Kazık yükleme deneyinin sonlu elemanlar yöntemiyle modellenmesi hakkında bir inceleme
A Study about modelling of pile load test by finite element method
- Tez No: 126830
- Danışmanlar: DOÇ. DR. TUĞRUL ÖZKAN
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2002
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Geoteknik Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 287
Özet
ÖZET Bu tez kapsamı içinde, kazığın taşıma gücünün belirlenmesinde ve kazığın gelecekteki davranışı hakkında gerçeğe çok yakın bilgiler veren kazık yükleme deneyleri üzerinde açıklamara gidilmiş ve bu kazık yükleme deneylerinde göçme yükünün doğru olarak tahmin edilmesi için tartışmalar yapılmıştır. Kazıkların göçme yükü araştırılarak, limit değerler bulunmuştur. Günümüzde en çok kullanılan ve geçerliklerini koruyan, norm ve şartnameler (ASTM, DEV, BSI, TSE) incelenerek, kazık yükleme deneylerinin doğru uygulanması için gerekenler açıklanmıştır. Çalışmanın sonunda yine günümüzde yapılan kazık yükleme deneylerinden örnekler verilmiş ve çeşitli yöntemlere göre göçme yükü bulunup bu kazıklar için limit değerler saptanmıştır. Bu yöntemlerin birbirlerine göre değerlendirilmesi yapılıp, gerçeğe en yakın tahminler belirlenmiştir. Değişik çaplardaki kil ve kum zeminler üzerinde çalışmalar yapılmış, bu çalışmalar yardımıyla kazığın çevre sürtünmesi ile deplasman arasında amprik formüller bulunmuştur. Kil ve kum zeminler için bulunan bu iki adet amprik formül yardımıyla kazığın çevre sürtünmesinin değişmediği andaki deplasman değeri bulunabilmektedir. Göçme yükünün doğru saptanması için kazık yükleme deneylerinden yük yerdeğiştirme eğrisi hatasız olarak elde edilmelidir. Bunun için, kazık yükleme deneylerinin büyük bir itina ile şartnamelere uygun olarak yapılması gerekir. Bir kazık için göçme yükü, kazığın dalmasıdır. Dalma büyük yerdeğiştirmelerden sonra meydana gelir. Bu zemin ile kazık sisteminin kabul edilebilir şeklini aşmasıdır. Bir başka deyişle uygulanan yük, arttırılmadan kazığın yerdeğiştirmesine devam etmesidir. Bu, yük yerdeğiştirme eğrisinde belirgin şekilde farkedilmelidir. Yükleme deneylerinin güvenilirliği ve uygulamada faydalı sonuçlarının sağladığı ekonomi, bu deney için harcanan para ve kaybedilen süreden daha önemlidir. ıx
Özet (Çeviri)
A STUDY ABOUT MODELLING OF PILE LOAD TEST BY FINITE ELEMENT METHOD SUMMARY The main function of bearing piles is to transfer the load to lower of the ground that are capable of sustaining the load with an adequate factor of safety and without settling at the working load by an amount damage to the structure that they support. When friction piles are installed in a deep deposit of fairly uniform consistency in order to transfer the foundation pressure to the lower levels, they should be long enough to ensure a substantial advantage over a shallow foundation. In these circumstances it should be borne in mind that for the same superficial area of pile surface, a few long piles forming a piled foundation are more effective and will support the load with smaller settlements than short piles. Piles should be installed to the prescribed depth, resistance or set per blow without damage to the pile shafts or the bearing stratum and records of the installation process should be maintained. The piles should be able to carry their design loads without exceeding the permissible working stresses in the material of the pile, but the stresses during driving may exceed these. The stresses during handling should be within the safe bending stresses prescribed in the design. An investigation of the ground should be carried out by talented and experienced persons. Boring should reach depths adequate to explore the nature of the soil both around and beneath the proposed piles, including all strata likely to contribute significantly to settlement. It is important that the nature and occurrence of groundwater should be investigated. If the standing levels vary from stratum to stratum or if there is a water table gradient between boreholes this should be noted. Groundwater or soil may contain harmful constituents in amounts sufficient to cause damage to Portland cement concrete or buried metals. Chemical analysis of samples of the groundwater and soil should be undertaken to assess the necessity for special precautions. On sites where the soil conditions, do not permit point bearing at economical depth, the available point resistance and skin friction should be determined separately at various depths before the pile foundation is designed. The bearing capacity of a pile dependent on the size, shape and type of pile and on the properties of the soil in which it is embedded. The ultimate bearing capacity is the load at which the resistance of the soil becomes fully mobilized. At a load greater than the ultimate bearing capacity the soil undergoes shear failure, allowing the pile to penetrate into the become, in effect, a different pilefrom that originally installed, with a greater embedded length and possibly different conditions. The ultimate bearing capacity of a pile may be calculated;. By means of a dynamic pile formula. By a static formula on the basis of soil test. By in-situ test (CPT, SPT). By pile-loading test By using a dynamic formula an estimate of the ultimate bearing capacity may be obtained from the driving characteristics of each individual pile, the accuracy being dependent on the reliability of the formula, the conditions of use and data used. By using a static formula an estimated value of the ultimate bearing capacity of a typical pile is obtained, the accuracy being dependent on the reliability of the formula and the soil strength data to which it is applied. The actual ultimate bearing capacity of any particular pile will differ from this value in so far as the soil surrounding it has properties different from those used in the calculation. The test loads may be applied by direct load from platform on which heavy weights are placed, by