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Sıkıştırılmış zeminlerde donma-çözülme olayının deneysel incelenmesi

Experimental i̇nvestigation of freeze and thaw on compacted soils

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  1. Tez No: 356143
  2. Yazar: ADEM IŞIK
  3. Danışmanlar: DOÇ. DR. RECEP İYİSAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2014
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
  13. Sayfa Sayısı: 155

Özet

Soğuk iklim koşullarının hâkim olduğu bölgelerdeki zeminler, mevsimsel sıcaklık farklılıklarından dolayı yılda birkaç kez donma-çözülmeye maruz kalmaktadır. Zeminler donma-çözülme etkisi altında yapısal olarak değişime uğradıkları için fiziksel ve mekanik özelliklerinde de değişimler meydana gelmektedir. Bu değişimler iklim şartlarına, zemin türüne ve özelliklerine göre farklılık göstermektedir. Yani zeminlerde donma-çözülme sonrası oluşacak fiziksel ve mekanik özelliklerin zemin mühendisliği ve yol muhendisliği açısından önemi büyüktür. Bu yüzden soğuk iklim bölgelerinde yapılan mühendislik tasarımlarında donma-çözülmenin zemin üzerindeki etkileri dikkate alınmalıdır. Donma olayının gerçekleşmesi ve donma-çözülmenin önemli değişimler meydana getirmesi için zeminin donmaya karşı hassas olması, zemin ortamında yeterli su bulunması ve ortamın donma sıcaklığında olması gerekmektedir. Ayrıca zeminlerde donma olayı esnasında hacimsel olarak artıştan dolayı donma kabarması meydana gelmektedir. Zeminlerde donma kabarması zemin cinsi, dane dağılımı, minerolojik yapısı, ince dane oranı, ortamdaki su miktarı, sürşarj yükü ve sıcaklık gibi birçok faktörden etkilenmektedir. Donma kabarması hem laboratuvar hemde arazide yapılan deneyler ile belirlenebilmektedir. Donma kabarması laboratuvarda yapılan donma-çözülme deneyi ile incelenmekte ve bu deneylerle zeminlerin donmaya karşı hassas olup olmadığı belirlenmektedir. Zeminlerin donmaya karşı hassaslığının belirlenmesi için birçok sınıflandırma yöntemi geliştirilmiştir. Bu çalışmada sıkıştırılmış 20 adet zemin numunesinin donmaya karşı hassaslığını belirlemek için farklı geoteknik özelliklere sahip numuneler üzerinde donma-çözülme deneyleri ve donma-çözülme sonrası numuneler üzerinde CBR deneyi yapılmıştır. Deneyler sonucunda belirlenen donma kabarma hızları ile zeminin endeks özellikleri arasındaki ilişkiler incelenmiştir. Pratik amaçlar doğrultusunda donma kabarma hızını, likit limit, plastik limit, plastisite indisi, deney sonu su muhtevası, maksimum kuru birim hacim ağırlık, optimum su muhtevası, boşluk oranı ve poroziteye bağlı olarak tahmin edebilmek için korelasyon bağıntıları geliştirilmiştir. Bu amaçla farklı numuneler üzerinde yapılan donma-çözülme deneyi sonucunda belirlenen donma kabarmasının artan deney sonu su muhtevasının likit limite oranı ile artığı gözlenmiştir. Ayrıca düşük ve yüksek plastisiteli diye iki ayrı gruba göre belirlenen donma kabarma hızları ile maksimum kuru birim hacim ağırlık, optimum su muhtevası ve boşluk oranı arasındaki ilişki incelenmiş ve sonuç olarak maksimum kuru birim hacim ağırlık artıkça donma kabarma hızlarının düştüğü ancak optimum su muhtevası ve boşluk oranı değerlerinin artması durumunda ise donma kabarma hızlarının arttığı belirlenmiştir. Donma-çözülme deneyi sonucunda elde edilen donma kabarma hızı değeri ile artan likit limit, plastik limit ve plastisite indisi arasında bir ilişki belirlenememiş ve sonuç olarak donma kabarma hızının daha çok siltli numunelerde daha yüksek değerlere ulaştığı belirlenmiştir. Bu çalışma kapsamında zemin numunelerinin donma-çözülme sonrası davranışı belirlemek için numuneler üzerinde donma-çözülme öncesi ve sonrası CBR deneyi yapılarak sonuçlar karşılaştırılmıştır. Ayrıca doygun halde hazırlanan numuneler üzerinde donma-çözülme sonrası CBR deneyi yapılarak doygun durumdaki sonuçlar ile doygun olmayan durumdaki sonuçlar karşılaştırılmıştır. CBR deneyleri sonucunda sırasıyla donma-çözülme öncesi, donma-çözülme sonrası ve doygun halde donma-çözülme sonrası CBR değerlerinin azaldığı belirlenmiştir. Donma-çözülme öncesi ve sonrasına göre belirlenen CBR değerlerindeki değişim miktarları ile zemin numunelerinin likit limit, plastik limit, plastisite indisi ve optimum su muhtevası gibi özellikleri arasındaki ilişkiler incelenmiştir. Sonuç olarak CBR değerlerindeki değişim miktarları artan likit limit, plastik limit, plastisite indisi ve optimum su muhtevası ile artmıştır.

