Kazıkların davranışlarının sonlu elemanlar metodu ile belirlenmesi
Başlık çevirisi mevcut değil.
- Tez No: 75220
- Danışmanlar: PROF. DR. M. ATİLLA ANSAL
- 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: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 188
Özet
ÖZET Kazıkların yatay, düşey ve tekrarlı yük etkisi altındaki davranışlarını tespit etmek için birçok yöntem geliştirilmiştir. Gelişen yöntemler ve bununla birlikte daha karmaşık hesapların bilgisayarlar tarafından rahatlıkla çözümlenmesinden dolayı, mühendislere problemin iyi bir şekilde analiz edilmesi ve doğru kabuller yapılarak en iyi yaklaşımın elde edilmesi için büyük görev düşmektedir. Bu çalışmada düşey, yatay yüklemeler ve tekrarlı yüklemeler durumunda kullanılabilecek yöntemler üzerinde durulmuş; düşey yükleme için kohezyonlu ve kohezyonsuz zemin ayrı ayrı incelenmiştir. Yatay yüklemeler için ise yöntemler düşey yüklemeden farklı bir şekilde incelenmiştir. Tekrarlı yük durumunun etkisi yatay yüklemede bahsedilmekle birlikte ayrı bir bölümde şaft direnci üzerindeki etkisinden bahsedilmiştir. Ayrıca, Galata köprüsü inşası sırasında çıkarılan zemin profilinde bulunan bir kazığı, taşıma kapasitelerini belirleyen yöntemlerinden birkaç tanesinin sonlu eleman programı LUSAS- yardımıyla modelleyip çözümü yolluna gidilmiştir. Değerlendirme kısmında sonuçlar ve modeller değerlendirilmiştir. xrv
Özet (Çeviri)
DETERMINING THE PILE BEARING CAPACITY WITH FINITE ELEMENT METHOD SUMMARY Piles and pile foundations have been used in use since prehistoric times. The neolithic inhabitants of Switzerland drove wooden poles in the soft bottoms of shallow lakes 12.000 years ago and erected their homes on them. Venice was built on timber piles in the marshy delta of the Po river to protect early Italians from the invaders of Eastern Europe and at the same time enable them to be close to the sea and their source of livelihood. In Venezuela, the indians lived in pile- supportedhuts in lagoons around the snores of lake Maracaibo. Today, Pile foundations serve the same purpose: to make to build in areas where the soil conditions are unfavorable for shallow foundations. Piles one columnar elements in a foundation which have the function of transferring load from the superstructure through weak compressible strata or through water, onto stiffer or more compact and less compresible soils onto rock. They may be requried to carry uplift loads when used to support tall structures subjected to overturning forces from winds or waves. Piles used in marine structures are subjected to lateral loads from the impact of berthing ships or from waves. Combination of vertical or horizontal loads are carried where piles are used to support retaining walls, bridge piers and abutments and machinery foundations. Piles are commonly used; 1. To carry the superstructure loads into or through a soil stratum. Both vertical and lateral loads may be involved. 2. To resist uplift, overturning^ forces as for basement mats below the water table or to support tower legs subjected to overturning. 3. To compact loose, cohesionless deposits through a combination of pile volume displacement and driving vibrations. These piles may be pulled. 4. To control settlements when spread footings or amat is on marginal soil or is underlain by a highly compressible stratum. 5. To stiffen the soil beneath machine foundations to control both amplitudes of vibration and the natural frequency of the system. 6. As an additional safety factor beneath bridge abutments and/or piers, particularly if scour is a potential problem. 7. In offshore constructon to transmit loads above the surface through the water and into the underlying soil. This is a case of partially embedded piling subjected to vertical (and buckling) as well as lateral loads. A pile foundation is much more expensive than spread footings and likely to be more expensive than a mat. On any case great care should be exercised in xvdetermining the soil properties at site for the depth of possible interest so that neither an excessive number nor lengths are specified. A cost analysis should be made to determine whether a mat or piles are used to control the settlement at marginal soil sites, care should be taken to utilize both the existing ground and the piles in parallel so that a minimum number are requried. Piles are inserted into the soil via a number of methods: 1. Driving with a steady succesion of blows on the top of the pile using a pile hammer. This produces both considerable noise and local vibrations which may be disallowed by local codes or enviromental agencies and, of course, may damage adjacent property. 2. Driving using a vibratory device attached to the top of the pile, This is usually a relatively quiet method and driving vibrations may not be excessive. The method is more applicable in deposits with little cohesion. 3. Jacking the pile. This is more applicable for short stiff members. 4. Drilling a hole and either inserting a pile into it or, more common, filling the cavity with concrete which produces a pile upon hardening. A number of methods exist for this technique. Piles materials are; Timber, because of its strength combined with lightness, durability and ease of cutting and handling, remained the only material used for piling until comparatively recent times. It was replaced by concrete and stell only because these newer materials could be fabricated into uniit that capable of sustaining compresive, bending and tensile forces far beyond the capacity of a timber pile of like dimension. Reinforced concrete, which was developed as a structural medium in late nineteenth and ealy twentieth centuries, largely replaced timber for high-capacity piling for works on land. It could be precast in various structual forms to suit the imposed loading and ground conditions and its durability was satisfactory for most soil and immersion conditions. Steel has been used to on increasing extent for piling due to its ease of fabriaction and handling and its ability to withstand hard driving problems of corrosion in marine structure have been overcome by the introduction of durable coating and cathodic protection. While materials for piles can be precisely specified and their fabrication and can be controlled to confirm to strict spesification and code of practice requnements, the conclusion of their load- carrying capacity is a complex matter which at the present time is based partly on theorical concepts derived from the science of soil and rock mechanics, but mainly on emprical methods based on experience. The commonest function of piles to transfer a load that cannot be adequately supported at shallow depths to a depth where adequate support becomes available. When pile passes through poor material and its tip penetrates a small XVIdistance into a stratum of good bearing capacity, it is called a bearing pile. When piles are installed in a deep stratum of limited supporting ability and these