Yumuşak zeminlerin kayma mukavemeti
Başlık çevirisi mevcut değil.
- Tez No: 55759
- Danışmanlar: PROF.DR. ERGUN TOĞRAL
- 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: 77
Özet
ÖZET Zeminlerin mekanik davranışlarının belirlenmesi ve modellenmesi üzerine yapılan çalışmalardan biri olan kritik durum teorisi, zeminin kayma gerilmelerinin kritik duruma ulaşıncaya kadar gösterdiği efektif davranışı incelemektedir. Zemin, kaygan bir sıvı haline gelinceye kadar zorlanınca kritik duruma erişmektedir ve kırılıncaya kadar gösterdiği davranış zeminin konsolidasyon tarihçesi ile orantılıdır. Bu çalışmada kohezyonlu bir zeminin mukavemet parametreleri, kritik durum teorisinden yararlanarak araştırılmış ve gerilme şartlarına bağlı olarak maksimum kırılması ifade edilmeye çalışılmıştır. Bunun için bir seri normal ve aşın konsolide numuneler ile drenajsız basınç deneyleri yapılmış ve numunelerin gerilme izleri sabit su muhtevası düzlemlerinde elde edilmiştir. Yapılan tüm deneyler deformasyon kontrollü üç eksenli basınç deneyleridir. Yapılan 14 adet konsolidasyonlu - drenajsız deneyler sonucunda, maksimum deviatör gerilmelerinin oluşturduğu kritik durum eğrisinin, q'-p! düzlemindeki izdüşümü bulunmuştur. Ayrıca w - log p' düzleminde kritik durum ve normal konsolidasyon eğrilerinin izdüşümleri çizilip, iki eğrinin birbirine paralel olduğu saptanmıştır. Ülkemizde zemin problemlerinin çözümünde Mohr - Coulomb kırılma hipotezinden elde edilen parametreler kullanılmaktadır. Bu çalışma, kritik durum teorisi ile elde edilen parametrelerin mühendislik çalışmalarında tasarım kriterleri haline getirebilinmesi için ileride yapılacak bir seri çalışmanın ilk basamağıdır. XI
Özet (Çeviri)
Soil, as a material is the composite of the rocks which are weathered under different environmental conditions. The mechanical or chemical agents, erosion, transportation, deposition and compression by later sediments are the natural cycle of weathering of Earth's crust. Since soils are formed by the weathering of rocks the soil grains will consist of the basic rock - forming minerals or their products after chemical alteration. If rocks are only physically degraded by the motion of ice, water or air the soil grains will have the same composition as the parent rock ; the size, shape and texture of the mineral grains will depend primarily on the history of degradation, transportation and deposition. When chemical changes occur, the basic rock - forming minerals may be changed to the day minerals. Civil engineers are interested in the design and the construction and are obliged to perform safety and serviceability of the engineering structure. A geotechnical engineer must be concerned with the mechanical behavior of the soil more than the microscopic properties. Testing the samples in the laboratory plays an important role in soil mechanics research and civil engineering practice. Laboratory tests are applied on small samples of soil to examine the characteristics of soil or on models of soil structures to examine how slopes, walls and foundation deform and collapse. The triaxial tests the most common and versatile test to study the stress - strain properties of soils. A cylindrical specimen of soil is fist subjected to a confining pressure, then the axial stress is increased until the specimen fails. In the field the major and minor principal stresses are not equal, so the soil is subjected to anizotropic consolidation. Therefore the soil samples consolidated in the triaxial tests before compressing to model the in - site conditions. In this study, consolidated - undrained triaxial compression tests are carried out on uniform remolded, saturated cohesive soils to analyze the effective stress - strain behavior of them. The chapters of this study are summarized as below:. The critical state of shear stresses, the minerolojic structure of clays, the behavior clays under various initial consolidation and drainaged conditions and the complete state boundary surface occurred by the stress paths are explained in chapter 2. Also in this chapter, the effective behavior of clays and changes of pure water pressures according to loadings are expressed in q' - p' - v space.. In chapter 3 preparing of triaxial specimens, the basic features of triaxial tests (consolidated - undrained) and the information's about the other tests used in the xii. study are given. The parameters used in the study and the methods of calculation of experiments, are explained also in this chapter. The whole results of tests and the discussions of them are in chapter 4 while chapter 5 involves the interpretation of results obtained from the tests and the characteristic soil parameters of the clay used in the study. The graphics of oedometer tests and the stress path of the triaxial specimens are presented in Appendix. The general concept in Soil Mechanics, failure occurs if the soil is compressed until the yielding point. The behavior till this yielding point is related with the consolidation history of the soil. The failure points obtained from drained and undrained tests form a single and unique line, defined as critical state line. This line is a function of mean effective stress, deviatoric stress and specific volume. The projection of this critical state line on the q' - p' plane is described by the equation : q = M* p where : q = Deviatoric stress M = The slope of critical state line when it is projected onto constant volume plane p = Mean effective stress The undrained tests at constant specific volume and the effective stress will alter to bring the sample into an ultimate state. In drained test the volume of the soil changes due to the drainage of water, because of absence of pore water pressure, the effective behavior of soil can not be observed. According to the initial consolidation conditions, the sample reaches the critical state line in two different ways : i. Normal consolidated and lightly over consolidated samples yields before the critical state line at T| < M stress ratio. If we continue loading with plastically hardening the sample reaches critical state under ı\ ~ M stress ratio. ii. Heavily over consolidated samples reaches critical state line before yielding, and the yielding occurs at r| > M stress ratio. As