Kazık taşıma gücünün sonlu elemanlar yöntemi kullanarak kohezyonlu lineer olmayan zemin davranışında incelenmesi
A study on the single pile bearing capacity computed with finite element method
- Tez No: 66669
- Danışmanlar: DOÇ. DR. M. TUĞRUL ÖZKAN
- 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: Geoteknik Bilim Dalı
- Sayfa Sayısı: 171
Özet
ÖZET Kazıklar ve kazıklı temeller, inşaat mühendisliğinde çok önemli bir yere sahiptir. Sorunlu ve taşıma gücü düşük olan zeminlere inşa edilecek binalarda, köprülerde, dolgularda, deniz yapılarında, şevlerin stabilitesinin sağlanmasında, otoyollarda, kazıların geçici veya kakçı olarak desteklenmesinde kazık veya kazıkların grup olarak değişik sekililerde ve amaçlarda kullanıldığını görmekteyiz. Geoteknik bir problemin çözümünün bir takım zorluklan vardır. Bunların başında zeminin üçfazlı ortam oluşu, homojen olmayışı ve elasto-plastik davranış göstermesidir. Bu özellikler zemini, diğer yapı elemanlarından ayırır. Zeminin malzeme özelliklerini iyi bir şekilde belirlemek ve bu özelliklerin yatay ve düşey mesafelerle değişimini gözlemek önemlidir. Zemin elasto-plastik bir malzeme olduğu için akma noktasını belirlemek önem kazanmaktadır. Çünkü bu noktadan sonra zemin taşana gücünün büyük bir bölümünü kaybetmekte, akıcı bir kıvam almakta ve büyük deformasyomar yapmaktadır. Kazık davranışı incelenirken, kazığın geçmiş olduğu tabakaların özelliklerim iyi bir şekilde belirlemek ve kazığın imalatı sırasında ki lokal özellik değişimlerini ve ekstra kuvvetleri göz önünde bulundurmak, kazık taşıma gücünü gerçeğe uygun olarak hesaplamamızı sağlayacaktır. Düşey yük altındaki bir kazığın taşıma gücü, kazığın taşıma gücünü etkileyen faktörlerden negatif çevre sürtünmesi ve yoğrulma etkisi Sonlu elemanlar programı LUSAS 11.3 kullanılarak yapılmıştır. Sonlu elamanlar programı, kullanıcıya çok değişik malzeme özellikleri ve yük durumları tanımlama kolaylığı sağlamaktadır. Lusas ile yapılan analizlerde, değişik çaplar da kazıklar kullanılmıştır. IX
Özet (Çeviri)
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 required to- carry uplift loads- when used to support taü 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. Combination of vertical or horizantal 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 art and science of the civil engineer. In Britain the are numerous example of timber piling in bridge works and river side settlements constructed by the Romans, la Ghin% timber piling was-used by bridge 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 steU only because these newer materials could be fabricated into units that were 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 the late nmeteenthr and early twentieth centuries, largely replaced timber for high-capaqity piling for works on land. It could be precast in various structural 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 fabrication and handling and its ability to withstand hard driving problems of corrosion in marine structure have beer* overcome by the introduction- of durable coating and cathodic protection. While materials for piles can be precisely specified and their fabricationr and can. be controlled to confirm to strict specification and code of practice requirements, 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 conditions which govern the supportting capacity of the piled foundation are quite different. No matter whether the pile is installed by driving withr 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 soi} or rock beneath the toe of pile is compressed ( or sometime loosened ) to an extent which may affect significantly its- end, bearing resistance. Changes take place in tiie conditions at the pile- soil interface over periods of days, months or year which in turn depend on tiie relative pile-to sou movement, and chemical: or electio-chemieal 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 operation of driving or drilling for adjacent piles can cause changes in the carriying capacity and load-settlement characteristic of the piles in the group-that have already been driven,. The soil parametres 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 check 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: design 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 asa means of cheeking- workmanship^ is a separete consideration. Where the effective overburden pressure is an important parameter for calculating the ultimate bearing capacity of piles ( as in the case for granular so£|s ) 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, ojj- a long term effect such as the predicted 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 pile ( say, three to five pile diameters ) at this instant, undrained sou- 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 ramain oceuring 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 consoliation ofthe sou 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 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 overconsohdated clays; in fact the capacity may decrease slightly with time as the high lateral pressure dissipates via creep over a period of time. xtWhere 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 sou become part of the pile shat as-the cement hydrates. The loss of Ko 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.& to 4,0 or more, depending on designer uncertaineties. 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 ia form,their suecesful use, to make a prediction of capacity which closely compares with a load test if often a fortunate event. This- is because of the difficulties in- determining the in situ- soil properties and which because in the vicinity of the pile after it is installed- driven or othervise. AdditionaHy,the sou, varibihry, both laterally, and- vertically, coupled with a complex pile-soil interaction creates a formidable problem for suecesful 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 sou-, where slip between pile and sou, where is defined as the accumulated differences in shaft strain from axial load and soil strain caused by the-load trasfered to it via, skin resistance. This-süp-progresses down-the pile shaft with increased load. Where limiting shear ressitance is developed at large slips in the upper zoneSj part of tike load is transferred back kt©^ the pile shaft which ia turn- produces larger relative slips and at progressively greater depths. A study of load-settlement and load-trasfer 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 tiae 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- 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 sou via resistance, the pile load will decrease from the top the point. The elastic shorthening ( end 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 cohesionless soils and the shape somewhat dependent on embedment depth in all materials. Generally a short pile will display a more linear load-trasfer curve a long pile ; however, this is somewhat speculative since not many very long have been instrumented for obvious reasons. sitGenerally, there will be a minimum of two or three piles under a foundation elemets- of footing to allow for nusaugnments-and other inadvertent eccentricities, T,he superimposed pressure intensity will depend on both pile load and spicing and if sufficiently large sour wiH fait in shear or the settlement wiH be excessive. The stress intensity from overlapping stressed zones will obviously decrease with increased pile spacing, s; however, large spacing are often-impractial since a pile cap-is-east 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 astrength or settlement estimate. These stresses are difficult to estimate for several reasons: 1. Infiuanee 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 püe, which are generally not known ; hence point load is also not known. 3-. The overlap of stress fromadjacent püe% which is- difficult to evaluate. 4. The infiuanee of driving the piles on the adjacent soil. 5. Time-dependent effects such as- consolidations, thjxotrophy, 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 deposit. As can be seen a pile group either transmits the load throughout a soil mass of depth Lf for friction piles or to-depth L for on end-bearing pie. The sou at or below these dephts-musfr carry the load without excessive deformation or the load must be transmitted to deeper strata. An analtieat method of evaluating the stresses in the strata undelying a- pile group uses authers extension of a method proposed by the Geddes adaption of the Mindlin soluation of a point load at tha- interior of an- elastic solid. As with- tiie Boussiness analysis, this method assumes the soil is semi-infinitive isotropic, homogenous and elastic. Sou does usually fit those assumptions; thus the soluations are in error, but they be as good as the Boussinesq soluation which is widely for footing- setiements. Geddes later made soluations for the Boussinsq case for subsurface loadings. These are generally less accurate than the Mindlin soluation. Poulos and Davis also used the Mindlin soluation to predict settlements^ Instead of presenting tables of stress coefficienct, they presented charts for settlement-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 single pile bearing capacitys, shaft resistance, tip resistance, negative skin friction and remolding effects on the pile bearing capacity, load- xiiidisplecament, stress-strain curves and the relations between them were made with the LUSAS Finite Element System. Results of these analysis are given in a comparative way. JQV
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