Normal konsolide killerin tekrarlı yükler etkisi altında davranışı

Abstract

Günümüz dünyasının enerji gereksiniminin hızlı artışı, açık denizlerde petrol ve doğal gaz işletilmesini, büyük boru hatları ve depolama tesisleri ile nükleer santrallerin yapımını, büyük baraj lar inşaasını zorunlu ve ekonomik hale getirmiştir. Tüm bu geliş melerin beraberinde getirdiği en önemli mühendislik problemlerinden biri de, yapıların temelini oluşturan zeminlerin tekrarlı kayma gerilmeleri altındaki gerilme-şekil değiştirme davranışının belir lenmesi, kayma mukavemetinde meydana gelebilecek değişimlerin bulun ması, elde edilen değerlerin mühendislik yapılarının projelendiril mesinde ve uzun süreli stabilite hesaplarında kullanılmasıdır. Ülkemizin, aktif bir deprem kuşağında bulunması, bu konuların önemini daha da arttırmaktadır. Bu araştırmada; yüksek su muhtevasında hazırlanarak konsolide edilen kaolin numunelerin dinamik basit kesme ve dinamik üç eksenli deney aletlerinde sinosoidal tekrarlı yüklemelere tabi tutulmuş, bu yükler etkisi altında gerilme-şekil değiştirme ve dinamik mukave met özellikleri zamana bağlı olarak incelenmiştir. Çalışmalarda, dinamik yükleme frekansı ve genliğinin, kohezyonlu zeminlerin dina mik özelliklerine etkisi ön planda tutulmuş, deney sonu kriteri olarak, düşük frekans ve genlikteki deneyler için boşluk suyu basınç larının sabitleşmesi, diğer deneylerde ise, birim- kayma veya eksenel birim boy değişiminin + %10 değerine ulaşması seçilmiştir. Her iki numune grubu ve deney sisteminde yapılan deneyler; çevrim sayısı Ve gerilme oranlarındaki artımlarla frekans değerlerin deki az boaların boşluk suyu basınçlarını ve deformasyonları artır dığını, dolayısıyle dinamik mukavemeti azalttığını göstermiştir. Düzlemsel izotropik numunelerde, izotropik numunelerden daha fazla deformasyon oluşurken, boşluk suyu basınçlarının daha düşük değer lerde kaldığı belirlenmiştir. Ayrıca, deney sonuçları yapılan regrasyon analizleri yardımiyle matematiksel olarak ifade edilmiştir.