Özet (Çeviri)

In cold regions, soils are exposed to freezing and thawing cycles which are important in geotechnical engineering. Most of the engineering properties of soil are affected by freezing and thawing period. Protection against frost action of a man-made fill deposits are essential for transportation road construction projects in cold regions. These soils are exposed to at least one freezing-thawing cycle every year in seasonally frozen area. The freezing and thawing process would change the structure and arrangement of soil particles and the mechanical properties of soil. For instance; strength, permeability and compressibility of soil could be changed considerably due to frost action. These changes depend on climate conditions, soil types and properties. Cold region engineers must take into account of effects of freezing and thawing process on soil properties. Damage due to frost action is essentially caused by the presence of ice lenses in soils. The formation of ice lenses requires a frost susceptible soil, a water supply, and subfreezing temperatures. In these conditions, for assessments of the frost susceptibility of soils are important to understand behaviour of soils during frost action. There are two phenomena about frost heave how occurs in soils. The first one which is early consideration is based on volume extension of water. The other is based on ice lenses that result from water migration through the soil voids toward the freezing front. In geotechnical considerations, frost heave results from ground freezing that occurs in cold region. Frost heave is a very important issue in design and construction throughout cold regions. Frost heave causes costly damage to roads, building foundations, airfields and pipelines.Frost heave is effected by most factor which are soil type, particle size distribution, minerology, the ratio of fine grained, availability of water, overburden pressure and tempeature gradient. Frost heave can be determined either laboratory or in-situ test. Also frost susceptibility of soils is determined by these tests. The frost heave common geotechnical problem in cold regions is typically observed in fine-grained soils. When the soil is fully frozen the volume of water, which is the between soil particles, increases approximately by 9% and then cracks are occurred in the soil. The strength of the soil decreases as the size and number of cracks increase. After winter seasons shear strength and stability of the soil generally are affected negatively, which is defined as thaw weakening. Frost susceptibility of soils which show us the effect of freezing and thawin process on soils is determined with classification method. There are three estimating levels for identification of frost susceptibility of soils. Level I is primarily based on the percentage of soil finer than a specified particle size commonly 0.075 mm or 0.02 mm. Level II classification method is based on grain size distribution, vater retention capacity, or minerology such as Atterberg limits and clay contents. Level III is based on dirct observations of frost heaving in the laboratory or in the field. The effect of freezing-thawing on engineering properties of soils varies by not only the soil type but also the local climate and site conditions . To see the effect of freeze and thaw weakening, a susceptible soil type, pore water in the porous soil media, and low and high temperature degrees for freezing and thawing cycle is required at the same time. While the grain size distribution, permeability and capillarity are important in the soil type, water content and ground water level verifies the presence of water in the pores of the soil. Furthermore, the density, degree of compaction, and overburden pressure may also illustrate the local site conditions. In geotechnical considerations, frost heave resulting from ground freezing is a very important issue in designing and construction in cold regions. It causes costly damage to roads, building foundations, airfields and pipelines. Many studies have been performed to predict and prevent frost heave in designing structures. Frost-susceptibility of the subgrade layer plays an important role on the selection of a soil material for a fill site. In this study, frost susceptibility and freezing and thawing weakening of 20 compacted soils which have different geotechnical properties are determined in laboratory. The soil samples are compacted their optimum water content by using standart Proktor energy. Frost heave and CBR test are conducted on soil samples for determination of freeze