Journal of Engineering Geology

---

Introduction
Improving the mechanical behavior of clay soil by stabilization agents is a mean of fulfilling geotechnical design criteria. The method of stabilization can be divided into chemical, mechanical, or a combination of both methods. Chemical stabilization is performed by adding chemical agents such as cement, lime or fly ash to the soil (Bahar et al., 2004). Soil reinforcement is one of the mechanical methods that is used for improving the behavior of soils (Tang et al., 2007). Reinforcement of soil achieved by either inclusion of strips, bars, grids and etc. within a soil mass in a preferred direction or mixing discrete fibers randomly with a soil mass.
Mixing of cement with soil is made a production that is called soil-cement and results in chemical reaction between soil, cement, and water. The compressive strength of soil-cement is increased by increasing the cement content and this leads to brittle behavior or sudden failure. On the other hand, by increasing the cement to soil ratio for cohesive soils, shrinkage micro-cracks may develop in the soil as a result of the loss of water content during drying or hydration of cement. Therefore, if the tensile strength of these materials is not sufficient cracks will develop under loading and damage will be resulted (Khattak and Alrashidi, 2006). Consoli et al. (2003) and Tang et al. (2007) indicated that adding the fiber to soil can prevent from occurrence of these cracks and increases the tensile strength of the soil.
The focus of this paper is on the statistical analysis of the results and development of regression models. Regression relationships are developed based on the experimental results that were presented by Estabragh et al. (2017). These relationships relate the compressive and tensile strengths of the soil to percent of used fiber, cement and curing time.
Material and methods of testing
Unconfined compression and tensile strength tests were carried on unreinforced and reinforced soil, soil cement according to ASTM standards. Samples of soil-cement were made by mixing a clay soil and two different weight percent of cement (8 and 10%). Reinforced soil samples were also prepared by mixing 0.5 and 1 weight percent of Polypropylene fibers with 10, 15, 20 and 25 mm lengths. The dry unit weight and water content of prepared samples were the same as optimum water content and maximum dry unit weight that were resulted from standard compaction test. The compressive and tensile strength tests were conducted on the samples by considering the curing time according to ASTM standards until the failure of the sample is achieved.
Results and discussion
The experimental tests showed that reinforcement of the soil and soil cement increase the peak compressive and tensile strength. The peak compressive strength of reinforced soil is increased by increasing the fiber content at a constant length of the fiber. It can be said that by increasing the percent of fiber, the number of fibers in the sample is increased and contact between soil particle and fibers is increased which result in increase in the strength (Maher 1994). However, by increasing the length of the constant fiber inclusion there will be no significant increase in strength because the number of shorter fiber is more than longer fiber in a specific sample (Ahmad et al., 2010). Inclusion of fibers can greatly increase the tensile strength of clay soil. In addition to reinforcement of soil cement showed the same trend. When fiber is added to soil cement, the surface of fiber adheres to the hydration products of cement and some clay particle. Therefore, this combination increases the efficiency of load transfer from the composition to the fibers which increase the peak strength (Tang et al., 2007). In addition, the tensile strength shows the same trend.
Based on the experimental data on the behavior of a randomly reinforced clay soil and soil cement multiple regression models (linear and non-linear) were developed for calculating the peak compressive and tensile strength (dependent variables) based on the value of the coefficient of determination (R²). The proposed regression models were functions of independent variables including weight percent of fiber, length of fiber (length/diameter of fiber), weight percent of cement, and curing time. Finally, the comparison is made between the predicted results from proposed models and experimental results. In order to investigate the model accuracy, the Root Mean Square Error (RMSE) and Normalized Root Mean Square Error (NRMSE) are used.
 The Multiple Linear Regression models (MLR) was very suitable for the study of the effect of independent variables on the quantitative analytic dependent variable. The NRSME for peak compressive and tensile strength is was 3.59% and 5.11% respectively for these models. Also, the Multiple Nonlinear Regression models (MNLR) had a much lower error than the linear model because of the quadratic equation, the equation will be able to predict the increase and decrease of the output variable in terms of the increase of the independent input variable. Therefore, The NRMSE for peak compressive and tensile strength was 1.02% and 4.04% for MNLR models respectively.
Conclusion
The following conclusions can be drawn from this study:
- The strength of reinforced soil and soil cement is increased by increasing the fiber content.
- Increasing the length of the fibers in the soil and soil cement has no significant effect on increasing the peak compressive strength, but it will be effective in increasing the tensile strength.
- The Multiple Nonlinear Regression models (MNLR) have more accuracy for prediction of output variable (peak strength) because of lower normalized root mean square error../files/site1/files/131/7Extended_Abstract.pdf


 

