Normal ve aşırı konsolide killerin kırılmadaki davranışları

Abstract

Zemin mühendisliğinde karşılaşılan stabilité problemlerinin çözümü, zeminlerin mukavemet parametrelerinin bilinmesini zorunlu kılar. Ancak zeminin özellikleri geniş bir aralık içinde değiştiği için, arazideki yüklemeler altında davranışlarını analiz edebilmek ve mühendislik tasarımlan yapabilmek, diğer inşaat malzemelerine göre daha karmaşık hale gelmektedir. Bu nedenle zemin davranışım belirlemede gerekli parametreleri elde etmek için deneysel çalışmalara önem verilmektedir. Bu tez çalışması da kohezyonlu zemin için deneysel bir araştırmadır. Bu çalışmaıun amacı, normal konsolide ve aşın konsolide killerin davranışlarını incelemek ve mühendislik çalışmalarım planlarken yardımcı fikirler sunabilmek olarak seçilmiştir. Burada kritik durum teorisinden yararlanılarak yuğrulmuş, suya doygun, her tarafında özellikleri üniform, Bentler Kaolini üzerinde drenajsız üç eksenli basınç deneyleri yapılarak mukavemet parametreleri araştırılmıştır. Gerilme şartlarına bağlı olarak kohezyonlu zeminin maksimum kırılması ifade edilmeye çalışılmıştır. Bu deneyler, deformasyon kontrollü basmç deneyleridir. Çeşitli basınçlar altında konsolide edilen ve drenajsız kesmeye maruz bırakılan numunelere ait gerilme izlerinden, uygulanan gerilme izi ile deney izinin çakıştığı yörünge aranılmıştır. Bu yörünge, boşluk suyu basıncının teorik olarak sıfir olabileceği yer olarak tasarlanılmıştır. Bu araştırma kapsamında normal konsolide killer için 22, aşın konsolide killer için 2 konsolidasyonlu drenajsız deneyler yapılmıştır.

