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Ground improvement by cement mixing have been applied extensively for structure foundation, excavation control and liquefaction mitigation. Problematic soils have been ignored for a long time in favor of more quality soils with reduced technical difficulties and lower construction costs. Alternative areas for construction have become important during the last decades, due to non availability of better quality soils for construction. Soils are the oldest and cheapest abundantly available complex natural engineering material used as a foundation or construction material. When we design foundation it is important to take into account the foundation soil condition. If foundation soil is not good enough to support the structural load, ground improvement is needed to be carried out. Soil improvement techniques can be classified in various ways: according to the nature of the process involved, the material added, the desired results, etc. For example, on the basis of the process, we have mechanical stabilization, chemical stabilization, thermal stabilization, and electrical stabilization. Usually the soil at a site to be developed is not ideal from the viewpoint of soil engineering. In some cases, the engineer can avoid potential soil problems by choosing another site, or by removing the undesirable soil and replacing it with desirable soil. A second approach to the problem of bad soils is to adapt the design for the conditions at hand. For example, floating foundation and deep foundations can be designed to avoid many of the settlement and stability problems associated with soft foundations. A third approach available to the engineer is to improve the soils.The techniques of ground improvement depend on the depth of the soil to be improved. Soil improvement may be permanent or temporary, depending on the purpose for which it is used. Generally the cost of adding/removal technique of stabilization can be low as compared with other techniques for soil improvement. The construction procedure, cost and results obtained from addition/removal stabilization technique depends mostly on the type of problem and nature of soil at field condition. Type of stabilizer depends on the treated soil, for example for sandy soil cement is preferred and for clayey soils lime is preferred to be used as a stabilizer because of their mineral composition. By using soil improvement techniques, density, shear strength is increased while compressibility, settlement and permeability are reduced making the soil more stable and durable.Ground improvement is becoming a necessary part of the infrastructure development projects both in the developed and developing countries. Foundations for buildings can be placed at shallow depth, if soil close to the ground possesses the sufficient bearing capacity. Foundation conditions often can be determined from a visual inspection of erosional features, of outcrops, and of excavations such as highways or rail road cuts, building excavation and abandoned pits. If top soil is weak or soft, the load from superstructure has to be transferred to deeper and firm strata. For such conditions pile foundation is used.Mixing of sand with cement is one of the fastest growing techniques in many coastal and offshore areas and is becoming a means of improving poor soil conditions. This technique is gaining popularity these days because of its relative ease of use. However, long term behavior of cement treated sand has not yet been clarified.When using cement-treated sand around existing/newly built pile foundation, the mechanical behaviour of the cement-treated sand around the structural members (piles) may influence the seismic behaviour of pile foundation. In these cases, localised deformations or failures in cement-treated soil do not always limit the capacity of the foundation, leaving sufficient capacity even after a localized failure of the cement-treated sand. Therefore, to evaluate the seismic performance of a structure, accounting for the post peak (softening) behaviour of cementtreated sand is important.The purpose of this study is to investigate the mechanical behavior of cementtreated sand and its use to improve the cyclic resistance of pile foundation. In this research, specimens were casted by using high early strength cement, sand and limestone powder for water to cement ratio (W/C) of 100%, 130%, 150%, 170% and 190 %, cement to sand ratio (C/S) of 30%, limestone powder to cement ratio (L/C) of 130%. Limestone powder was used to increase the viscosity of the mix. The