Zemine gömülü borulara etkileyen yükler
The Loads acting on buried pipes into the soil
- Tez No: 66723
- Danışmanlar: PROF. DR. AHMET SAĞLAMER
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 154
Özet
ÖZET Modern yaşamın vazgeçilmez bir parçası olan, suların, petrol gibi sıvı şeylerin ve doğal gazların taşınmasında kullanılan, gömülü borulara, yeraltında etkiyen yüklerin büyüklüğünü ve etkisini tespit etmek için geliştirilen bir çok hesap metodu vardır. Bu metotlar içinde en yaygın olarak kullanılanı, Marston ve Spangler'ın geliştirdikleri hesap metotlarıdır. Bu metotlar gömülü boruların projelendirilmesinde Avrupa, A.B.D. ve Türkiye'de yaygın olarak kullanılmaktadır. Ülkemizde yaygın olarak kullanılmasına rağmen, bu metotların dayandığı teoriler hakkında kapsamlı bir araştırma yapılmamış olması; hatta gömülü boruların yerleşim şartlan, maruz kaldığı yüklerin hesabı ve tasarım metotları ile ilgili kapsamlı bir çalışma yapılmamış olması ve devamlı yabancı kaynaklardan yararlanılması bizi böyle bir çalışmaya itmiştir. Marston ve Spangler, Iowa Eyalet Üniversitesi'nde uzun ve planlı araştırmalar kapsamında yaptıkları deneyler ve testler sonucunda elde ettikleri katsayılar ve sabitleri kullanarak, gömülü boruların yapısal tasarımı için hesap metotları geliştirmişlerdir. Bu çalışmada, Marston ve Spangler'ın geliştirdikleri bu iki hesap metodu hakkında bilgi verilmiş ve dayandığı teoriler irdelenmiştir. Ayrıca, formüllerde kullanılan deneysel sabitler ve katsayılar, tablolar halinde verilmiştir. Buna ek olarak boruların yataklanma şartlan, yük faktörleri, yerleşim şartlan ve yapısal tasarımında kullanılan güvenlik faktörleri verilmiştir. Aynı zamanda, dolgu ve hareketli yüklerin pratik olarak tespiti için, Young ve O'Reilly'nin İngiliz şartnamesi için Marston yükünü kullanarak hazırlamış olduklan yük grafikleri verilmiştir. Bu sayede, bu çalışma, bir ölçüde gömülü boruların yapısal tasarımında bir kaynak özelliği taşımaktadır. Son yıllarda, zemin mekaniği problemlerinde sonlu elemanlar metodu artan bir şekilde kullanılmaktadır. Bu nedenle, sonlu elemanlar metodunun gömülü boruların analizinde ve tasarımında kullanılması ile ilgili bir araştırıma yapılmış ve bu amaçla, bir sonlu elemanlar programı olan SIGMA/W yazılımı ile bir hendek ortamının modellemesi ve analizi yapılmıştır. xn
Özet (Çeviri)
SUMMARY THE LOADS ACTING ON BURIED PIPES INTO THE SOIL The increasingly successful of scientific principles of mechanics to the analysis of soil has characterized engineering development during the first twoscore years of the twentieth century. The problems which concern soils have begun to be analyzed, the result of organized researches. One notable example of problems in this category concerns the design of underground structures such as conduits, culverts, water mains, and gas lines. The analysis and design of buried pipes is one of the most complicated and difficult subject in the structural aspects of geotechnical engineering. It is now possible to use engineering science to design these underground conduits with a degree of precision comparable with that obtained in designing buildings and bridges. Underground conduits of this general type have been used for transmission drinking water and sewage by mankind at least 3000 years. Today, underground conduits serve in diverse applications such as water mains, gas lines, sewer lines, drain lines, telephone and electrical conduits, culverts, oil lines, and heat distribution lines. But not until about 1900 was a serious effort directed toward the development of a rational method for detennining the magnitude and character of the loads to which underground conduits were subjected in service due to the earth overburden and other load sources like vehicle loads on surface. In the early 1900s, Anson Marston, who was dean of engineering at Iowa State University in the US., inaugurated a research program which has been continuously active under his direction. In the result of the organized research program, Anson Marston developed a method of calculating the earth load to which a buried conduit is subjected in service. This method which is called Marston Load Theory, serves to predict supporting strength of pipe under various installation conditions. M.G.Spangler, who was a student of Marston, developed a theory for flexible pipe design. In addition, much testing and research have produced quantities of empirical data which also can be used in the design process. The most widely used method of estimating external loads on a buried pipeline was pioneered by Marston, Spangler and Schlick in the US. It was further developed and extended for the UK practice by Young and O'Reilly and is generally termed the“Marston”or“computed load”method. The method is to some extent empirical and uses a soil model based on Rankine's theories of soil behavior. This has been the cause of some criticism, but considerable testing and successful XIIIpractical experience has demonstrated its merits, and it remains