PARAMETRIC STUDY OF SHALLOW FOUNDATION BEARING CAPACITY IN CLAYEY SOIL

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 10, October 2018, pp , Article ID: IJCIET_09_10_121 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed PARAMETRIC STUDY OF SHALLOW FOUNDATION BEARING CAPACITY IN CLAYEY SOIL Mohanned Q. Waheed Civil Engineering Department University of Technology, Baghdad, Iraq Noor M. Asmael Highway and Transportation Engineering Department Faculty of Engineering, Mustansiriyah University, Baghdad, Iraq. ABSTRACT In the present study, a numerical analysis was carried out on the behavior of shallow foundation subjected to vertical axial loading in clayey soil. The analysis is performed using the Plaxis-3D program for developing the finite element model. The purpose of this study was to assess the effect of some parameters on the loadsettlement relationship and evaluate the (Nc) factor of bearing capacity equation. The parameters considered included Poisson s ratio of soil, soil modulus of elasticity, foundation width and soil cohesion. It was found that an increasing of the Poisson s ratio of soil increase slightly the load capacity at small settlements of the foundations, whereas it showed significant effect at large settlements to reach about (25%) at final stages of settlement. The foundation load capacity increases significantly with the increase in the soil modulus of elasticity. Increasing the foundation width does not have a significant effect on the value of the bearing pressure, so that the scale effect could not be observed. Variation undrained cohesion of soil had no effect on the (Nc) factor of bearing capacity at a certain value of settlement ratio (S/B), however, it is found equal (6), while with the increase in settlement ratio the (Nc) factor increases. Key words: Bearing capacity, Shallow foundation, Plaxis, Clay. Cite this Article: Mohanned Q. Waheed and Noor M. Asmael, Parametric Study of Shallow Foundation Bearing Capacity in Clayey Soil, International Journal of Civil Engineering and Technology (IJCIET) 9(10), 2018, pp INTRODUCTION Soil is the most important material which is used for construction of engineering structures, therefore, it is necessary to understand and study the behaviour of the soil under loads and the editor@iaeme.com

2 Mohanned Q. Waheed and Noor M. Asmael factors that affect it. In comparison with other materials, soil has a more complex material due to the nature of their components; the need to analytical in the last decades arises through the development in computer technology and the finite element program. The selected computer program should be including the proper material model and it is important to know how the parameters should be selected for the analysis to give results closer to reality and more logical. The development constitutive models for simulate the soil behaviour represent the key aspect in analysis of geotechnical structures, because soil is in complex nature. However, despite the complex soil nature there was a recent development of new constitutive relations, [1-4]. There is a lot of effort, increasing day after day, to develop more accurate models in business codes to help the designers to solve geotechnical problems considering more realistic of the soil behavior [5]. Dixit and Patil [6] performed an experimental study to investigate the effect of different parameters on bearing capacity of soil, it was concluded that the most parameters are cohesion, friction angle and unit weight of soil in addition to depth and width of foundation. It was observed from previous studies that most of the experimental researches for studying the behavior of foundation in clay soil are carried out in the remolded soils, therefore, the trend in this research is studying the behavior of footing in undisturbed soil at natural conditions to represent the real case. The purpose was to study the effect of some parameters on the load-settlement relationship and evaluate the (Nc) factor of bearing capacity equation. 2. MODELING AND MODEL VALIDATION The present paper deals with studying the behaviour of shallow foundation subjected to vertical axial loading in clayey soil.the parametric study for analyzing raft foundations is carried out by finite element method using the Plaxis - 3D program. Plaxis is a finite element computer program and it is specially prepared for analysis the geotechnical problems. Plaxis - 3D Foundation software is a family member of PLAXIS, which is a special purpose threedimensional finite element software used to simulate deformation analyses for various types of foundations in soil and rock. It is possible in this program for automatic generation of two and three dimension finite element meshes, which enables users to quickly generate a true three-dimensional finite element mesh based on materials properties of horizontal work planes at different vertical levels. The validation is carried out to evaluate the efficiency analysis of the numerical results using the program for predicting the behaviour of the experimental data. The case study for validation was selected from the field tests performed by Ornek et al.[7] of natural clay deposits including large-scale footing diameters, where the footing diameter equal to (0.9 m). The test was performed at the Adana Metropolitan Municipality s (AMM) Water Treatment Facility Centre (WTFC) located in the western part of Adana, Turkey. The data used to run the numerical models have been obtained from these results. Three subsoil layers were observed in the soil profile, as shown in Fig. (1), the first top layer with (0.8 m ) in depth and it was removed before the tests. A second layer of (6.2 m) thickness of high plasticity silty clay (CH). A third silty clay with sand of low plasticity (CL), was observed in the bottom of the previous layer to a depth of (10 m). Standard Penetration Test (SPT) was conducted during the drilling of boreholes, the distribution of (SPT) values with depth from boreholes and soil profile of the site is shown in Fig. (1). Conventional laboratory tests were performed on the soil, where the clay content, water content and the unconfined shear strength (qu) of the soil along the depths are presented in editor@iaeme.com

