A Research on the Effect of the Reinforcement Geometry on the Soil Bearing Capacity Under the Strip

Abstract

Keywords: Geotextile; wraparound ends; strip footing; settlement; reinforced soil; PLAXIS.

1. Introduction

The reinforced soil is consists of a mixture of the reinforcement materials and the soil. The reinforcements from metals or synthetic fibers are resistance components against tensile forces, which are created in the form of bars, strips, network fabrics, layers, etc.[1,2]. The friction between the soil and the reinforcing element plays a remarkable role on the shear and pressure forces in the reinforced soils [3]. The reinforced soil may attain a superior behavior, as a consequence of the stress transfer from the soil to the reinforcement in the contact area [4]. Therefore, an appropriate interaction in the contact area of the reinforced soil is essential for sufficient soil resistance. The advantage of the reinforced soil behavior, compared with the unreinforced soil, is often as the result of an increase in the shear strength. The shear strength of the soil reinforced with geosynthetics is due to the higher modulus of the soil, as well as the high tensile strength in the reinforcement. This is the result of the frictional resistance between the soil and the reinforcement and has been developed due to the passive resistance between the cross-reinforcement component [5,6]. The reinforcements are often used in parallel with each other because of the stress transfer between them and the soil. Up to now, a variety of researches have been carried out on the applicability and the beneficial effects of using the reinforced soil in various geotechnical works and in this domain, several methods have been appreciably extended for soil reinforcement [718]. Along with the other available options, they have been employed in most projects, as an economic approach. Many investigations have been accomplished on the behavior of the shallow foundation, based on the context of the reinforced soils, using a variety of reinforcements. One single result, which has been achieved by all these researchers, is the large increase in the soil bearing capacity, by using the reinforcement in the depth of the soil. Kazi et al [19] conducted a laboratory study on a strip foundation, located on the bed of the geotextile-reinforced sand. They concluded that using a geotextile layer with wraparound ends can increase the load capacity, compared to the simple mode. Azzam and Nasr [20] performed a laboratory study on the bearing capacity of the strip footing, located on the geotextile-reinforced sand with different relative densities. Moreover, Al-Saeed et al. [21] completed several laboratory studies on the behavior of the circular foundation, located on the reinforced sand with side constraints. After the respective investigations and tests, they claimed that the maximum improvement happens, when the diameter of the cylinder is the same as the foundation diameter. Naderi and Hatef [22] worked on the interaction between the circular and ring-shaped foundations, placed on the reinforced sand. They stated that the bearing capacity of the reinforced soil was increased due to the effect of the overlap and the interference. Chakraborty and Kumar [23] used numerical methods to evaluate the bearing capacity of the circular foundations, located on the reinforced granular soil. They declared that the optimum depth of the reinforcement was approximately between 0.15d and 0.43d (d: the diameter of the foundation). Lai et al. [24] carried out a numerical study to investigate the increase of the bearing capacity of shallow foundations using geosynthetics. They explored the effect of different parameters, including the depth of the buried length of the reinforcement material, and the number and arrangement of the reinforcement layers on the bearing capacity. Marandi et al. [25] carried out a numerical study on the behavior of the circular shallow foundations, located on the geogrid-reinforced sandy bed. They showed that the bearing capacity ratio of the geogridreinforced sand depends on several parameters, such as the location of the reinforcement, the number of the reinforcement layers, the reinforcement resistance, and the geometrical properties of the circular foundation. Moreover, they proposed new curves to estimate the bearing capacity of the circular foundation. According to the expression of records, it can be witnessed that in all studies, the geosynthetic reinforcement is among the soil layers, and no investigation has been done on the geosynthetic

reinforcement with wrapround ends. Therefore, this can prove the need for the investigation into this matter. In the current study, it has been tried to propose a new approach to increase the bearing capacity of the soil by changing the geometry of the reinforcement. In all previous studies, the researchers have used the reinforcement layers horizontally and flat layers (i.e., the normal mode), which is exhibited in Figure 1.

Figure 1. A schematic view of normal reinforced sand [19] However, in this new idea, during the installation, the geometry of the reinforcement is changed and converted into the wraparound ends. For this purpose, the experimental results, presented by Kazi et al., [19] were exploited as the basis for managing the obtained results by the numerical method. They changed the reinforcement geometry into wraparound ends, when placed in the soil. The idea is portrayed in Figure 2.

