Halka kesme aleti ve kalıcı mukavemetin belirlenmesinde kullanımı
A Ring shear apparatus and its application to the measurement of residual strength
- Tez No: 66871
- Danışmanlar: PROF. DR. ATİLLA ANSAL
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Geoteknik Mühendisliği Bilim Dalı
- Sayfa Sayısı: 114
Özet
ÖZET Bu çalışmada kohezyonlu zeminlerin kalıcı mukavemetleri ile ilgili incelemeler yapılmış ve yüksek kesme hızlarında numuneyi kesebilen bir halka kesme deney aleti kullanılarak, kalıcı mukavemet değerleri elde edilmiştir. Burada kalıcı mukavemet kavramı, kohezyonlu zeminlerin kalıcı mukavemetleri, zemin özelliklerinin ve dane yapısının kalıcı mukavemet değerleri üzerindeki etkisi araştırılmış, geçmişte kalıcı mukavemet parametrelerinin belirlenmesinde kullanılan yöntemler incelenerek, deney sonuçlan kaydedilmiştir. Ardından, deneylerde kullanılan halka kesme aleti tanıtılmış ve bu aletten yararlanılarak gerçekleştirilen deney sonuçlan karşılaştırmalı olarak ele alınmıştır. Başlangıçta araştırmacılar kalıcı mukavemet parametrelerinin belirlenmesinde kesme kutusu deneyini kullanmışlar, daha sonraları, zemin numunelerinin bir doğrultuda sürekli kesilmesini, dolayısıyla kayma doğrultusuna paralel olarak kil parçacıklarının yönlenmesini sağlayan, halka kesme aletini tasarlamışlardır. Bu tasannu kullanarak boşluk suyu basıncının sönümlenmesine izin verecek kadar düşük kesme hızlarında çalışmış ve kalıcı mukavemet parametrelerini elde etmişlerdir. İlk kalıcı mukavemet bağıntılarına sadece zemin indeks özellikleri bir temel oluşturmuşsa da, bu deneyler sonucunda drenajlı kalıcı mukavemetin, zeminlerin içerdiği kil minerallerinin tipi ve kil boyutundaki parçacıkların miktarı ile kontrol edildiğini, tekil bir parametreye bağlı olmayıp zemine etkiyen normal gerilme ile değiştiğini saptamışlardır. İ.T.Ü zemin mekaniği laboratuvarında yapılan halka kesme deneylerinde, yeni birleşik zemin sınıflandırma yöntemi kullanılarak sınıflandırılan üç ayrı plastisiteye sahip kohezyonlu zemin numuneleri kullanılmıştır. Bu numuneler standart Proktor enerjisi kullanılarak en büyük kuru birim hacim ağırlığın %95 ıslak tarafında, deney aletinin kesme kutusu boyutlarına uygun olacak şekilde hazırlanmış, su içeren ortamda alete yerleştirilen numuneler, çok aşamalı yüklemeler yapılarak üç ayrı kesme hızında kesilmiştir. Elde edilen kalıcı mukavemet değerleri karşılaştırılarak, önceki araştırmalarla olan ilgileri incelenmiş, kesme hızının ve zemin sıkılığının kalıcı mukavemete olan etkileri kaydedilmiştir.