direct load from a platform on which water tanks are placed, by jacking against a loaded platform, by jacking against an existing structure, thus reducing the amount of load needed. Alternatively, the test pile may be driven in by hydraulic jack bearing against a rijit beam supported on two or more firmly driven anchor piles. The anchor piles should be a clear distance from the test pile or pile group at least five times the maximum diameter of the largest anchor or test pile but not less than 2m. Whichever method of loading is used, it should be possible to load the pile gradually and to remove the load completely after each increment has been applied. Two different loading procedures can be used, mainly maintained loading (ML) and constant rate of penetration (CRP). At maintained loading the load is applied in increment and each load increment, the settlement of pile head is measured. This should be measured by a calibrated load gauge and also by a calibrated pressure gauge in the hydraulic system. The jack and load gauge should be carefully aligned so that the load applied is axial with the pile. The engineer requiring the test to be performed should state the working load and prescribe the stages of loading. For each stage the load or increment of load should be applied as smoothly and expeditiously as possible. A reading of the time and settlement should be taken when loading commences and readings should be made at suitable intervals the load increases. When the load for the stage is reached, time and settlement observations should again be made. The load should then be held constant and settlement reading continued. A graph of settlement versus time should be plotted as the test proceeds, and the trend of the curve will indicate when movement has decreased to an acceptably small rate. The settlement at this xi V^T£&point should be taken as the settlement under the applied load for the purpose of this test. In the constant rate of penetration (CRP) test the pile is made to penetrate the soil at a constant speed while the force applied at the top of the pile to maintain the rate of penetration is continuously measured. As a result of the movement the soil is progressively stressed until in shear; when that occurs the ultimate bearing capacity of the pile has been reached. It should be understood that the purpose of the test is the determination of the ultimate bearing capacity of the pile and that the force curve obtained in the test does not represent an equilibrium relationship between load and settlement. A rate of penetration of about 0.75 mm/min. is suitable for friction piles in clay. For end-bearing piles in sand or gravel, rates of penetration of 1.5 mm/min. may be used. The test usually proceeds very rapidly and requires the services of several observers for taking simultaneous reading. The rate of penetration should be chosen to suit the available pumping equipment. To be preferred are piles tests made in advance of the planning of the structures. Then, besides information indicating load bearing of capacity, these test may yield valuable data regarding the best type of pile for particular site, the length of pile needed, the pile driving equipment required, and whether jetting is necessary. All these will probably enable the designer to develop a safe and economical design for the structure. Movement at the pile head is caused by elastic deformation of pile and soil and plastic deformation of soil. This is the value that is the significant one to be obtained from load test. The theoretical elastic deformation can be computed from the formula, and will be a straight line. 8=(Q x 1) / (A x E) (S.1) Q is the applied load, 1 is pile length, A is pile cross-sectional area and E is modulus of elasticity of the pile material. It is important that the ultimate bearing capacity of a pile is to determine by test loading, because of finding the value truer than others methods. Pile loading tests must be carried out in the following cases by the Commission of the European Communities;. When using a pile system which is outside local experience and has not been tested under similar soil and loading conditions.. When using of piling system is outside the experience of the operatives carrying out the work. Xll. When the piles in the foundation will be subjected to severe transient loading conditions (e.g. heavy cyclic loading, alternative tension and compression). Pile testing must then contain similar loading cycles.. When the pile behavior during installation is below the critical bound which has been anticipated from the site investigation and previous experience. Generally, load and settlement test data are plotted. The plotted settlement could either be gross (the total movement of pile butt under full test load) or the net (the distance the pile has permanently moved after it has rebounded upon removal of the test load). These plotted data are then used to estimate the failure load so that allowable pile capacity can be calculated. Other failure definitions consider arbitrary settlement limits such as the pile is considered to have failed when the pile head has moved 10 percent of the pile end diameter or the gross settlement of 38 mm. and net settlement of 19 mm. occurs under two times the design load. Many engineers define the failure load at the point of intersection of the initial tangent to the load movement curve and the tangent to or extension of the final portion of the curve. By the help of studies on varies diameters clay and sand soils, empirical formulas have been obtained between friction resistance and settlement. The settlement value when the friction resistance has maximum value can be found by these formulas. In this thesis author proposes to state the methods of predicting the failure load on the pile-loading test, so that the current pile capacity can be calculated. Xlll
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