International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 7, July 217, pp. 653 659, Article ID: IJCIET_8_7_7 Available online at http://www.ia aeme.com/ijciet/issues.asp?jtype=ijciet&vtyp pe=8&itype=7 ISSN Print: 976-638 and ISSN Online: 976-6316 IAEME Publication Scopus Indexed RISING WATER LEVEL EFFECT ON THE GEOTECHNICAL BEHAVIOR OF ARID ZONES SOIL-NUMERICAL SIMULATION Y. Sadek, A. Berga, S. Benayad Laboratoire de Fiabilité des Matériaux et des structures (FIMAS). Tahri Mohammed University Béchar. BP 417 - (8), Algeria ABSTRACT This paper was to evaluate the effect of water table rise on settlement of footing. The alarming evidence of rising shallow ground water level and the widespread structural problem have mitigate the extra demand to develop special geotechnical awareness, standard testing and designing procedures to minimize the impact of wetting on soil behavior in the semi-arid region. Terzaghi (1943) suggested that the submergence of soil mass reduces the soil stiffness to half, which in turn doubles the settlement. Geotechnical procedures to estimate collapse potential with means to control and limit the rising ground water level have been addressed. Key words: Soil behavior; Semi-arid region; FEM; PLAXIS; Ground water level; Settlement. Cite this Article: Y. Sadek, A. Berga, S. Benayad. Rising Water Level Effect on the Geotechnical Behavior of Arid Zones Soil-Numerical Simulation. International Journal of Civil Engineering and Technology, 8(7), 217, pp. 653 659. http://www.iaeme.com/ijci IET/issues.asp?JType=IJCIET&VType=8&ITy ype=7 1. INTRODUCTION Evaporation rates in hot climate regions of the south part of North African countries are very high and exceeding precipitation by a factor as high as thirty. Soil stratigraphy in the region is depicted typically by a layer of wind blown fine sand underlain by firmly compacted fine sand and shale deposits on weathered limestone. The massive and extensive use and exploitation of the ground water have brought about significant changes to the ground water equilibrium. The shallow ground water level in most of the urbanized areas is rising in an increasing rate. The changes in the shallow ground water level lead to adverse geotechnical problems, particularly with the so called water sensitive soils of the hot climate regions. The problems are mainly related to soil volume change and foundation movement. The variation of moisture content stored in the ground and earth structures under varying environmental conditions is an important aspect closely related to the mechanical behavior of partially saturated soils. Change in the degree of saturation can cause significant changes in volume, shear strength and hydraulic properties, consequently bearing capacity. Eventually the soil may get submerged and bearing capacity will get reduced. (A. S. Stipho). So the http://www.iaeme.com/ijciet/index.asp 653 editor@iaeme.com
Rising Water Level Effect on the Geotechnical Behavior of Arid Zones Soil-Numerical Simulation problems are mainly related to soil volume change and foundation movement, the continuous evaporation pumping within the arid region introduces major chemical changes and salt accumulation in both dissolved and crystallized form within the capillary fringe. Soil bearing capacity and soil modulus of subgrade reaction are some various measures of strength-deformation properties of soil. To perform the structural analysis of footings, we must know the principles of evaluating the coefficient of subgrade reaction ks. The coefficient of subgrade reaction ks is the ratio between the pressure p at any given point and the settlement w produced by load application at that point (equation 1). Biot (1937) [3], Terzaghi (1955) [1], Vesic (1961)[11], Meyerhof [4] and Baike (1965), Selvadurai (1984) [8] and Bowles (1998)[2] have investigated the factors affect the determination of ks.. k = (1) Biot (1937) [3]solved the problem for an infinite beam with a concentrated load resting on a 3D elastic soil continuum. Biot found a correlation of the continuum elastic theory and Winkler model (M. A. Biot)[3]. Vesic (1961)[11] tried to develop a value for ks, by matching the maximum displacement of the beam. He obtained an equation for ks to be used in the Winkler model (A. B. Vesic, J. E. Bowles)[11], [2]. However, different formulas to calculate the modulus of subgrade reaction ks by some different authors are presented in Table 1. Table 1 Some different formulas to calculate the modulus of subgrade reaction, ks. No. Investigator Year Suggested formula 1 Biot 1937 =.95 (1 ) (1 ) 2 Terzaghi 1955. Sand: =, Clay : = 3 Vesic 1961 =.65 (1 ) 4 Meyerhof and Baike 1965 = (1 ) 5 Selvadurai 1984 =.65 (1 ) 6 Bowles 1998 = (1 ) ks = the coefficient of subgrade reaction. p = the pressure per unit of area. w = the settlement produced by load application. B1 = side dimension of square base