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Settlement of Shallow Foundations Near Reinforced Slopes

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Settlement of Shallow Foundations Near Reinforced Slopes Settlement of Shallow Foundations near Reinforced Slopes Mehdi Raftari Department of Civil Engineering, Khorramabad Branch, Islamic Azad University, Khorramabad, Iran [email protected] Prof. Dr. Khairul Anuar Kassim Department of Geotechnics & Transportation, Faculty of Civil Engineering, UTM, Johor, Malaysia [email protected] Dr. Ahmad Safuan A.Rashid Department of Geotechnics & Transportation, Faculty of Civil Engineering, UTM, Johor, Malaysia [email protected] Hossein Moayedi Faculty of Engineering, Kermanshah University of Technology Kermanshah, Iran [email protected] ABSTRACT Nowadays, there are many situations that foundations are built near the slopes. To design such foundations large settlement towards the slope are expected. To reduce the settlement as well as increasing the bearing ratio, using the geosynthetic reinforcement is common. In some cases the slope is reinforced and this reinforcement could have a significant impact on decrease of shallow foundation settlement on it. In this paper we selected five different slopes from the Lorestan province located in Iran and reinforced it with the appropriate reinforcement. To analyze the models using the Finite Element Method (FEM), the Plaxis 8.2 was used. Firstly, the settlements of shallow foundation on both reinforced and unreinforced slope were compared. Then other important parameters such as number of layers, vertical spacing between layers, and distance from foundation to slope crest during the design of reinforced structures were tested. As a result, the nearer placement of the footing to crest of the slope increases the settlement, however using the reinforcement could significantly decreases the mentioned settlement. KEYWORDS: Geosynthetic; Reinforced slope; Plaxis; Finite Element analysis INTRODUCTION Increase in the bearing capacity of shallow foundations on slopes is being always one of the concerns for designers to design appropriate structures such as building’s foundations, roads, and railways[1]. Nowadays, researchers investigate methods with least cost that they cause increase in - 797 - Vol. 18 [2013], Bund. D 798 bearing capacity of shallow foundations. The slope stability analysis has been offered by Fellenius in 1937. There are some little studies carried out about bearing capacity of foundations near the slopes that most of them are about sandy soils[2-6]. Also, there are many researches in field of reinforced layer soil[7-12]. Some of these researches have been studied on reinforced clayey soils and their behavior [13-18]. Otani [13] says that bearing capacity of reinforced soils is changed by changing in length and depth of reinforced layers, then he suggested an optimum depth and length for the most effectiveness of reinforcement operations on it. Investigations of footing located on slopes are very limit, and all of them have been concentrated on sandy soils[19-20]. Consequently, variety types of limit equilibrium methods or even numerical methods are used in stability analysis of slip surfaces. Furthermore, using reinforcements such as geotextile, Geogrid, and geonet is very important against deep and costly foundations because of their advantages like long life, lower cost, filtration, drainable features, and scour. Using these artificial stuff is similar to rebar in concrete for reinforce that it causes considerable increment in shear strength soil[3]. This case is very vital for areas with earthquake potential like Iran. It is necessary to consider two criteria for design of every foundation; Firstly, final bearing capacity and secondly foundation settlement. The main objective of the present research is obtaining the optimum value for depth, number of reinforced layers, and distance of shallow foundation from crest of slope in the soils of Lorestan province in Iran. THE KINDS OF SOIL IN LORESTAN PROVINCE In the present research five different soil samples were collected from the five different site investigations in the Lorestan province. Type of Soil in Azna city (sample A), in south of Khorramabad city (sample B), in north of Khorrmabad city (sample C), in Aleshtar city(sample D), and soil in Zagheh city (sample E). Azna city This city is located at east of Lorestan province with geographical coordinates 33° 27′ 21″ N, 49° 27′ 20″ E. In the based on Unified Classification System (UCS) the type of soil in this area is SC. In the based on results of Hansen’s method have obtained physical and mechanical properties of soil in this area as follow: Figure 1: Gradation curve – sample A Vol. 18 [2013], Bund. D 799 Khorramabad city Khorramabad city is located at the center of Lorestan with geographical coordinates 33° 29′ 16″ N, 48° 21′ 21″ E. It is situated in the Zagros Mountain