İnce daneli zeminlerde deplasmana bağlı mukavemet kaybının aşırı konsolidasyon oranı ve gerilme seviyesine göre değişimi
Effect of overconsolidation ratio and stress level on the displacement induced strength loss in fine grained soils
Yazar: OĞUZHAN SALTALI
Danışman: DR. ÖĞR. ÜYESİ MUSTAFA HATİPOĞLU
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Kayma dayanımı = Shear strength ; İnce daneli zemin = Fine grained soil

Özet:
Dış yükler ve doğal süreçlerin etkisiyle zeminlerin maruz kaldıkları gerilmeler zemin kitlesinde şekil değiştirmelere neden olmaktadır. Zemin ortamında oluşan kayma gerilmeleri ise zeminin kendi direncine ulaştığı ya da aştığı zaman kırılma (göçme) meydana gelmektedir. Kayma mukavemeti ise zeminin dayanabileceği en büyük kayma gerilmesi olarak adlandırılmaktadır. Bu kavram ise stabilite (denge) ve deformasyon problemlerinin çözümü için önem arz etmektedir. Kayma mukavemeti kavramı deformasyon seviyesiyle ilişkilidir. İleri deformasyon seviyelerinde erişilen ve danelerin kesme yüzeyine paralel olarak yönlendiği durumda kalıcı kayma mukavemeti olarak adlandırılan sabit bir mukavemet değeri gözlenmektedir. Erişilen yüksek deformasyonla birlikte daneler arasındaki bağlar kopmakta ya da sıfıra yaklaşmakta dolayısıyla mukavemet danelerin sürtünmesiyle elde edilmektedir. Özellikle kil içeriği yüksek aşırı konsolide zeminlerin hâkim birim olarak yer aldığı şevlerin stabilite analizlerinde kalıcı mukavemet kavramı önemlidir. Laboratuvarda,
kullanım olarak yaygın olan ve yüksek deformasyon seviyelerine erişilebilen bir yöntem olan Tekrarlı Kesme Kutusu Deneyi ile kalıcı kayma mukavemeti belirlenebilmektedir. Genel manada, kalıcı kayma mukavemeti mineroloji, kesme hızı ve gerilme seviyesinden etkilenmektedir. Literatürde etkisi çeşitli araştırmalara konu olmuş aşırı konsolidasyon oranı ise kalıcı mukavemeti belirgin bir etkilememekle birlikte pik mukavemeti büyük bir ölçüde etkilemektedir. Bu durumda pik ve kalıcı mukavemet arasında ciddi bir fark söz konusu olmaktadır. Bu çalışma kapsamında farklı kıvam özelliklerine sahip ince daneli zeminler standart bir numune hazırlama metodu kullanılarak belirli aşırı konsolidasyon oranlarında (1, 2, 3, 5, 7 ve 10) hazırlanmıştır. Konsolidasyon aşamasının ardından hedef aşırı konsolidasyon oranlarında hazırlanan numuneler üzerinde 50 kPa, 100 kPa ve 200 kPa gerilme altında Tekrarlı Kesme Kutusu deneyleri yapılmıştır. Numunelerin pik ve kalıcı mukavemetleri arasındaki fark/oran ile sekant açılarının aşırı konsolidasyon oranı ve gerilme seviyesine göre değişimi incelenmiştir. Zeminin kıvam limitleri ile sekant açıları ve mukavemet oranları ilişkilendirilerek pratik olarak kullanılabilecek çeşitli korelasyonlar elde edilmiştir. Sayısal analizlerle regresyon bağıntıları elde edilmiş ve farklı değişkenler arasındaki ilişkiler belirlenmiştir.

