Geotekstil üzerine bir inceleme

Abstract

Çalışma, inşaat mühendisliğinin tüm dallarında önemi ve kullanımı hızla artan geotekstiileri içermekte ve* üç bölümden meydana gelmektedir. Birinci bölümde; geotekstillerin hammaddeleri, üretim metotları ve kullanım fonksiyonları hakkında genel bilgiler özetlenmiştir. İkinci bölümde; konu ile ilgili yapılan çalışmalara örnekler verilmekte ve bu amaçla; geotekstille güçlendirilmiş şev ve dolgu yapıları üzerinde, limit denge ve sonlu elemanlar yaklaşımları kullanılarak yapılan analizlerin sonucunda geliştirilen dizayn metotları ve geotekstille güçlendirilmiş kum numuneleri üzerinde yapılan direkt kesme ve üç eksenli basınç deneylerinin sonuçları sunulmuştur. Son bölümde ise; geotekstille güçlendirilmiş bir şev, elastik olarak sonlu elemanlar programı kullanılarak incelenmiştir. İncelemede; sadece zemin ve geotekstil elastisite modülleri değiştirilerek, geotekstilsiz halde ve çeşitli geotekstil serilme hallerinde oluşan yer değiştirme ve gerilmeler hesaplanmış ve birbirileri ile mukayese edilmiştir. Mukayese sonucunda; yer değiştirme ve gerilmelerin, zemin ve geotekstil elastisite modülüne ve serilme tipine bağlı olarak nasıl değiştiği saptanmış, böylece en uygun serilme tipi ve boyunun nasıl olabileceği hakkında bilgi toplanıp, yorum yapılmıştır.

