Modern yol inşaatında geotekstil ve geogrid uygulaması konularına araştıma

Abstract

Bu çalışmada, modern yol inşaatlarında kullanımı giderek yaygınlaşan geotekstil ve geogrid malzemeleri in celenmiştir. Genel anlamda geosentetikler, klasik geotek- nik problemlerin çağdaş çözümüdür. Yapılan araştırmada, geotekstillerin tarihçesi, işlevleri, üretim metodları, hammaddeleri, uygulama alanları, uzun vadede özellik de ğişimleri, arazide dikkat edilmesi gereken hususlar gibi konulara yer verilmiştir. Bu çalışma sonucunda, örgüsüz iğne delgili geotekstillerin diğer tip geotekstillere göre daha üstün geoteknik özelliklere sahip olduğu ve problemlere daha kalıcı ve pragmatik çözümler getirdiği görülmüştür. Geogridler ise, bu çalışmanın ikinci ana konusunu kapsamaktadır. Geogridler konusunda da tanıtıcı detaylı bilgiler sunulmuş, yol kaplaması içerisinde geogrid yer leştirilebilir bölgeler detaylı olarak incelenmiştir. Geogrid destekli, zayıf taşıma gücüne sahip zeminlerde ya pılan laboratuar çalışmalarından örnekler verilmiş ve 1986 yılında Meksika Havameydanı * nda gerçekleştirilen geogrid uygulaması incelenmiştir. Konunun gelişimine baz teşkil etmesi amacıyla, yol altı zemin stabilizasyonunda kullanılmak üzere geogrid teknik şartnamesi düzenlenmiştir.

Geotextiles and geogrids now play a major role in geotechnical engineering. Thirty years ago geotextiles did not exist and twenty years ago they existed but were not recognized. The geotechnical community has advanced considerably in the last twenty years in terms of usage and attitudes concerning geotextiles. This thesis reviews the past and the future of geotextiles in geotechnical engineering. First, the history of geotextiles is presented and basic concepts are discussed with emphasis on the functions performed by geotextiles and later by geogrids. Fundamental reasons for the growth of geotextiles in geotechnical engineering are discussed. The transformation of geotechnical engineering through the increased usage of geotextiles is reviewed. The last part of the first section is the expected evolution of the geotextile discipline. Forms of geotextiles have existed for thousands of years. Reinforced soil was used by the Babylonians more than three thousand years ago to built the ziggurats. One famous ziggurate, the Tower of Babel, collepsed, perhaps. because it was not reinforced. For thousands of years, the Chinese have used wood, bambov and straw to strengthen the soil. The importance of soil reinforcement in ancient China is demonstrated by the fact that the Chinese symbol for Civil Engineering translates as "earth and wood". During World War II, the British Army used armored vehicles specially designed to carry and lay on the ground, rolls of fascines or canvas. This technique was used in 1944 for the invasion of Normandy. At first glance, it seems clear that the mats of palm fronds used to construct the ziggurats were intended to reinforce the structure. However, some of the mats used were thick enough to convey a significant amount of water within their plane. Such drainage would have accelerated consolidation of the clay and minimized the risk of failure during construction. In the second section, the functions and properties of geotextiles are presented. As function of the geotextiles, seperation, filtration, drainage, reinforcement, protection and sealing can be defined. The most dominants are seperation and filtration. Here we present the functions of geotextiles: i) Drainage: A geotextile provides fluid transmission when it collects a liquid or a gas and conveys it within its own plane toward an outlet. This function is ussually called "drainage" which creates a lot of confusion with the "drainage aplications", therefore the terms" fluid transmission" or simply "transmission" are recommended. ii) Filtration: A geotextile acts as a filter when it allows liquid to pass normal to its plane, while preventing most soil particles from being carried away by the liquid current. iii) Seperation: A geotextile acts as a seperator when placed between a fine soil and a coarse material (gravel, stones, blocks, boards, etc.), it prevents the fine soil and the coarse material from mixing under the action of repeated applied loads. iv) Protection: A geotextile protects a material when it alleviates or distributes stresses and strains transmitted to the protected material. v) Reinforcement: A geotextile acts as a tensile member when it provides tensile modulus and strength to the soil with which it is interacting through interface shear strength (i.e., friction, cohesion-adhesion and interlocking between geotextile and soil). vi) Sealing: A geotextile acts as a sealing material in order to protect the soil structure from water. Bitumen is used for sealing the geotextile. The amount of bitumen should be varied between 900 gr/m and 1100 gr/m for a fabulous results. In the third section, the fabric reinforced road design methods and parametric studies are presented. Using geotextiles