Kumlu zeminlerin kayma direncine bazalt fiberin katkısı = Contribution of basalt fiber to shear strength of sand soils

Abstract

Anahtar Kelimeler: zemin iyileştirme, bazalt fiber, kumlu zemin, kayma direnci, kesme kutusu. Nüfus yoğunluğunun beraberinde kentleşmenin giderek artması büyük projelere (yüksek katlı binalar, yeraltı ve yerüstü ulaşım yolları, barajlar v.b) olan ihtiyacı meydana getirmiştir. İnşa edilecek yapı yüklerinin artması, elverişli zemin arazilerinin yetersizliği, zayıf zeminin terkedilememesi ve projenin zemin koşullarına uygun hale getirilememesi gibi durumlar elverişsiz zeminlerde yapılaşmayı zorunlu hale getirmiştir. Bu olumsuz koşullar zemin iyileştirme yöntemlerini çözüm olarak ortaya çıkarmaktadır. Zemin iyileştirme, zeminin geoteknik özelliklerinin farklı fiziksel, kimyasal veya biyolojik yöntemler kullanılarak iyileştirilmesi olarak tanımlanabilir. İyileştirmenin amacı zeminin taşıma gücünü ve kayma direnci parametrelerini arttırmak, oturmaları, sıvılaşma potansiyelini ve boşluk oranını azaltmaktır. Son yıllarda geoteknik mühendisliği uygulamalarında zemine katkı maddeleri katılarak iyileştirme yöntemi oldukça yaygındır. Bu katkı maddelerine doğal lif olarak sınıflandırılan yüksek mukavemetli bazalt fiberler örnek olarak verilebilir. Bazalt fiberler ucuz hammadde ve düşük üretim maliyetine sahip olmasının yanı sıra çevre dostu olması sebebiyle zemin iyileştirme çalışmalarındaki değeri literatürde her geçen gün artmaktadır. Bu çalışmada, bazalt fiberin kumlu zeminlerin kayma direnci parametreleri üzerindeki etkisi araştırılmıştır. Deneylerde kullanılan kum Sakarya Üniversitesi İnşaat Mühendisliği Geoteknik Laboratuvarı'ndan alınmıştır. Doğal kum zeminine ağırlıkça %0, %1, %1.5, %2, %2.5 oranında ve boy olarak 6 mm, 12 mm, 24 mm uzunluğundaki bazalt fiberler katılarak numuneler hazırlanmıştır. Hazırlanan numuneler sabit bir rölatif sıkılık değerinde olacak şekilde kare kesitli kesme kutusuna hassasiyetle yerleştirilmiştir. Deneyler 50, 100 ve 150 kPa normal gerilmeler altında yükleme hızı 0.5 mm/dk olacak şekilde kesme kuvvetine maruz bırakılarak gerçekleştirilmiştir. Bazalt fiber katkılı ve katkısız kum numuneleri üzerinde toplamda 39 adet kesme kutusu deneyi gerçekleştirilmiştir. Elde edilen deney sonuçlarına göre, artan fiber oranı ve boyu ile kohezyon arasında sistematik bir ilişki saptanamamıştır. Bununla birlikte kohezyon maksimum değerine %2 oranında 12 mm boyunda bazalt fiber katılmasıyla elde edilen numunede ulaşmıştır. Kum zemine %1.5 oranında bazalt fiber katılmasıyla tüm boylarda kayma direnci açıları en yüksek değerine sahipken bu orandan sonra radikal bir biçimde düşüşe geçmiştir ve maksimum değerine %1.5 oranında 24 mm boyunda bazalt fiber katılmasıyla elde edilen numunede ulaşmıştır. Sonuçlardan anlaşılacağı üzere kum zeminlere bazalt fiber eklenmesi kayma direncini arttırıcı bir etki göstermiştir ve katkı malzemeleriyle zemin iyileştirme uygulamalarında kullanılan malzemelere alternatif olabileceği sonucuna varılmıştır.