3 Parametric Study of Shallow Foundation Bearing Capacity in Clayey Soil Fig. (2), it can be seen the values of the undrained shear strengths (cu) determined using the unconfined compression test in the range of (60 80 kpa). Figure 1 SPT values with depth from boreholes of the site (From Ornek et al.[7]) Figure 2 The properties of natural clay soil (a) Clay content, (b) Water content and (c) Shear strength (From Ornek et al.[7]). The input properties of the soil used in the analysis of the case study are shown in Table (1). It is of interest to note that the circular footings with diameters of (0.9 m) of the case study was approximated as square footing in numerical modeling with equivalent contact areas of circular shape according to Lee and Salgado [8] that used square width equal to (0.9) diameter of circular shape, therefore it was used a footing of width (0.8 m) equivalent to (0.9 m) circular footing in numerical modeling. Fig. (3) Explain the geometry of footing models by Plaxis and the distribution of vertical displacement of the case study. The load-settlement response obtained from numerical analysis is compared with experimental test, shown in Fig. (4), where a node is selected in the center of the foundation. It can be seen that the experimental and the numerical curve are more or less similar for the footing investigated, editor@iaeme.com

4 Mohanned Q. Waheed and Noor M. Asmael thus, the results of the Plaxis program fit with measured results and the finite element results is validated. Table 1 The input properties of the soil used in the analysis of the case study. Description Symbol Unit Clay F.E. Model HS - Hardening Type of model behavior - - Drained Youngˋs modulus E ref 50 MPa 10 Oedometer modulus E ref oed MPa 29.4 (Auto. calculate) Unloading modulus E ref ur MPa 30 (Default = 3 E ref 50) Cohesion c kpa 70 Friction angle Ø ᴼ 10 Poisson s Ratio ŋ Exponential Power m - 1 Figure 3 Geometry of footing model by Plaxis and the distribution of vertical displacement under load in the case study. Figure 4 The load-settlement response of experimental test and finite element analysis of the case study. 3. PARAMETRIC STUDY In parametric study a rectangular shape of foundation is considered. The parameters considered in this study can be summarized in Table (2), where the effect of these parameters on the behavior of raft foundation subjected to a uniform pressure was studied. A constant value is selected for each parameter when investigating the effect of other parameters editor@iaeme.com

5 Parametric Study of Shallow Foundation Bearing Capacity in Clayey Soil Table 2 Parameters considered in this study. Parameter Range 1 Poisson s ratio of soil (ŋ) 0.25, 0.3, 0.35* and Modulus of elasticity of soil (kpa) 4000, 7000, * and Foundation width (m) 0.8*, 3, 7 and 10 4 Soil cohesion (kpa) 40, 70*, 100 and 200 * Constant value of this parameter when study other parameters. 4. PARAMETRIC STUDY RESULTS The effect of variation of the four parameters on the behavior of shallow foundation under loading is conducted and the results are shown below Effect of Soil Poisson s Ratio The effect of Poisson s ratio of soil on the load settlement response of foundation is shown in Fig. (5), it was observed that the effect is slight at small settlements of the foundations, whereas it showed significant effect at large settlements which is in agreement with the results found by Omeman [9]. It seems that increasing of Poisson s ratio from (0.25 to 0.4) increases gradually the load capacity of footing to reach about ( 25%) at the final stages of settlement. Figure 5 influence Poisson s ratio on the load settlement relationship Effect of Soil Modulus of Elasticity The effect of changing the soil modulus of elasticity on the load settlement response is shown in Fig. (6). The load capacity of the footing increases significantly up to twice with increasing in the soil modulus of elasticity from (4 to 13 MPa) editor@iaeme.com

