Yumuşak zeminler üzerine inşa edilen dolguların geotekstil ile güçlendirilmesi

Abstract

Bu çalışmada, yumuşak zeminler üzerine inşa edilen geotekstil-donatılı dolgunun davranışını incelemek amacı ile yapılan iki seri CBR (California Bearing Ratio) deneyi ve CBR deneyinin sonlu elemanlar analizi ile modellenmesi konu edilmiştir. Deneylerde ve analizlerde kum dolgu zemini ile yumuşak kil zemini arasına serilen geoteksilin, kum zemin içerisinde muhtelif derinliklere yerleştirilen ikinci bir geotekstil levhası yerleşim bölgesinin ve kil zemin mukavemetinin, kum-kil zemin sistemin taşıma kapasitesine etkisi incelenmiştir. Ayrıca, bu parametrelere ek olarak, sonlu elemanlar yöntemiyle de geotekstil elastisite modülünün kum-geotekstil-kil zemin sisteminin taşıma kapasitesine etkisi araştırılmıştır. Deneylerden ve analizlerden elde edilen sonuçlar, kum dolgu zemin ile yumuşak kil zemin arasına serilen geotekstil levhasının sadece ayırıcı olarak çalıştığını donatı olarak bir katkısının olmadığım ortaya koymuştur. Kum içerisineki geotekstil levhası yerleşim derinliğinin dolgu-geotekstil- yumuşak zemin sisteminin taşıma kapasitesini önemli derecede etkilediği görülmüştür. Geotekstilin yüzeye yakın yerleştirilmesi durumunda, kum-geotekstil-kil zemin sisteminin taşıma kapasitesi en fazla bulunmuştur. Ancak, bazı donatı gömme derinliklerinde geotekstil, kum-geotekstil-kil zemin sisteminin taşıma kapasitesini düşürmüştür. Buna sebep olarak, geotekstil ile zemin arasında yeterli sürtünme kuvvetinin mobilize olamadığı ve bu bölgede kayma yüzeyinin oluştuğu düşünülmektedir. İleri penetrasyon miktarlarında geotekstilin taşıma kapasitesine katkısı başlangıç penetrasyon miktarlarına göre daha büyük elde edilmiştir. Kil zeminin düşük mukavemet parametrelerinde kum-geotekstil-kil zemin sistemi taşıma kapasitesinin daha fazla arttığı görülmüştür. Nümerik analiz sonuçlarından, ve geotekstilin yüksek elastisite modülü değerlerinde, geotekstilin kum-geotekstil-kil zemin sisteminin taşıma kapasitesini daha fazla artırdığı görülmüştür. Ancak, belirli bir değerden sonra elastisite modülünün artması ile taşıma kapasitesindeki bu artış azalan bir hızla devam etmiştir.