the strain continues the sample reaches the critical state under r\ = M stress ratio with plastic softening. The behavior of heavily over consolidated samples on triaxial tests are examined in two states : a. The first state is the“failure state”at which the deviator stress reaches a maximum. This state is clearly of interest if we wish to know how much load an element of soil can sustain, and is the state most often sought for in tests whose results are to be used in engineering practice. xiiib. The second state can be called the“ultimate state”, and it is that state at which large shear strains can occur with no change in stress or in volume. A surface followed by all isotropically normal consolidated samples which are loaded by axial compression in the triaxial apparatus and this surface is called“Roscoe surface”. The Roscoe surface forms a boundary beyond which the test paths do not go therefore this surface is a surface which separates states which samples can achieve from states which samples can never achieve. As the failure occurs after the stress path passes the critical state line on heavily over consolidated samples, the failure points forms a surface called as“Hvorslev Surface”which is a state boundary surface for soils. We note that the Hvorslev and the Roscoe state boundary surfaces intersect and that the line of intersection is the critical state line. In undrained tests on normal consolidated and lightly over consolidated samples, the change in pore water occurs positive and in tests heavily over consolidated samples, negative pore water pressure is developed. This shows that the pore water pressure at failure are considerably affected by the initial over consolidation ratios of the specimens. In the study, the triaxial compression test used with the cylinder soil samples 50 mm. in diameter and twice the diameter 100 mm height. In the compression tests back pressure system is used and the pore water pressures are observed by transducer. The clay used in the tests is obtained from the Sarıyer - Kilyos road ( Istanbul ) and dried in room temperature. The dust obtained from the No. 40 sieve kept in the moisture room. Index properties the clay are listed below: w>l=65% wp = 40% Ip = 25 % Gs = 2.70 The triaxial specimens are prepared from the slurry (clay + water) in two ways: 1. The slurry is dried in the air until it reaches a plastic form and plastered into the cylindrical ring without air inside. Eventually the triaxial specimen is obtained by removing the soil from the ring 2. The slurry placed inside of the large diameter slurry consolidation apparatus, consolidated under 100 kPa until the water drainage is over. The samples are obtained from the cylindrical soil mass which is removed from large diameter slurry consolidation apparatus. The sample is enclosed in a thin rubber sleeve sealed to the top platen and to the base pedestal by rubber O-rings. This is contained in a water - filled cell with a cell pressure ct'c A frictionless piston passes through the top of the cell and applies a force P to the top platen. The cell and the sample assembly are placed inside a loading frame and a xivmotor drive applies a constant rate of strain loading. If the drainage valve, connected to flushing is open the sample is drained and if it is closed the sample is undrained. Standard consolidated - undrained tests are performed on normal and over consolidated samples with the measurement of pore water pressure to observe the effective behavior of clay under compression due to the various initial consolidation conditions. The triaxial apparatus is used in all process of tests (consolidation stage and undrained stage) and the results are evaluated in the terms of stress invariants and water content. The stress invariants used are as follows: q = d - a3 = a'ı - a'3 p=l/3(CT'1 + 2a'3) p = l/3(cr,1-a'3) + a,3 Where; a'ı and ct'3 = effective principal stresses In triaxial test G3 is the consolidation pressure which is the difference between constant cell pressure and back pressure, d is calculated from the increment of axial loading divided by the average cross - sectional area of the specimen. The effective stresses (a'ı, c'3 ) are calculated with the help of the pore water pressure obtained from the transducer during the test. The stress invariants forms the stress paths that show the total and effective behavior of sample on q' - p1 plane. The horizontal difference between the two paths (total and effective) on q' - p' plane shows the pore water pressure changes of the samples during the undrained tests. In this study as it is mentioned before on normal consolidated samples the pore water pressure formed positive values while negative values are observed on over consolidated samples at failure. The state line on q' - p' -v space is formed with the points obtained from the maximum values of deviatoric stresses. The projection of this line on q' - p' plane and w - log p' plane are drawn and soil parameters M, X, T are found for the clay used in this study. Also the normal consolidation line is drawn on w - log p' plane to find the parameter N and it is observed that the both lines are similar. The stress paths of normal and over consolidated samples are not similar, because of the pore water pressure changes during the test in each phase. Some of the specimens have failed before they reached the critical state line. As the consolidation pressure increases the permeability decreases, therefore the water content around the shear surface increases and forms a slippery surface. Furthermore, pore water pressure and water content at shear surface can not be measured accurately and the stress path of this samples do not reach the critical state line. XVGenerally the basic parameters of Mohr - Coulomb failure criterion which are angle of friction (|) and cohesion c, are used in geotechnical engineering projects in our country. The parameters of critical state which analyzes the effective behavior of soil take place in theoretical studies. To use this critical theory as a practical engineering theory, the parameters obtained from the critical state must be converted to the engineering design parameters. XVI
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