BEHAVIOR IMPROVEMENT OF FOOTINGS ON SOFT CLAY UTILIZING GEOFOAM

Transcription

1 BEHAVIOR IMPROVEMENT OF FOOTINGS ON SOFT CLAY UTILIZING GEOFOAM G. E. ABDELRAHMAN AND A. F. ELRAGI Department of Civil Engineering, Fayoum University Fayoum, Egypt ABSTRACT: EPS, expanded poly-styrene geofoam in the last three decades appeared to be the lightweight solution of a number of civil engineering problems. One the most important problems are to construct on soft clay soil, which extends in Egypt in many areas. Soil replacement can be fulfilled by using EPS blocks underlay footings. Three different EPS thicknesses are used in this study as soft clay replacement. A numerical finite element analysis program, PLAXIS, was used to simulate the problem. The decrease in settlement under the footing and the increase in bearing stresses were compared to those without EPS. An experimental model was used in the laboratory using white Kaolin with three different strengths as soft clay soil underneath square footings with fixed dimensions. Load settlement relationship was measured. Comparison was made between analytical and experimental results. Results showed that utilizing EPS decreases the settlement and increases the footing load. EPS thickness appeared to be an important factor in improving footing behavior on soft clay. INTRODUCTION Horvath 1 classified the applications utilizing EPS blocks by (their function). The four functions of EPS are lightweight fill, compressible inclusion, thermal insulation and small amplitude wave damping (ground vibration and acoustic). Horvath 1 add two more function, drainage and structural another way to classify the applications is by engineering properties. Five EPS properties appear to be very useful when utilizing EPS. These properties are: density, compressibility, thermal resistance, vibration damping and self-supporting nature of the EPS. These properties can solve many important engineering problems such as settlement problems, slope stability problems and bearing capacity problems such as embankments bridge approaches, earth retaining structures, bridge abutment, buried pipes, landscape architectural plaza, deck basement and insulation railways. Conventional geotechnical solutions for such problems (e.g., deep foundations, sheet piles, retaining walls or other solutions) may be economically unfeasible. In this research, EPS, as a lightweight material, was used as replacement material of soft clay underlain a footing foundation in order to decrease footing settlement and to increase the footing stress as the geofoam layer redistribute the footing stress over the clay layer. An experimental program, included, a model were embedded in Egyptian white kaolin that was reconstituted and pre-stressed to different clay consistencies and the EPS which have different thicknesses and a constant footing dimensions. Comprehensive finite-element analysis, where the footing underlay foam are represented in the mesh is adopted here using the nonlinear elasto-plastic finite element program PLAXIS. The program is plane strain, finite element program for soil modeling. The soil is modeled using 6 nodded triangular elements. The soil model is Mohr-Columb method with nonlinear failure envelope. The experimental and analytical models showed that EPS decrease the settlement under the footing as a replacement layer for soft clay and underlay the footing which allowing the 333

2 designer to increase the stress load. Also the results showed that the EPS layer thickness compared to footing thickness has great effect on the analysis. EXPERIMENTAL MODEL The experimental study described herein is concerned with the effect of EPS on the footing behavior on homogenous soft to firm clay. The experimental program, included, the model were embedded in Egyptian white Kaolin that was reconstituted and pre-stressed to different clay consistencies and the geofoam which have different thicknesses and constant footing dimensions. 1 Model Description To provide a soil deposit of high uniformity in water content, and degree of saturation throughout the bed, than can be usually found in nature, a special construction method was, therefore, required for forming artificial beds of clay that had homogeneous profiles of shear strength as practically possible. The apparatus used was developed in order to prepare uniform kaolin-clay beds in rectangular steel containers. The dimensions were sufficient to minimize any effect of the walls or the bottom of the containers on the soil resistance. The apparatus was built to act as a vertical consolidation cell which is composed of the following: 1.1 The Steel Tank A steel chamber 1050 x 200 x 600 mm deep was used with wall thickness of 5 mm. A geotextile filter covered the inner wall to allow the water movement in and out of the clay bed without the soil particles. During the vertical consolidation process, a vertical settlement occurred in the clay bed. A rectangular steel cover with dimensions 1030 mm and 183 mm and 20 mm in thickness, it weigh 31.4 kg covered with geo-textile filter, was placed over the clay slurry to distribute the vertical load during the vertical consolidation as shown in Figure Footing and Geofoam Dimensions Square blocks of wood were used as footing model with fixed dimension 70 x70mm and with 15 mm depth. EPS density was 16 kg/m 3 and its dimensions were the same as footing with three different depths, 30, 50, and 70 mm 1.3 Vertical Loading System Pre-stress pressure was used during the vertical consolidation phase. Two criteria controlled the pre-stress pressure; first to get the clay bed consolidated in the shortest possible time, and the second is getting desired shear strength, including soft to firm clay. A load of 320,177 and 55 kg was chosen to give a pre-stress clay of bearing capacity equal to 0.6, 0.3 and kg/cm 2, respectively. A steel bar was calibrated to transform vertical load to the top cover plate to allow bed consolidation, one kilogram on loading bar equal to 10 kilogram on the soil bed as shown in Figure