Figure 2. A schematic view of wraparound end reinforced sand [19]

2. The Geometry And Properties Of Model

For the modeling phase, the data provided by Kazi et al. [19] were modeled in PLAXIS software, which is one of the the finite element analysis softwares, involved in the field of geotechnical engineering. Consequently, the results of the numerical and experimental models were compared. The finite element analysis was performed for the plane strain. In addition, the model boundary conditions were considered based on the true size of the models in the laboratory

tests. Moreover, the soil behavior was modeled based on the Mohr-Coulomb condition. The studies were accomplished for different conditions, for the exposure of the reinforcement (3 conditions), the soil relative density (3 conditions), and the depth of the reinforcement (4 conditions). The parameters presented in Tables 1 to 3 were used to perform the numerical modeling. A sample of the model geometry and the modified model are shown in Figure 3. Table 1. Sand properties used in numerical analysis [19] Parameter of soil γd (kN/m3) φ [˚] ψ [˚] E (kN/m2) c (kPa) υ Rinter

Table 2. Insertion depth of geotextile [19] Geometric parameter D (mm) d (mm)

Table 3. Properties of geotextile and footing [19] Parameter Axial stiffness (kN/m) B (mm) b (mm) b’ (mm) l (mm) Thickness (m) EA (kN/m) EI (kNm2/m)

Figure 3. (a) Typical geometry of model and (b) Typical deformed mesh of model

3. Results And Discussion

In this study, two dimentional plane strain finite element analyses were done to calculate stresses and total settlements under the footing rested on poorly graded sand. PLAXIS version 8.5 is used to simulate the model.PLAXIS has been verified previously by comparing solutions achieved from it with measurements taken in actual case histories. PLAXIS is able to model different types of soils, footings [26], geogrid sheets [27], cavity and excavation [28], retaining walls [29] and dynamic analysis [30]. In current study, one example of the results of the laboratory and numerical methods are compared in Figure 4. As shown in the figure, the values obtained from the experimental and numerical methods are close to each other and they all fit well together. This verifies the accuracy and acceptability of the software outputs.

Figure 1. Load-Settlement curve for Dr=70% and wrapround reinforcement The results of the numerical methods and laboratory tests demonstrated a better match after the increase in the relative density of the soil in the unreinforced soil, the normally reinforced soil, and the wrapround end reinforcement case. Also, the results of the numerical and laboratory experiments indicated that by using the wrapround reinforcement in different relative densities of soils, a better adaptation was observed, compared to the unreinforced and the normally reinforced situations.

4. Conclusion

In the present study, the numerical modeling with the finite element method was carried out using PLAXIS code in order to analyze the strip foundation, located on the sand bed, reinforced with geotextiles. In this work, the geometrical shape of the reinforcement was changed during the

installation and was transformed into the wrapround form. The effects of the changes in the geometry were investigated and compared with the normally reinforced and unreinforced conditions. According to the mentioned cases and researches, the following results were acquired: The obtained results in the unreinforced, normally reinforced, and wraparound reinforcement conditions, in the respective load ranges, disclosed that the results of the numerical and experimental tests have a better compliance, when the relative density of the soil was increased. The findings of both the numerical methods and the laboratory experiments confirmed a better compliance, at the relative densities of 50%, 70%, 90%, when using the wraparound reinforcement over the other two modes (i.e., the normally reinforced soil and the unreinforced soil). In fact, using the reinforcement in the wraparound form, instead of the normal reinforcement in different relative densities, led to reduce the settlement from 4% to 42%. This confirmed that the ideas can be introduced as a new way for using the reinforcements. The ratio of the depth of insertion (D/B= 0.3) made a large reduction in the settlements. Therefore, in this depth, if the wraparound reinforcement is used, rather than the normal reinforcement, the percentage of the settlement for the relative densities of 50%, 70%, and 90%, was 25-26%, 22-34%, and 24-36%, respectively, which is remarkable. In the ratio of the depth of insertion (D/B= 0.3), if the wraparound reinforcement is used, compared to the unreinforced soil, the percentage of the settlement, for the relative densities of 50%, 70%, and 90%, was 29-37%, 58-62%, and 56-58%, respectively, which is significant.

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