Abstract:
Due to external loads and natural processes, soils are exposed to stresses which inevitably cause deformations in the soil mass. When the soil reaches or exceeds its resistance, failure can occur in the soil mass. Shear strength is known as the greatest shear stress that the soil can tolerate/withstand. It is of significant importance for solving stability (equilibrium) and deformation problems. Stability problems, which are the mostly encountered in the discipline of geotechnical engineering, are highly interrelated to shear stress development in the soil mass, as well as the safety of
structures that have been built or are planned to be built. In this case, the equilibrium state deteriorates if deformations exceed the allowable limits. Thus, shear strength should be investigated in detail for the design and application phases of slopes. In this context, it is necessary to determine the shear strength paramaters in an
appropriate way. The concept of the shear strength is directly linked to the deformation level. Especially at the large deformation levels, a constant strength value which is known as residual shear strength is formed in the case where grains are oriented parallel to the shear surface. When the large deformation levels are achieved, the bond between the grains break and the cohesion value approaches to zero; therefore, the strength is obtained by the frictional forces among the particles. In particular, residual shear strength is important in the stability analysis of slopes, on
those where overconsolited fissured clays with high clay content are dominant. The primary factors that influence on residual shear strength are mineralogical structure, shear rate, and effective normal stress. On the other hand, overconsolidation ratio does not have a significant effect on the residual shear strength, but greatly increases
the peak shear strength with respect to the characteristic feature of grains. In the laboratory, residual shear strength could be determined by the Reversal Shear Box Test which is not only widely used, but also could reach large deformation
levels. In the test, the sample is placed in a circular or a square box section and consolidated with an axial stress. It is forced to slide along a pre-determined shear surface by pushing the lower part steadily with respect to upper part. This process is sustained until required deformations are reached. Then, the soil sample should be returned to its initial position and one reversal/cycle movement could be obtained in this way. Such cycles are repeated until a minimum constant shear stress is attained. Practically, the residual shear strength could be reached after at least three
reversals/cycles depending on the soil properties. The experimental studies within the scope of this thesis were carried out on four fine grained soils with different consistency properties. The liquid limits values vary between 24% and 93% and the plasticity index values are in between 7% and 60%. The samples have fine content values ranging from 75% to 100% as well. According to Unified Soil Classification System (USCS), the soil samples are categorized as low plasticity clay (CL) and high plasticity clay (CH). For the preparation of samples, the soils were grinded and sieved through #40 sieve
at first. Then, water was added at approximately 1.5 times the liquid limit and mixed until it becomes homogenous in order to prevent air bubbles in the soil samples. In order to acquire samples with different overconsolidation ratios, the consolidation process was started. The samples were consolidated with normal stresses ranging from 50 kPa to 2000 kPa. It should be noted that the desired stresses were reached gradually, not in single stage. After the completion of the primary consolidation, the normal stress level was decreased to 50 kPa, 100 kPa and 200 kPa in single stage.
Through using this operation, fine grained soil samples were acquired at an overconsolidation ratio determined as 1, 2, 3, 5, 7, and 10 and were sheared under different vertical stresses. Since large deformation levels need to be reached to obtain residual shear strength and the deformation level of shear box test in one direction limited to, selected samples were sheared in five cycles so as to obtain residual shear strength. Moreover, the peak shear strengths were attained at least in two shearing cycles. The purpose of this approach is to save time and increase the number of
experiments that could be performed since the consolidation and shearing processes take plenty of time. A total of 72 experiments were conducted, performing 18 test for each of the four samples. The shear rate used in the reversal shear box tests has a pivotal role on the result of experiment. In this context, the shear rate should be at a low level which do not lead an excessive pore water pressure with object of reflecting field/in-situ conditions. Hence, as for the shear rate, 0.01 mm/sec was selected as a rate which could be acceptable as a suitable value according to studies given in the literature. Due to the discontinuity of the slip surface area, which is a limitation of the shear box test, area
correction was not employed in the calculations done within the scope of this study, and the stress values were determined as reported in Head (2006). In this experimental study, the variation of the strength loss and strength parameters of samples prepared at different overconsolidation ratios were investigated. In line with this target, the secant friction angle, the peak shear strength, and the peak and the residual shear strength ratios were determined. Then, trends and changes in peak and residual strength ratios expressing strength loss were examined and correlations
with consistency limits were derived for practical use. The existence and power of the relationships between variables were investigated by multivariate regression analysis. The relationship between consistency limits with the secant angle and the strength ratio were determined by using experimental results. Additionally, regression properties of the correlations were compared to each other. Such relations and correlations could provide useful estimative approach for the strength loss of overconsolidated clays. Consequently, the consistency limits and the soil mineralogy are significant in the post-peak behavior of soil. Samples with low plasticity and low clay percentage indicate that there is a slight decrease in the residual shear strength after reaching the peak strength value. However, a sample with high plasticity and low clay percentage shows remarkable decrease towards the value of residual shear strength value from
the peak strength. As expected, the overconsolidation ratio and the normal stress level have a crucial role on the peak shear strength. However, the overconsolidation ratio does not impact the residual shear strength regardless of the plasticity of soils.In addition to this, strength loss in samples with high plasticity and high percentage of clay is greater than a soil with low plasticity. In such samples, there may be nearly 80% strength loss at especially low stress levels, while it can give rise to 40% loss at high stress levels. Higher correlation coefficients were obtained in the peak and residual strength ratio approach compared with secant angle approach; therefore, the former shows close proximity to the experimental results than the latter.