Özet (Çeviri)
SUMMARY A RING SHEAR APPARATUS AND ITS APPLICATION TO THE MEASUREMENT OF RESIDUAL STRENGTH For a given set of stress conditions, a soil will, if deformed under drained conditions and given sufficent time to consolidate or swell, arrive ultimately at the same final water content (saturated clays) or void ratio (sands), regardless of the initial state of the soil. The strength of the soil is referred to as the“ultimate”or“residual”strength, and the friction angle corresponding to the strength is termed the residual friction angle 'r (2.2) Where c'r and tyT are the residual shear strength parameters in terms of effective stress. For many soils the value of c'r is relatively low and can be taken to be zero. In general, the value of 'r decreases with increasing clay content. The relative valuses of peak and residual strength can be expressed by the brittleness index (Ib), defined as follows: hrXf-Xt/xt (2.3) The brittleness index for a particular soil is dependent on the level of efective normal stress. The peak strength of normally consolidated clay exceeds the ultimate value because of rupture of cemented bonds, particle reorientations, and other factorscontributing to sensivity. The shear of a heavily overconsolidated clay strength loss beyond the peak. The behavior of dense sand is similar to that of overconsolidated clays. Loose sands behave somewhat like normally consolidated clays, except the peak and ultimate strengths of a loose sand are the same. Differences between the peak and residual strengths of an initially dense sand are attributable to differences in void ratio, dilatancy effects, and fabric. The residual strength of the massive nonclay minerals is not much different than the peak strength if interlocking and dilatancy are accounted for, and quartz, feldspar, and calcite all have the same value of 'r. Particle rearrengements are relatively unimportant in these minerals at the residual state. The values of residual strength were determined by shearing samples in direct shear back and forth through a displacement of 2 to 2,5mm each side of center until minimum values were obtained. In some cases, a total displacement of several inches in the same direction may be required before minimum residual strength can be obtained. A series of small back and forth displacements may not be equivalent to a test with a displacement of the same total magnitude always in the same direction. In the drained triatrial test, most clays would evetually show a decrease in shear strength with increasing strain after the peak strength has been reached. However in the triaxial test there is a limit to the strain which can be applied to the specimen. The most satisfactory method of investigating the shear strength of clays at large strains is by means of the specialized ring shear apparatus, an annular direct shear apparatus. The annular specimen is sheared, under a given normal stress, on a horizontal plane by the rotation of one half of the apparatus relative to the other; there is no restriction to the magnitude of shear displacement between the two halves of the specimen. The rate of rotation must be slow enough to ensure that the specimen remains in a drained condition. Chattopadhyay (1972) show stress dependency of residual friction angle of the clay minerals. It is noted that residual strength can be obtained after only 1 or 2mm of displacement for clay materials in contact with hard surfaces, such as steel; Where as, a displacemet of several centimeters may be required for clay aganist clay. Fabric studies of kaolin subjected to direct shear show that platy particles must be in perfect parallel orientation before minimum ultimate strength can be obtained (Morgenstern and Tchalenko, 1967). xvThese findings suggest that just a few particles projecting into the plane of shear are sufficient to provide enough interlocking to account for an increment of several degrees in the friction angle. One explanation for the stress-dependency could be that under low normal stress less work is required to shear the clay in the absence of perfect orientation of clay plates in the shear zone than would be required to develop parallelism during shear. Under higher normal stresses, however, least work is expended if perfect particle reorientations develop during shear displacement. If a hard, smooth surface is on the opposite side of the shear plane, less displacement is needed to effect the orientation of clay plates because during shear: a) There will not be particles projecting across the shear plane and catching on each other. b) The hard, smooth surface provides a fixed plane to quide the alignment of particles. An alternative explanation of the stress-dependency of (j)'r for clays can be based on the elastic junction theory. On this basis, the area of real contact between sliding surfaces increases less than proportionally in normal effective stress. Data for several clays shows agreement with this theory in the low pressure range but at higher stresses, o-'r is independent of stress indicating that the solid contact area varies load. The drained residual shear strength of cohesive soils is a crucial parameter in evaluating the stability of preexisting slip surfaces in new and existing slopes and the design of remedial measures. At present, the reversal direct shear test is widely used to measure the drained residual strength of clays and clayshales, eventhough it has several limitations. The primary limitation is that the soil is sheared forward and then backward until a minimum shear resistance is measured. Each reversal of the shear box results in a horizontal displacement that is usually less than 0,5cm. As a result, the specimen is not subjected to continuous shear deformation in one