3 0.2m 2 Clay Deposit Figure 1 Experimental Model Kaolin based artificial soils have been particularly popular for laboratory work, and has been widely used both in fundamental studies of soil behavior and in physical model tests. The properties of pure kaolin are somewhat a typical of natural clay soils. The physical properties of Egyptian white Kaolin were measured. It has a liquid limit, w L, of 47.6 %, plastic limit, w p, of 25 %, plasticity index, PI, of 22.6 %, dry density, γ d, of 1.7 kg/cm 3, wet density, γ w, =1.9 kg/cm 3, Poisson s ratio, υ, = 0.49, and specific gravity, G s, of 2.7. Hydrometer test result shows that 93.5 percent of the material passes a No 200 sieve and that the clay fraction is 50 percent, the silt is 46.5 percent, and the fine sand is 3.5 percent. Modulus of elasticity, E, of 1.89,1.51,1.36 kg/cm 2 for bearing capacities 0.6, 0.3, and respectively. White kaolin was used to prepare three different bearing capacities clay deposits. In order to reach full saturation and homogeneity, the clay slurry was mixed at initial water content close to the value of its liquid limit, 10 liters water added to 30kg kaolin powder to give 33% water content in a concrete mixer for 15 minutes. Before placing the clay mixture inside the model, clean fine sand was used to fill the bottom to serve as a filter. The thickness of the bottom layer was for 35 mm thick. The sand layer was saturated. The mixed clay was then transferred from the concrete mixer to the tank up to the top level of the model. The top steel cover was placed on the clay surface. The consolidation load was then distributed on the cover. Clay deposits were consolidated vertically using distributed pressure to reach different clay bearing capacities, 0.125, 0.3, 0.6 kg/cm 2. During the vertical consolidation, the settlement of the steel plate was measured. 3 EPS Properties Expanded Polystyrene, EPS is a plastic polymeric material with chemical composition of C 8 H 8. Material prices vary depending on the type and density of EPS as well as the job size and location. EPS is a lightweight material with a good insulation and energy absorption characteristics. Its strength and stiffness are comparable to medium clay. EPS density appears to be the main parameter that correlates with most of its mechanical properties. Compression strength, shear strength, tension strength, flexural strength, stiffness, creep behavior and other mechanical properties depend on the density. Non mechanical properties like insulation coefficients are also density dependent. EPS densities for practical civil application range between 335

4 11 and 30 kg/m for other applications like insulation higher densities are more efficient, Van Drop 2, with its lightweight property, EPS blocks can be easily handled after manufacturing, during curing, transportation or placement in the field. Sun 3 reported that, EPS under confining compression the strength and initial tangent modulus decrease with the increase in confining stress. Sun 3 concluded these results based on axial deviator stress strain curves, which are important for submerged geofoam. The elastic modulus of EPS is small compared to the elastic modulus of some other engineering materials such as soil, concrete, and wood. EPS initial modulus is a function of the density. For EPS Young s modulus, there is no agreement from the researchers on a constant value of Young s modulus for each density. In this research Young s modulus was choose to be equal to 4 MPa (40 kg/cm 2 ) Elragy and Negussy 4 Table 1. ASTM-C 578 EPS densities and compressive strength Type XI I VIII II IX Density (kg / m 3 ) Compressive strength at 10% strain ( kg/m Poisson s ratio is an index of the lateral pressure of EPS in contact on adjacent structural elements for a certain applied vertical load on the geofoam. Most of the researchers measured different value for Poisson s ratio ranges from 0. 5 to 0.05). In this study Poisson s ratio, υ, was assumed to be equal 0.08, and dry density γ dry of 16 kg/ m 3. 4 EPS DESIGN CONSEDRATION Foundation engineers often use an approximate method to determine the increase of stress with depth caused by the construction of a foundation. Generally the load distribution from footing to soil is referred to as the 2:1 method. In case of using EPS layers underlay the footing, Nishi 5 suggested that, the load distributed at an angle θ which equal 20 for EPS layers as shown in Figure 2. Vertical stress (t/m2) Stress by Field Test Boussines 4 q's Eq. Laboratory 3.5 Test Theta= Theta=30 Theta= Deapth from loading point (m) Figure 2 Vertical stress in the EPS (after Nishi, et al., 1996) 336

