Tekil kazık davranışının lineer olmayan zemin modelinde incelenmesi
Başlık çevirisi mevcut değil.
- Tez No: 55996
- Danışmanlar: DOÇ.DR. M. TUĞRUL ÖZKAN
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 99
Özet
ÖZET Günümüzde, nüfusun,sanayileşmenin ve ulaşımın artmasıyla birlikte yeni yapılanma bölgelerine ihtiyaç artmıştır. Bu ihtiyaç nedeniyle şehirlerde ve çevrelerinde önceden yapı yapılmaktan kaçınılan zayıf zemin özellikli yerlere yönelinmiştir. Bu bölgelerde planlanan yapılar içinde uygun temel çeşitlerinden biride KAZIKLI TEMELLER'dir. Kazıkların diğer temel çeşitlerine göre hem daha pahalı hem de uygulanmasının daha zor olduğu açıktır. Bu yüzden, kazıklı bir temelin yapılmasına karar verilmeden önce diğer çözümler de göz önünde tutulmalıdır. Kazıklı temelin yapılmasına da karar verildikten sonra çeşitli kazık dizaynları arasında en uygunu seçilmelidir. Değişik kazık projeleri hazırlamak ve bunları çözümlemek bir hayli emek ve zaman harcamayı gerektirmektedir.Bilgisayar teknolojisinin hızlı gelişimiyle birlikte kullanım olanağı artan sonlu elemanlar yöntemiyle çalışarak emek ve zaman kaybından ekonomi sağlanabilir. Bu amaçla tezimde Sonlu Elemanlar Yöntemini kullanan Lusas isimli programla tekil kazıkları inceledim. İlk olarak kazıklar hakkında bilgi verdim. Bu bilgileri kazıkların gruplandırılması, statik taşıma gücü, statik kazık formülleri, düşey tekil kazıkların yanal kuvvetlere karşı hesabı, kazığın yanal direnci, yük oturma eğrisinden göçme yükünün hesaplanması ana başlıkları altında verdim ve son olarak Lusas programıyla tekil bir kazığı çeşitli yükler altında davranışını ve zeminle etkileşimini irdeledim. XIII
Özet (Çeviri)
BEHAVIOUR OF A SINGLE PILE IN NONLINEAR SOIL MODELS SUMMARY Piles are columnar elements in a foundation which have the function of transfering 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 are subject to overturning forces from winds or waves. Piles used in marine structures are subjected to lareral loads from the impact of berthing ships and from waves. Combinations of vertical and horizantal loads are carried where piles are used to support retaining walls, bridge piers and abutments, and machinary foundations. The driving of bearing piles to support structures is one of the earliest example of art and science ofcivil engineer. In Britain there are numerous example of timber piling in bridge works and riverside settlements constructed by the romans. In nediaeval times, piles of oak and alder were usedin the foundations of great monasteries constructed in the fenlands of East Anglia. In China, timber piling was used by the bridge builders of the Han Dynasty (200 BC to AD 200). The carrying capacity of timber piles is limeted by the girth of natural timbers and the ability of the material to withstand driving by hammer without suffering damage due to splitting or splintering. Thus primitive rules must have been established in the earliest days of piling by which the allowable load on a pile was determined from its resistance to driving by a hammer of known height of drop. Knowledge was also accumulated regarding the durability of piles of different species of wood, and measures takento prevent decay by charring the timber or by building masonary rafts on pile heads cut off below water level. Timber, because of its strength combined with lightness, durabilityand 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 compresive, bending and tensile forces far beyond the capacity of a timber pile of like dimensions. Concrete, in particular, was adaptable to in-situ forms of construction which facilitated the installation of piled foundations in drilled holes in situations where noise, vibration and ground heave had to be avoided.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 precast in various structural forms to supt the imposed loading and ground conditions, and its durability was satisfactory for most soil and immersion conditions. The partial replacements of deriven precast concrete piles by numerous forms of cast-in-situ piles has been due XIVmore to development of highly efficient machines for drilling pile boreholes of large diameter and greath depth in wide range of soil and rock conditions, than to any deficiency in the performance of the precast concrete element. Stell has been used to an increasing extent for piling due to its ease of fabrication and handling and its ability to withstand hard driving. Problems of corrosion in marine structures have been overcome by the introduction of durable coatings and cathodic protection. While materials piles can be precisely specified, and their fabrication and installation can be controlled to conform to strict specification and code of practice requirements, the calculation of their load-carryingcapacity is complex matter which at the present time is based partly on theoritival concepts derived form the sciences of soil and rock mechanics, but mainly on emprical methods based on experience. Practice in calculating the ultimate carrying capacity of piles based on the principles of soil mechanics differs greatly from the applications of these principles to shallow spread foundations. In the latter case the entire area of soil supporting the foundation is exposed and can be inspected and sampled to ensurethat ists bearing characteristics conform to those deduced from the results