Yatay yük etkisindeki model kazıkların davranışı

Abstract

Dünya'da pek çok ülkenin en önemli sorunlarından birisi enerji açığıdır. Enerji sorununu halletmek üzere bazı ülkeler petrol arama ça lışmalarını denizlerde yapmak gereğini duymuştur. Bu çalışmaların sonu cunda Dünya petrol ihtiyacının önemli bir kısmının denizlerden karşıla nabileceği anlaşılmıştır. Bu amaçla, petrol üretimi için inşa edilen ya pılar çoğunlukla kazıklı platform tipinde olmuştur. Ayrıca, deniz içer sinde teşkil -edilen deniz fenerleri, askeri ve sivil havacılık radarları, derin su rıhtımları, nükleer enerji santralları gibi yapılar genellikle kazıklı temel üzerine oturmaktadır. Deniz yapılarının geoteknik tasarı mında en önemli husus rüzgâr ve dalganın oluşturduğu tekrarlı yanal kuv vetlerdir. Ayrıca tankerlerin platforma yanaşmasından ötürü kısa süreli statik yüklemeler de söz konusudur. Bu tezde, problemin tanımı yapıldıktan sonra yatay kuvvet etki sindeki kazıkların davranışını açıklayan mevcut yöntemlere yer verilmiş tir. Daha sonra yanal yüklü kazıkların eğilme momentleri veya yerdeğiş- tirmelerinin deneysel belirlenmesinde ortaya çıkan sorunlardan bahsedil miştir. Tezin dördüncü bölümünde yatay yüklü kazık probleminin çözümü için (2), (3) no'lu referanslarda verilen yöntemden bahsedilmiştir. Ayrı ca, bu bölümde çeşitli model' kazıklar üzerinde yapılan deneylerde kulla nılan deney düzenine yer verilmiştir. Beşinci bölümde ise kazık davranı şının analizi, tekrarlı yatay kuvvet etkisiyle yatay kazık yerdeğ iş t irme sinde meydana gelen artışlar, yük artımının ve tekrarlı yüklemenin eğilme momentine etkisi ayrıntılı olarak incelenmiştir. Altıncı bölümde ise ge nel sonuçlardan bahsedilmiştir.

Energy gap is a vary serious problem concerning many countries of the World today. There have been many operations including exploring, drilling and production in the seas by some countries. As a result of these operations, it is clearly seen that an important and wide portion of the World's petroleum need could be supplied by the offshore operations today. The petroleum platforms erected for drilling and production purpose are generally constructed in the form structures on piles. Many other structures such as military and civil aviation radars, nuclear power plants are generally based on pile foundations. In the geotechnical designs of the offshore structures must be taken into account the cyclic lateral forces exerted by winds and waves. Also there are other short-term horizontal loadings such as berthing loads and accidental impacts caused by the ships. The behaviour of a pile under cyclic-lateral loading has been studied by many investigators and the results of these studies have been reported in the literature. Such techniques include linear subgrade reaction analysis, non linear subgrade reaction analysis or p-y analysis, elastic analysis and finite element analysis. For the analysis of pile behaviour under cyclic loading, p-y method has certain advantages. The design of offshore structures should take into account the three types of lateral loadings that are given below: o Short-term static loading, o Cyclic loading, o Cyclic loading followed by static or cyclic loading. Laterally loaded piles are generally classified as below considering the pile head condition: o Free-head piles, o Fixed-head piles. They may also be classified according to the pile penetration: o Rigid or short piles, o Flexibil or long piles, o Semi-rigid or intermediate piles. It may be said. that long piles are usually preferred in practice. For this reason, in this thesis, the behaviour of long piles under cyclic and static lateral loading have been studied. The problem of laterally loaded piles is closely relat ed to the familiar problem of a beam on an elastic foundation; however, in one respect, it represents a more specialized case. All external forces and moments applied to the pile- soil system are introduced through boundary conditions existing at one point, the top of the pile, while loading may be applied at many points along a beam. On the other hand, rational solutions of pile-soil interaction problems require generalization of the beam-on-elastic-foundation theory to account for the non-linear characteristics of real soils. By writing the differential equation of a pile affected by lateral forces, the classical beam on an elastic foundation theory can be adopted(21). In this case it can be assumed that. - VI - dx where ; y: Pile deflection x: The depth below soil surface EI: Flexural rigidity of the pile p: Soil reaction __The solution of this equation depends upon the develop- ment of"~â mathematically convenient function for the soil reaction p. The soil reaction may be a function of the pile properties, the stress-strain relationships of the soil, the effective unit weight of the soil, the depth of the overburden at the point considered, the deflection of the pile, the rate of loading, the number of cycles of loading, and perhaps other parameters. A comprehensive solution to the problem of the laterally loaded pile has two major parts: 1- It is necessary to obtain complete information describing the behavior of the surrounding soil, 2- It is necessary to solve the differential equation. If the soil resistance-pile deflection relationship is not lineer line then in the solution the elasto-plast ic behaviour of the soil must be considered. In this case, either the iterative applications of one of the elastic methods, or the behavior of the laterally loaded pile is examined by con sidering the physical model of the pile-soil system. In this thesis, after summarizing the methods explaining the behaviour of piles affected by the horizontal forces, the problems observed in tne experimental determination of the moments or deflection has been explained. Later, the writer of this thesis has studied the behaviour of the model piles made of two different materials affected by static and cyclic horizontal forces. The laboratory experiments have been carried out in two groups. In the first group of experiments, araldite sheets were used as a model pile and the behaviour of a pile inside the soil was studied by photo-elastic method. In the second group of experiments, a model pile was formed by duraliminium. The deformation of pile under horizontal forces in the soil was detected by strain gages. In the following parts and paragraphs of the thesis, the data from- the model experiments, the analysis of data, and the evaluation of data are presented. The general conclusions reached by this work are listed below: 1- The Pile deflection bending moment, shear force and soil reaction curves obtained from the experiments are in harmony. The harmonization of these curves that were determined by the analysis of Galerkin method of pile behaviour shows the validity of the method used. 2- The cyclic nature of a horizontal force has made important increases in the pile deflections that are measured on the soil surface. In a pile group, it is known that if the approximate horizontal force acting on each pile is considered to act on a single pile, the horizontal deflection of t~he single pile will be less than the horizontal deflection of the pile group. For this reason, the deflections of the pile groups of offshore structures under the cyclic horizontal forces must be carefully investigated. 3- The points that the bending moment is maximum, change from 3b to 5b from the soil surface where (b) shows the pile width (diameter). This value is in agreement with the values given by the literature. The cyclic loading of the pile doesn't change practically the place where bending moment passes through the maximum. - V1I1.4- Cyclic loading increases the maximum bending moment which affects the pile about 1% to 5%. This increase of the maximum bending point can be neglected. Cyclic loading has made an important increase at the bending moments at points of the pile's bottom middle. This increase is almost 30% of the bending moment observed under static loading at the mentioned cross-sections. With all these, because the pile design is made according to the maximum bending moment, the increase of bending moment which is at the bottom middle of the pile, has not been important for projection of the constant cross-sectioned piles but important for the variable cross-sectioned piles. 5- The applied load level of the horizontal force or the increase of cycle number in the cyclic loading experiments has caused the soil displacements around the pile to extend to a larger and a deeper region. During the cyclic loading cavities may form around a pile because of the rolling of sand grains at sections near the surface. Because of this reason, due to the cavity and separation and the decrease of soil reactions, the pile deflections have been increased. At the design of offshore constructions, this point must be taken into account.