Sitli ve killi kumlarda sıvılaşma

Abstract

Bu araştırmada suya doygun, ince dane içeren kumlu zeminlerin dinamik davranış biçimleri ve bu davranış biçimi üzerinde dinamik gerilme seviyesinin, plastisitenin ve örselenmenin etkisi incelenmiştir. Laboratuvarda hazırlanmış temiz kum, plastik ve plastik olmayan ince dane içeren kum numuneleri ile örselenmemiş ince daneli kumlar üzerinde, gerilme kontrollü olan dinamik basit kesme deney sisteminde bir seri deney yapılmıştır. Sonuçlar, uygulanan dinamik gerilme seviyesinin artması durumunda ince daneli kumların sıvılaşabilirliğinin artmakta olduğunu göstermiştir. Ayrıca, plastisitenin dinamik mukavemet üzerinde oldukça etkili olduğu gözlenmiştir. Plastik ince danelerin dinamik mukavemeti, plastik olmayan ince danelerin dinamik mukavemetinden daha yüksek bulunmuştur. Diğer bir sonuç ise örselenmenin etkisidir. Laboratuvarda hazırlanan plastik ince dane içeren kum numunelerin dinamik mukavemetinin, örselenmemiş numunelerin dinamik mukavemetine oranla daha düşük değerlerde kaldığı gözlenmiştir.