the standard method in Europe, and the US. The design methods for buried pipes of Marston and Spangler are widely used to design for buried pipes in Turkey. But not until now, there is a serious study about the Marston's theory for external loads on buried pipes. The purpose of this study is to review Marston's theory of loads on, and supporting strengths of, buried pipes and to record the experimental background, together with working values of many of the constants required for the practical application of the theory. In addition, the study contains general design procedures for various piping products and installation conditions. This study also gives the knowledge about the using the finite element method for design of buried pipes. For this reason, the study may be useful a guide for the installation design of buried pipes. In Chapter 2, methods for calculating soil loads and surface loads are given. Marston's theory for soil loads on buried pipes is discussed along with the various factors which contribute to these loads. The loads acting on shallow buried pipe and conduits are influenced by the relative rigidity of the pipe and surrounding soil, depth of cover, type of loading, span (maximum width) of structure, method of construction, compaction density of backfill, and shape of pipe. Buried pipes, in general, may be divided into four main classes on the basis of construction conditions under which they are installed. These four main installations are considered when earth loads acting on a buried pipe are calculated: 1. Narrow trench condition, 2. Embankment or valley fill with positive projection condition, 3. Embankment or valley fill with negative projection condition, 4. Tunnel or heading condition. 1. Narrow trench condition: Pipes are installed and completely buried in narrow trench in relatively passive or undisturbed soil. The fill load on the pipe is the weight of the prism of the fill at the top of the pipe level minus the friction of the fill on the adjacent soil. The theory ignores the effect of cohesion on the shear surface since considerable time would have to elapse before cohesion could develop and the assumption of no cohesion would yield the maximum load on the pipe. Trench width can be maximum three times the lateral width of pipe. Examples of this class of conduits are sewers, drains, and water mains. 2. Embankment or valley fill with positive projection condition: Pipes are installed in a bedding above the surface of the natural ground, and then covered with an embankment. Railway and highway culverts are good illustrations of this class of conduits. Pipes installed in trenches wider than about two or three times their maximum horizontal breadth may also be treated as the positive projection condition. XIV3. Embankment or valley fill with negative projection condition: This case differs from positive projection in that the pipe is laid in a subtrench below the natural ground level. 4. Tunnel or heading condition: In urban areas the use of trenches techniques can reduce to a large extent the disruption caused by trench construction and also allow obstructions such as rivers, railways, major roads, etc. to be negotiated conveniently. The techniques currently, used may be roughly divided into microtunelling and conventional tunneling. Marston's theory may be used to determine soil loads on pipes that are in tunnels or that are jacked into place through undisturbed soil. Buried pipes are sometimes installed under highways, railways, and airports. For this reason, buried pipes are subjected to such applied loads produced by ground transporation traffic. The French mathematician, Boussinesq, calculated the distribution of stress in a semi-infinite elastic medium due to a point load applied at its surface. This solution assumes an elastic, homogeneous, isotropic medium which soil certainly is not. However, experiments have shown that the classical Boussinesq solution, when prop'erly applied, gives reasonably good results for soil. In chapter 2, methods are also given for the determination of loads that are imposed on pipes in these and other applications. Design methods that are used to determine an installation design for buried pipes are described in Chapter 3. Bedding conditions and bedding factors are given by presented and recommended Young and O'Reilly. İn addition, pipe performance limits are given and recommended safety factors are reviewed. The finite element method for design of buried piping systems is relatively new. During the past decade, digital computers combined with finite element techniques and sophisticated soil models have given the engineering profession another design tool which will undoubtedly produce even more precise designs. The use of this powerful tool is increasing with time. In this chapter, a detailed discussion of this method is also included. A buried pipe and soil surrounding it are interactive structures. Soil-pipe interaction influences pipe performance and is a