6 Mohanned Q. Waheed and Noor M. Asmael Figure 6 Influence modulus of elasticity on the load settlement relationship Effect of Foundation Size In order to analyze the results of the effect of foundation size on the applied stress, the results have been demonstrated as the settlement ratio (S/B) and a relation of the bearing pressure (q), as can be seen in Fig. (7), where the settlement ratio represent the ratio of settlement of foundation to its width (B), it can be observed that increasing the width does not have a significant effect on the value of the bearing pressure, although the width increased from (0.8 to 20 m) which means that scale effect due to variation the footing sizes in clay soils cannot be observed from these results, similar findings are reported by [10], [11] and [12]. Figure 7 Relationship between bearing pressure & settlement ratio for different foundation width Effect of Soil Cohesion In Fig. (8), the effect of variation soil cohesion on bearing pressure (q) against the settlement ratio (S/B) curve is shown, and it is obvious that the ultimate bearing capacity increased significantly with variation undrained cohesion of soil from (40 to 200 kpa), Table (3) shows the values of undrained cohesion used with the corresponding modulus of elasticity that represent (E ref 50) in the program. Fig. (9) shows the relationship between bearing ratio (q/cu) and settlement ratio (S/B), and the results clearly show variation undrained cohesion of soil editor@iaeme.com

7 Parametric Study of Shallow Foundation Bearing Capacity in Clayey Soil has no effect. The bearing ratio represents the bearing capacity factor (Nc) and according to the observation of Ornek et al. [7] that the natural clay soil deposit collapsed and there was no longer load can applied after settlement ratio (3%), therefore, the (Nc) is (6), where this value is within the acceptable range (4 to 6.28) for saturated clay [13]. It can be concluded that the (Nc) factor affected by the settlement criterion used for defining the ultimate capacity, however, with the increase in settlement the (Nc) factor increases. Table 3 Values of undrained cohesion with the corresponding modulus of elasticity used in the analysis. Undrained cohesion (kpa ) Modulus of elasticity (kpa) Figure 8 Relationship between bearing pressure & settlement ratio for different soil cohesion. Figure 9 Relationship between settlement ratio and Bearing ratio for different soil cohesion editor@iaeme.com

8 Mohanned Q. Waheed and Noor M. Asmael 5. CONCLUSIONS The results of this study yielded the following conclusions: The results of the analysis using Plaxis program of raft foundation in clay are in good agreement with measured experimental results; hence, it can be used for studies of other parametric study. Variation the Poisson s ratio of soil from (0.25 to 0.4) increase slightly the load capacity of footing at small settlements of the foundations, whereas it showed significant effect to reach about (25%) at the final stages of settlement. The load capacity of the footing increases significantly up to twice with increasing in the soil modulus of elasticity from (4 to 13 MPa). Increasing the foundation width does not have a significant effect on the value of the bearing pressure, so that scale effect cannot be observed. Variation undrained cohesion of soil has no effect on the (Nc) factor at a certain value of the settlement ratio (S/B), however, it is found equal (6), where this value is within the acceptable range, while with the increase in settlement ratio the (Nc) factor increases. Conflict of Interest On behalf of all authors, the corresponding author states that there is no conflict of interest. REFERENCES [1] Darve, F. (2014) Incrementally non-linear constitutive relationships in geomaterials: constitutive equations and modeling. CRC Press, [2] Chambon, R. (2000).Uniqueness, second order work and bifurcation in hypoplasticity in Constitutive Modelling of Granular Materials. Springer, [3] Dafalias, Y. F. and Manzari, M. T. (2004). Simple plasticity sand model accounting for fabric change effects. Journal of Engineering Mechanics, 130(6), [4] Wood, D. M. (2014), Geotechnical modelling. CRC Press. [5] Abate, G., Caruso, C., Massimino, M. R., and Maugeri, M. (2008).Evaluation of shallow foundation settlements by an elasto-plastic kinematic-isotropic hardening numerical model for granular soil. Geomechanics and Geoengineering, 3(1), [6] Dixit, M.S. and Patil, K.A. (2010). Study of Effect of Different Parameters on Bearing Capacity of Soil, IGC 2009, Guntur, INDIA. [7] Ornek M., Laman M., Demir A., Prediction of bearing capacity of circular footings on soft clay stabilized with granular soil. Soils Found 2012; 52: [8] Lee J, and Salgado R. Estimation of footing settlement in sand. Int J Geomech 2002; 2: [9] Omeman Z.M. Load sharing of piled-raft foundations in sand subjected to vertical load. PhD Thesis 2012; CONCORDIA: 149. [10] Consoli N.C., Schnaid F, Milititsky J. Interpretation of plate load tests on residual soil site. J Geotech Geoenvironmental Eng 1998; 124: [11] Fellenius B.H., Altaee A. Stress and settlement of footings in sand. Proc Conf Vert Horiz Deform Found Embankments Part ; 2: [12] Ismael N.F. Allowable pressure from loading tests on Kuwaiti soils. Can Geotech J 1985; 22: [13] Bowles L.E. Foundation analysis and design. McGraw-hill, editor@iaeme.com