of exploraty boreholes and soil tests. Provided that the correct constructional techniques are used the disturbance to the soil is limited to a depth of only a few centimeters below the excavation level for a spread foundation. Virtually the whole mass of soil influenced by the bearing pressure remains undisturbed and unaffected bt the constructional operations. Thus the safety factor againest general shear failure of the spread foundation and its settlement under the design working load can be predicted from a knowledge of the physical characteristics of the undisturbed soil with a degree of certainty which depends only on the complexiy of the soil stratifacation. The conditions which govern the supporting capacity of the piled foundation are quite different. No natter whether the pile is installed by driving by 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 completly disturbed by the method of installation. Similarry the soil rock beneath the toe of a pile is compressed (or sometimes loosened to an extent which may affect significantly its end-bearing resistance. Changes take place in conditions at the pile-soil interface over periods of days, months or years which materially affect the skin-friction resistance of a pile. These changes may be due to the dissipationof excess pore pressure set up by installing the pile, to the relative effects of friction and cohesion 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 wiyh soil. Where piles are installedin groups to carry heavy foundation loads, the operation of driving or drilling for adjacent piles can cause changes in the carrying capacity and load-settelment characteristics of the piles in the group yhat have already been deriven. In the present state of knowledge, the effects of various methods of pile installation on the carrying capacity and deformation characteristics cannot be calculated the strict application of soil or rock mechanics theory. The general procedure is to apply to simple empirical factors to the strength density, andcompressibility properties of the undisturbed soil or rock. The various factors which can be used depend on particular method of installation and are based on experience and on the results of field loading tests. The basis of the 'soil mechanics approach' to calculating the carrying capacity of piles is that the total resistance of the pile to compression loads is sum of two components, namely skin friction and end resistance. A pile in which the skin frictioanal component predominates is known as an friction pile, while a pile bearing on rock or some other hard incompressible material is known as an end-bearing pile. However, even if it is possible to make a reliable estimate of total pile resistance a further difficulty arises in predicting the problems involved in installing the piles to the depths indicated by the emprical or semempirical calculations. It is one problem to calculate that a precast concrete pile must be deriven to a depth of, say, 20 meters to carry safely a certain working load, but quite anather problem to dicede on the energy of the hanner required to drive the pileto this depth, and yet another problem to decide whether or not the pile will be irredeemably shattered while driving it to the required depth. In the case of driven and cast-in- place piles the ability to drive the pilling tube to the required depth and then to extract it within the pulling capacitiy of pilling rig must be correctly predicted. Bjerrum has drawn attention to the importance of time effects include the rate of applying load to a pile, and the time interval between installing and testing pile. The skin frictional resistance of a pile in clay loaded very slowly may only be one-half of that which is measured under the rate at which load is normally applied during a pile loading test. Tha slow rate of loading may correspond to that of a building under construction, yet the ability of a pile to carry its load is judged on its behaviour under a comparatively rapid loading test made only a few days after installation. The carrying capacity of a pile in sands may also diminish with time, but in spite of the importance of such time effects both in cohesive and cohesionless soilsthe only practicable way of determining the load-carrying capacity of a piled foundation is to confirm The design calculations by short-term tests on isolated single piles, and then to allow in the safety factor for any reduction in the carrying capacity with time. The effects of grouping piles can be taken into account by considering the pile group to act as a block foundation. Piles are commonly used: 1. To carry the superstructure loads into or through a soil stratum. Both vertical and laeral loads may be involved. 2. To resist uplift, or 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 later pulled. 4. To control settlements when spread footings or a mat is on a marginal soil or is underline by 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 xviscour is a potential problem. 