direction of shear may not be obtained. In order to obtain the residual shear strength parameters, multiple reversal direct shear box tests and ring shear tests were performed. In ring shear test, there is no change in the area of the cross section of the shear plane as the test proceeds and that the sample can be sheared through an uninterrupted displacement of any magnitude. Because of these, the results obtained from ring shear test are more suitable than the results obtained from multiple reversal direct shear box test. xviThe main advantage of the torsional ring shear apparatus is that it shears the specimen continuously in one direction for any magnitude of displacement. This allows clay particles to be oriented parallel to the direction of shear and a residual strength condition to develop. Other adventages of the ring shear apparatus include minimal laboratory supervision during shear, and the possible use of data acquisition techniques. Results of torsional ring shear tests on cohesive soils reveal that the drained residual strength is related to the type of clay mineral and quantity of clay size particles. The liquid limit is used as an indicator of clay minerology, and the clay size fraction indicates quantity of particles smaller than 0,002mm. Therefore increasing the liquid limit and clay size fraction decreases the drained residual strength. The ring shear tests also reveal that the drained residual failure envelope is nonlinear, and the nonlinearity is significant for cohesive soils with a clay size fraction greater than 50% and a liquid limit between 60% and 220%. This nonlinearity should be incorporated into stability analyses. An empirical correlation for residual friction angle can be described that is a function of liquid limit, clay size fraction, and effective normal stress. For slope stability analyses, it is recommended that the residual strength be modeled using the entire nonlinear residual strength envelope or a residual friction angle that corresponds to the average effective normal stress on the slip surface. The values of residual strength to be used for analysis of particular problems should be made under stress conditions approximating those in the field. Displacement in one direction should be provided for of a magnitude that have or will develop in the field. ' Correlations between both peak and residual friction angle with the plasticity index exist but in the case of !p aganist Ip curve, although its shape remains practically unchanged. Variations in the stress level and soil structure have an almost negligible effect on the values of tyt. A good correlation is obtained for the residual friction angle in terms of the plasticity index. In the laboratory tests were used the ring shear test apparatus, an annular ring- shaped specimen, subjected to a constant normal stress a'n, is confined laterally, and ultimately caused to rupture on a plane of relative rotary motion. The apparatus was designed so that the total normal load and shear torque being transferred through the soil across the plane of relative rotary motion are accurately known, i.e. friction forces in the apparatus are measured where appropriate. XVIIThe ring shear apparatus is a controlled rate of displacement. The gross normal load is maintained constant during shear and the net normal load is obtained at any stage of the test by subtracting the measured side friction force. The test was conducted by shearing the same sample under a series of different normal stresses. After the residual strength under one normal stress had been determined, the motor was stopped and the new normal stress was applied. The sample was then allowed to consolidate under the new load before shearing was restarted. When the sample was sheared under a reduced normal stress a peak was first observed and the difference between the peak and the residual strengths increased as the effective normal stress decreased. When the normal stress was increased no such peak occurs. Test samples compacted with standrad Proctor compactive effort and used big proctor mold. Maximum dry density and optimum water moisture content were obtained. Then, samples were compacted 95% wet of optimum water content. The samples which had o.d=T5cm, i.d=10cm, initial thickness=2,0cm, were placed in the shear box of the ring shear apparatus. After allowing the sample to consolidate under a constant normal load. The gap between the upper and lower pairs of confining rings was opened by means of upper pulling jack. A speed which was too high to permit adequate dissipation of excess pore water pressure, selected to give a constant rate of strain. The clutch was engaged and shearing commenced. The beginning shearing rate was 2cm/min. During test, the rate was increased by 2-5 times the initial speed in order to study rate effects. The readings were normally taken with a nominal gap between the upper and lower confining rings over a period of time to confirm whether or not a steady state had been achieved. Development of the slip surface often results in uneven vertical deformation of the sample and the upper loading platen and confining rings will tilt slightly until restrained by the loading mechanism. This would introduce a friction component if metal to metal friction occured at any point or if squeezed-out clay was jammed in the gap. The vertical deformations of samples were consistent but subject to some in accuracy since they were affected by loss of clay by squeezing through the confining ring gaps. As a result, residual shear strength parameters were obtained and compared with each other. The effects of shearing rate and compaction effort at the residual shear strength were examined. xviu
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