used in the plate load test. B = width of footing. ksp = the value of subgrade reaction for.3.3 bearing plate. ksf = value of modulus of subgrade reaction for the full-size foundation. Es = modulus of elasticity. υs = poisson s ratio. EI = flexural rigidity of footing, M = takes 1, 2 and 4 for edges, sides and center of footing, respectively. IS and IF = influence factors depend on the shape of footing. This paper investigate the major potential problems of the water sensitive soil in south of Bechar province, southern west of Algeria. It is directed towards understanding the behavior such soil manifesting itself on an increasing contact with ground water. According to the former, using finite element (FE) software; PLAXIS 2D, the effect of side dimension of foundation and ground water level, and derived values of geotechnical parameters are investigated for diameters D = (.3,...,3.) m. http://www.iaeme.com/ijciet/index.asp 654 editor@iaeme.com
Y. Sadek, A. Berga, S. Benayad 2. FINITE ELEMENT MODEL All FE analysis were performed with an axis-symmetric mesh, because of the problem symmetry. The domain radius and height are 1D and 6D respectively. 15-noded triangular elements with a fourth order interpolation for displacements and twelve Gauss points for the numerical integration were used to define the finite element mesh shown in Fig. 2. Near the edges of a loaded area were stress concentrations are expected, mesh is refined by reducing the size of the elements (Desai C.S., Christian J)[1]. Analysis is performed under displacement control by a prescribed vertical displacement boundary condition applied to the soil surface below the position of the loading line (Potts D.M., Zdravkovic L), (Potts D.M., Zdravkovic L.) [6-7]. In order to prevent any rigid body motions of the whole problem domain, it is assumed that both the displacement in the horizontal and vertical direction are zero for all nodes along the bottom boundary of the mesh. On the vertical side boundaries, the horizontal displacement have been assumed to be zero too (Potts D.M., Zdravkovic L), (Potts D.M., Zdravkovic L.)[6-7]. FE calculation is based on cyclic loading on soil in two phases. The behaviour of the ground depends on the current stresses and strains. In the first phase the loading begin. In the second phase the loading were set to zero. The loading is simulated by a distributed load as described above D/2 D/2 9D 1D 5D Figure 1 Mesh and geometry for finite element model. The soil behavior it is assumed to be described by the Mohr-Coulomb model, having the properties below (table2); Table 2 Soil parameters Parameter C[kPa] φ ( ) ν E[MPa] γ[kn/m3] γsat[kn/m3] Value 23 15.3 39 19.7 22.1 3. RESULTS AND DISCUSSIONS Result from twenty vertical cyclic loading tests analysis are performed by plaxis software using the mesh shown on figure 1, with the previous soil parameters in table 1. The vertical settlement (y) for each analysis obtained, according to the constant contact pressure (p) about http://www.iaeme.com/ijciet/index.asp 655 editor@iaeme.com
Rising Water Level Effect on the Geotechnical Behavior of Arid Zones Soil-Numerical Simulation (kpa) plotted and Then the secant modulus of each graph (ks) is determined. The finite element analysis is performed both with ground water level and without it. In the first loading stage, a slight curved deformation was observed, and almost typical in all tests, and continued to various load intensity levels (Figure 2, 3). In the second stage, collapse took place in a rate. As it is known, the bearing capacity of soils decreases with the increase in size of the loading area and thus is essentially depenedent of the size of the loading area. Therefore the scale effect is another explanation for the larger error in evaluation of stress strain modulus, Es, by PLT with (relatively) small plates. Dia.3-WOGwL Dia.6-WOGwL Dia.9-WOGwL Dia.12-WOGwL Dia.15-WOGwL Dia.18-WOGwL Dia.21-WOGwL Dia.24-WOGwL Dia.27-WOGwL Dia.3-WOGwL 2 2 1 15 1 1 5-2.E-3 1.E-3 4.E-3 7.E-3 1.E-2 1.3E-2 1.6E-2 1.9E-2 2.2E-2 2.5E-2 Figure 2 Numerical load vs. settlement curves with water level case. Dia.3-WGwL Dia.6-WGwL Dia.9-WGwL Dia.12-WGwL Dia.15-WGwL Dia.18-WGwL Dia.21-WGwL Dia.24-WGwL Dia.27-WGwL Dia.3-WGwL 2 2 1 15 1 1 5.E+5.E-31.E-21.5E-22.E-2 2.5E-23.E-23.5E-24.E-24.5E-.E-2 Figure 3 Numerical load vs. settlement curves without water level case. There is a marked increase in the plate deformation associated with the high ground water level. The relatively higher settlement in this case may not only attribute to the high level of ground water, but also in part, to the softer mild winter weather, that decreased evaporation and desiccation. The high ground water level acts to extend the capillary zone upward, while the lower levels develop suction in the soil pores and negative pore water pressure that causes soil particles to move closer to each other. One of the uncertainties about Terzaghi's formula is that it neglects the effect of water table in the soil. Figure 4 represents the effect of ground water level. Full results are shown. http://www.iaeme.com/ijciet/index.asp 656 editor@iaeme.com
