CONVENTIONAL AND FINITE ELEMENT METHOD ANALYSIS OF FOUNDATION SOIL INETERACTION. Universiti Malaysia Sabah, Jalan UMS, Kota Kinabalu, Sabah.

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1 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah CONVENTIONAL AND FINITE ELEMENT METHOD ANALYSIS OF FOUNDATION SOIL INETERACTION Z.S. Tang 1*, N.S.V.K. Rao 1, Jodin Makinda 1 & Nurmin Bolong 1 1 Civil Engineering Program, School of Engineering and Information Technology, Universiti Malaysia Sabah, Jalan UMS, 8800 Kota Kinabalu, Sabah. ABSTRACT. The objective of this research is to study the soil-structure interaction using different laboratory models. Conventionally, four models were tested; steel beam, aluminium beam, steel square plate and concrete square plate under shallow foundation condition and sand is selected as the soil medium. The soil model used in this research is Winkler s model which provides the rational analysis. Analyses using Finite Element Method (FEM) computer software and theoretical calculations were then performed for data comparison. From the results obtained, it can be concluded that steel beam and aluminium beam with thickness of 3 mm are flexible member due to the deflection of the beams at the middle where the load applied is greater than the other positions. On the other hand, the steel and concrete square plates with thickness of 3 mm are rigid member due to the uniform settlements for each edge of the plates. The deflection results obtained using conventional analyses were found to be smaller compared to the computerized FEM analyses probably due to the determination of modulus of elasticity of soil. In addition, results from computerized FEM matched the theoretical calculations. KEYWORDS: Foundation soil interaction, sand, shallow foundation, FEM INTRODUCTION Soil structure interaction describes the effect of the foundation and soil on the behaviour of the structure. Soil structure interaction can be analysed either by using substructure or direct methods. Guttierrez and Chopra (1978) used substructure method analyses for both structure and soil region separately and produced computationally efficient findings. Previous studies on soil structure interaction also include the prediction of stresses in structure and supporting soil analysed at macroscopic or microscopic level (Ismail, 005). At macroscopic level, Winkler s model is commonly adopted, where soil medium is mutually independent elastic spring which is capable to resist compressive forces only. The settlement characteristics of foundations in Winkler model are identical, independent, closely spaced, discrete and linearly elastic spring (Thangaraj and Ilamparuthi, 010). At microscopic level, finite element method is used to model the superstructure and the soil media (Ismail, 005). This particular research studied the deflection behaviour of four different structures (steel beam, aluminium beam, steel square plate and concrete square plate) using laboratory models under shallow foundation conditions and sand as soil medium. The deflection performances of the models were then compared to the theoretical calculation and Finite Element Method (FEM) software analysis. 50

2 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah MATERIALS AND METHODS Soil Medium The soil medium selected is sand collected from the construction area of School of Engineering and Information Technology. Sample Preparation About 1000 g of the collected soil sample was oven dried for -hour at temperature of 105 o C according to BS The rest was sieved to remove foreign particles, packed and stored properly to maintain the moisture content. Soil Classification The dried sample was then sieved to determine its gradation. The test was done according to ASTM C136. Soil Characteristics The properties of the soil samples were determined for inputs in theoretical calculation and software analysis. The properties are: specific gravity (ASTM D5550), shear strength (ASTM D3080), modulus of elasticity (Rao, 011) and relative strength (BS ). At the same time, unit weight, moisture content and void ratio were also obtained. Model Laboratory Test A model of beam foundation was constructed as shown in Figure 1. The beam was prepared for 500 mm (l) x 50 mm (b) x 3 mm (t) for steel and aluminium. After that, the beam was replaced by 100 mm x 100 mm x 3 mm (t) square concrete and steel footing. All the tests were carried out with the load of 1 kn, kn and kn. Figure 1: Schematic diagram for model laboratory test The theoretical vertical deflections of the beams and footings were calculated by using equations (1), () and (3) (Rao, 011). 51

3 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah w o =P/Lk M o =P/ (1) () (3) Where, L is length of beam, w o is deflection, M o is bending moment, P is concentrated load, k is spring constant, k s b, k s is modulus of subgrade reaction and b is width of beam. FEM Software Analysis Plaxis 3D Foundation was used for FEM analysis. The inputs were the soil properties obtained from the laboratory test. Linear elastic model was used to represents Hooke s law of isotropic linear elasticity. The model involved Young s modulus E s and Poisson s ratio v. RESULTS AND DISCUSSION Soil Classification Sieve analysis data show a total mass loss of the experiment is 0.% which less than 0.5% from the total and thus fulfilled the ASTM C136 reliability requirement. From Figure, the soil sample is classified as poorly graded sand, SP (D 10 = 0.085, D 30 = 0.16, D 60 = 0.00, Cv =.35, Cc = 1.58). Based on US Bureau of soil classification the sand sample is also classified as fine sand as more than 70% of the samples are less than 0.5 mm (Rao, 011). Figure : Grain size distribution curve for collected soil sample Soil Characteristics Table 1 shows the soil characteristics of the sand. From the result of plate load test and CBR test, the modulus of elasticity for the soil sample is much lower than the theoretical value Thus, the subgrade reaction of the soil sample is assumed as 30 x 10 6 N/m (Rao, 011). 5