The extensive damages resulting from soil liquefaction in recent earthquakes have re-emphasized the need for reliable procedures for predicting the possible development of this phenomenon. Studies of the types of soil which have liquefied during earthquakes, laboratory investigations of the factors inducing liquefaction of saturated sands under cyclic loading conditions, and analytical procedures for predicting the liquefaction potential of sands deposits have thrown considerable light on the subject. As the pore water pressure approaches a value equal to the applied confining pressure, the sand begins to undergo deformations. If the sand is loose, the pore pressure will increase suddenly to a value equal to the applied confining pressure, and the sand will rapidly begin to undergo large deformations with shear strains which may exceed ±20 percent or more. If the sand will undergo unlimited deformations without mobilizing significant resistance to deformation, it can be said to be liquefied. If, on the other hand, the sand is dense, it may develop a residual pore pressure, on completion of a full stress cycle, which is equal to the confining pressure (a condition of initial liquefaction), but when the cyclic stress is reapplied on the next stress cycle, or if the sand is subjected to monotonic loading, the soil tend to dilate, the pore pressure will drop if the sand is undrained, and the soil will ultimately develop enough resistance to withstand the applied stress. However, it will have to undergo some degree of deformation to develop the resistance, and as the cyclic loading continues, the amount of deformation required to produce a stable condition may increase. Ultimately, however, for any cyclic loading condition, there appears to be a cyclic strain level at which the soil will be able to withstand any number of cycles of a given stress without further deformation.. This is the type of behavior termed "cyclic mobility" (Castro, 1975) or "initial liquefaction with a limited strain potential" (Seed, Arango and Chan, 1975). It should be noted, however, that once the cyclic stress applications stop, if they return to a zero stress condition, there will be a residual pore pressure in the soil equal to the overburden pressure, and this will inevitably lead to an upward flow of water in the soil which could have deleterious consequences for overlying layers. For purposes of clarification, the following terminology will be used herein: Liquefaction denotes a condition where a soil will undergo continued deformation at a constant low residual stress or with no residual resistance, du to the buildup and maintenance of high pore-water pressures that reduce the effective confining pressure to a very low value; pore pressure buildup may be due either to static or cyclic stress application. Initial Liquefaction donates a condition where, during the course of cyclic stress applications, the residual pore-water pressure on completion of any full stress cycle becomes equal to the applied initial effective confining pressure; the development of initial liquefaction has no implications concerning the magnitude of the deformations which the soil might subsequently undergo; however, it defines a condition that is a useful basis for assessing various possible forms of subsequent soil behavior. Initial Liquefaction with Limited Strain Potential or Cyclic Mobility denotes a condition in which cyclic stress applications cause limited strains to develop either because of remaining resistance of the soil to deformation or because the soil dilates, the pore pressure drops, and the soil stabilizes under applied loads. Based on recent investigations it seems reasonable to conclude that cyclic liquefaction characteristics of a sand, in-situ are influenced by the various factors. 1. Relative density 2. Method of S oil Formation (Soil Structure) 3. Period under Sustained Load 4. Previous Strain History 5. Lateral Earth Pressure Coefficient and Overconsolidation 6. Fines Content and Plasticity 7. Grain Characteristics According to the increment of the relative density, stability of structure, lateral earth pressure coefficient, K0, in time under pressure, stress ratio for liquefaction increases. At the same time stress ratio for liquefaction increases with prior seismic strains. The increase in plastic fines increase the liquefaction resistance, the increase in non plastic fines reduces the liquefaction resistance. In order to explain the mechanism of liquefaction, extensive experimental studies have been conducted on reconstituted sand samples(Seed pt al.,1971; Martin et al.,1975; Mulilis et al.,1977; Castro et al.,1977; Ladd, 1977). In this studies it was observed that the method of sample preparation strongly affects the cyclic mobility of sands and there are some difficulties about sample preparation for silty sands in a wide range of gradation and density. In addition, another problem associated with reconstituted samples is the lack of in situ stress history which leads to overestimation of liquefaction resistance. VI In recent years, in the light of previous findings, the studies of the liquefaction phenomenon of undisturbed sandy, silty soils have been received an increasing attention to eliminate the effects of the factors mentioned above. Various advanced undisturbed soil sampling techniques have been develop by Hatanaka et al. (1986), Goto et al.,(1987), and Yoshimi et al.,(1989). In these techniques, undisturbed soils samples are obtained by in situ freezing. Hatanaka et al. (1988), have shown that liquefaction resistance of reconstituted samples are about 50 % less than that of undisturbed samples even though they have the same density. In some studies, frozen Shelby tube samples (Ishihara, 1985) and block sampling (Ishihara et al, 1977 ) have been used instead of in situ freezing. In these investigations, it is observed that the reconstituted sand samples have also lower liquefaction resistance compared to undisturbed sand specimens. In this research, undrained behavior of saturated sands that contain fines under dynamic loads and the effects of dynamic stress level, plasticity index and disturbance have been studied based on undisturbed and reconstituted samples. A series of tests are carried out with dynamic simple shear test device. The cyclic simple shear testing system used in this study is modified version of Norwegian simple shear apparatus, developed by Prof. Ishihara and Prof. Silver. The cyclic shear stresses are controlled by a pneumatic system, applied at frequencies between 0.0001 Hz and 5 Hz. The horizontal shear stresses are applied as stress controlled at the top cap connected to a horizontally moveable shaft going through the cell. The test sample has 70 mm diameter and a 30 mm height, and can be consolidated under both isotropic stresses and anisotropic stresses. The shear stresses are measured by a load cell located in the chamber. The pore pressure transducer is connected to the bottom platen, axial and horizontal deformations are measured by sensitive displacement transducers located outside of the chamber. In this study, the method adopted to obtain undisturbed sand samples is based on piston samplers and shelby tubes. When samples are taken from a sand deposit below the ground water table, water in the pores of saturated sand is drained and the capillary tension within the pores becomes effective in developing a temporary strength necessary for sample handling. In order to prevent disturbance of sands, shelby tubes are frozen before transportation and are kept frozen until they are used in tests. Water in pores of clayey sand shelby tube samples were frozen at -10 ° C in a deep freezer. Since the samples were not fully saturated the freezing would not produce any volume change. First a frozen shelby tube was cut longitudinally with an electric saw to the specified test length. Then they were trimmed as cylindrical samples with 70 mm in diameter and 30 mm in height. After trimming, the test sample was placed in the test chamber using 0.3 mm thick membrane with O-rings. Then the size of the test samples was measured. A vacuum of about 90 kPa was applied to both ends of the test sample and the sample was allowed to completely thaw for two hours. vn Dimensions of the sample was again measured to calculate the thawed unit weight. The test chamber was assembled and filled with viscous oil. The air in the pores are removed by circulating water from bottom to top under vacuum. Then vacuum gradually reduced to zero while simultaneously increasing the cell pressure to a value of 100 kPa. Cell pressure and back pressure were increased while maintaining the constant effective confining stress. The sample was allowed to stabilize at this stress for 24 hours. In most cases, B value was obtained as 0.98 or greater after 24 hours. The sample was consolidated to the desired effective confining stress and sample was cyclically loaded as stress controlled at a frequency of 1 Hz. Undisturbed samples are clayey sands, liquid limit values are 34%-49% and index of plasticity 14%-26%. Reconstituted samples are prepared by the method of pluviation in water. A vacuum approximately 20~30 kPa was applied to clean sand samples. In order to have the samples saturated, long term vacuum is applied. Vacuum applied to the clean sands samples containing 20% silt and clay particles around 10 kPa value. Vacuum applied for a short time. Thus prevention of washing away the fine grained materials. Uniform clean sand has mean grain size, D50, 0.60 mm. Silt has liquid limir value 35%. It has plasticity index 13%. Plasticity index and liquid limit of clay are 44% and 66%. In this study, aim was to investigate the effects of cyclic shear stress level, fines content and plasticity and disturbance. The results of undrained dynamic simple shear tests on undisturbed and reconstituted sandy, silty sand and clayey sands samples the following conclusions. 1. The liquefaction resistance of undisturbed samples is more than the resistance of disturbed samples. 2. The increase in plastic fines increase the liquefaction resistance. 3. The increase in non-plastic fines reduces the liquefaction resistance. 4. The liquefaction resistance decreases as the dynamic stress level increases. Finally, it would seem that the design engineering confronted with the need to evaluate the possible liquefaction potential of a deposit has three choices: 1. To take the best possible undisturbed samples and then try to reconstruct their true field characteristics, 2. To devise and utilize a procedure such as freezing the samples during sampling and handling and thaw them prior to testing in order to possibly maintain soil density and structure, however, the use of such as procedure. still requires investigation as to its usefulness, VIII 3. To be guided by the known field performance of sand deposits correlated with some measure of in-situ characteristics, such as the standard penetration test, on the grounds that most factors that tend to improve liquefaction resistance also tend to increase the standard penetration resistance or the results of any other in-situ test that may be adopted as a possible indicator of field liquefaction behavior. In the best situations it would hopefully be possible indicator reasonable agreement on the potential for liquefaction using all of these approaches, or at least approaches 1 and 3 (since approach 2 is still under investigation). However without the exercise of considerable judgment or a serious attempt to maintain or recreate the in-situ soil characteristics, the direct use of laboratory test data from tests or even undisturbed samples of moderately dense to dense deposits seems likely to lead to severe overdesign in many cases involving clean medium and coarse sands or some degree of overdesign in slightly cohesive sands or sands with a sufficient fines content to create substantial apparent cohesion due to capillary effects.