and there is distance between its north and south. The experimental tests have shown that there is difference between type of soil in the north and south. South of Khorramabad city: In the based on UCS the type of soil in this area is low plasticity silt (ML). Figure 2: Gradation curve – sample B North of Khorramabad city: In the based on UCS the type of soil in this area is clayey sand (SC). The results of experimental tests showed properties of soil in this region as follow: Figure 3: Gradation curve – sample C Vol. 18 [2013], Bund. D 800 Aleshtar city This city is located at the north of Lorestan province with geographical coordinates 35° 6′ 0″ N, 47° 26′ 0″ E. In the based on UCS the type of soil in this area is clayey sand (SC). The results of experimental tests have obtained as follow: Figure 4: Gradation curve – sample D Zagheh city This city is located at the center of Lorestan province with geographical coordinates 33° 29′ 56″ N, 48° 42′ 31″ E. In the based on UCS the type of soil in this area is low plasticity clay (CL). The results of experimental tests have obtained as follow: Figure 5: Gradation curve – sample E Vol. 18 [2013], Bund. D 801 Table 1: Physical and mechanical properties of soil in the Lorestan province Sample C φ ɤ Hansen(1970) (kg/cm2 ) (degree) (kN/m3 ) Nc Nq Nɤ A 0.13 30 20.2 38.64 26.09 24.44 B 0.06 26 18.1 22.25 11.8 7.9 C 0.05 30 17.8 35.49 23.18 20.79 D 0.05 27 20.4 25.80 13.2 10.2 E 0.15 30 15.3 27.86 16.44 12.84 PROPERTIES OF REINFORCEMENT The used material reinforcement in this study is CE121 with high density Polyethylene polymer. Its Mesh aperture size is 8*6 mm with mesh thickness 3.3. Elongation at one-half peak strength is 3.2 and the flexural strength at maximum strain is 35 MPa. Also, the impact strength is 13.2 kJ·m −2 .The mechanical and physical features of CE121 are shown in Table 2. We modeled Geogrid by using elastic “Geogrid elements” via Plaxis software. Table 2: Physical and mechanical properties of Geogrid (ce121) Properties CE121 Width (m) 2.5 Length (m) 50-100 Aperture (mm) 8*6 Tensile Strength (KPa) 7.68 Elongation at maximum strain (%) 20.2 Tensile strength at 10% strain (kN/m) 6.80 NUMERICAL PROCESS The two- dimensional FEM analyses are performed by Plaxis 8.2 software. Plaxis software is a two-dimensional FEM computer program that is used for stability analysis and deformation of soils in variety geotechnical problems [3]. In this method structure of analysis are divided to smaller two- dimensional elements in mesh and displacements of each point are related to displacement of nodes through these elements. Finally, strain-stress relationship is calculated by caused strain values. In contrast to displacements, stresses and strains are calculated at individual Gaussian integration points rather than at the nodes. The automatic generation of 6 or 15 node triangle plane strain elements are done for the soil via Plaxis software that we used of 15 nodes to more accuracy. Displacements in nodes are related to displacements of points inside the element by interpolation function. In this analysis we have investigated affecting factors of settlement such as the value of load, the number of reinforcement layers, the space between reinforcement layers, and the distance between foundation and slope crest. The FEM has been analyzed by plane strain conditions. Also Mohr-coulomb model are used for the simulation of soil behavior. Our modeling is a shallow foundation with dimensions 1*1 meter on a slope angle of 45 degree. The height of this slope is 5 meters. The slope consists of homogeneous soil layout. In the center of this foundation we have different loads with these values: 100, 150, 200, 250, and 300 kN. Vol. 18 [2013], Bund. D 802 Figure 6: The schematic view of the slope section RESULTS OF ANALYSIS Effect of using Geogrid We analyzed one shallow foundation on a slope in both reinforced and unreinforced situations by loads with 100, 150, 200, 250, and 300 kN. A series of 40 difffferent FEM analyses were performed according to mechanical properties of the studied soils. The reinforced sample has been modeled by using three Geogrid layers with vertical spacings of one meeter from each other. Then, the results of this analysis have been offered in both reinforced and unreinforced samples in load- settlement charts in Figure7. Vol. 18 [2013], Bund. D 803 Figure 7: Load- settlement chart for reinforced & unreinforced slopes As can be seen from the figures the samples A, and C, and D, the settlement under the footing was improved significantly. The charts show that the settlement in the reinforced sample is approximately 50 percent better than unreinforced system in the all of samples. The incline of these charts have been decreased (settlement procedure) in the loads higher than 200 kN. That means the reinforced operations has better performance in the loads higher than 200 kN.
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