for building roads on soft subgrades is now well accepted construction technique. Besides providing seperation, filtration and drainage, the geotextile acts as a reinforcement by increasing the subgrade bearing capacity, restraining subgrade and aggregate and supporting the load with membrane action. According to most design methods, the aggregate height required is a function of the subgrade strength, permissible rut depth, wheel load, traffic, fabric modulus and load spreading capacity. Result presented show the relative importance of these input parameters and demonstrate that a significant increase in modulus is required ix in order to benefit from the membrane effect. In this section, however, the calculation methods of geotextiles are presented. In the fourth section of this thesis, the aplication of geogrid is discribed. In recent years, the concept of reinforcing pavements using geogrids has received considerably attention. Assuming that the reinforcement function is established, a geogrid reinforced pavement will be assigned a longer design lifetime compared to a similar pavement without reinforcement. Moreover, designing for a specific lifetime, a reinforced pavement will require a reduced section compared to an unreinforced section. In both cases, the use of geogrids can be cost-effective. This section of the thesis (fourth section) is concerned with the potential reinforcement functions of geogrids in paved roads. It also includes a literature review investigation the potential uses of geogrids in paved roads. Published field and laboratory data are presented together with a description of testing techniques used. Available method of analysis and design of geogrid reinforced pavements are presented. The following three applications are considered: (i) geogrids beneath or within stone base course, (ii) geogrids within surface course and (iii) geogrids beneath or within pavement overlays. Laboratory and analytical studies indicate that the placement of geogrids in stone base course layers can lead to improved performance of flexible pavements. The greatest improvement appears to be associated with following two conditions :. Weak soil subgrades with CBR of 3% or lower.. Poor quality aggregate base material. For improved performance with weak subgrade soil, the optimum location of the geogrid is, in general, at the bottom of the stone base course. However, laboratory tests indicate that some degree of contamination of the aggregate is likely to occur with weak subgrade (CBR 2 or 3). For good subgrade and low quality aggregate bases, the optimum location may be at the middle of the base layer. The same may be true for thick bases. Experimental and analytical data show the use of geogrids can be beneficial particularly for weak subgrade and low quality aggregate layers. It should prove to be economical under these conditions to incorporate geogrid reinforcement. However, due to the limited available data, laboratory as well as full-scale traffic tests are still needed in order to substantiate the reinforcement effect and to establish design criteria. The available data regarding improvements of pavements performance or economy for geogrid reinforcement of pavement surface courses are very limited. Laboratory tests under realistic loading conditions and pavement sections are gereatly needed in order to establish the reinforcement function of geogrid. Full-scale field traffictests can then be performed in order to substantiate the laboratory data after establishing that the use of geogrids proves to be benefical. The reinforcing effect of geogrids in asphalt overlays to prevent reflective cracking is not yet established despite the successfull performance in the laboratory. To simulate the variable field conditions in the laboratory promises to be very difficult. Scale effects are always present and are difficult to guantify using scaling laws for difficult and complex cross- sections and materials. Therefore, its is essential that full-scale field traffic tests be performed to evaluate the potential benefit of incorporating geogrids into overlays. In the fourth section of this thesis, also, the types of polymeric geogrids are described. Polymeric geogrids consist basically of two sets of straight ribs intersecting at right angles and forming an open structure. The ribs in the direction of the manufactured product, usually of higher strength, are referred to as longitudinal ribs, while the perpendicular ribs are called transverse ribs. Various types of geogrids (many having different styles) are commercially available. They differ considerably in geometry and mechanical properties. However, they may be classified into two general categories with respect to their physical and geometric structure. The first category (called unitized stiff geogrids) is produced essentially from a sheet of polyolefin (polyethylene or polypropylene) polymer having a uniform and controlled pattern of holes. The sheet is then