5 TESTING PROCEDURES The main objective of this study is to measure the settlement under sustained loads for different soil consistencies with EPS existence. To achieve this goal, an accurate measurements of footing settlement, and soil movement changes with time had been recorded. After the footing was placed over the foam footing inside the clay bed, and the dial gauges placed around it, for each load, dial gages (0.01-mm accuracy), was read till the end of primary consolidation is finished, around 3 days after applying the load. Dial gauges were placed at the footing top. After the test was completed, dial gauges were removed, the footing was pulled out carefully from the clay bed. The tank was then cleaned out from all the clay and filters sand and washed with clean water in preparation for the next test. EXPERIMENTAL AND NUMERICAL RESULTS From the experimental model and as shown in Figure 3, the existence of EPS decrease the settlement for different bearing capacities depending on EPS thickness E, compared to footing depth, F. Higher EPS thickness, less settlement decrease proportional to soil bearing capacity as shown in table 2. From measured results it is concluded that, EPS layer as clay replacement decrees the settlement under the footing for two reason, the first is because it carries the load and absorb the energy ( 97% is air) of footing stress, and the second reason that the EPS redistribute the stress footing so less stress reach the clay soil. If the EPS layer has a small thickness, it does not show a good effect. Numerical program, PLAXIS shows good results which give similar as experimental results. These percentages show typical results as measured from the experimental model and analyzed using PLAXIS as shown in Figure3. Table 2 Percentage of settlement decrees at different stress and different EPS thickness. E/F PLAXIS 25-45% 9-32% 4-12% Experimental results 26-34% 5-15% 5% 337

6 0.7 Total Stress (kg/cm 2 ) Measured Plaxis E/F=4.663 E/F=3.33 E/F=2 E/F=Zero E/F=3.33 E/F=Zero E/F=4.66 E/F=2 E/F=3.33 E/F=3.33 E/F=2 E/F=zero E/F=2 0.1 E/F=4.66 E/F=4.66 E/F=zero Total Settlement (mm) Figure 3 Results of experimental and numerical for different EPS thickness. CONCLUSIONS (1) EPS decrease the settlement under the footing. (2) Using EPS as a replacement layer for soft clay and underlay the footing allow the designer to increase the stress load (3) Finite element analysis using PLAXIS program, shows good agreement with that measured with the experimental model results. (4) EPS thickness compared to footing depth is important factor. REFERENCES 1. Horvath, J., S., Geofoam Geosynthetic Horvath Engineering, P. C, Scarsdale, New York, USA 1995a. 2. van Dorp, T., Expanded Polystyrene Foam as Light Fill and Foundation Material in Road Structures, International Congress on Expanded Polystyrene, Milan, Italy, Sun, M., Engineering Behavior of Geofoam (Expanded Polystyrene) and Lateral Pressure Reduction in Substructures, Master s Thesis, Syracuse University, Syracuse, NY, USA, Negussey, D., and Elragi, A., EPS Geofoam, an Overview, Internal Report AE1-00, Geofoam Research Center, Syracuse University, Syracuse, NY, USA, 2000a. 5. Nishi, T., Hotta, H., Kuroda, S., and Hasegawa, H., Feedback to Design Based on Results of Field Observations of EPS Embankments Proceedings of International Symposium on EPS Construction Method, Tokyo,