---

Introduction
Expansive soils are a very common cause of extreme damages because they are susceptible to volume change due to a change in water content. Geotechnical problems associated with the expansive soils are well documented in different literature. As a result, a clear understanding of the behavior of such soils is required for the effective design of structures and infrastructures on these soils. The effects of hydrocarbon pollutants as a flooding fluid on the swelling potential of an expansive soil during wetting and drying cycles have not been considered in the previous researches. The aim of this research is to study the properties of an expansive soil with different flooding fluids, i.e. distilled water and solutions of glycerol with 10 and 20% through a number of cycles of wetting and drying tests under constant surcharge pressure.
Material and methods
The soil that was used in this work was a highly expansive clay soil (according to the classification by McKeen (1992)). It was prepared by mixing 20% bentonite and 80% kaolin. This soil was classified as a clay with high plasticity according to the Unified Soil Classification System (USCS). The optimum water content in the standard compaction test was 18.11% and the maximum dry unit weight was 16.27 kN/m³.
Distilled water and solutions of glycerol with concentrations of 10 and 20% were used for flooding the samples. To prepare the glycerol solutions, the required amount of glycerol was mixed with distilled water.
For making compacted samples for testing, the needed air-dried soil was weighed and the required water was added to it to reach the desired water content (4% below the optimum water content according to the compaction curve). The soil and water were mixed by hand and then was kept in a plastic bag for 24 hours to allow the uniform distribution of moisture in the soil. Samples were prepared by static compaction of the moist soil in a special mould.
A conventional oedometer was modified to allow the wetting and drying tests to be conducted under controlled surcharge pressure and temperature. During wetting and drying, the vertical deformation of the sample was measured by using a dial gauge. The variation of water content with void ratio during wetting and drying cycles was determined by using the information from the duplicated samples.
Results and discussion
Fig. 1 shows the variations of vertical deformation during wetting and drying cycles for samples that were flooded with distilled water and solutions of 10 and 20% glycerol. This figure illustrates that by increasing the number of cycles the amount of irreversible deformation is reduced until the equilibrium condition is achieved where the deformation due to wetting and drying is nearly the same. These results indicate that by increasing the concentration of glycerol the equilibrium condition with reversible deformation is reached in a fewer cycle of wetting and drying than the sample that was flooded with distilled water.

Figure 1. Wetting and drying cycles for different quality of flooding fluids
The results of void ratio versus water content at the equilibrium conditions for the samples flooded with distilled water and solutions of 10 and 20% glycerol (that were obtained from duplicated samples) are shown in Fig. 2. This figure displays that the paths of drying-wetting for different flooding fluids are nearly S-shaped curves. It is also seen in this figure that the order of the curves in this space is dependent on the percent of glycerol, the curves for the sample flooded with distilled water and 20% glycerol are located at the top and bottom of the space of void ratio against water content.

Figure 2. Water content-void ratio paths for different quality of flooding fluids
The change in the thickness of the diffuse double layer (DDL) affects on the swelling behavior of soil. The thickness of DDL is dependent on factors such as valency and concentration of cations, temperature, and dielectric constant. The value of dielectric constant for water is 80 and for solutions of 10 and 20% glycerol are 74.9 and 71.8, respectively. The magnitude of the attractive and repulsive forces between clay particles are inversely and directly depended on the value of the dielectric constant. The reduction in the value of the dielectric constant causes an increase in the attractive forces and leads to a reduction in the thickness of DDL. When the flooding fluid is a solution of glycerol, the initial chemical composition of pore fluid in the sample is changed. The chemical composition of pore fluid has different effects on the structure of clay soil such as changes in the thickness of DDL. When the flooding fluid is distilled water the pore fluid of samples has a dielectric constant of about 80. Therefore, the values of attractive and repulsive forces are not changed because of the same dielectric constant of flooding fluid and pore fluid. The results of tests on these samples (flooded with distilled water) show that by repeating the wetting and drying cycles the potential of swelling is reduced and after several cycles a reversible equilibrium condition is attained as depicted in Fig.1. When the pore fluid is the solution of glycerol, the attractive forces are increased due to the reduction of the dielectric constant of pore fluid and causes a reduction in the thickness of DDL. The shrinking of DDL is led to the formation of flocculated structure in the soil and results in pasting of particles together leading to the reduction potential of swelling. When the concentration solution of glycerol is increased the dielectric constant is decreased, the magnitude of attractive forces is increased and the degree of flocculation of the soil structure is increased that is yielded to a reduction of swelling potential.
Conclusion
Effect of different flooding fluids on the properties of an expansive soil during wetting and drying cycles were studied. The following conclusions can be drawn from the present research:
-After a number of wetting and drying cycles, the observed irreversible          deformation was diminished and equilibrium was achieved. The solution of glycerol causes more reduction in the potential of swelling than distilled water.
-The wetting and drying paths in the space of void ratio and water content are S-shaped curves. The variations in the void ratio of samples flooded with the solution of glycerol are smaller than distilled water../files/site1/files/142/babaei.pdf