İnce daneli zeminlerde deplasmana bağlı mukavemet kaybının aşırı konsolidasyon oranına göre değişimi
Effect of overconsolidation ratio on the displacement induced strength loss in fine grained soils
Yazar: BURAK ÖZÇÖREKÇİ
Danışman: DR. ÖĞR. ÜYESİ MUSTAFA HATİPOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: İnce daneli zemin = Fine grained soil

Özet:
Zeminin göçme durumunun görülmesi, mevcut zemindeki olası kayma  yüzeyinin sahip olduğu dirençten daha büyük bir gerilmenin etkimesiyle aşılması olarak ifade edilebilir. Kayma mukavemeti de bu aşım gerçekleşmeden hemen önce zeminin sahip olduğu etkiyen yüke karşı en büyük dayanımdır. Mohr-Coulomb göçme hipotezine göre  farklı asal gerilmeler altında zeminin mohr dairesinin kırılma zarfına teğet olduğu durumda göçmenin gerçekleştiğini belirtir. Kırılma zarfının denklemi, zeminin sahip olduğu kohezyon ve kayma mukavemeti açısı ile ifade edilir. Zeminin gerilme şekil değiştirme  ilişkisi göz önünde bulundurulduğunda şekil değiştirme miktarı arttıkça, sıkılık veya sertlik durumuna bağlı olarak  göçmeye uğrar. Bu durumun devamında görülen büyük deplasmanlar sonucu zemindeki yüke karşı koyan mukavemet seviyesi sabit bir değere yaklaşır.Bu sabit değere kalıcı kayma mukavemeti adı verilir. Artan bu yer değiştirmeler sonucu daneler arası kohezyonun kalmaması yada sıfıra yaklaşması ile etkiyen yüke karşı oluşan bu sabit dayanım sadece kayma yüzeyindeki danelerin sürtünmesi ile elde edilir. 
Kalıcı kayma mukavemetine kesme hızı, mineralojik yapı, efektif normal gerilme, kil yüzdesi gibi değişkenler etkilemektedir. Özellikle ince daneli zeminlerin dane yapısının yassılaşması ile göçme sonrası kohezyon teriminin etkisinin azalması ya da tamamen sıfırlanması ile beraber daneler arası yüzey etkileşimi olarak ifade edilen sürtünmede de azalma meydana gelir. Bu nedenle aşırı konsolidasyon oranının artması, göçme öncesi görülen kayma mukavemeti değerini belirgin biçimde arttırsa bile göçme sonrası elde edilen kalıcı kayma mukavemeti değeri için etkisini önemli derecede azalmakta, danelerin şekliyle ilişkili olarak pik kayma mukavemeti ile kalıcı kayma mukavemeti arasında oluşan bu farkı arttırmaktadır.  İTÜ Zemin Mekaniği Laboratuvarı'nda kesme kutusu deney düzeneği kullanılarak farklı kıvam limitleri değerlerine sahip ince daneli numuneler üzerinde deneyler yapılmıştır. Bu deneyler yardımı ile aşırı konsolidasyon oranının pik kayma mukavemetiyle kalıcı kayma mukavemetinin oranına olan etkisi incelenmiştir. Bu doğrultuda, pratik amaçlar için pik kayma mukavemetini ve  pik kayma mukavemeti ile kalıcı kayma mukavemetinin birbirine olan oranını, kıvam limitleri ve aşırı konsolidasyon oranına bağlı olarak veren korelasyonlar oluşturulmuştur.

Abstract:
Soil failure is observed when the strength of the soil is exceeded by affecting stress on possible slip surface of the soil. Shear strength is also defined as the greatest strength against to the effecting load of the soil just before failure occurs. According to the Mohr-Coulomb failure hypothesis, the failure occurs when the mohr circle of the soil, under different principal stresses, becomes tangent to the failure envelope. The equation of failure envelope is expressed with the cohesion and shear stregth angle of the soil. Considering the stress-strain relationship of the soil, as the deformation increases, failure takes place depending on the firmness or hardness. As a result of the large displacements seen in the continuation of this movement, the resistance to the load of the soil approaches a constant value. This constant value is called residual shear strength. Due to these large displacements, this constant strength to the load acting upon the loss of cohesion between the grains is obtained only by the friction of the grains on the shear surface. The main factors such as shear rate, mineralogical structure, effective normal stress and clay content affect the residual shear strength. The difference between peak shear strength and residual shear strength increases with respect to the charactheristics of the grains.