drawn unidirectionally or biaxially depending on the required reinforcing function of the final product. The drawing process forces the polymer into a post-yield state cousing its molecular structure to elongate. The structure remains in this state after removal of the external drawing forces. As a result, strength, modulus and resistance to creep are increased substantially over the original sheet. Because the junctions between the longitudinal and transverse ribs are part of the original sheet, they are referred to as unitized geogrids. Uniaxially drawn geogrids of this category are made from polyethylene while those biaxially drawn are manufactured from polypropylene. Two types of geogrids, i.e. those manufactured by Tensar and Tenax companies classified into this first category. Only biaxially drawn geogrids are used in pavement applications since the direction of major principal stress is not clearly identified. xx Geogrids of the second category (called woven flexible geogrids) have individual longitudinal and transverse ribs which are joined together by knitting, weaving, bonding or other process to form the junctions. A number of geogrids, those manufactured by Mirafi, Nicolon, Conwed, Huesker and etc., are made from high tenacity polyester fibers or yarns and are then coated with polyvinyl chloride (PVC), latex or bitumen. They involve weaving and/or knitting to forming a junctions. They can be fabricated such that the major strength is in one direction or in a biaxial ly uniform strength. It may be noted that still another variation is a fiberglass geogrid produced in Canada by the Bay Mills Co. The junctions of the intersecting longitudinal and transverse ribs are made by weaving. Because of not having a market in the USA and European contries, there is no sufficient knowledge about this type of geogrids. In order to distinguish between the two types, they are conveniently referred to as stiff and flexible respectively. It must be recognized, however, that the above flexural rigidity refers to the products flexibility and not to the modulus of elasticity of the rib elements. In closing, it should be added that the author has not found any instance where the inclusion of the geogrids has been harmful to the performance of any aspect of the pavement cross-section. The use of geogrids in pavement sections should be encouraged. The target locations of the geogrids should be those that where discussed in this thesis;. beneath or within the stone base course,. within the surface course, and. beneath or within pavement overlays In all cases, the reinforced pavement sections should be compared to a nonreinforced section to note the degree of improvement. If possible and within the constraints of the funding level, the geogrids should be instrumented and monitored. Strain gages are well within the state of the art. Additional instrumentation may be considered for the other materials that are involved, i.e. soil subgrades, stone base course or asphalt pavement. As with many early uses of geosynthetics, positive feedback of the result of successful field installations may actually preceed the analysis and design methodologies. Hopefully, the recommended field trials suggested will be successful and will then lead to rational desing methods for using geogrids in the subject areas of this thesis. In the fifth section of this thesis, load tests on geogrid reinforced gravel fills constructed on weak subgrades is presented. The use of geotextiles and geogrids in the construction of roads and embankments on soft, highly compressible organic soils is now widespread. In xix the reinforcement of asphalt mix at the international airport of Mexico. Natural soil conditions within Mexico City and its surroundings comprise a constant problem for designers and engineers. One of the most characteristic cases is the constant re-paving of the asphalt surfaces at the International Airport of Mexico. Cracking reflection made it necessary in order to prevent possible hazzards to aircrafts but constant increase of asphalt base thickness also increased its weight and increased the causes of cracking reflection. Reinforcement of surface layer by the inclusion of high density polyethilene geogrids, not only decreased maintenance requirements with consequential economy of time and money, but also opened a new field of investigation on application of synthetic products for pavement optimization. In the conclusion, the benefits of needle-punched nonwoven geotextiles are described and also proposed. According to seperation, filtration, transmission, reinforcement and sealing functions, the utilization of nonwoven needle-punched geotextile is an advantage. In the conclusion, also the optimum locations for geogrids are presented. Moreover, the specification for geogrids in enlargement the road embankments is prepared and compiled. This is the first presentation of specification on geogrids in Turkey (1993), presented by the author of this thesis.