Y. Sadek, A. Berga, S. Benayad Dia.3-WGwL Dia.3-WOGwL Dia.9-WGwL Dia.9-WOGwL (a) (c) 2 2 1 15 1 1 5.E+ 2.E-3 4.E-3 2 2 1 15 1 1 5 Dia.15-WGwL.E+ 1.E-2 2.E-2 3.E-2 Dia.15-WOGwL (b) (d) 2 2 1 15 1 1 5 2 2 1 15 1 1 5.E+ 5.E-3 1.E-2 1.5E-2 Dia.3-WGwL.E+2.E-2 4.E-2 6.E-2 Figure 4 Numerical load vs. settlement curves with ground wate level variation. In Fig. 5 this is illustrated by plotting the settlement versus plate diameter relationship for various loading magnitude. As it can be seen, only for large diametres (D 12 cm) the settlement increases proportional with the size of the loading surface, a polynomial eaquation is representing this variation: Dry case: Uz = 6E-11D 3-8E-8D 2 + 1E-4D +.1 (2) Wet case: Uz = 4E-11D 3 + 2E-7D 2 + 1E-4D +.2 (3) Settlement [m].5.4.3.2.1 With GWL 3 6 9 12 15 18 21 24 27 3 33 Plate diameter [cm] Figure 5 Relation between plate diameter and settlement. It is evident from Figure that the value of the ks, varies according to the size of the plate. Thus ks has no unique value and depends on the size of the loaded area, it decreases with increasing size of plate. The use of values for ks, usually recommended in literature, seems to be, therefore, meaningless. Also the effect of groundwater level is clearly observed as http://www.iaeme.com/ijciet/index.asp 657 editor@iaeme.com
Rising Water Level Effect on the Geotechnical Behavior of Arid Zones Soil-Numerical Simulation decreasing proportionally with plate zise effect. This variation is represented by a simple power equation for both cases as shown in fig 6: Dry k s = 5E+6 D -1.186 (4) Wet: k s = 2E+6 D -.94 (5) ks [kn/m] 9.E+4 8.E+4 7.E+4 6.E+4 5.E+4 4.E+4 3.E+4 2.E+4 1.E+4.E+ ks-wgwl ks-wogwl 3 6 9 12 15 18 21 24 27 3 33 Plate diameter [cm] Figure 6 Variation of subgrade reaction coefficient vs. plate diameter. 4. CONCLUSIONS The soil in the hot climate conditions can be considered special for its high desiccation, sensitivity to ingress water and complex heterogeneity. Addition of water to the arid sensitive soil reduces its strength and releases the confining stresses, thus the soil fails. The geotechnical investigation and the engineering sampling and testing procedures in the hot climate regions must be different to those normally employed in temperate areas. They should always include an appraisal of geology, ground chemistry and ground water movements. Soil water characteristics in hot climate regions are changing as salts are being dissolved, precipitated and chemically reacted with other materials. This may result in larger construction depths and sizes, but should not be overlooked to avoid potential unfavorable and future surprises. REFERENCES [1] Desai C.S., Christian J.: Numerical Methods in Geotech Eng. McGrawHill, New York, 1997. [2] J. E. Bowles: Foundation Analysis and Design 6 th Edition, McGrow-Hill Interrnational Press, 1998. [3] M. A. Biot: Bending of Infinite Beams on an Elastic Foundation Journal of Applied Mechanics Trans. Am.Soc. Mech. Eng, 1937(Vol. 59) pp A1-A7. [4] Meyerhof, G.G.: Shallow foundations. Journal of the Soil Mechanics and Foundations Division, ASCE, 1965(Vol. 91(2)) pp 21-31. [5] Plaxis Manual 2d 212: PLAXIS Material Models Manual. Available: https://cunspterkonfper.files.wordpress.com/215/11/plaxis-manual-2d-212.pdf,. [6] Potts D.M., Zdravkovic L.: Finite Element Analysis In Geotechnical Engineering Theory. Thomas Telford Ltd., London, 1999. http://www.iaeme.com/ijciet/index.asp 658 editor@iaeme.com
Y. Sadek, A. Berga, S. Benayad [7] Potts D.M., Zdravkovic L.: Finite Element Analysis In Geotechnical Engineering Application. Thomas Telford Ltd., London, 21. [8] Selvadurai, A.P.S.: The flexure of an infinite strip of finite width embedded in an isotropic elastic medium of finite extent Int. J. Numer. Anal. Meth. Geomech, 1984(Vol. 8) pp 157-166. [9] Terzaghi K: Theoretical Soil Mechanics John Wiley and Sons. New York, NY, USA, 1943. [1] Terzaghi, K.V.: Evaluation of coefficient of subgrade reaction Geotechnique, 1955(V.5, No.4) pp 297-326. [11] Vesic, A. S.: Beams on elastic subgrade and the Winkler's hypothesis fifth ICSMFE, 1961(Vol. 1) pp 845-85. [12] Dr. V.S Pradeepan, V.P Reethi and N. Namitha, Effect of Diesel Contamination on Geotechnical Properties of Clay Near BPCL, International Journal of Civil Engineering and Technology, 7(2), 216, pp. 152 158 [13] Mohammad Fariyaz Ahmed, SS. Asadi and S. Srikanth Reddy, Evaluation of Geotechnical properties of Pond Ash For Economic Alternative Construction Materials For Fills. International Journal of Civil Engineering and Technology, 8(1), 217, pp. 287 29. http://www.iaeme.com/ijciet/index.asp 659 editor@iaeme.com