As a civil engineering material, soil is just as important as steel and concrete. Civil engineers are concerned with the design and construction by taking into account of safety and serviceability of any structure. But, before these calculations can be performed, the mechanical behavior of engineering materials such as steel, concrete, and soil must be understood. Any change in the loading must produce distortions in the structure. The necessary links between load and distortion are found using physical theories for the mechanical behaviour of the materials in the structure. A physical theory, whether it be for soil, steel, concrete, must describe all aspects of the mechanical behavior of the material. Here is the study of the behaviour of cohesive soils under various stress systems. An estimate must be made of the equilibrium pore water pressures in the soil. Since the pore water pressure gradually dissipates with time until equilibrium values of pore pressure, this process takes an important role in immediate and long term stability for soils. The triaxial test has become a popular method to determine the shear properties of a soil. In the field consolidation of the soil generally occurs under conditions in which the major and minor principal stresses are not equal. It is necessary to consolidate the specimens under stress ratios similar to those occurring in the field. Therefore anisotropic consolidation may be prefed, but in this study the normally consolidated and over consolidated clays will be isotropically consolidated before shear. In this study, triaxial teste are carried out under various initial consolidation and drainage conditions. The factors controlling the state of maximum shear stress at failure are analyzed. Tests in this research are conducted on a remoulded, saturated cohesive soil which has highly uniform characteristics (Bender Kaolin ). Undrained triaxial compression tests are performed on normally consolidated and overconsolidated clays. The aims are: to prove a relationship between the maximum shear stress at failure and effective normal stress and the water content, to find this relationship as a curve in the devaitor stress ( q ), the mean principle effective stress ( p ), and the water content (w) space, to find out the specific point which develops zero excess pore pressure in p-q space, when maximum deviator stress at failure occurs. In the year 1958 Roscoe, Schofield and Wroth brought a new understanding to the relation of water content, shear stress and mean effective stress; and explained many aspects of existing theories with just one single theory, known as "critical state" concept. The critical state concept is defined by the help of critical voids ratio. The continuos yielding of a sample can be represented by a loading path which rises to the yield surface and then remains on that surface. The problem arises as to whether the path ends at any specific point. If such a point exists, then it will be appropriate to say that a sample is in a critical voids ratio state when the loading path reaches that point. In a drained test the critical voids ratio state can be defined as that ultimate state of a sample will not result in any change of voids ratio. In any series of drained tests the set of critical voids ratio points can be expected to lie in or near a line on the drained yield surface. In an undrained tests the sample remains at a constant voids ratio but the effective stress will alter to bring the sample into an ultimate state. In any series of undrained tests the set of critical voids ratio points can be expected to lie in or near a line on the undrained yield surface. If the results of drained and undrained tests show that, there is one unique line to which all loading paths in ( p, e, q ) space, then it is called as" the critical voids ratio line’’. The projection of this critical state line onto the p-q plane is described by an equation q = M*p where; M = The gradient of deviator stress and effective normal stress. The projection of the critical state line onto the w-p plane is curved. However, if the same data are replotted with axes w-lnp, the points fall close to a straight line. Positive excess pore pressure is developed during tests on normally consolidated specimens. A normally consolidated specimen must fail without change in specific volume at a point on critical state line. We assume that at the beginning the excess pore pressure is zero. Mean effective normal stress increases during tests after a slight increase at the start of test. The decrease in mean effective normal stress, which usually means an increase in volume, was enough to compensate for the decrease in volume that usually accompanies the shearing of a normally consolidated clay. In the heavily overconsolidated specimen, negative excess pore pressure is developed and effective stress increases during this test. Thus the tendency toward volume, increase during shear of a heavily overconsolidated. In tests on overconsolidated samples, the positive change in excess pore pressure is less than in test on normally consolidated samples. Isotropically consolidated undrained triaxial compression tests with pore water pressure measurements using back pressure technique are conducted on samples of SO mm. in diameter and 100 mm. in height. The triaxial apparatus used in this research consists of the triaxial cell, constant pressure application system, loading system, proving ring, pores filter stone, rubber membrane. The materials used in the tests have been brought from a kaolinite mine in Bentler (Buyvikdere, Istanbul). In laboratory, the materials are sufficiently dried in the air not in the oven and then sieved from a No:40 sieved. The materials in dust form are stored in moisture room until they have been used in preparing the clay-water slurry. Index properties of Bentler Kaolin obtained from laboratory tests are given below: Wl = 55% WP = 27 % IP =23% Gs = 2.65 The clay-water slurry is approximately prepared at the same water content. It must be mixed not less than 30 minutes. The oven is not used for drying the slurry, because crystal structure of kaolinite is damaged in oven. It is left in air so that the water can disperse trough the clay particles. In preparing triaxial specimens, a cylindrical steel ring and the piston which moves the specimen out of the ring are used. A piece of slurry is plastered in the cylindrical ring so that no air is settled. Then the triaxial specimen is completed by plastering the slurry into the ring. The prepared specimen is placed on a porous stone. A filter paper separates the soil and the stone so that the specimen will not clog the pores of the stone. The rubber membrane prevents the fluid permeating into the sample. After placing the specimen in the triaxial cell, the cell is filled with oil because of its higher density. The pressure of oil acts uniformly on the soil sample. By definition, the cell fluid can not transmit shear stresses, and the rubber membrane, is also incapable of transmitting shear stresses. Therefore, only normal stresses are applied to all vertical surfaces of the soil. It can be assumed that all the vertical places are principal plane. The triaxial principle stresses acting on the specimen are each equal to the stress applied by the cell fluid-oii. This is the cell pressure and there is a back pressure also. The purpose of a back pressure is to saturate the specimen by dissolving any air present in the pore water. The consolidation process is started when a consolidation pressure which is the difference between cell pressure and back pressure is applied. This period occurs at least one week in this study. To understand whether it has finished or not, the values of pore pressure are measured by pore pressure transducer. This measurement is done while the drainage canals are open and close positions. There must be no difference between them. This means that the water in the sample has been dissipated. The saturation of the sample must be controlled by B control. B is mostly calculated 95% for these tests. The standard consolidated-undrained tests are performed on normally consolidated and over consolidated clays with the measurement of pore pressure. Both consolidation stage and undrained shear stage are represented in the triaxial cell. Throughout all tests in the standard triaxial apparatus it will be assumed that aj is the stress on the end caps of the sample and 02 = 03 is the cell pressure. Triaxial test results are given in terms of water content ( w ), mean principal effective stress ( p ), and deviator stress ( q ). The parameters used are as follows; p = 1/3 (pi + 2ct3’) p = 1/3 (0^ - o3') + o3' q = o1-a3 = a1'-a3’ In consolidation stage, the independent parameter is p, the dependent parameter is w. In q = 0 plane, w-lnp plane is shown. In undrained test, the specimen remains at constant voids ratio. The independent parameter is q, the dependent parameter is p. The deviator stress is calculated from axial load increment which is divided by the average crossectional area of the specimen. The deviatoric loads and pore pressure values are performed due to the axial strain values, p and q are calculated from the values of Oj, a3, u. The relationship q versus p, q versus e, u versus e are determined for normally consolidated and overconsolidated Bentler Kaolin. Normal consolidation line and critical state line are drawn and M, N, T, X soil parameters are found for Bentler Kaolin. The behaviour of normally consolidated and overconsolidated clay at failure is analyzed by isotropically consolidated undrained triaxial compression tests. After the attainment of maximum deviator stress at Mure, the shapes of test paths do not represent a regular behaviour, probably due to the inadequacy of the testing apparatus; excess pore pressure transducer. Some of the specimens have not failed at the critical state line. They have failed before they reached the critical state line. We can say that when the consolidation pressure increases, the permeability of the specimen decreases. Therefore, relative increase of water content at the surrounding of the shear surface become in a thinner surface than specimen consolidated at low pressures. Furthermore, pore pressure and water content at shear surface cannot be measured as accurately. Thus, the loading paths do not reach critical state line. Sometimes normally consolidated specimens which are subjected to shear behave like over consolidated. This means that the consolidation pressure is greater than that they have been subjected since remolding. For normally consolidated and lightly overconsolidated specimens develop positive excess pore pressure at failure. For heavily overconsolidated specimens, the tendency towards volume expansion exists out to large strains. As it can be seen from the test results; there may be a test path which has zero excess pore pressure. This path represents an indecisive behaviour between normally and lightly overconsolidated soils and heavily overconsolidated soils. For this reason; a path, where zero excess pore pressure will be zero, should be found out for different overconsolidation ratio values.