Kil oranının siltli zeminlerde konsolidasyon özellikleri üzerindeki etkileri = The effects of clay content on consolidation properties in silty soils

Abstract

Zemin mekaniğinde oldukça önemli konular olan, üst yapı yükü, dolgu, kazı, yer altı su seviyesindeki değişim ve titreşim gibi çok sayıda etki ile oluşabilecek zemindeki sıkışma, şişme ve oturma miktarları ile geçirimlilik katsayısı, konsolidasyon (ödometre) deney verileri ile hesaplanabilmektedir. Siltli zeminlerde konsolidasyon konusunda yapılan çalışmaların az sayıda olması, bu tezde siltli zeminlerin konsolidasyon ve geçirimlilik karakteristiklerinin ödometre deneyleri ve diğer ek deneyler yardımıyla incelenmesinin motivasyonu olmuştur. Bu hedef kapsamında, Adapazarı ilçesinden temin edilen doğal silt zemin öncelikle içerdiği kil ve kum boyutlarındaki danelerden ayrıştırılmış, sonrasında yine Adapazarı kiline benzer özellik gösteren bentonit kili katkısıyla 9 adet silt-kil karışımı deney numunesi hazırlanmıştır. Hazırlanan bu numunelerin Casagrande ve Düşen Koni yöntemleri ile likit limit değerleri, yuvarlama (roll on) yöntemi ile plastik limit değerleri, hidrometre ve pipet yöntemleri ile kil içerikleri, piknometre deneyi ile özgül ağırlıkları bulunmuş ve dane dağılım eğrileri çizilmiştir. Fiziksel özellikleri belirlenmiş bu 9 adet numune laboratuvar ortamında su muhtevaları likit limit değerlerinin 1.3 katı oranında olan bulamaçlar halinde hazırlanmış ve 100 kPa gerilme uygulanması ile konsolide edilerek yeniden oluşturulmuşlardır. Bu ön konsolide edilerek yeniden oluşturulan numuneler, klasik ödometre aletinde konsolidasyon deneyine, düşen seviyeli permeabilite düzeneğinde geçirimlilik deneyine ve basınç plakası deneyine tabi tutulmuşlardır. Deney sonuçlarından konsolidasyon eğrileri çizilmiş, sıkışma katsayısı, konsolidasyon katsayısı, sıkışma-yeniden sıkışma indisleri, geçirimlilik katsayısı ve ön konsolidasyon basıncı gibi değerler elde edilmiştir. Bu elde edilen değerler düşen seviyeli geçirimlilik deneyi yapılmış aynı numunelerden gelen geçirimlilik değerleriyle ve SWC eğrilerinden elde edilen boşluk boyutu dağılımları ile de karşılaştırılmıştır. Bu sonuçlar doğrultusunda öncelikle Adapazarı bölgesinden temin edilen doğal silt zemin ve ona yakın kil yüzdesine sahip bentonit katkılı karışım numunesi farklı açılardan karşılaştırılmıştır. Adapazarı kiline benzer özellik gösteren bentonit katkılı bu karışım ile Adapazarı doğal silt zeminin, boşluk boyutu geometrisi açısından ve birçok deney sonucunda benzer özellik gösterdiği anlaşılmış ve böylece yapılan karışımların doğala uygun özellikte davranış gösterdiği doğrulanmıştır. Sonrasında Adapazarı silt zemininin kil içeriğinden olabildiğince arındırılmasıyla elde edilen ve sıfır numunesi olarak adlandırılan numuneye, %2, %4, %8, %10, %15, %20, %30 ve %40 oranlarında bentonit kili ilave edilerek yeni karışımlar hazırlanmıştır. Bu karışımlar üzerinde yapılan deneyler sonrasında, sıkışma davranışları incelenerek hangi kil oranının sınır değer olabileceğinin bulunması hedeflenmiştir.