The gradual increase in urbanization along with the overpopulation has created the need for large projects (high-rise buildings, infrastructure and surface transportation roads, dams, etc.). Situations such as the increase in structural loads, the inadequacy of suitable soil sites, the inability to abandon the weak soil and adapt the project to soil conditions have made it necessary to build the structures on weak soils. These adverse conditions reveal soil improvement methods as a solution. Improvement methods can range from lowering the pore water level of the soil to increasing the shear resistance of the soil with chemical or biological additive agents. Therefore, it is not possible to classify these methods under a single title. As a matter of fact, many researchers have categorized the improvement methods differently. When choosing the method to be applied, the type of soil (clay, silt, organic matter content, etc.), the geological condition of the site and the groundwater level must be taken into consideration. However, while determining the method to be applied in the principle of geotechnical engineering, it is also very important that it is safe, fast in terms of supply, economical and functional. Soil improvement can be defined as the improvement of the geotechnical properties of the soil by using different physical, chemical or biological methods. In other words, it means to increase strength and stiffness of the soil. The aim of the improvement is to increase the bearing capacity and shear resistance parameters of the soil, to reduce settlements, liquefaction potential and void ratio. If the soil properties are not sufficient in terms of bearing capacity and settlement, the simplest and easiest solution is to design the foundation system according to the soil conditions. In these systems, the first thing that comes to mind is to create a logical study with shallow foundation applications by taking the uniform load distribution of the soil structure as a criterion. If the structural loads cannot be transmitted safely to the soil through the shallow foundation system, deep foundations can be fabricated. However, due to the increase in the height of the buildings constructed nowadays, there is an increase in the structural loads to be transmitted to the soil. Therefore, advanced technology applications are required for the new structures to be built compared to the old ones. As a result, construction costs increase extremely. This reveals the disadvantages of deep foundation applications. Different methods are being developed and researched to reduce costs and provide ease of application. For example, to improve the physical and mechanical properties of the soils, soil improvement studies are constantly increasing by adding natural and artificial additives to the soil. For many years, lime, cement and bitumen has been used as additives. However, in recent years, different alternatives have been created with the addition of natural or artificial fibers to the soils. In recent years, the soil improvement method by utilizng additives to the soil is quite common in geotechnical engineering applications. High-strength basalt fibers that are classified as natural fibers, can be given as examples of these additives. Basalt fibers are obtained by melting and drawing basalt, a type of volcanic rock, at high temperatures. In brief the production process; it can be listed as preparing melt from basalt, shrinking the melt, fiber formation, coating the fiber surface with resin and wrapping it into tubes with a winding device. The production technique of basalt fibers is similar to the production technique of glass fibers, except for the differences in temperature and viscosity parameters. Since basalt rock is widely found in the earth's crust, the raw material of basalt fiber is cheap and the production cost is low. The dense and hard structure of basalt provides the its fiber to have superior mechanical properties and a durable structure. It also shows strong resistance against heat, impact load and chemical factors. In addition to these properties, basalt fibers can be used in many areas in civil engineering as they are environmentally friendly and they show high tensile strength. Especially its value in soil improvement studies is increasing day by day in the literature. In this study, the effect of basalt fiber on the shear strength parameters of sandy soils was investigated. Within the scope of the thesis, soil improvement methods with additives were explained and supported by literature examples. In addition, the properties of the materials used in the thesis, the testing apparatus and methods were introduced, and the test results were presented in tables and graphics. The effect of basalt fiber additive on sandy soil, depending on the fiber length and ratio parameters, was determined by directs shear testing apparatus. Direct shear tests are one of the oldest and fastest methods used to determine the shear resistance parameters of soils. In the principle of the experiment, a soil sample is placed in a box with square or circular cross-section and consisting of two parts. The upper part of these boxes is kept fixed and the lower part is moved at a constant speed and by applying a certain horizontal force. Meanwhile, a certain normal force is applied to the sample in the box and it is forced to fail along a predetermined horizontal plane. All experiments within the scope of the study were carried out in Sakarya University Civil Engineering Geotechnical Laboratory. By evaluating the data obtained from the experiments, it is aimed to facilitate the applications for improving the soils by using additives. The sand used in the experiments was taken from Sakarya University Civil Engineering Geotechnical Laboratory. First of all, physical experiments were applied to determine the geotechnical properties of the natural sandy soil. TS 1500/2000 standard was used and a sieve analysis test was carried out to determine the class of the soil. A pycnometer test was carried out to determine the specific gravity of the soil. Finally, the relative density test was carried out in order to determine the natural, maximum and minimum void ratio. Thereafter, samples were prepared by adding 0%, 1%, 1.5%, 2%, 2.5% of 6 mm, 12 mm, 24 mm long basalt fibers to the natural sandy soil. The prepared samples were sensitively placed in a square direct shear box with a constant relative density value. The experiments were carried out under normal stresses of 50, 100 and 150 kPa, by subjecting them to a shear force at a loading speed of 0.5 mm/min. A total of 39 shear box tests were carried out on the samples with and without basalt fiber. According to the test results obtained, the shear resistance angle (ϕ) and the cohesion value (c) of the natural sand was found to be 34o and 23.1 kPa, respectively. No systematic relationship was found between increased fiber content and cohesion. However, with the addition of basalt fiber at the rate of 1.5% to the samples, the cohesion values in all lengths had the lowest value, but after this rate, it increased and reached its maximum value in the sample obtained by adding 2% of basalt fiber in the length of 12 mm. With the addition of 1.5% basalt fiber to the samples, the shear resistance angles had the highest value in all lengths, but after this ratio, it decreased radically and reached its maximum value in the sample obtained by adding 1.5% basalt fiber with a length of 24 mm.The results has shown that, the addition of 1.5% basalt fiber to sandy soils played a role in increasing the shear strength. Above this ratio, the fibers tend to agglomerate among themselves and cannot be distributed homogeneously in the soil. When the tables and graphs of the maximum shear stress values obtained from the direct shear tests carried out on basalt fiber reinforced sand samples are examined together; higher shear stresses were observed mostly in 24 mm long fibers. This positive effect is followed by fibers of 12 mm length, while fibers of 6 mm length have much less effect on shear stress. This is associated with an increase in the contact area between the fiber and the soil as the fiber length increases. Thus, it is thought the fibers that enter between the soil particles and show better resistance to lateral stresses and prevent the particles from slipping easily over each other. As the basalt fiber ratio and length increased during the experimental processes, homogeneous mixing and maintaining the homogeneity during the fiber mixtures were transferred to the box became difficult. In order to avoid agglomeration, the fibers can be studied in their original form not separated by air pressure and in larger scale direct shear tests. Direct shear tests were carried out under three different stresses 50, 100 and 150 kPa and outliers were detected in the results of some 150 kPa normal stresses. Therefore, it is recommended to repeat similar studies under lower and more number (at least 4) normal stresses.