İnce daneli zeminlerde kil içeriğinin kayma direncine etkisi = Effect of the clay content on shear strength in fine-grained soils

Abstract

Temel taşıma kapasitesi, istinat duvarları ve heyelan analizi gibi stabilite problemlerinin çözümünde zeminlerin kayma direnci parametrelerinin kullanılması gerekmektedir. Zemin ve yükleme hızıyla ilgili olarak, kohezyon ve içsel sürtünme açısı olan kayma direnci parametreleri, toplam (drenajsız durum) veya efektif (drenajlı durum) gerilmeler cinsinden olabilir. Drenajlı ve drenajsız koşullarda parametrelerin doğru olarak belirlenmesi geoteknik yapıların tasarımında büyük önem taşımaktadır. Özellikle, doygun ince daneli zeminlerin (kil ve silt) yükleme altındaki kısa dönem davranışına hâkim olan parametre drenajsız kayma direncidir ve bu parametrenin elde edildiği koşullar tasarımların alt sınırını oluşturur. Geçmişten bu yana geoteknik mühendisleri için düşük plastisiteli veya plastik olmayan siltlerin davranışını tahmin etmek kum ve killere kıyasla oldukça zor olmuştur. Siltlerin öngörülemez davranışı genleşme ve büzülme özelliklerinden dolayıdır. Siltler genellikle birçok fiziksel sürecin etkisiyle kuvars kristallerinin parçalanması sonucunda oluşur. Adapazarı siltleri, Sakarya Nehri tarafından Adapazarı Ovası'na taşınmış akarsu kökenli malzemelerdir. Bu tez çalışması, doğal Adapazarı siltinin laboratuvar ortamında kil oranı azaltılarak elde edilmiş yapay siltli zemine ilave edilen kil miktarının drenajsız kayma direnci üzerindeki etkisine ilişkin deneysel araştırmayı sunmaktadır. Farklı bentonit içeriğine sahip (ağrılıkça %0, 2, 4, 8, 10, 15, 20, 30, 40) siltli zemin üzerinde bir dizi konsolidasyonsuz-drenajsız (UU) üç eksenli testleri yapılmıştır. Ana hedef, ince daneli zeminlerin kil içeriği ile drenajsız kayma direnci parametrelerinin nasıl değiştiğini belirlemek, ayrıca siltin kum benzeri ve kil benzeri davranışı arasındaki eşik değeri değerlendirmektir. Siltli zemine bentonit ilavesi ile ML'den CH'a kadar farklı zemin sınıfları oluşturulmuştur. Üç eksenli deney düzeneğinde test edilmeden önce başlangıç doygunluk koşulunu sağlamak ve özdeş numuneler elde etmek için bulamaç yöntemi ile hazırlanan numunelerin 50, 100 ve 150 kPa basınç altında ön konsolidasyonu yapılmıştır. UU üç eksenli testlerde, bentonit oranının %10 ve daha fazla olması durumunda siltli zeminin davranışının kum benzeriden kil benzeri davranışa geçtiği görülmüştür. Bu eşik değerden önce baskın olan silt, numunelerin daha yüksek dayanıma sahip olmasına neden olurken, artan bentonit içeriği dayanımın azalmasına neden olmuştur. Fiziksel özellik değişimlerine dayanan çıkarımlar, ince daneli zeminlerin drenajsız kayma direncinin belirlenmesi için bir kılavuz görevi görebilir.

