Zemin-boru etkileşimi ve gömülü rijit borular üzerindeki toprak yükleri ile ilgili karşılaştırmalı bir inceleme
A comparative study on soil-pipe interaction and loads on rigid underground conduits
- Tez No: 75513
- Danışmanlar: DOÇ. DR. TUĞRUL ÖZKAN
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Geoteknik Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 150
Özet
ÖZET Rijit borular üzerine etkiyen toprak yükü hesabında dolaylı hesap yöntemleri denilen teorik hesaplar gerçek zemin ve imalat koşullarını yansıtmamaktadır. Bu yüzden rijit boruların boyutlandırılmasında yanılgılara düşülebilmektedir. Günümüzde matematik modellerin gelişmesi tüm zemin mühendisliği prob lemlerinde olduğu gibi zemin-boru etkileşimlerinde ve boru üzerindeki toprak yükü hesaplarında sonlu elemanlar yönteminin kullanılmasını geçerli kılmıştır. Çeşitli ze min koşullarında kullanılabilecek zemin parametrelerini belirlemek için laboratuar deneyleri yapılmış ve bulunan parametreler sonlu elemanlar yöntemini kullanan programlarda denenmiştir. Zemin parametreleri gerçek zemin ve imalat koşulları modelleyebilmekte ve boru-zemin ara yüzeyindeki normal-kayma gerilmeleri dağılımı, boru cidarındaki moment-eksenel kuvvet dağılımı ve çevre zemindeki gerilme-deformayon bağıntıları bulunabilmektedir. Boru üzerindeki yükleri doğrudan belirlediği için bu hesap yön temlerine doğrudan hesap yöntemleri denir. Bu çalışmada dolaylı ve doğrudan hesap yöntemleri anlatılmış ve karşılaş tırmaları yapılmıştır. Zemin ve yerleştirilmiş zemin parametrelerini belirlemek için yapılan laboratuar deneylerinden bahsedilmiştir. LUSAS sonlu elemanlar sistemi tanıtılmış ve bu sistem ile hendek içindeki borularda boru-zemin etkileşimi modellenmiştir. Oluşturulan model üzerinde dolgu yüksekliği, hendek genişliği, geri dolgu sıkılığı ve malzeme cinsi gibi faktörlerin etkisi incelenmiştir. Son olarak LUSAS sonuçları ve diğer hesap yöntemleri karşılaştı rılm ıştır. XIV
Özet (Çeviri)
SUMMARY Underground conduits of the types used for sewers, drains, culverts, water mains, gas lines and the like have served to improve the standard of living of man kind since the dawn of civilization. Remnants of structures of this kind have been found among the earliest examples of the practice of the engineering arts, but only within the past several decades has it been possible to design conduits on a ra tional basis with a degree of precision comparable with that obtained in the design of bridges and buildings. The loads to which buried conduits are subjected in serv ice and their supporting strength under various installation conditions may be deter mined by means of the Marston Theory of Loads on Underground Conduits. For purposes of load computation, underground conduits are divided into two major classes, known as ditch conduits and projecting conduits, the classifica tion being based on the construction or environmental conditions which influence the load. Projecting conduits are further subdivided into positive projecting conduits and negative projecting conduits. Also there are several special cases having char acteristics which are similar to those of both of the major classes. A ditch conduit is defined as one which is installed in a relatively narrow ditch dug in passive or undisturbed soil and which is then covered with earth back fill. A positive projecting conduit is one which is installed in shallow bedding with its top projecting above the surface of the natural ground and which is then covered with an embankment. A negative projecting conduit is one which is installed in rela tively narrow and shallow ditch with its top at an elevation below the natural ground surface and which is then covered with an embankment. This is a very favorable method of installing a railway or highway culvert, since the load produced by a given height of fill is generally less than it would be in the case of a positive pro jecting conduit. Design of buried concrete pipe installations usually involves determining the expected load to be carried by the pipe and then establishing pipe strength re quirements, taking into consideration installation conditions. The most common and best known method of design is based on an approach conceived by Marston over 70 years ago and further developed by Spangler. The method, including de sign parameters, has essentially remained unchanged for more than three decades. The Marston-Spangler Method is based on certain assumptions of soil be havior and principles of mechanics. The earth load on a pipe varies not only with the soil properties, but also with the installation geometry. Therefore, separate load equations must be derived for each installation type. xvThe Marston-Spangler method for designing pipe for a particular installation involves the following general steps: (1) Calculate the earth load and live loads on the pipe, (2) Determine the bedding factor for the bedding to be used, (3) Designate the appropriate factor of safety, (4) Select the required pipe 3-edge bearing strength based on the first three steps. In a trench installation, the earth load on the pipe developes as the backfill soil is placed in the trench. When each layer is added, the soil settles, which in turn causes shear stresses to develop along the trench wall. These soil shear stresses are assumed to be frictional only. The soil weight supported by the pipe is assumed to be the weight of all the soil in the trench minus the vertical shear force along the trench walls from the soil surface to the pipe crown. The support provided by the trench backfill at the sides of the pipe is neglected. The assumption is made that the pipe supports all of the soil load in the trench that reaches the crown elevation, i.e., the sidefill supports no load. Thus the pipe diameter is not a factor of the soil assumed to be supported by the pipe (Wd). This assumption