and thaw behaviour of soils. After tests the frost heave rate and reduction of CBR value have been associated with geotechnical properties of soil samples such as liquid limits, plastic limitsi plasticity index, optimum water content, maximum dry unit weight, void ratio and porozite. Frost susceptibility of soils was investigated by using frost heave test which conducted according to ASTM D5918. In this test, assuming that no source of water was available during the freezing process except originally kept in the voids of the soil, a closed system was used. In a cold chamber two freeze and thaw cycles were conducted on the samples lasting totally 120 hours at temperatures varying between -12°C and +12°C as suggested by ASTM D5918. To observe both the amounts of frost heaving during freezing and settlements at thawing processes, the vertical displacements were observed by 0.01 mm precision micrometers attached on the top of a 3.2 kN/m2 surcharge load placed on the soil. The frost heave versus time of sample was ploted after the test. Frost heave rates were determined at initial 8 hours intervals of the freezing periods. Comparing the rates heave rate potential of the soils were generally determined by second freeze-thaw cycle. After the test, an average value of the final moisture content of the soil sample was determined on specimens. Thaw weakening susceptibility of the compacted soil samples prepared at maximum dry unit weight and optimum water content were also determined by comparing their California Bearing Ratio (CBR) results performed before the saturation process and after saturation and the freeze and thaw process. The dead load acting at the top of the soil sample in the CBR mold was same as the freeze and thaw test. During the test the vertical loads were collected while a piston with a 4.96 cm in diameter is penetrating into the specimen at a uniform rate of 1.27 mm/min (ASTM D1883). To utilize for practice objectives frost heave and reduction of CBR value of soil samples which have different geotechnical properties have been associated with their index properties. For this purpose frost heave rate that determined after the tests increase with increasing the rate of final water content to liquid limit. Moreover, we know that frost susceptibility of soils differ from its plasticity in the literature. Therefore, frost heave rate has assessed as two group of soil samples which are low and high plasticity group. In this case the frost heave rate of soil samples has been associated with maximum dry unit weight, optimum water content, void ratio and porozite. The result of tests show us when maximum dry unit weight of soil samples is higher the frost heave rate has become lower value for either low plasticity or high plasticity soils samples, but frost heave rate increases with the increasing the optimum water content, void ratio and porozite. Also it could not be directly associated between the frost heave rate and liquid limit or plasticity index. In the scope of this study, CBR tests were performed on soil samples before and after freezing and thawing process in order to determine freeze and thaw behaviour of soils. Also the soil samples were saturated and subjected to freeze and thaw process. CBR tests were conducted after freeze and thaw process of saturated soil samples for assesstment of effect of water on soil in freezing and thawing conditions. The results of CBR tests it have seen that after freeze and thaw process the CBR values mostly effected by saturated conditions. In unsaturated conditions that prepared optimum water content by using standard Proktor energy effects of freeze and thaw on soil samples less than saturated conditions. After freeze and thaw process soil samples lost their bearing capacity that determined CBR test. Before and after freeze and thaw process the reduction of CBR value determined. The reduction of CBR value has been associated with geotechnical properties of soil which are liquid limit, plastic limit, plasticity index and optimum water content. The results of tests shown that the reduction of CBR value increase with increasing liquid limit, plastic limit, plasticity index and optimum water content in saturated and unsaturated conditions. CBR values for all the samples illustrated a reduction in the range of 21% and 86% after freeze and thaw processes. It was observed that the maximum reduction in the CBR value was belongs to the soil which has the highest liquid limit value because of its high water retention capacity.

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