4 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah Table 1: Soil Characteristics Properties Parameters Specific gravity.61 Dry unit weight kn/m 3 Bulk unit weight kn/m 3 Saturated unit weight 0.01 kn/m 3 Moisture content.% Internal friction 9.07 o Cohesion 0 Deflection of Beam on Elastic Foundation Table, 3 and show the theoretical, experimental and software analysis of the vertical deflection of steel and aluminum beams. The deflections were measured at three locations; end gauge 1, middle gauge and end gauge. It was found that the differences in the experimental and theoretical result ranging from 18.09% to 38.1% and an average of about 30% at all of the points. The experimental deflections measured were relatively smaller than the theoretical result, while the software analysis shows closer similarity to the theoretical values. Table : Theoretical value of vertical deflection of beam on different loads. Beam Load End gauge 1 Middle Gauge End Gauge (kn) Steel Beam Aluminium Table 3: Experimental value of vertical deflection of beam on different loads. Beam Load (kn) End gauge 1 Middle Gauge End Gauge Steel Aluminium

5 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah Table : Vertical deflection of beam on different load using FEM Beam Load (kn) End 1 Middle End Steel Aluminium From the findings obtained, the beam is considered as flexible foundation for steel beam and aluminium beam because the downward deflection is greater in the middle and upward deflections at both end of the beam (Das, 009). The Young modulus of the materials seems to affect the deflections and this explains why the middle deflections (download deflection) of the aluminium beam are greater than the middle deflections of the steel beam. Inversely, the deflections of ends are smaller for aluminium. Figure 3, and 5 shows the output of the FEM analysis. The deflections trends observed proved the flexibility of the beam Figure 3: Typical deflection of the beam on elastic foundation Figure : Deflection of beam (side view) Figure 5. Deflection of beam (top view) Plate on Elastic Foundation 5

6 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah Table 5 and 6 show the results obtained from the laboratory model test and the theoretical result for steel and concrete plates, while Table 7 shows the FEM analysis. It was found that the settlements of the steel and concrete plates at each edge are almost similar for all three methods. Thus, it can be concluded that the steel plate and concrete plate are rigid foundation with the uniform settlements for all the edges (Das, 009). Table 5: Theoretical value of vertical deflection for steel and concrete plate. Load (kn) Gauge 1 Gauge Gauge Table 6: Experimental value of vertical deflection for steel and concrete plate. Plate Load (kn) Gauge 1 Gauge Gauge 3 Steel Concrete Table 7: Vertical deflection using FEM for steel and concrete plate Load (kn) Edge 1 Edge Edge This findings show that for steel and concrete plates, the major factor in settlement rate is the dimension and not the materials of the footing. Figure 6, 7 and 8 show the output of the FEM analysis. The deflections trends observed proved the rigidity of the plates. Figure 6: Typical settlements for square plate 55

7 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah Figure 7: Uniform settlements of square plate Figure 8: Settlements of square plate (top view) CONCLUSION Based on the deflection and settlement values using laboratory test, theoretical calculation and FEM analysis, steel and concrete beam found to act as flexible foundation with maximum deflection at the middle point while square steel and concrete plates found to act as rigid foundation with uniform deflection at any point of the plates. 56

8 Proceeding of the 10 th Seminar of Science & Technology 1- December 01, Kota Kinabalu Sabah ACKNOWLEDGEMENTS The authors would like to thank the School of Engineering and Information Technology, University Malaysia Sabah for the support in this research. Apart from this, authors would like to express the gratitude to all civil engineering lecturers for providing useful information and advices in completing the project. REFERENCES ASTM C Standard Test Method for Sieve Analysis for Fine and Coarse Aggregate. Pennsylvania: American Society for Testing and Materials International. ASTM D Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions. Pennsylvania: American Society for Testing and Materials International. ASTM D Standard Test Method for Specific Gravity of Soil Solids by Gas Pycnometer. Pennsylvania: American Society for Testing and Materials International. BS Part Methods of Test for Soils for Civil Engineering Purposes: Compactionrelated tests. London: British Standards Institution. Das, B. M Shallow Foundations Bearing Capacity and Settlement. nd Ed. United States: CRC Press.F Guttierrez, J. A. & Chopra, A. K A substructure method for earthquake analysis of structures including structure-soil interaction. Earthquake Engineering and Structural Dynamics: 6(1): Mohamed Abdulla Ismail Al-Gografi Inelastic Analysis of Soil Structure Interaction System. Degree of Master dissertation. Universiti Putra Malaysia. Rao, N. S. V. K Foundation design: theory and practice. Singapore: John Wiley and Sons. Thangaraj, D. D. & Ilamparuthi, K Parametric Study on the Performance of Raft Foundation with Interaction of Frame. Electronic Journal of Geotechnical Engineering: 15 (Bund. H): Retrieved September 0, 010 from 57