Kazıklı temellerin sonlu elemanlar yöntemi kullanılarak hesaplanması ile ilgili bir inceleme
Başlık çevirisi mevcut değil.
- Tez No: 55935
- Danışmanlar: DOÇ.DR. TUĞRUL ÖZKAN
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Jeoteknik, Kazıklı temeller, Sonlu elemanlar yöntemi, Geotechnics, Piled foundations, Finite element method
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Günümüzde kazıklar ve kazıklı temeller inşaat mühendisliğinde çok önemli yere sahiptir. Sorunlu ve düşük taşıma gücü olan zeminlerde inşaa edilecek binalarda, köprülerde, dolgularda, deniz yapılarında şevlerin stabilitesinin sağlanmasında, otoyollarda, kazıların geçici veya kalıcı olarak desteklenmesinde kazık veya kazıkların grup olarak değişik şekillerde ve amaçlarda kullanıldığını görmekteyiz. Bu çalışma kazıklı temellerin sonlu elemanlar analizi yapan LUSAS ile genel bir incelemesini kapsamaktadır.Kazık gruplarının davranışı hakkında (oturmalar, gerilmeler ve taşıma gücü) çok kesin bilgiler halen yok bu yüzden sonlu elemanlar yöntemi, bu kesin olmayan bilgilerin doğrulanması ve yeni katkılar yapması bakımından, çok fazla öneme sahiptir. Kazıkların ve kazık gruplarının genel olarak tanıtıldığı ve dayanışlarının, hesaplarının verildiği bu tez çalışması, kazıklı temeller için sonlu elemanlar program LUSAS 11.3 kullanılarak incelemeler ve bu incelemelerin sonuçlarım içermektedir. Kazıklı temellerde yükün kazıklar ve radye tarafından nasıl ve hangi oranla zemine aktarıldığı ve bu durumu etkileyen kazık aralığı, s ve kazık çapı, D ilişkileri mcelenmiştir. İncelemeler kaya, kiL kum vb. zeminler için kdelenmiştir. xıu
Özet (Çeviri)
A STUDY ON THE PILED FOUNDATIONS COMPUTED WITH FINITE ELEMENT METHOD SUMMARY 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 suffer or more compact and less compressible soils or onto rock. They may be required 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 and from waves. Combinations of vertical and horizontal loads are carried where piles are used to support retaining walls, bridge piers and abutments and machinery foundations. The driving of bearing piles to support structures is one of the earliest examples of the art and science of the civil engineer. In Britain the are numerous examples of timber piling in birdge works and riverside settlements constructed by the Romans. In China, timber piling was used by the birdge builders of the Han Dynasty (200 BC to AD 200) 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 steel only because these newer materials could be fabricated into units that were capable of sustaining compressive, bending and tensile forces far beyond the capacity of a timber pile of like dimensions. Reinforced concrete, which was developed as a structural medium in the late nineteenth and early twentieth centuries, largely replaced timber for high-capacity piling for works on land. It could be preast in various structural forms tu suit the imposed loading and graund 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 fabrication and handling and its ability to withstand hard driving problems of corrosion in marine structure have been overcome by the introduction of durable xivcoatings and cathodic protection While materials for piles can be precisely specified and their fabrication and can be controlled to confirm to strict specification and code of practice requirements, the conciliation of their load-carrying capacity is a complex matter which at the present time is based partly on theoretical concepts derived from the science of soil and rock mechanics, but mainly on empirical methods based on experience. The conditions which govern the supportting 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 the pile face, from which the pile derives its support by skin friction, and its resistance to lateral loads, is completely distrubed by the method of installation. Similarly the soil or rock beneath the toe of pile is compressed ( or sometimed loosened) to an extent which may affect significantly its end, bearing resistance. Changes take place in the conditions at the pile-soil interface over periods of days, months or years which in turn depend on the relative pile-to soil movement, and to chemical or electro chemical effects caused by the hardening of the concrete or the corrosion of the steel in contact with the soil where piles are installed in groups to carry heavy foundation loads, the operation of driving or drilling for adjacent piles can cause changes in the carrying capacity and load-settlement characteristic of the piles in the group that have already been driven. The soil parameters for static (and group) capacity analysis consist in the angle of internal friction § and the cohesion c. Controversy arises since some designers use undrained (or total) stress where others-particulary more recently-use effective stress values. The engineer is often presented with inadequate 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 chech by loads testing or merely to use the design methods to give a preliminary guide to pile diameter and length and then to base the final designs on an extensive fields testing programme with loading tests to failure. Such testing is always justified an a large-scale piling project. Proof-load testing as a means of checking workmanship is a seperate consideration. Where the effective overburden pressure is an important parameter for calculating the ultimate bearing capatiy of piles (as in the case for granular sails) 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 a major river, or a long-term effect such as the predisted large general rise in ground-water levels in Greater London. Piles driven into the mass always produce same-to a very considerable remolding of the soil in the immediate vicinity of the pile (say, three to five pile xvdiameters) At this instant, undrained soil-strength parameters are produced, which may approach remailed drained values if the degree of saturation is low. In general, however, there is some considerable time lapse (several months to years) before full design loads are applied. In this interval the excess pore pressures dissipate and drained, remolded, soil parameters best decribe the soil behaviour. The pile capacity for soft clays increases with time, with most strength remain occurring 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. In fact the soil very near the pile (a zone of perhaps 50 to 200 mm) tends to consolidate to such a high value that the effective diameters of the pile is 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 