function of both the pipe properties and embedment soil properties. The designer must consider soil- structure interaction in the design of buried pipes. Of course, the soil exerts pressure on the pipe, and if the structure is relatively noncompressible compared to the soil, the pipe will feel pressure concentrations; a hard spot develops in the soil. On the other hand, if the pipe compresses or deforms under the soil load, soil pressure on the pipe may be relieved by the protective strength of the soil. The soil actually arches over the pipe like a masonry arch. Arching action of the soil thus reduces the pressure on the pipe. Arching action depends on soil strength and on relative compression of soil and pipe. The soil-pipe system is highly statically ^determinate. This means that the interface pressure between the soil and pipe cannot be calculated by statics alone- XVthe stiffness properties of both and soil pipe must also be considered. The ratio of pipe stiffness determines to a large degree the load imposed on the pipe. For the purposes of design, it is convenient to classify the buried pipes as either rigid or flexible. For example, a rigid pipe will have a much greater load than a flexible pipe installed under the same or similar conditions. Concrete, reinforced concrete, clay and asbestos pipes are examples of materials which are usually considered to be rigid. Thin steel and plastic pipes are usually considered to be flexible. The design of a rigit pipe is based on its wall resistance to against external loads. The rigid pipe is assumed to not deforme under external loads. Rigid pipes are tested for strength in the laboratory by the three-edge bearing test. The three-edge bearing strength is the load per length required to cause crushing or critical cracking of the pipe test specimen. Experience has shown that the Marston load to cause failure is usually greater than the three-edge bearing strength and depends on how the pipe was bedded. The Marston load to cause failure is called the field strength. The ratio of field strength to three-edge bearing strength is termed the“bedding factor”or“load factor”since it is dependent upon how the pipe was bedded. Field strength Beddingfactor = --:(1) Three - edge bearing strength The field strength is the Marston load that will cause failure in the field. Most designers and specifications require a factor of safety in the design. Thus the required strength is as fallows:.. Design load* Factor of safety Re quired three - edge bearing strength =^,,. -. (2) M s s s Beddingfactor. v ' A design procedure to select the appropriate pipe classification or strength is outlined as follows: 1. Determine the earth load 2. Determine the live load 3. Select the bedding requirement 4. Determine the load factor 5. Apply the safety factor 6. Select the appropriate pipe strength The flexible pipe tends to deflect under soil load. The parameters are most essential in the design or the analysis of a flexible pipe installation: 1. Load (depth of burial) 2. Soil stiffness in pipe zone 3. Pipe stiffness XVIThe design load on the pipe is easily calculated using the prism-load. Theory as discussed in Chapter 2. This load is simply the product of the soil unit weight and the height of cover. Research has shown that the long term load on a flexible pipe approaches the prism load. The soil modulus is a function of soil properties such as soil density, soil type, and moisture content. Experience has shown that soil density is the most important parameter influencing soil stiflhess. For flexible pipes, pipe stifrhess rather than crush strength is usually the controlling pipe material property. Design follows three simple steps: 1. Determine the external loads 2. Determine adequate wall thickness 3. Check ring deflection (allowable ring deflection is %5) In recent years, a large number of constitutive models have been proposed to predict the behavior of soil structure. For this reason, the use of finite element method in soil-pipe interaction mechanics is given at the end of the Chapter 3. The soil-structure interaction finite element system, the salient characteristics of the soil and pipe models, the soil and pipe properties and the principal features of the soil meshes are described. In chapter 4, an analytical study of a trench condition excavated in soft soil is presented. A finite element software program-SIGMA/W- that can be used in stress-deformation analysis of geotechnical problems is used to simulate the problem. For this reason, first, SIGMA/W program is described, than trench condition is simulate to analyse by SIGMA/W program. The analysis contains the following step so as to simulate the incremantal construction. 1. Insitu stress analysis 2. Installation of steel sheet piles and excavation step by step 3. Installation of embedment 4. Installation of the pipe 5. Installation of backfill step by step 6. Extraction of the sheet piles xvn
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