In this study, it was aimed to determine the loss of strength between peak shear strength and residual shear strength value by performing reversal shear box tests on fine grained samples at ITU Soil Mechanics Laboratory. It is one of the most common test systems used in geotechnical engineering for the determination of residual shear strength. The sample is placed in a two-piece rigid box, and consolidated under a certain axial stress, before it is forced to be shear by pushing it from the bottom part while the upper part is fixed. In this system, it is aimed to determine the residual shear strength on the horizontal plane by moving it back and forth. This test system, which allows the determination of the failure envelope of the soil by repeating the test under different normal stresses, cannot provide the orientation of all flat clay particles parallel to the shear direction since it can be shear in only one direction at a limited distance.
Reversal shear box test is applied to determine the residual shear strength parameters with the same parameters with the same device used in shear box test. After determining the peak shear strength of the shear movement, sheared sample is retracted to the initial position without lifting vertical load which is defined as a cycle. These cycles are repeated until a constant shear stress value is observed. 
In this study, 9 different fine-grained soil samples with different consistency properties were used. Liquid limit values which were acquired from the samples that were used in experiments have ranged from 24% to 113%, plastic limit values which were acquired from the samples that were used in experiments have ranged from 17% to 42% and plasticity index values which were acquired from the samples that were used in experiments have ranged from 7% to 79%. The fine contents of the samples are between 75% and 100% and the clay contents are between 7% and 69%.  In order to investigate the effect of overconsolidation ratio on peak shear strength, residual shear strength and the strength loss, these samples having different consistency limits were prepared at different overconsolidation ratios and sheared under different constant vertical stress.
For the preparation of the samples, the grinded soil sample which is passed through the No. 40 sieve, was mixed with water at least 1.5 times the liquid limit value. In this way, it is aimed that the water-soil sample mixture can be easily processed and placed in the cell without air voids. Provide to maintain saturation during the consolidation and shearing processes, water is added to the cell where the sample is placed. 
During the consolidation process, the sample is consolidated under 50 kPa, 100 kPa, 150 kPa, 250 kPa, 350 kPa and 500 kPa stresses in order to have different overconsolidation ratios. The desired stress was reached by raising in steps, not at once. At each stage, desired vertical stress is applied when the completion of the primary consolidation was observed. Each stress stage was kept under load at that stage for a period ranging from several hours to one day. The total consolidation times of the samples vary from 3 to 10 days at high stresses and 1 to 3 days at low stress levels, depending on the number of stages until the final vertical stress levels are reached. After loading process, stress level was decreased to 50 kPa in order to make all samples prepared at desired different overconsolidation ratios.
It is known that overconsolidation ratio has a large role on peak strength. Although residual shear strength is not affected completely by this ratio. It gains great importance in determining the loss of strength between peak strength and residual strength. In order to examine the relationship between residual shear strength and overconsolidation ratio, experiments were performed on the shear box device within the scope of this study. After the loading and unloading phase in order to obtain different overconsolidation ratios, shearing process was started. 
In reversal shear box test, the sample should not create excess pore water pressure during the test to satisfy the drained conditions. For this purpose, with the information obtained from the consolidation process of the sample, a shear rate is determined so as not to create excess pore water pressure in the sample during the shearing process of the sample. In this study, due to the fine grains of the soil samples used, the shear rate was selected as 0.01.mm/min in order to avoid excess pore pressure. In this selection, shear velocity values which was given by Skempton (1985), for the re-acted slope under static conditions in the field, have been taken into account. The displacement amount of the sample to be made in one direction during the shearing process was determined as 9 mm due to the limitations of the test system. To consider  the specified shear rate, one cycle takes place within 30 hours. Within the scope of this study, the samples which had five cycles were sheared in approximately six days, while the samples taken for three cycles were approximately four days. 
Experiments were performed by repeating the same process in all samples with different plasticity properties with different overconsolidation ratios. As a result of the experiments, the peak shear strength increases due to the magnitude of the overconsolidation ratio, but the effect of this ratio on the amount of the residual shear strength is very limited.
In the calculations made within the scope of this study, area correction was not applied due to the discontinuity of the slip surface area which is a disadvantage of the shear box test, while the stress values were found as given in Head (2006). Since the failure was observed at low diplacement levels for samples having higher over consolidation levels, area correction has not a significant effect on the stress values. 
In this study, the relationship between peak shear strength and residual shear strength with respect to overconsolidation ratio was investigated. In the literature, it is stated that the residual shear strength value reached at the end of the large displacement values does not depend on overconsolidation ratio. Conclusions that supporting the argument in the literature have been obtained and these results have been supported with relations that can be used for practical purposes.