Sabit deformasyon hızlı konsolidasyon deneyleri
Constat rate of strain tests
- Tez No: 39648
- Danışmanlar: DOÇ.DR. HÜSEYİN YILDIRIM
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 64
Özet
ÖZET Yapılardan dolayı zeminde meydana gelecek oturmaların hesabı zemin mühendisliğinin önemli konularından birisini oluşturmaktadır. Bu nedenle, laboratuvarda tek boyutlu deformasyona maruz bir zeminin oturma miktarını ve hızını tahmin etmek için, 1925 yılında Terzaghi tarafından önerilen ve halen yaygın bir şekilde kullanılan kademeli yüklemeli klasik konsolidasyon deneyleri yapılmaktadır. Klasik konsolidasyon deneyi; uzun deney süresi, oturma eğrisinin sürekli bir şekilde elde edilememesi, her yük artışı esnasında uniform olmayan gerilme dağılımı, yüksek ve değişebilir deformasyon hızlan, suya doygunluk için ters basınç uygulanamayışı gibi dezavantajlara sahiptir. Bu aksaklıktan ortadan kaldırmak ve mümkün olduğunca arazideki şartlan uygulayabilmek amacı ile günümüze kadar bir takım yeni deney sistemleri ve analiz yöntemleri geliştirilmiştir. Bu çalışmada, bu yeni deney yöntemlerinden birisi olan sabit deformasyon hızlı konsolidasyon deneyi ele alınmış ve belli bir su muhtevasında çamur konsolidometrede suni olarak çökeltilen numuneler üzerinde bir seri sabit deformasyon hızlı konsolidasyon deneyi yapılmıştır. Deneylerde Birleştirilmiş Zemin Sınıflandırma Sistemi 'ne göre CH ve ML olan iki tip zemin kullanılmış, farklı deformasyon hızlannda konsolide edilerek, yapılan klasik deneylerle uyumu gözlenmiş ve zeminlerin konsolidasyon karakteristikleri belirlenmeye çalışılmıştır. VII
Özet (Çeviri)
CONSTANT RATE OF STRATN TESTS ABSTRACT In nature, the loads applied due to engineering structures such as building, fill, bridges etc. occur settlements in soil layers. When a soil deposit is subjected to an increase in total stress by the construction of a building or an embankment, excess pore water pressures are set up in the soil mass. Since water cannot sustain shear stress, these excess pore water pressures dissipate by water flowing out of the soil. The rate at which this process can occur is controlled principally by the permeability of the soil mass. For granular soil such as sands, the permeability is relatively high so that excess pore water pressures can dissipate virtually instantaneously, that is, settlement of a structure is generally complete by the end of construction. In contrast, clay soils generally have a very low permeability and therefore the dissipation of excess pore water pressures is a very slow process. Consequently, an engineering structure may continue to undergo settlement for many years after construction is completed. The dissipation of excess pore water pressures by the outflow of water from the soil is referred to as“consolidation”. The conventional method for estimating both the rate and amount of settlement of a compressible soil subjected to one - dimensional strain is to take samples of the soil and perform standard consolidation tests. Estimates of this type are of key importance in the design of engineered structures (selecting a foundation type) and the evaluation of their performance. The testing procedure of consolidation was first proposed by Terzaghi and has been widely used since 1925. The odometer test is probably the simplest load - deformation experiment conducted in all engineering areas. In the conventional consolidation test method, a relatively thin cylindrical soil specimen is restrained laterally and loaded axially with total stress increments. Each stress increment is maintained until excess pore water pressures are completely dissipated. According to the Terzaghi theory, as the excess pore water pressure dissipates, the effective stress (stress in the soil structure) increases at the same rate i.e., the sum of the effective stress and the excess pore water pressure is equal to the total externally applied pressure. VIIIDuring the consolidation process, measurements are made of change in the specimen height and these data are used to determine the relationship between the effective stress and void ratio or strain, and the rate at which consolidation can occur by evaluating the coefficient to consolidation. In addition, time - deformation readings are required to determine the time for completion of primary consolidation and for evaluating the coefficient of consolidation. Since the coefficient of consolidation varies with stress level and load increment (loading and unloading), the load increments with timed readings must be selected with specific reference to the individual project. The main advantage of the incremental compressibility test is the simplicity of equipment that can be used. The quality of the test will be higher or lower according to the type of equipment used. To obtain directly applicable test results, the consolidation test should simulate as closely as possible the conditions which exist for an element of the prototype in the field. It is known that the incremental loading consolidation test has several deficiencies including the following ones : - Long testing time, - Incomplete definition of compression curve, - Non - uniform stress state during each increment, - High and variable rates of strain, - Difficulties in data interpretation, - Lack of back pressure saturation. As researchers became aware of these practical problems associated with traditional incremental odometer tests and electronic measuring