In the solution of stability problems such as foundation bearing capacity, retaining walls and landslide analysis, the shear strenght parameters of soils should be used. Depending on the soil and loading speed, the shear strength parameters which are internal friction angle and cohesion, can be in terms of total (undrained state) or effective (drained state) stresses. Accurate determination of parameters in drained and undrained conditions is of great importance in the design of geotechnical structures. Especially, the parameter that dominates the short-term behavior of saturated fine-grained soils (clay and silt) under loading is the undrained shear resistance, and the conditions under which this parameter is obtained form the lower limit of the designs. Determination of this parameter is usually carried out in the laboratory by using different types of tests such as triaxial compression, direct shear, and unconfined compression test. It can also be measured directly in situ by pressuremeter, vane shear and cone penetration tests. Empirical correlations using the physical properties are also available. Historically, it has been difficult for geotechnical engineers to predict the behavior of low-plastic or non-plastic silts compared to sands and clays. Geotechnical engineer meets difficulties in dealing with silty soils because it can exhibit two different behaviours. Differentiating sand-like and clay-like response especially in liquefaction problems becomes a crucial issue. The unpredictable behavior of silts is due to their dilatant and shrinking properties. Silts are generally formed as a result of the split of quartz crystals under the influence of many physical processes. Adapazarı silts are fluvial-origin materials removed to the Adapazarı Plain by the Sakarya River. This thesis presents the experimental research on the effect of the amount of clay added to the artificial silty soil obtained by reducing the clay content of the natural Adapazarı silt in the laboratory on the undrained shear strength. A series of unconsolidated-undrained (UU) triaxial tests were conducted on silty soil with different bentonite content (0, 2, 4, 8, 10, 15, 20, 30, 40 wt%). Within the scope of this study, it was aimed to add certain percentages of bentonite clay to the zero sample in order to obtain specimens with different physical properties. Hence, the zero-sample prepared for the mixtures was divided into 9 equal parts. Then, bentonite clay was added to the zero sample in previously mentioned proportions and mixed until homogeneity was achieved. The reason for adding bentonite clay is based in its mineralogic composition that is similar to the clay minerals in Adapazarı soils and its high activity may allow to obtain samples covering a wide range of physical properties. Moreover, Unconsolidated-Undrained (UU) triaxial test is widely used to determine the undrained shear strength of saturated clays. Theoretically, for a saturated fine-grained soil with under undrained conditions, if loading is so fast that the water cannot drain, the increases in stress are resisted by the pore water pressure, that is, there is no change in the effective stress in the soil. If the effective stress in the soil does not change, the shear strength value of the soil at failure remains constant. If this is the case, in terms of total stresses the change in load applied to the soil causes the shifting of the stress circles to the right, but diameters are unchanged, particularly in normally consolidated clayey soils. Under these conditions considering the total stresses, a horizontal failure envelope is obtained and the angle of shearing resistance () becomes zero ( = 0 concept). The value at which the envelope crosses the vertical axis, half the difference between the principal stresses, indicates the undrained shear strength (cu). However, this basic principle, which is valid for saturated normally consolidated clays, becomes inapplicable for silts. Several investigators reported that the angle of shearing resistance angle () was measured to be considerably greater than zero in silts. The main objective is determining how the clay content of fine-grained soils and undrained shear strenght parameters change, as well as to evaluate the threshold between sand-like and clay-like behavior of silt. With the addition of bentonite to the silty soil, different soil classes from ML to CH were created. Since saturation has a great impact on determining the undrained shear strength, it is also important to prepare the samples at equal degrees of saturation. Preparing identical samples is also essential for the reliability of test results. It is difficult to ignore the influence of the sample preparation method on obtaining identical samples. The properties controlled during preparation of reconstituted samples in laboratory are water content, unit weight or void ratio. Various sample preparation techniques have thus been introduced depending on the initial and desired soil parameters. Preferred sample preparation techniques for triaxial experiments can be summarized as: moist tamping, air pluviation, water pluviation and slurry consolidation. Therefore, samples were prepared by the slurry method to ensure the initial saturation condition and obtain identical samples in this thesis. Before testing in the triaxial system, the samples prepared by the slurry method were pre-consolidated under 50, 100 and 150 kPa pressures in order to simulate the in-situ conditions in the laboratory. After pre-consolidation stage, unconsolidated and undrained (UU) triaxial compression tests were performed on all the mixtures in order to find the undrained shear strength (cu) parameter of the samples. This procedure basically consists of two parts: applying a limiting effective stress (cell pressure) and shearing the sample under this pressure until failure occurs. In the first part, the sample prepared for UU test is placed in the triaxial cell. The sample was weighed before and after the test to find out the physical properties and water content of the soil. Based on the index tests, the fact that the fundamental properties are highly correlated addresses to what extent which bentonite additive can change both the clay percentage and the liquid limit. Adding UU evaluations made it possible to obtain the sand-like and clay-like threshold of silts in terms of bentonite percentage, clay amount and liquid limit, by examining failure envelopes, shear resistance angle and undrained cohesion, separately. Failure envelopes with an apparent slope, lost this feature after a certain percentage of bentonite and started to indicate only cohesion as it should be in UU tests. UU test results show that low clay content samples indicate angles of shearing resistance >0 contrary to the traditional approach. This means that silt with low clay contents will need to be evaluated as sand. It is shown that angle  which is mobilised during the decrease of the bentonite content from 40% to zero, starts from 21 and falls to zero, while c does not show such a trend. However, it can be stated that 12% clay content (zero sample + %10 bentonite) forms a certain limit here. Test results show that the undrained shearing resistance of silt-clay mixtures is significantly affected by the clay content and confining pressure. The sample may behave like sand or clay under undrained conditions depending on the clay content. Furthermore, test results show that the undrained shear strength of silt-clay mixtures is significantly affected by clay content and environmental pressure. When the shear resistance of the mixtures was compared at a certain confining pressure value, increasing clay ratio had a decreasing effect on the shear resistance. However, it has been observed that the shear resistance value gives higher results in samples with high pre-consolidation pressure. Inferences based on physical property changes can serve as a guide for determining the undrained shear strenght of fine-grained soils.