7. In offshore construction to transmit loads above the water surface through the water and into the underlying soil. This is a case of partially embedded pilling subjected to vertical (and bucling) as well as lateral loads. Piles are sometimes used to control earth movements (as landslides). The reader should note that power poles and may outdoor sign poles may be considered as partiaaly embedded piles subject to lateral loads. Vertical loads may not be significant, altough buckling may require investigation for very tall members. A pile foundation is much more expensive than spread footings and likely to be more expensive than a mat. In any case great care should be exercised in determinig the soil properties at the site for the depth of possible interest so that it can be accurately determined that a pile foundation is needed and, if so, that neither an execisive number nor lengths are specified. A cost analysis should be made to determine whether a mat or piles are used to control the settelment 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 required. Piles are inserted into the soil via number of methods: 1. Driving with steady succession 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 environmental agencies and, of course, may damage adjacent propery. 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 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 tecnique. When a pile foundation is decided to opun, it is necessary to compute the required pile cross section and length based on the load from the superstructure, allowable stress in the pile material (usually a code value), and the in-situ soil properties. This is so that the necessary number and lengths of piles can be ordered by the foundation contractor. Dynamic formulas, pile-loda tests, or a combination are used to determine if the piles are adequaetly designed and placed. It is generally accepted that a load test is the most reliable means of determining the actual pile capacity. Pile capacity determinations are very difficult. A large number of different equations are used, and seldom will any two give the same computed capacity. Organizations which have been using a particular equation tend to stick with it particularly if a successful data base has been established. It is for the reason that a number of what are believed to be the most widely used (or currenly xviiaccepted) equations are included in this text. In a design situationone might compute the pile capacity by several equations using the required emperical factors suitably adjusted (or estimated) and observe the computed capacity. From a number of these computations some 'feel' for the probable capacity will develop so that a design recommmendation/proposal can be made. We should note that although all the pile-capacity equations are for a single pile, rarely is a single pile used; rather two or three (or more) piles are used in a group. We should further note that the soil properties which are used in the design are those from the initial soil-exploraiton program, and the soil properties which exist when the foundation is in service may be very different depending on how the piles have been placed and the number of piles in the group. The British Standard Code of Practice For Foundations (BS 8004) places piles in three categories. These are as follows. Large displecements piles copmrise solid-section piles with closed end, which are driven or jacked into the ground and thus displace soil. All types of driven and cast-in-place piles come into this category. Small displecements piles are also driven or jacked into the ground but have relatively small cross sectional area. They include rolled stell H orl sections, and pipe or box sections driven with an open end such that the soil enters the hollow section. Where these pile types plug with soil during driving they become large displacement types. Replacement piles are formed by first removing the soil by boring using wide range of drilling techniques. Concrete may be placed into an unlined or lined hole, or the lining may be withdrawn as the concrete is placed. Performed elements of timber, concrete, or steel may be placed in drilled holes. Types of piles in each of these categories can be listed as follows. Large displacement piles (driven types) 1. Timber (round or square sections, jointed or continuous). 2. Precast concrete (solid or tubular section in continuous or jointed units). 3. Prestressed concrete (solid or tubular section). 4. Steel tube (driven with closed end). 5. Steel box (driven with closed end). 6. Jacked-down stell tube with closed end. 7. Jacked-down solid concrete cylinder. Large displacement piles (driven and cast-in-place types) 1. Steel tube driven and withdrawn after placing concrete. xviii2. Precast concrete shell filled with concrete. Thin-walled stell shell driven by withdrawable mandrel and then filled with concrete. Small displacement piles 1. Prekast concrete (tubular section driven with open end). 2. Prestressed concrete (tubular section driven with open end). 3. Steel H section 4. Steel tube section (driven with open end and soil removed as required) 5. Steel box section (driven with open end and soil removed as required) Replacement piles 1. Concrete placed in hole drilled by rotary auger, baling, grabbing, airlift orreverse- circulation methods (bored and cast-in-place) 2. Tubes olaced in hole drilled as above and filled with concrete as necessary. 3. Precast concrete units placed in drilled hole. 4. Cement mortar or concrete injected into drilled hole. 5. Steel sections placed in drilled hole. 6. Stell tube drilled down XIX
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