influence of material strength and height to diameter ratio (H/D) on Compressive strength and Stress strain relation are investigated. Young Modulus, poisson’s ratio, compressive and bending strength are measured and the relationship between them is suggested. The custom made silicone bars were used in order to investigate the compression fracture zone length of cement-treated sand. Plastic strain and fracture behavior are studied and models for cement treated sand are suggested.Results have indicated that max compressive strength for (W/C) of 100%, 130%, 150%, 170% and 190 % is about 19 MPa, 10 MPa, 7MPa, 4MPa and 3.5 MPa respectively and maximum compressive stress of cement-treated sand is independent of the specimen size. The compression fracture zone length extends throughout the height of specimens with H/D ratios less than or equal to one and it increases with the height of the specimen before becoming constant at a height of approximately 250 mm.In order to investigate the effect of soil improvement on lateral resistance of pile foundations, model tests were carried out. Aluminum and Steel piles having 2.5 cm diameter and 45 cm long were used to investigate the effect of flexural stiffness on the lateral capacity of the piles. Piles were embedded in cementtreated sand, which was cast as 30 x 30 cm block having uniaxial compressive strength of 3.7 MPa. The steel box dimension was 50 x 50 x 45 cm. Displacement controlled lateral load test were performed. Cyclic loads were applied until pile displacement was 4%, 10%, 20%, 35%, 50%, 75% and 100% of pile diameter. The experimental results are compared with commonly used methods like Brinch Hansen, Brom, Poulos etc. and analysis was carried out to investigate the ultimate soil resistance. The 3D finite element model of piles embedded in sand and cement-treated sand is developed in Marc Mentat and analyzed using COM 3.Results have indicated that the stiffness of piles have a significant role in the lateral capacity of the piles. It is observed that the lateral capacity of piles embedded in cement-treated sand was increased to 5 times when compared with piles surrounded by sand, which is compacted to 70% of relative density.The model for plastic strain and fracture parameter, stress-strain relation suggested in this research will be helpful in analyzing the behavior of such cement treated sands. The cement-treated sand can be used to increase the lateral capacity of existing/ newly build structures. The use of such soil under cyclic loading is good, as degradation of stiffness is not rapid compared with other materials like concrete.
ACKNOWLEDGEMENT iABSTRACT iiLIST OF FIGURES xLIST OF TABLES xvLIST OF SYMBOLS xvi1. INTRODUCTION1.1 General 21.2 Problematic Soils 31.2.1 Leaning tower of Pisa 41.2.2 Foundations damages due to earthquakes in Japan 41.3 Ground Improvement 71.4 Objectives of Research Study 91.5 Utilization of Research Results 102. LITERATURE REVIEW2.1 General 122.2 Soil Improvement 122.2.1 Compaction Techniques 122.2.2 Grouts 132.2.3 Soil Stabilization 172.2.4 Foundation Improvement-Case Studies 192.3 Previous Studies on Laterally Loaded Piles 252.3.1 Lateral Resistance of Piles 263. EXPERIMENTAL INVESTIGATION ON MECHANICAL BEHAVIOUROF CEMENT TREATED SAND3.1 General 353.2 Experimental Program 353.3 Behaviour in Compression 373.3.1 Uniaxial compression test 373.3.2 Compression fracture zone length, Lp 423.3.3 Compression fracture energy, Gfc 473.4 Direct shear and Triaxial test 483.5 Behaviour in Tension 503.5.1 Tensile fracture energy, Gft 533.6 Relationship between Gfc and Gft 553.7 Conclusions 564. MODELLING OF STRESS-STRAIN RELATION OF CEMENTTREATED SAND4.1 General 584.2 Existing Concrete Model under Compression 594.3 Proposed Compressive Stress-Strain model for Cement treated sand 614.4 Uniaxial Compressive Stress-Strain relation based on the element size 644.5 Tensile Stress-Strain relations based on element size 674.6 Conclusions 685. EFFECT OF CEMENT-TREATED SAND ON LATERAL CAPACITYOF PILES5.1 General 715.2 Experimental Setup 725.2.1 Sand and Cement treated sand 725.2.2 Piles and Testing equipment 735.2.3 Measurements 745.3 Lateral load test on piles embedded in sand 765.4 Lateral load test on piles embedded in cement treated sand 795.5 Estimation of soil reaction 855.6 Conclusions 866. FINITE ELEMENT ANALYSIS OF LATERALLY LOADED PILESIN CEMENT TREATED SAND6.1 General 896.2 Analysis of laterally loaded piles 906.2.1 FEM Model 906.2.2 Results of analysis 936.3 Conclusions 1007. CONCLUSIONS7.1 Summary 1027.2 Overall Conclusions 102REFERENCES
主指導教員 : 牧剛史准教授