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Siltli kumlar ve Kumlu siltlerde sıvılaşma

The Liquefaction of silty sands and sandy silts

  1. Tez No: 46369
  2. Yazar: CENGİZ KILIÇ
  3. Danışmanlar: DOÇ. DR. AYFER ERKEN
  4. Tez Türü: Yüksek Lisans
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1995
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 74

Özet

Bu araştırmada Erzincan ovasmda yer alan Ekşisu bölgesinden alınmış örselenmemiş süfli kum ve kumlu süt zeminlerin dinamik davranış biçimleri ve bu davranış biçimi üzerinde dinamik gerilme seviyesinin, plastik ve plastik olmayan süt oranlarının, sıkılığın ve deneylerde uygulanan konsolidasyon basınçlarının etkisi incelenmiştir. Ekşisu bölgesinde depremden sonra sondajlar sırasında tüplerle ve pistonlu numune alıcılarla alman örselenmemiş numuneler üzerinde laboratuvarda genime kontrollü Dinamik Basit Kesme deney sisteminde drenajsız koşullar altında deneyler yapılmıştır. Deneylerin sonunda numunelere uygulanan dinamik genime seviyesinin artması durumunda ince daneli kumların dinamik mukavemetinin azalarak srvilasabilirliğinin artmakta olduğu gözlenmiştir. Ayrıca süfli kum numuneler üzerinde yapılan deneyler sonunda arazide bulundukları ortamın sıkılığının dinamik mukavemetleri üzerinde içerdikleri ince dane oranlarından daha etkili olduğu bulunmuştur. Diğer bir sonuç ise deneylerde uygulanan konsolidasyon basınçlarının etkisidir. Süfli kum numunelerde laboratuvarda uygulanan konsolidasyon basmçlan arttıkça dinamik mukavemetlerinde bir azalma olduğu gözlenmiştir. x

Özet (Çeviri)