The use of geotextiles has developed dramatically in all construction segments, such as earthworks, foundati ons, hydraulic works, road, rail and draninage works. The success of geotextiles is due to the variety of functions that they can perform simultaneously(f iltrati - on, separation, reinforcement etc. ) and to their properties (mechanical and chemical resistance, ease of handling, cost, etc.). A precise theoretical design of the necessary geotextile parametres, like in structural engineering, is still not available, mainly because of the complexity of the soil -geotextile interactions. However, the knowledge of the different properties and behavior of the three main types of fabrics, the understanding of the functions that the geotextile will perform, and the application of criteria resulting from already 20 years field and laboratory experience will greatly simplify the selec tion. The raw materials of the geotextiles are polypropy- lene(PP), polyethylene(PE), polyester(PET) and polyami- de(PA). Polypropylene has melting point of 165 C. It tends to suffer from creep. It requires addition of a stabiliser to cope with sunlight and its melting point is too low for it to be used with hot bitumen. Polyethylene has melting point of 110 C. Therefore it is used as a binder between fibers. Polyester is resistant to all substances accuring naturally in the soil. It is not susceptible to creep under continuous loading and, with a melting point of 260 C. It can be used in contact with hot bitumen. Its stability to light is good and testing showed three months open air weathering on site does not lead to any loss of strength. Polyamide's mechanical properties are affected by soil moisture. The higher water content the higher creep. These polymers are then transformed into three forms. These forms are continuous filaments, staple fibers (fila ments cutted into 50~150 mm long fibers) and slitted films(tapes) of variable width. The manufacturing techniques of geotextile are wo- vens, nonwovens needlepunched, nonwoven thermally bonded and other(staple fibers fibers chamically bonded, knitted fabrics, composites, etc.) Woven geotextiles have a weft element along the length. Threads may be flat or circular in cross-sec tion, and produce fairly uniform rectangular openings in the mesh they form. Nonwoven geotextiles have a haphazard orientation of fibres, usually fused together by heat. These are made in one of three ways; by using a small percentage of low melting point filaments in the web to acts as an adhesive, by using core fibres with a sheat which becomes tacky on heating and melts at a lower temperature than the core of individual fibres, or in the case of thin nonwovens and by using staple fibers that can be bonded by applying pressure. The major functions of geotextiles are seperation, reinforcement, filtration and drainage. Seperation is to prevent contamination of good quality materials by fine-grained subsoil. To optimally fulfill the separation function, the geotextile should exhibit follwing properties: 1) Sufficient elongation at break, to withstand lo cally important deformations 2) High "work to break", to oppose penetration by VI individual elements 3) Good tear and puncture resistance, to withstand sharp aggregate elements 4>Addequate filtration characteristics, to retain fine soil particles Reinforcement is to enable concentrated forced to be evenly distributed, and to reinforce the soil mass by making it resistant to tensile stresses. To perform an adeguate reinforcement function a geotextile should have: 1) High initral modulus 2) High "work to break "(toughness ) 3) Sufficient elongation at maximum load Filtration i s to allow the water to pass through, but in the same time to retain the soil particles. To perform a long term filtration function geotextiles must have following properties: 1) Adequate maximum pore size, to prevent continuous soil piping 2) Large number of pores (percent open area), to pre vent pore blocking 3) Sufficient water permeability 4> Low sensibility to compression Drainage is to allow transport of excess the plane(thi ckness ) of the geotextile. To good drainage function, a geotextile must have: water in fulfill * a 1) High in-plane permeability (transmissi vity) 2) High resistance to compression 3) Good filter properties A numerical technique for the analysis of geotextile reinforced embankments is outlined by Rowe, Booker and Balaam. This technique permits consideration of soil-re inforcement interaction, slip at the soil -geotextile interface, plastic failure within the soil and large deformations. The applicability of the approach has been assessed by examining the observed and predicted performance of a number of reinforced embankments const ructed on soft foundations. In Rowe' s study, the application of the approach is illustrated by reference to an embankment constructed on a soft peat deposit. Finally, Rowe's study presents a practical design procedure which involves the use of simple design charts. The use of this design procedure is illustrated by means of a worked example usign a typical set of desing charts. Vll Arı analytical approach to geotextile reinforced slope is presented by Dov Leshchinsky. It is based on limit- equilibrium and variational extremizati on. The results indicate that the potential failure surfaces are either planar or log-spiral. The analysis utilies a reinforcing membrane sheet that is orthogonal to the radius vector defining its intersection with the slip surface. Results of a closed-form solution imply that : (1) The stronger the geotextile the deeper the failure; (2) the geotextile's elevation has little effect on the stability or on the location of the slip surface provided that failure is passing through it; (3) the presence of a geotextile increases the compressive stress over the critical slip surface, and (4) the presence of a geotextile decreases the soil's tensile stress that tends to develop near the crest. The results are presented in a convenient format of stability charts. An approach for stability analysis of geotextile reinforced earth structures over firm foundations is presented by Don Leshchinsky and Ralph H.Boedeker. This approach involves both internal and external stability analyses. The internal stability analysis is based on variational limiting equilibrium and satisfies all equi librium requirements. Two extreme inclinations of reinforcement tensile resistance are investigated ; ort hogonal to the radius defining the geotextile sheet, and horizontal, signifying the as-installed position. Although a horizontal positioning requires slightly lon ger anchorage to assure pullout resistance, the slip surface is shallower when compared to the orthogonal case. As a result, the required total embedment length is longer for the orthogonal inclination. The external stability is an extension of the bilinear wedge method and it allows a slip plane to propagate horizontally along a reinforcing sheet. The results for both the internal and external stability analyses are conveniently presented in the form of design charts. Given a slope and a design safety factor, the geotextile sheet' profile as well as their required tensile resistance can be determined utilizing these charts. The results are summarized for an experimental program involving over 450 direct shear tests of sand- polymer interfaces by T.D. O'Rourke and S. J. Druschel. The interface frictional strength was found to increase with soil density and decrease with hardness of the polymer. The shear strength characteristics were found to vary as a funtion of the type of sand, but were independent of repeated loading, at least insofar as polyethylene piping and linings are concerned. This model allows for rapid evaluation of interface frictional strength and applies to plastic piping, linings, soil strip reinforcement, and a variety of other soil -polymer systems. Vlll Tri axial compression tests were run to compare the stress-strain response of a sand reinforced with conti nuous, geotextile lagers as opposed to randomly disribu- ted, discrete fibers by Donald H.Gray and Talal Al-Refeai. The influence of various test parameters such as amount of reinforcement, confining stress, and inclusion modulus and surface friction were also investigated. Test results, showed that both types of reinforcement improved strength, increased the axial strain at failure, and in most cases reduced post-peak loss of strength. At very low strains (<1%) geotextile inclusions resulted in a loss of compressive stiffness. This effect was not observed in the case of fiber reinforcement. The exis tence of a critical confining stress was common to both systems. Failure envelopes for reinforced sand paralle led the unreinforced envelope above this stress. Streng th increase was generally proportional to the amount of reinforcement, i.e, the number of geototextile layers or weight fraction of fibers, up to some limiting content. Thereafter, the strength increase approached an asymptotic upper limit. Fiber-reinforced samples failed along a classic planar shear plane, whereas fabric-rein forced sand failed by bulging between layers. In this study, a slope mesh is solved with the finite element program which is based on elastic method. In this program; soil is defined with elastic modulus, poisson ratio and unit weight and geotextile is defined with elastic modulus and section area. The geotextile is placed a t^ eight different types in slope. During the solution; the elastic modulus of soil is changed 1500 t/m, 3000 t/m and 6000 t/m respectively, the elastic modulus of geotextile is changed 37.500 t/m 62.500 t/m and 125.000 t/m respectively and poisson ratio and unit weight of soil, section area of geotextile and slope inc lination is constant 0.3, 1.8 t/m, 0,002 m and 60 respecti vely. In result of this solutions, how displacement and stress change with elastise modulus of soil and geotextile was observed. When the geotextile was not used; the greater the elastic modulus of soil the smaller displacements, but stresses were not affected by elastic modulus of soil. When geotextile was used; displacements and stresses were affected by elastic modulus and placed types of geotextiles. The higher elastic modulus, the lower displacements and stresses and, the lower the dis tance between two geotextile layers, the lower displace ments and stresses.