piles develop their carrying capacity by friction on the sides of the pile, they are called friction piles. Many times, the load carrying of piles results from a combination of point resistance and skin friction. The conditions which govern the supporting capacity of the piled foundation are quite different. No matter whether the pile is installed by driving with a hammer, by jetting, by vibration, by jacking, screwing or drilling, the soil in contact with pile face, from which the pile derives its support by skin friction, and its resistsnce to lateral loads, is completely distrubed by the method of installiation. Similarly the soil or rock beneath the toe of pile is compresed to an extent may affect significiantly its end, bearing resistance. Changes take place in the conditions at the pile soil interface overperiods of days, months or year which in turn depend on the relative pile to soil movement, and chemical or electro chemical effects caused by the hardening of concrete or the corrosion of the steel in contact with the soil where piles are installed in groups to carry heavy foundation loads, the opreation of driving or drilling for adjacent piles cancause changes in the carrying capacity and load - settlement characteristic. The soil parametres for static capacity analysis consist in the angle of internal friction and the cohesion c. Controversy arises since some designers use undrained stress where others- particulary more recently-use effective stress values. The engineer is often presented with adequate information on the soil properties. He then has to decide whether to base his designs on conservative values with an appropriate safety factor without any check by loads testing or merely to use the design methods to give a preliminary guide to pile diameter with loading tests to failure. Such testing is always justified a large scale piling project. Proof load testing as means of checking is a separator consideration. Where the effective overburden pressure is an important parameter for calculating the ultimate bearing capacity of piles account must be taken of the effects of a rise ground water levels. This may be local or may be general rise, due for example to seasonable flooding of major river, or a long term effect such as the predicted large general rise in ground water levels Greater London. Pile driven into the mass always produce same to very considerable remolding of the soil in the immediate vicinity of pile (say, three to pile diameters) at this instant, undranied soil-strength parameters are produced, which may approach remailed drained values if the degree of saturation low. In general,, however, there is some considerable time lapse before full design loads are apllied In this interval the excees porepressures dissipate and drained, remolded, soil parametersbest describe the soil behaviour. The pile capacity for soft clays increases with time, with most strength remain occuring in from.1 to 3 months. This is somewhat explained by the high pore pressures and the displaced volume effect producing a rapid drainage and consolidation of the soil very near the pile. In fact the soil very near the pile tends to consolidate to such a high value that effective diameters of the pileis increased 5 to 7 percent over actual value. The reduced water content resulting from consolidation in this zone has been observed for some time. The increase is likely to be- marginal in very stiff and/or overconsolidated days; in fact the capacity may XVIIdecrease slightly with time as the high lateral pressure dissipates via creep over a period of time. Where piles are placed in predrilled holes, the existing soil state remains at wearly the drained conditions. Possible deterioration of the cohesion at the interface of the wet concrete and soil may occur but this may be partially offset by the slight increase in pile diameter as grains in the surrounding soil become part of the pile shat as the cement hydrates. The loss of K> from expansion into the cavity may be partially offset by the latearl pressure developed from the wet concrete which has a higher density than the soil. Safety factors which commonly range from 2.0 to 4.0 or more, depending on designer uncertanities. In general, the safety factors are larger than for spread foundations because of the greater uncertanities in pile-soil interaction and the fact the foundation is likely to be more expensive when piles are used. While equations are certainly not highly complex in form, their succesful use to make a prediction of the capacity which closely compares with aload test if often a fortunate event. This is beacuse in the vicinity of the pile after it is installed-driven or othervise. Addditionally the soil varibility, both laterally, and vertically, coupled with a complex pile-soil inreaction creates a formidable problem for succesful analysis. There are a few general approaches that are avaiable for the analysis of single piles subjected to axial loads. In general following methods estimate pile capacity: Static pile capacities can be computed by the following equations; 1. Static analysis by utilizing soil strength 2.Emprical analysis by utilizing standart field tests a)Standart penetration test b)Cone penetration test cjPresuremeter tests 3. Dynamic driving resistance a)By pile driving formulas b)By wave equation 4. Full-scale pile load tests Lateral pile capacity can be computed by the following equations; I.Winkler or subgrade reaction approach 2.Broms method 3.Brinch Hansen method 4.Poulos method 5.P-Y curves method Winkler to studying the lateral resistance of piles, the soil pressure (p) is related to the lateral deflection (y) through the modulus of subgrade reaction" (kh). The modulus of subgrade reaction has units of force/length, if is multiplied by the length and diameter of a given pile segment, the equivalent spring- stiffness is obtained, in this approach, the pile is considered to be supported bay an array of uncoupled springs. These springs can be taken to be nonlinear. The nonlinear XVIIbehaviourof the soil springs is represented by p-y curves which relate soil reaction and pile deflection at points along the pile length. Finite element or finite difference techniques can be used to determine response of the pile and spring system to applied loads. Winkler's method can be apply for axial and lateral loads with LUSAS finite element computer programme. in the study, non-linear behaviour of the pile-soil system is solved by LUSAS computer programme, and the results are summarized. In the seventh chapter, pile- soil system modelled three models; These models are, 1 -Winkler or subgrade conditions 2-2-D Continum model 3-3-D Continum model in the seventh chapter, non-linear, of the pile-soil system is analysied with lateral loads, axial load- and lateral+axial loads. Results of the analysis are given in the following chapters. XVIII
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