Compression, swelling, settlement and permeability values of the soil, which can occur due to many effects such as superstructure load, filling, excavation, change in ground water level and vibration, which are very important issues in soil mechanics, can be assessed through the parameters acquired in consolidation (oedometer) tests. The settlements that cause architectural and/or static damage to the building and sometimes irreversible problems due to the loss of function of the building should be determined before entering the construction phase, it is essential to undertake all required precautions in this regard. Compression and therefore settlement occur at different rates and amounts depending on the properties of the soil such as cohesive, cohesionless, saturated, dry, etc. Sudden and time-dependent settlement is observed depending on the stresses applied to the soil. Due to the high permeability of coarse-grained soils, immediate settlement by absorbing the pore water pressure faster than fine-grained soils, and slower damping in fine-grained soils, a more time-dependent settlement effect will occur compared to immediate settlement. Silty and clayey soils are grouped as fine-grained soils, but it is known as a result of studies that silty soils show similar behavior to sand soils at limit values. The limited number of studies on silty soils was important in examining the consolidation and permeability characteristics of silty soils with the help of oedometer tests and other additional tests. Within the scope of this target, after separating the clay and sand sized grains contained in the natural silty soil sample obtained from Adapazarı district, the test sample was prepared by making 9 different bentonite added silt-clay mixtures with the additive of bentonite clay, which has similar properties to Adapazarı clay. Liquid limit values of these prepared samples were determined by Casagrande and Falling cone tests, plastic limit values by rolling on method, clay contents by hydrometer and pipette test, specific gravity by pycnometer test and granulometer curves were drawn. These 9 samples, whose physical properties were determined, were reconstituted by loading them gradually in the form of slurries prepared by adding water at a rate of 1.3 times the liquid limit values in the laboratory environment, and consolidated under 100 kPa tension. These pre-consolidated reconstructed samples were subjected to consolidation test in conventional oedometer device, permeability test in falling level permeability apparatus suitable for fine-grained soils, and pressure plate test in pre-consolidated state. From the consolidation test results, consolidation curves, coefficient of compression, coefficient of volumetric compression, coefficient of consolidation, compression index, re-compression index, permeability coefficient and pre-consolidation pressure of these specimens with known pre-consolidation values were obtained by existing methods.The permeability coefficient value of the soil was found from the falling level permeability test, which is one of the preferred permeability tests for fine-grained soils. Matric suction-water content graphs were drawn with the pressure plate test, and then average pore size-cumulative pore volume curves were drawn by moving to the pore size distribution. These obtained values were also compared with the permeability values from the same samples where the falling level permeability test was performed and with the pore size distributions obtained from the SWC (Soil Water Characteristic) curves. In line with these results, the percentage of clay in the natural silty soil obtained from the Adapazarı region and the mixture sample with physical properties close to this content were compared. This mixture with bentonite additive, which has similar properties to Adapazarı soil, has been confirmed to behave in accordance with the Adapazarı soil (natural condition) as a result of its pore size distribution and many physical experiments. Following this verification, the objective was to obtain specimens representing various percentages of bentonite clay added to the zero sample (designated as 0%), which was as purified as possible using the method of floating amounts of clay content from Adapazari natural silty soil. Specimens were prepared with the addition of bentonite clay at proportions of 2%, 4%, 8%, 10%, 15%, 20%, 30%, and 40%, aiming for comprehensive investigation. All mentioned experimental procedures were conducted to determine the relationships and boundary behavior of the mixed percentages. As a result of the studies, preliminary consolidation methods whose applicability is problematic in silty soils, the consistency and relationship of the permeability coefficient found by the logarithm-time method and square root-time method from the consolidation test with the permeability coefficient found as a result of the falling level permeability test, the fact that the compaction coefficient differs in silty soils compared to other soil types. The aim is to find out the properties, whether there is a difference in the void geometries of the soil samples that may affect the consolidation, and the relationships between the physical parameters and the parameters obtained from these test results. It is known that Adapazarı silt behaves like clay within certain boundaries and sand-like at certain boundaries. The study conducted supports this approach. Looking at the consolidation results, the consolidation curves of 0% (zero sample), 2%, 4%, 8%, 10% and 15% bentonite added samples formed a different curve compared to the 20%, 30% and 40% samples. In line with these results, if we accept the 15% sample permeability values as the limit value, bentonite additives above this rate show more clay-like sample properties. Again, the consolidation parameters, in terms of deformation and compression coefficient values, started to exhibit more clay-like behavior after 15% contribution rate, and this limit value is also seen in the gap size geometry. In terms of permeability determination, when the consolidation permeability coefficients obtained from the consolidation test parameters were compared with the permeability values found using the parameters obtained from the Casagrande and Taylor methods, the permeability coefficient found with the Taylor square root method parameters gave more consistent results. These values are also consistent with the values obtained by the falling level permeability test. For this reason, the falling level permeability test is recommended for the permeability coefficient in silty and silty-clayey soils within the scope of this study. The reasons for this are that the test is less prone to error, is easier and time-consuming. It is a more economical experiment. Ingeotechnical science, where pre-consolidation value and over-consolidation rate are very important, most pre-consolidation determination methods in the literature are on clays. This does not give sufficient confidence in the applicability of these methods in silty soils or soils containing silt. As a result of the study, general methods gave normal results on samples containing 10% or more bentonite additives, but the inconsistency was seen quite high in samples below this limit. Among these methods, the Tavenas method gives the best and consistent results for the samples.