and control devices become a practical reality, three new testing methods were developed in a three year period. These new methods are the constant hydraulic gradient test (Lowe, Jones, and Ocrician, 1969), the constant rate of strain test (Smith and Wahls, 1969 and Wissa, Christian, Davis, and Heiberg, 1971), and the constant rate of stress test (Aboshi, Yoshikume, andMaruyama, 1970). The main subject of our study is the constant rate of strain tests. Prior to the study, review of the previous work, and development of the CRS consolidometer and analysis methods will be briefly reported. In a paper devoted to a discussion of preconsolidation pressure of sensitive clays, Hamilton and Crawford (1959) made one of the earliest mentions of the constant rate of strain consolidation test. They indicated that this type of test showed promise as a rapid means of determining both the preconsolidation pressure and the void ratio- IXeffective stress relationship. When compared with conventional tests, the CRS tests showed lower compressibility, but the general shapes of the curves were somewhat similar. Although they realized that some excess pore pressures were developed in the CRS tests, they didn't measure them. The results of such pressures would have been to cause an apparent decrease in compressibility. The range of strain rates in this series varied from 0.15 % per min to 0.005 % per min, but even this large difference did not greatly affect the data. The data for the slower strain rates did show a tendency for closer conformity to the standard test. Observation of the test specimens after they were dried indicated that side friction apparently caused more serious stress variations through the specimens in the standard test than in the CRS test. Crawford (1964) reported more data from the CRS test. Rather than justifying the test as a rapid procedure, he presented the effect of strain rate as a factor that had been too long ignored in settlement analysis. As a result of the thin sample used in the laboratory, Crawford noted that laboratory consolidation rates are frequently several million times larger than rates experienced in the field. Data from the CRS test appeared quite similar to that of standard tests conducting using different load durations. During this series of tests excess pore pressure were obtained at the base of the sample. The maximum excess pore pressure in the CRS test was approximately 5 % of the applied pressure. Crawford concluded that in the CRS test the consolidation was primarily due to a plastic rather than a hydrodynamic effect. For this reason Crawford classified the compression in the CRS test as being totally secondary compression. Crawford (1965) reported a further investigation in which the rates of strain varied from 0.133 % per min to 0.0027 % per min, and the maximum excess pore pressure measured at the base of the sample was equal to 15 % of the applied pressure. Pore pressures were measured at the base of the samples throughout the duration of the test and the average effective stress on the sample was calculated by subtracting 1/2 of the excess pore pressure at the base from the total vertical stress. For comparison standard incremental consolidation tests were conducted. From these tests, Crawford concluded that the soil structure had an important time-dependent resistance to compression. Test data showed that the higher the rate of strain, the lower the compressibility or the greater the plastic resistance. He also suggested that the final void ratio for a particular load is mainly dependent on the average rate of compression and not the method by which the load is applied. This was indicated by the marked similarity of results between the incremental tests and the CRS test. In general agreement with the work done by Crawford was the study conducted on remolded samples and reported by Wahls and DeGodoy. The strain rates in this series of tests varied from 0.23 % per min to 0.053 % per min., and, as was the case in Crawford' s studies, pore pressures were measured at the base of the sample. As was the case in Crawford' s work, there was an increase in compressibility with decreasing strain rates. When the CRS results were compared with standard test result, Wahls and DeGodoy observed that for the range of strain rates used all of the CRS test Xshowed more compressibility than the standard test. This is in contrast to Crawford' s work. Smith and Wahls (1969), developed approximate interpretation formulas for CRS tests based on the observed pore water pressure at the impervious sample base, and an assumed pore pressure distribution throughout the sample. Also, the CRS consolidometer first developed by them with the intended purpose of performing quicker consolidation tests. The formulation of the mathematical model for the CRS test is similar to Terzaghi 's one dimensional theory. The basic model were obtained by introducing some, but not all of the Terzaghi Assumptions. The assumptions used in their theory were: 1. The soil is both homogeneous and saturated. 2. Both the water and the solids are incompressible relative to the soil skeleton. 3. Darcy 's law is valid for flow through the soil. 4. The soil is laterally confined and drainage occurs only in the vertical direction. 