Reinforced earth, is a composite material obtained by the inclusion of reinforced elements in the form of fibers, bars, strips or sheets inside a soil mass. The concept of reinforcing soils with tensile members is not new. As early as 1000 B.C. reeds and vines were used extensively to reinforce clay bricks and granular soils in the construction of many large earth structures (EXXON Chemical, 1989). However, the first scientific study on reinforced soil had been patented by the French engineer Henri Vidal and since then increasing number of studies on this subject have been conducted by several researchers (Schlosser and Guilloux, 1982). There are three main requirements for geotextile reinforcement materials (EXXON Chemical, 1989):. Strength. Stiffness. Bond First, and the most important, reinforcement must have sufficient strength to support the force required to achieve equilibrium in the soil. Secondly, the geotextile reinforcement must have sufficient stiffness so that the required force can be mobilised at a tensile strain which is compatible with the allowable deformation in the soil. Thirdly, the geotextile reinforcement has to remain in equilibrium with the surrounded soils and must bond sufficiently well to trasmit the required reinforcement force to the soil. The reinforcement elements can be manufactured from both metal and polymer raw materials. However, it was found from a set of laboratuary tests that polymer reinforcements had been employed much more effectively in the soil than metal reinforcements, because greater surface friction occurs at the interface of the IX reinforcement and the soil (Tümay et al., 1979). Corrosion is also a major problem for metal reinforcemens. There are two main types of conventional geotextile (Jewell, 1996):. woven geotextiles. non-woven geotextiles Geotextile-related products have a coarser structure than conventional geotextiles and those used for soil reinforcement include:. geogrids ^. alternative geogrid products. geomeshes. strips. webbings. knitted geotextiles. cellular products Besides serving as reinforcement, geotextiles can serve many other functions, such as seperation, drainage and filtration. However, the effects of ultraviolet light on getextiles are generally more severe. Also, long-term behavior of geotextiles is still not fully understood. Numerous techiques within the general category of "ground improvement" are available, e.g. grouting, freezing, dewatering, compacting etc. Most, however, are site specific, often costly and generally time consuming (Koerner et al., 1987). The use of geotextiles, compared with the conventional techniques, facilitates ease, speed of construction and can reduce costs. This is the reason for the increasing use of geotextiles reinforcing the earth sturctures. Building upon experience, geotechnical engineers nation wide are designing larger and more critical embankments using stronger specially designed reinforcing geotextiles-saving owners millions of dollars over conventional embankment construction techniques (Sprague and Koutsourais, 1992). The use of geotextiles to reinforce soils can be adopted for a wide variety of applications, such as reinforced soil walls, reinforced soil slopes and reinforced embankments constructed over soft or unstable foundation soils. Geotextiles are being increasingly used in the design and construction of embankments on soft foundations. The inclusion of the geotextile as an embankment reinforcement may reduce settlement and increase the embankment stability (Rowe, 1984). However, it should also be emphasized that geotextile can not reduce the soil's natural compressibility (Wewerka, 1982). It was stated that geotextile decreased the lateral movements under the embankment, but it had a very little influence on the vertical movements (Rowe et al., 1984). This study was undertaken to investigate the behavior of geotextile-reinforced embankment constructed on a soft subgrade by performing CBR tests as well as finite element analysis. The geotextile was not only placed as a single layer between the sand fill and soft clay subgrade, but also placed as double layer between sand and clay at different depths into the sand fill. The objective of this study is to determine:. the effect of geotextile, which is placed betwen sand and clay layers, as a reinforcement,. the effect of depth of the geotextile which is placed in the sand layer,. the effect of strength of clay layer,. the effect of elastic modulus of geotextile on the bearing capacity of geotextile-reinforced system. In Chapter 1, an introduction and the objectives of this study were presented. The results of tests and the analysis are also summarized. In Chapter 2, the history of geotextiles, geotextiles as materials and different applications of geotextiles are presented. Experimental and theoretical studies on this subject are also reviewed in detail. In Chapter 3, two sets of CBR tests and their results are discussed. Poorly graded sand and clay with high plasticity index were used to represent the fill and the soft subgrade respectively. The soil was placed into the CBR mold by conducting modified proctor test. 6074 Propex nonwoven geotextile, manufactured by Amaco fabrics, was used in the tests and it was placed horizontally into the mold. The different depth ratios (u/d) for the second geotextile was taken as 0.125, 0.25, 0.50, 0.75, 1.0 where; u: depth of the geotextile from the surface of the mold, d: diameter of the piston. XI 26 CBR tests were conducted at the laboratuary. The CBR tests were performed by recording the stress induced at the piston for 41 penetration steps. By performing CBR tests, load-penetration curves for sand and clay were obtained. At the end of each test, torque veyn was applied to clay in order to find its undrained shear strength. The only difference between the two sets of tests was the strength of clay. The test results indicated that the geotextile placed between. sand and clay, acts more likely as a separator than a reinforcement. Geotextile improved the bearing capacity of the composite material by seperating the two different layers. It was found that depth of the second geotextile had a very important effect on bearing capacity of geotextile-reinforced system. When the geotextile was placed closer to the surface (u/d=0. 125), bearing capacity ratio (BCR) reached the value of 7. Here; BCR=q/q0 (1) where, q: Load at the piston of reinforced system, qo: Load at the piston of unreinforced system. However, it should be emphasized that geotextile reduced the bearing capacity of sand-geotextile-clay system at particular depths. It was thought that the friction induced at the interface of geotextile and soil was not sufficient to mobilize the tensile strength at geotextile. Although it wasn't possible to have a general decision on the effect of depth of geotextile with the test results, it might be said that, in general, BCR value decreased by increasing value of geotextile' s placement depth. Maximum BCR value was obtained with lower value in strength of clay. However, the number of tests to investigate the effect of strength was not enough to have a general decision on the test results. Chapter 4 presents the finite element analysis and its results. SIGMAAV computer program was used for computation of the analysis. Numerical analyses were performed upto 8 mm of penetration with 19 penetration increments. Sand and clay were simulated by elastic-plastic, piston and geotextile was modelled by lineer elastic soil models. Engineering parameters of the soil and the reinforcement, loading and boundary conditions were tried to keep similar with those of the tests performed. Analysis were conducted under axi-symmetric conditions. Insitu stresses of the soil elements were computed before performing each of the load-deformation analysis. In the analysis, geotextile could not be modelled as a single layer, instead geotextile' s parameters were applied to soil elements which were placed over and beneath the geotextile in the finite element mesh. Numerical analysis, indicated that, BCR had its maximum value when the geotextile was placed closer to the surface (u/d=0. 1 25) of xii the mold. In contrast with the test results, geotextile did not decrease the bearing capacity of sand-geotextile-clay system at any of placement depths. In additon, as the geotextile's elastic modulus increased, BCR value increased. However, at higher values of elastic modulus (E=1000 Mpa), the increase in BCR value continued with decreasing rate. As the strength of clay decreased, BCR value increased. However, this increase in BCR value was not very much. In comparision with the tests, three different types of clay were used in the analysis. In Chapter 5, experiment and numerical analysis results were compared for reinforced and unreinforced systems. The discussions about the findings were also presented. Chaper 6 summarizes the main findinigs of this investigation. It should be noted that in order to establish more accurate design criteria for geotextile-reinforced earth embankments on soft soils, large-scale model tests are proposed. In addition, further studies are needed to determine the effect of strength of clay.