECIMENS
- Clayey Soil-Fibre Specimens
- SUMMARY
The coefficient of uniformity (Cu) and curvature (Cc) of the soil is 1.58 and 0.97, respectively. Standard Proctor compaction tests were conducted in accordance with ASTM D698-12 to determine the maximum dry weight unit (MDU) and optimum moisture content (OMC) of the clayey soil and soil fiber mixture. The minimum and maximum dry unit weight of sandy soil was determined according to ASTM D4254-06.
The details of the materials used in the laboratory tests are presented in this chapter.
UNCONFINED COMPRESSIVE STRENGTH BEHAVIOUR OF FIBRE-REINFORCED CLAYEY SOIL
INTRODUCTION
EXPERIMENTAL PROGRAMME
The test was performed until specimen failure or 15% axial strain, whichever occurred first. For each set of parameters, three samples were tested and the results were analyzed to assess the reproducibility of the test results. For each soil-fiber mixture, the values of UCS, failure strain, secant modulus of failure, and energy absorption capacity are the average of three tested samples.
RESULTS AND DISCUSSION .1 Strength Characteristics
- Strength Ratio
- Deformation Characteristics
- Failure Secant Modulus
- Energy Absorption Capability
The effect of variation of moisture content on stress-strain behavior of samples with the same dry unit weight is shown in Fig. 4.11 shown, the effect of dry unit weight on the variation of UCS is depicted for all reinforced samples with 20 mm fiber length and formed at OMC. The SR values are tabulated in Table 4.4 for specimens from Series 1 and 3 reinforced with 20 mm fibers and formed at OMC with different dry unit weights.
4.19 shows the effect of dry unit weight on the variation of the secant failure modulus for all reinforced samples with a fiber length of 20 mm (series 1 and 3).
SUMMARY
At fiber content up to 0.5%, the EAC value shows a continuous increase with dry unit weight up to 17.6 kN/m3. However, at any dry unit weight, the secant modulus at failure strain level only increases with increasing moisture content up to OMC and then decreases. For constant dry unit weight of the reinforced samples, the energy absorption capacity (EAC) increases with higher fiber content or fiber length or moisture content, and maintains it.
However, at the highest dry unit weight (γd = 17.6 kN/m3), the reinforced specimen undergoes a distinct shear failure.
INTRODUCTION
EXPERIMENTAL PROGRAMME
RESULTS AND DISCUSSION .1 Load-Penetration Response
- California Bearing Ratio
- Bearing Pressure Ratio
- Secant Modulus and Subgrade Modulus
The effect of fiber content on secant modulus variation of reinforced specimens formed at OMC and MDU with 20 mm fibers is presented in Fig 5.6 for both unsoaked and 4 day soaked conditions. The effect of fiber length on secant modulus variation of specimens reinforced with 0.75% fibers is shown in Fig 5.7 for both unsoaked and 4 days soaked conditions. The typical effect of molded moisture content on the secant modulus is shown in Fig.
At any penetration depth, the secant modulus with 20 mm fibers is greater regardless of the poured moisture content.
Subgrade modulus, k s (MN/m3)
CBR (%)ks = 1.536CBR1.162
Effect of Fibre Reinforcement in Reduction of Pavement Thickness
The corresponding reductions in thickness are 29, 28.6 and 26.6%, respectively, which is greater than a quarter of the required pavement thickness over the unreinforced soil.
SUMMARY
As the soaking time increases, the CBR value of soaked soil decreases for both unreinforced and reinforced soil samples. The inclusion of 0.75% of 20 mm long fibers effectively improves the quality of unreinforced soil as a base material from very poor (soaked CBR value of 2.89%) to good (soaked CBR value of 8.23%). When this special soil fiber blend is compacted at OMC as a base layer in the field, the top pressure thickness for flexible low volume pressures can be reduced by up to 25% even at a traffic value of 150msa.
Since the minimum required soaked CBR value is 6% for application in the underlay of such pavements, the soil mixture with 0.75% fibers and longer length of 30 mm (soaked CBR value 6.93%) is also practical.
SHEAR STRENGTH BEHAVIOUR OF FIBRE-REINFORCED CLAYEY SOIL
INTRODUCTION
EXPERIMENTAL PROGRAMME
RESULTS AND DISCUSSION .1 Stress-Strain Response
- Failure Deviator Stress Ratio
- Shear Strength Parameters
Typical effect of confinement pressure on deviator stress-strain and pore water pressure-strain variation of reinforced specimens from series 1, with 0.75% fibers of 20 mm length, is shown in Fig. The combined effect of fiber content and confinement pressure on the DSR of specimens reinforced with. The combined effect of fiber length and confining pressure on the DSR of Series 1 specimens reinforced with 0.75% fibers is depicted in Fig.
Typical effect of dry unit weight and confining pressure on DSR of specimens reinforced with 0.75% fibers of 20 mm length is shown in Fig.
Secant Modulus Response
Typical effect of fiber content on secant modulus variation under 100 kPa confining pressure, for Series 1 specimens reinforced with 20 mm fibers, is shown in Fig. Typical effect of fiber length on secant modulus variation under 100 kPa confining pressure, for Series 1 specimens reinforced with 0.75% fibers, is shown in Fig. Typical effect of confining pressure on secant modulus variation for Series 1 specimens reinforced with 0.75% fibers of 20 mm length is shown in Fig.
We observe that the secant modulus increases with increasing confining pressure at any dry unit weight.
Energy Absorption Capability
At any dry unit weight, the EAC increases with increasing fiber content to a maximum value of 0.75%. The effect of confining pressure on the EAC of a sample with 0.75% fibers of 20 mm length at different dry weight units (series 1 and 2) is shown in Fig. confining pressure.
The variation of (EAC)f with fiber content under 100 kPa confining pressure for specimens reinforced with 20 mm fibers and cast with different dry unit weights is shown in Fig.
Regression Analysis
The values of R2, Ra2 and Es for the best fit expression obtained from the regression analysis are given in Table 6.11. For the F-test, the null hypothesis implies that none of the independent variables is related to the dependent variable, whereas the alternative hypothesis implies that at least one of the independent variables is related to. Since Fcrt is less than the calculated value of F, the null hypothesis is rejected, indicating that at least one of the independent variables in Eqn.
The summary of t value for various individual predictor parameters is given in Tables 6.13 for Eq.
SUMMARY
SHEAR STRENGTH BEHAVIOUR OF FIBRE-REINFORCED SANDY SOIL
INTRODUCTION
EXPERIMENTAL PROGRAMME
RESULTS AND DISCUSSION .1 Stress-Strain Response
From the volume response of strain to axial strain (Figures 7.1b and 7.2b), it can be seen that the samples with 35% relative density underwent mainly shrinkage, and the size increases with fiber content. A typical effect of fiber length on the deviator stress-axial strain and volumetric strain-axial strain response of sand samples at 35% and 85% relative density is presented in Figs. With increasing fiber length, the axial failure strain of reinforced specimens did not change noticeably at the same relative density and confining pressure (Figures 7.5a and 7.6a).
Specimens of 85% relative density show contraction at small axial stress followed by dilation at greater axial stress (Fig. 7.5b & 7.6b).