clays; in fact the capacity may decrease 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 offsett 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 lateral 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 uncertainties. In general, the safety factors are larger than for spread foundations because of the greater uncertainties 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 successful use to make a prediction of capacity which closely compares with a load test if often a fortunate event. This is because of the diffuculties in determining the in situ soil properties and which because in the vicinity of the pile after it is installed-driven or otherwise. Additionally, the soil variability, both laterally, both laterally and vertically, coupled with a complex pile-soil interaction creates a formidable problem for succesful analysis. The ultimate pile capacity Qu is not the sum of the ultimate skin resistance plus the ultimate point resistance. Ultimate skin resistance is produced at some small value of relative slip between pile and soil, where slip between pile and soil, where slip is defined as the accumulated differences in shaft strain from axial load and soil strain caused by the load transfered to it via skin resistance. This slip progresses xvidown the pile shaft with increased load. Where limiting shear ressitance is developed at large slips in the upper zones, part of the load is transferred back into the pile shaft which in turn produces larger relative slips and at progressively greater depths. A study of load-settlement and load-transfer curves from a number of load tests indicates that slip to developed maximum skin resistance is on the order 5 to 10 mm and is relatively independent of shaft diameter and embedment length, but may depend upon soil parameters. Mobilization of the ultimate point resistance requires a point displacement on the order of 10 percent of the tip diameter for driven piles and up to 30 percent of the base diameter for bored piles and caissons. This is a total point displacement and in material other than rock may include point displacement caused by skin resistance stress transferred through the soil to produce settlement of the soil beneath the point. It is highly probable that in the usual range of working loads, skin resistance is the principal load-carrying mechanism in all but the softest soil. Since the pile unloads to the surrounding soil via resistance, the pile load will decrease from the top to the point. The elastic shorthening (and relative slip) will be larger in the upper shaft length from the larger axial load being comied. Examination in the literature shows that the load transfer is approximately parabolic and decreasing with depth for cohosive soils. The load transfer, however, be nearly linear for cohesionlesss soils and the shape somewhat dependent on embedment depth in all materials. Generally a short pile will display a more linear load-transfer curve a long pile;however, this is somewhat speculative since not many very long have been instrumented for obvious reasons. Generally, there will be a minimum of two or three piles under a foundation element of footing to allow for misalignments and other inadvertent eccentricities. The superimposed pressure intensity will depend on both pile load and spacing and if sufficiently large the soil will fail in shear or the settlement will be excessive. The stress intensity from overlapping stressed zones will obviously decrease with increased pile pile spacing, s; however, large spacing are often impractical since a pile cap is cast over the pile group for the column base and /or to spread the load to the several piles in the group. The soil stresses on underlying strata produced by the several piles in a group are often required to make a strength or settlement estimate, These stresses are difficult to estimate for several reasons: 1) Influnce of pile cap-usually in direct contact with ground except on expansive soils. The results in both the contact soil and the pile carrying the load with the interaction highly intermediate. 2) The distribution of friction effects along the pile, which are generally not xvnknown; hence point load is also not known. 3) The overlap of stress from adjacent piles, which is diffucult to evaluate. 4) The influence of driving the piles on the adjacent soil 5) Time-dependent effects such as consolidation, thixotropy, varying loads, and change in groundwater level. Considering all these variables, it is common practice to simplify the stress computations in a few way. These analysises are to necessary to avoid overstressing the underlying deposits or consolidation settlements in clay deposits. As can be seen a pile group either transmits the load throughout a soil mass of depth depth Lf for friction piles or to depth L for on end-bearing pile. The soil at or below these depths must carry the load without excessive deformation or the load must be transmitted to deeper strata. An analtical method of evaluating the stresses in the strata underlying a pile group uses aouthers extension of a method proposed by the Geddes adaption of the Mindlin solution of a point load at the interior of an elastic solid. As with the Boussines analysis, this method assumes the soil is semi-infinitive isotropic, homogenous and elastic. Soil does usually fit those assumptions; thus the solutions are in error, but thay be as good as the Boussinesq solution which is widely for footing setlements. Geddes later made solutions for the Boussinsq case for subsurface loadings. These are generally less accurate than the Mindlin solution.Poulos and Davis also used the Mindlin solution to predict settlements. Instead of presenting tables of stress coefficienct, they presented charts for settelement-influence factors. Either the Geddes or the Poulos and Davis solutions should provide the same deflection if properly used, since one can easily compute deflections from stresses, but stresses are not as easily back-computed from deflections, stresses may be needed for consolidation settlements Analysis of the stresses beneath the pile groups, load-displacement, stress-strain curves and the relations between them were made with the LUSAS Finite Element System. Result of these analysis are given in a comparative way. The effect of pile spacing, s and pile diameter, D are studied in rock, clay, sand etc. in piled foundations. Distribution of load between the raft and piles examined with the LUSAS Finite Element System. xvni
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