causes the value of Wd to be increasingly too conservative as the trench is widened. To account for this effect, the maximum load for the trench case is assumed to be the soil load determined for an embankment installation for the same type and height of backfill. The trench width producing this same load is designated the“transition width”. There are two general categories of loading conditions to be considered in positive projecting embankment installations. One of these, designated the complete condition, consists of installations in which the plane of equal settlement is above or at the top of the fill. The plane of equal settlement is the horizontal plane above the pipe along which all vertical deflections are equal. The other, designated the incomplete condition, occurs when the plane of equal settlement is below the top of the fill. Both the complete and incomplete conditions are subdivided into two categories. One is a trench condition in which the shearing stresses on the side of the soil prism over the pipe act to remove some of the load on the pipe, much like a trench installation. This condition results when the soil zone adjacent to the pipe is more stiff than the pipe. The second is a projection condition in which the shearing stresses on the sides of the prism over the pipe add additional load to the pipe. This occurs when the pipe does not deflect as much as the adjacent soil. For the incomplete conditions, the height above the pipe crown, (He), to the plane of equal settlement must be found in order to determine the height over which the vertical soil shearing stresses act. This is done by equating the settlements of the interior and the exterior soil prisms, assuming that the behavior of the soil may be approximated with elastic theory. The height He, is expressed as a function of the product of the settlement ratio (rsd) and projection ratio, p. The negative projecting installations have both a complete and an incomplete condition. The adjacent natural ground is assumed to support part of the soil load above the pipe. Thus, in a negative projecting installation, the soil shearing stresses are assumed to act so as to always reduce load on the pipe, i.e., like a trench condition. XVIOnce the pipe load has been determined, the next step in pipe design for a particular installation involves defining the bedding factor. The bedding factor is the ratio between the supporting strength of the buried pipe and the strength of the pipe in a three edge bearing test. The bedding factor is dependent on a number of conditions, including the width of the bedding area, the quality of contact between the bedding and the pipe, the magnitude of supporting lateral pressure on the pipe, and the area over which this lateral pressure acts. The bedding factor for different classes of beddings varies between 1.1 to 2. Earth load and pressure distribution on buried rigid (concrete) pipe are the result of the effects of interaction between the buried pipe structure and the surrounding earth. The pipe-soil system constitutes an indeterminate structure, both the magnitude and the distribution of loads on the pipe are a function of the relative rigidity of the pipe and the stiffness of its surrounding soil. Currently used procedures for determining earth loads on buried rigid pipe are based on assumptions about the relative rigidity of the pipe and the surrounding soil. The Marston-Spangler theory for determining load on a buried concrete pipe is based on assumptions that the pipe is very rigid relative to the surrounding soil. Furthermore, for trench installations the in situ soil in the trench wall is assumed to be relatively rigid compared to the soil placed in the trench adjacent to the pipe and over the pipe. These are rather crude procedures to account for soil-structure interaction effects that result in earth loads on a buried pipe of up to about 1.5 times the weight of the earth prism directly over the pipe outside diameter for an embankment installation and as little as 0.7 times the prism load for some narrow trench installations. The predominant current methods for designing buried rigid pipe are termed“indirect”methods because they are based on certain long established empirical methods for determining the total earth plus surface load on the buried pipe and then converting these loads for the field installed condition to equivalent loads in a 3- edge bearing test. The availability of mathematical models that characterize the placed soil stiffness properties (modulus of elasticity and Poisson's ratio) with reasonable accuracy has made possible the development of more accurate analysis for earth load and pressure distributions on buried pipe. These require a nonlinear characterization of soil stiffness that changes with soil pressure, and also of pipe stiffness that changes as cracks develop in the pipe wall. As it is not practical to conduct extensive laboratory tests to obtain placed soil properties required by the finite element programs for each design, laboratory testing was carried out on different soil types (sand, silt, clay) and compaction levels. These represent a coarse-grained gravelly sand soil, a silty soil and a clayey soil, which give a reasonable range of characteristics representative of those frequently encountered in the field. Accurate modelling of soils such as pea gravel and highly plastic clays, ar unusual soils, will require additional testing. A consistent set of design parameters was obtained by fitting the test results to a hyperbolic soil model representing Young's modulus and bulk modulus as a function of stress state. As a result of these tests; XVII(1) much less effort is required to obtain %90 standart proctor density for gravelly sand soil than for the clay soil, (2) soil parameters c and
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