5. Both the total and the effective stresses are uniform along a horizontal plane, i.e., stress differentials occur only between different horizontal planes. Smith and Wahls tested three materials that were an undisturbed Massena clay, a remolded kaolinite, and a remolded calcium montmorillonite and obtain good results until the pore pressures exceeded 50 % of the applied load. Soon after, Wissa et al. (1971), developed the more general CRS apparatus also with the objective of reducing test time and to provide more continuous data. They elaborated further on the CRS test, examining the transient period in which the base pore pressure is adjusting itself to imposed rate of deformation. The solution includes both the initial transient portion of the test and the steady state conditions. Although the mathematical derivations were based on somewhat different assumptions, they reported similar results. As for our study, a series of constant rate of strain test have been carried out on samples artificially sedimented in slurry consolidometer in the water content of w= 250 %. In tests, two types of soils that are CH and ML according to the Unified Soil Classification System have been used and tried to find out the consolidation characteristics. Fat clay (CH) is gray colored and silt (ML) is red-brown colored. For classification and to obtain index properties of the soils required tests (sieve analysis, atterberg limits etc.) have been made. According to the results of the tests, CH and ML have the following properties, For CH, - Liquid limit LI = 63 %, plastic limit PL = 21 %, plasticity index PI = 42 % - sand = 5 %, silt = 59 %, clay = 36 %. For ML, - Liquid limit LI = 48 %, plastic limit PL = 30 %, plasticity index PI = 18 % - sand = 1 %, silt = 72 %, clay = 27 %. XIFirst of all, the soils were sieved from No.40 ASTM sieve and the passing part was mixed into a slurry at a water content (250 %) above the liquid limit. The soils were sedimented and consolidated to a vertical effective stress of about 1 kg per sq cm in the slurry consolidometer. CH, three times and ML, two times were prepared in the slurry consolidometer. Then, the samples were carefully removed as a block and stored. Prior to telling the test, it is useful to give general description and some features of the CRS consolidometer. The following items are required for the operation of the odometer: - The odometer cell including pore pressure transducer in its base. - Constant pressure application system and its gauges. - A load frame or loading platform. - A load cell. - A dial gauge to measure vertical displacement. - Trimming equipment including wire saw. - Silicon grease and wrenches. - Distilled water. The apparatus consists of two chambers, a cell chamber and a test specimen chamber, which hydraulically isolated from each other by means of a rolling diaphragm that seals the loading cap to the outer retaining ring. The loading piston uses a rolling diaphragm seal. Top surface drainage from the test specimen occurs through a coarse porous stone located on the underside of the loading cap. There are five valves on the consolidometer: for two back pressure, for two cell pressure and for one pore pressure drainage at the base of the consolidometer. The soil specimen is loaded by means of a piston head. The clearance between the piston head (loading cap) and the test chamber wall is small in order to apply a uniform displacement to the top surface and to prevent soil from squeezing out during loading. Load may be applied to the piston head by any combination of two mechanisms: 1. By applying an external force to the piston shaft. 2. By applying an hydraulic pressure in the cell chamber. This chamber is isolated from the test chamber by a reinforced rubber membrane (rolling diaphragm). The characteristics of this diaphragm require that the pressure in the cell chamber always be higher than pore water pressure at the top surface of the sample (i.e. the back pressure during the test). After trimming, the test specimen which is in the stainless steel ring (69.8 mm in diameter and 19.1 mm thick) is placed in the test chamber. Due to the tight fit XIIbetween the sample ring and the test chamber it is important that the test chamber and the part of the ring not occupied by the specimen be absolutely free of soil and dirt. To help remove some of the air within the specimen, a back pressure is applied. After that, the degree of saturation should be checked using the pore pressure parameter C. The loading sequence is not started until the value of C reached at least 0.95. Furthermore, before starting the actual test, a small load should be applied to the piston head by increasing the cell pressure to insure that it is in contact with the soil. In our tests, the difference between the cell pressure and the back pressure was 0.25 kg per sq cm. The specimens were mounted in the consolidometer and the saturated under back pressure of 4.25 kg per sq cm (this is a general value). They were then loaded to a vertical effective stress approximately 8 kg per sq cm. The strain rates used in tests varied from 8.7*10~5 sec-1 to 2.2*10~6 sec-1 and pore pressures were measured at the base of the sample. In addition, incremental loading tests were conducted for comparative purposes. Twenty seven CRS tests were conducted on the two materials. The whole standard tests have showed more compressibility than for the range of strain rates used all of the CRS test. The general shapes of the void ratio-the effective stress curves are similar. XIII
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