Modern life can be summarized in big engineering structures and cities. However, our civilization is always under threat of the mother nature. Earthquake is the main natural phenomena among this natural forces. During an earthquake, liquefaction which is developed in the saturated soil layers is the main source of the damage affecting to the construction. Since, the useful land is limited, high population growth and high land prices have driven the industry and housing from earthquake safe areas to more risky areas. This earthquake risky areas are becoming densely populated by the heavy damage risks on structures. The risk of danger has increased both for human life an engineering structures. Thus, need of research to understand liquefaction phenomenon has gained importance. As a result, researches on liquefaction have enormously increased. The main reason behind the liquefaction in saturated sandy layers is the shaking which is caused by earth movements during an earthquake. Shaking is causing cyclic shear stresses and as a result pore-water pressure is increased to the level of the confining pressure. Earthquake compresses saturated sandy layers and as a result of compression, the volume of layer decreases. Since, compression is instantaneously the water does not have enough time to leave the pores, this increases pore-water pressure. When pore-water pressure reaches confining pressure, on the sand starts to deforme. If the sand is loose, pore-water increases rapidly and becomes equal to confining pressure. Instantaneously, enormous deformation occurs in loose sand. In this situation, If the strength of the sand is very low and strain ratio is very high. When the soil satisfies this condition, it could be assumed that the soil is liquefied. In densed sands the pore-water pressure changes are very slow. This type of sand pore-water pressure equalizes to the confining pressure for a short time. From this point up because of the deformation, sand starts to dilation. Pore- water pressure decreases and the soil regains its strength against applied stress. For any cyclic loading condition, their appears to be a cyclic strain level at which the soil is able to with stand any number of cycles of a given stress without further deformation. This behavior is termed as“cyclic mobility”(Castro, 1975) or as“initial liquefaction with a limited strain potential”(Seed, Arango and Chan, 1975) XIThe term liquefaction describes a condition where a soil will undergo continuos deformation at a constant low residual or with non residual stress. An increase in pore-water pressure reduces the effective confining pressure to a low value. Generation of pore pressure is caused either by static and cyclic stress application. However in engineering practice liquefaction is usually associated with the development of high pore-water pressures due to seismic loading irrespective of whether large deformations occur or not. Indeed the empirical procedures for assessing liquefaction potential are all based primarily on surface manifestations of high pore-water pressure such as sand boils without consideration of whether the soil would behave in a contractive or dilative mode during shearing. Due to the importance of liquefaction most of the research is conducted on soil strength and consequent damage to soil structure. Even though, loose, saturated cohesionless soils are more vurnable to lose of its strength. Strong evidences such as soils containing fines or silty sand are also vurnable to liquefaction during earthquakes. Natural and artificial sand deposits which liquefied during ground shaking contain fine silty layer. Soil investigation has gained great importance. Different are been conducted different kind of soil samples. Especially, some of the recent earthquakes have forced the studies on liquefaction of silty sand. For example, in the earthquake which shook central Chile in 1985, Saguenay earthquake in Quebec in 1988 and the 1987 Superstition Hills earthquakes in California. Show that there was clear evidence of silty sand and sandy silt liquefaction effect in those all earthquakes. In general, the liquefaction potential of soil containing fines largely depend on percentage of fine and plasticity of fines content has to be tested before construction. Therefore a clear understanding of the behavior of silty sand undergoing earthquake loading is need prior to design and the improvement of structures. Based on recent investigations it seems that cyclic liquefaction characteristics of a sand, insitu are influenced by the following factors. 1. Relative Density 2. Fines Content and Plasticity 3. Method of Soil Formation (Soil Structure) 4. Period under Sustained Load 5. Previous Strain History 6. Lateral Earth Pressure Coefficient and Over consolidation 7. Consolidation Pressure 8. Grain Characteristic xnAt high relative density, and under lateral earth pressure coefficient, Kq, condition liquefaction increases. At the same time stress ratio for liquefaction resistance increases with prior seismic strains. The increase plastic fines increase the liquefaction resistance, the increase in non plastic fines reduces the liquefaction resistance. Laboratory tests has shown the liquefaction resistance disproportional to consolidation pressure. When the consolidation pressure is increased the liquefaction resistance decreases. In this study, Erzincan Ekşisu region is chosen as the research area, samples has been taken from this region. A series of cyclic tests have been carried out on undisturbed sandy silt and silty sand specimens obtained by piston sampler and shelby tubes with 70-80 mm in diameter. A series of tests were carried out with dynamic simple shear device. The cyclic simple shear testing system which is 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. Test samples which are used in this test configuration are undisturbed samples. When sample are taken from a sand deposits 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. All sandy silt samples have been kept in the moisture room until they been used in tests. In order to prevent disturbance of silty sands, shelby tubes are frozen before transportation and are kept frozen until they are used in tests. Water in pores silty sand shelby tube samples are frozen -10 ° C in a deep freezer. Since the samples were not fully saturated the freezing would not produce any volume change. Before test, first a frozen shelby tube was cut longitudinally with an electric chain 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 sample was measured. A vacuum of about 30 kPa was applied to both ends of the test sample and the sample was allowed to completely thaw for two hours. The test chamber was assembled and filled with viscous oil. The air in pores are removed by circulating water from bottom to top under vacuum. Then vacuum gradually reduced to zero while simultaneously increasing the cell pressure xiiito a value of 30 kPa. Cell pressure and back pressure were increased while maintaining the constant effective confining stress. The sample was allowed to consolidate at this stress for 24 hours. Before testing for the samples the following procedures should be conducted. First step, saturation level should be measured. To measure the saturation level of the samples the specimens should be kept for 24 hours. There are different saturation level for soil samples for example acceptable saturation level for sandy silt samples B>%95, silty sand samples B>%98. For dynamic tests, on silty sand samples consolidation pressure is increased to 100 kPa and the sample was left in pressured test chamber about 4 hours. This consolidation period is enough for silty sand samples. After this waiting period drainage valves were closed and pore-water pressure are checked, when they reached as stable value, the cyclic test is conducted at the frequency equal to 1 Hz. The test samples were obtained from Erzincan Ekşisu region boreholes samples. These undisturbed samples were taken approximately from a drilling depth of 16 m. The layer of samples were set as follows. From the sample fill 11-13 m the soil composition shows a mixture of soft sandy silt, loose and medium densed silty sand layer. SPT tests conducted on the field has shown in a range of N30=2-l 1 for silty sand and sandy silt layers. In the inspected drillings the ground water level is approximately 1 m below the surface. In our tests, the sandy silt samples had a water contents of wn= %36-%38, liquid limit value of wL^%31-%36, plasticity index of Ip= %10, fines content of %62-%83. In the silty sand samples the ratios were varying as follows, water content wn= %18-%41, liquid limit wL=%25-%31, fines content %1 1-%49 respectively. In this study, we tested the dynamic behavior of silty sand and sandy silt layers. We also investigated effect cyclic shear stress level, fines content, relative density and applied consolidation pressure effects on the test samples. From the test results we observed the following patterns: * In sands with fines the dynamic strength is disproportional to its dynamic stress level. When dynamic stress level is increased it is more susceptible for liquefaction. * Test performed on silty sand samples has shown that the relative density of the surrounding environment has great impact on liquefaction resistances than their fine content. xiv*From the test made under different consolidation pressures has guided us to this conclusion. The dynamic strength of the silty sandy decreased with the increasing consolidation pressure. The time scarcity has limited has limited us tremendously. Therefore, we could a limited number of drilling samples which were taken from Erzincan Ekşisu region. Our suggestion is that: * The number of samples from the region should be increased and drilling should be spread on different areas in the same region. * Silty sand and sandy silt layer deserve more attention to understand their dynamic behavior fully more detailed elaborated research in laboratories should be made. "?Laboratory tests should be complimented with insitu tests. *Erzincan Ekşisu region liquefaction potential could be extracted precisely and correctly from the evaluation of the field and laboratory data. xv

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