Because of the continuously increasing energy demands. in the world, it became necessary to construct big dams, nuclear power plants and off shore petroleum platforms in oceans. The soil layers under such buildings dams, power plants, off shore platforms are subjected to cyclic and transient loadings due to earthquakes, wind, wave action. It is very crucial to know the effects of cyclic loadings on soil layers and to implement these effects in the design and construction of these structures. Our country is on an active earthquake zone, therefore it is necessary to predict the behaviour of structures under earthquake loads. The behaviour of soil layers under cyclic loading is important for the structures that are built on these layers. The decrease in the bearing capacity of these soil layers and the probable settlements under cyclic loading roust be predicted before. For this purpose. dynamic laboratory, in-'situ ¦experimental methods have been developed and the stress-strain and dynamic strength properties of. the foil layers- have: been predicted with these methods, The main purpose in this thesis is to study the behaviour of cohesive soils subjected to cyclic loads based on dynamic shear and dynamic triaxial tests. The dynamic simple shear system is designed by M. Silver and K. Ishihara and it is possible to carry out stress controlled cyclic shear tests. The dynamic triaxial system is composed of a modified cell that strain --:nd stress. can be measured in side the cell and a loading unit capable of applying stress and strain controlled cyclic loading. Kaolinite clay used in the experiments is Uşak Kaolin and it is taken from Istanbul Yxldiz Porcelain factory. Liquidity limits and plasticity limits of the kaolin are 65 % and 25 % respectively. Kaolin samples are prepared in mud consolidometers starting initially with apporoximately 250 % moisture content where samples were consolidated under p = 10.0 kN/m^ axial pressure. Under these conditions the duration of consolidation takes about one month and soil samples obtained would have 45 % misture content. Xll In program two kinds of samples are used: undisturbed (transversly isotropic) and remolded (isotropic). Soil samples were placed in the cell and were subjected to 400 kN/nr confining pressure and 300 kN/mr back pressure. Then samples were left for consolidation under 100 kN/m'' effective stress. After the completion of consolidation saturation of the sample is controlled and B ^ 0.95 condition is obtained in all of the tests. Uniform sinusoidal cyclic loading were used in all experiments with frequencies 1.00 Hz., 0.50. Hz., 0.10 Hz., 0.01 Hz. and dynamic stress ratios 0.30, 0.40, 0.50, 0.60, 0.70, 0.80. Total number of experiments done were 66; 40 of them were performed with dynamic simple shear system on remolded and undisturbed soil samples, "26 of them were dynamic triaxial tests. In low cyclic stress levels, experiments were stopped when pore water pressure has reached a constand value. In other cases, experiments were continued until strains has reached 20 %. The results obtained from the dynamic simple shear and dynamic triaxial tests can be summarized as given below:. In these two series of tests conducted on two ^yp?-3 ~:f soil samples at the same frequency it was observed that. increase in the stress ratio e&a^e.i p.t increase in pore water pressure and deformations and led to a reelection in the dynamic stregth. The same effect coula be enhanced by decreasing the loading frequency and increasing number of cycles..a..... T/T,. =0.50 ratio can be defined as the critical stress f t ratio based on dynamic simple shear tests. In experiments conducted at cyclic stress larger. then the critical stress ratio, important increase in pore water pressure and limited deformations were observed. The increase in the cyclic strain amplitude in terms of number of cycles with respect to the dynamic stress ratio was observed to be hyperbolic for tests performed at the same frequency. xm The curves for different number of cycles were focused at T /t =0.30 and üj/a =0.20 values for transversly isotropic samples, On isotropic samples, the curves were focused t/t =0.40 and a /a =0.10 values. Deformations are constant or limited at these ratios. Shear strains obtained from the tests on transversly isotropic and isotropic samples are compared.lt was observed that shear strains of transversly isotropic samples are 25 % more than shear strains of isotropic samples. Axial strains for transversly isotropic samples are 17 % - 20 % more during initial cycles and 77 % - 84 % more for large number of cycles than axiaL strains of isotropic samples.. Pore water pressures obtained from two types of soil samples prepared differently in dynamic simple shear tests can be shown by the following formula: u. = (0.75 - 0.90) u. tr,is is In dynamic triaxial tests the expression was as; u. = 0.95 u. tr,is xs In dynamic triaxial tests, pore-water pressures obtained from transversly isotropic soil samples are 15 % - 20 % more than those obtained in dynamic simple shear tests. For isotropic soil. samples no significant difference was. observed in pore-water pressure, pi transversly isotropic soil samples pore, water pressure reaches maximum value around 100 cycles and for isotropic samples this value is 1000 cycles. Reduction in frequency increases pore-water pressure build up and strain amplitude at the same stress ratio. The effect of. frequency on transversly isotropic soil samples was more important. The effect of frequency was more pronounced for stress ratios T/x < 0.50. But this effect looses its importance at t/t - 0.50 ratio for transversly. isotropic samples and at stress ratios t/t ^ 0.70 for isotropic samples. xiv In two systems of tests with samples prepared differently the effect of frequency on the pore water pressure can be observed if the increase in number of cycles linear. If increase in number of cycle is slow and reaches to a maximum value, than differences are not important. Important differences are also observed on stress-strain properties in the tests at the frequency of 0.50 Hz. For this reason, it is very appropriate to select the tests frequency value as 0.10 Hz. at these laboratory tests. The results of the tests have been used to model the behaviour of normally consolidated clays under cyclic loading with mathematical expressions. The formulas that -liave been derived for a certain frequency value have expressed suitably the pore water pressures, shear strain and axial strains at different stress ratios and number of cycles. At the below expressions the data for the experiment that has the frequency of 0.10 Hz. have been used. At both of test systems, with the correlation of the test results, that have been done on the differently prepared samples the equations given below have been found for pore water pressures: At dynamic simple shear tests; u. = -3.66 + 19.93 (t/t.) * [-1.11 + 20.85(t/tJ | In N u. = -24.67 + 65.85(t/t,) + İ2.12 + 14.04(t/tJ| In N is f ' f ' At dynamic three axial tests; u, = -23.47 + 60.59 (ojo ) + | 9.62 * 4. 15 (a,/a )| In N u. = -21.74 + 43.92 (a, /a ) + [7.64 +10.59(a,/a ) | In N is a c ' a c ' xv The expressions for the shear strain and axial strains are given below: At the dynamic simple shear tests; Y. = 8.69 (T/xf)2-33. N0.72(x/xf)2-88 tr,ıs f Y. = 13.74 (T/xf)3-10. N0.42(T/x p1-76 is ?. f - At the dynamic three axial tests; er. - 3.31 (a Jo )1'93. N°-63(Vac)1,15 tr,is d c = 4.01 (aja )2-33. N°-33(Vffc)0'42 iz d c The pore water pressures, shear strains aud axial strains for certain test conditions and required number of cycles can be expressed with the help of these expressions.