Doygun olmayan siltli zeminlere oturan yüzeysel temellerin taşıma gücü
Bearing capacity of shallow foundations on unsaturated silty soils
Yazar: MEHMET UĞUR YILMAZOĞLU
Danışman: PROF. DR. AŞKIN ÖZOCAK
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Geoteknik Bilim Dalı
Derece: Doktora, 2023
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Deney tasarımı = Experimental design ; Kayma dayanımı = Shear strength ; Silt = Silt ; Sığ temeller = Shallow foundations ; Zemin mekaniği = Soil mechanics

Özet:
Genel olarak, doygun olmayan zeminler üzerinde yer alan yüzeysel temellerin nihai taşıma gücü, doygun zemin mekniği teorilerinin uygulandığı geleneksel kayma direnci parametreleri ile karakterize edilmektedir. Ancak bu durumda doygun durumda elde edilen zemin parameterleri kullanılarak tasarlanan temellerin ekonomik olmaktan uzak olacağı açıktır. Son yıllarda, drenajlı ve drenajsız yükleme koşullarını dikkate alarak doygun olmayan zeminler üzerinde yer alan temellerin taşıma kapasitesini tahmin etmek için prosedürler geliştirilmiştir. Ancak bu çalışmalar genellikle kumlu zeminlerde yoğunlaşmaktadır. Literatürde önerilen sonuçların diğer zemin cinsleri için de geçerliliğinin test edilmesi gerekliliği açıktır.
Bu çalışma, doygun olmayan siltli zemin üzerine yerleştirilen sığ temellerin taşıma gücü değerinin incelenmesine odaklanmıştır. Bu kapsamda siltli zeminin, boşluk oranı, doygunluk derecesi ve kılcal gerilme büyüklüğünün etkisi incelenmiştir. Zemin-su karakteristik eğrileri, basınç plakası ve filtre kağıdı yöntemleri kullanılarak oluşturulmuştur. Ayrıca, baskın boşluk boyutunun doygun olmayan zeminlerin davranışı üzerindeki etkisi, kılcal gerilme ve boşluk boyutu arasındaki ilişkinin analizi yoluyla incelenmiştir.
Doygun olmayan siltli zeminlerin kayma direnci literatürde önerilen geleneksel kesme kutusu deneyi ve su muhtevası-kılcal gerilme eğrilerinden (zemin-su karakteristik eğrisi) tahmin edilen kılcal gerilme değerleri yardımıyla belirlenmiştir. Basit bir deneysel prosedürüne sahip ve hemen hemen her zemin laboratuvarında gerçekleştirilebilecek olan bu yöntemin güvenilir sonuçlar verdiği gösterilmiştir Deneyler sonucunda doygun olmayan koşullardaki silt zemin davranışının diğer zeminlere göre farklı olduğu ve doygun kabuluyle yapılan hesaplamaların gerçek zemin davranışına kısayla kabul edilemez düzeyde hatalı olduğu ortaya çıkarılmıştır.
Doygun olmayan silt zeminlere olturan yüzeysel temellerin taşıma kapasitesi ve davranışının belirlenmesine yönelik yapılan model deneylerinde doygun ve doygun olmayan koşullar oluşturulmuştur. Drenajsız yükleme koşulları altında 1.15 mm/dak' lık bir yükleme oranı kullanılarak model temel testleri gerçekleştirilmiştir. Model temel boyut ve şekillerinin etkisini araştırmak için 50 mm daire, 50 mm x 50 mm kare, 42.3 mm daire ve 37.5 mm x 37.5 mm kare model temellerin taşıma gücü değerleri ölçülmüştür. Model deneylerden elde edilen sonuçları, Skempton (1948) tarafından doygun zeminler için önerilen taşıma kapasitesi teorisi uyarlanarak yorumlanmıştır. Ayrıca çalışmada, doygun olmayan zemin numuneleri üzerinde ölçülen serbest basma testleri ile doygunluk derecesine bağlı uydurma parametresi (𝑆𝛹) kullanılarak doygun olmayan silt zeminlere oturan yüzeysel temellerin taşıma güçü değerinin tahmini için yeni bir eşitlik önerilmiştir.

Abstract:
The bearing capacity of shallow foundations on unsaturated soils is typically determined by conventional shear resistance parameters, which are based on saturated soil mechanics theories. However, designing such foundations using soil parameters obtained in the saturated state is often uneconomical. Recently, procedures have been developed to estimate the bearing capacity of foundations on unsaturated soils, taking into account both drained and undrained loading conditions. However, these studies have mainly focused on sandy soils, and the validity of the results suggested in the literature should be tested for other soil types as well.
This research is focused on examining the bearing capacity of shallow foundations that are situated on unsaturated silty soil. The study investigates the impact of void ratio, saturation degree, and matric suction value of silty soil. The creation of soil-water characteristic curves was achieved by utilizing both the pressure plate and filter paper methods. Moreover, the study examined the correlation between matric suction and void size to analyze the effect of dominant void size on the behavior of unsaturated soils.
Soil samples were collected from Adapazarı and were used in experimental studies to investigate the capacity of shallow foundations built on unsaturated soils. The material used in the experiments was silty soil with low clay content. The grain distribution curve of the soil sample was obtained through sieve analysis and hydrometer method according to TS 1900-1/2006 (TS: Turkish Standard). The soil sample was classified as MI (medium plasticity silt) according to TS 1500/2000. The natural void ratio values of the natural silts in Adapazarı were examined, and it was decided that the void ratio values of the samples to be used in the experiments would be 0.65, 0.70, and 0.75. In selecting the saturation degrees, the seasonal groundwater level changes in Adapazarı were taken into account, and saturation degrees of 65%, 70%, 75%, and 85% were chosen.
The soil sample was made homogeneous and air-dried using various equipment for the upcoming tests. The samples were divided into three groups for Soil-Water Characteristic Curves (SWCC) tests, shear box e tests, and model tests.
The sieved air-dry soil material was wetted by spraying deionized and de-aerated water to the desired saturation level in 2 kg groups and mixed with a sample preparation mixer. The mixtures were placed in airtight containers and left at room temperature for 24 hours to cure. After the 24-hour period, three different samples were taken from each container to check the water content. The drying oven and microwave methods controlled the water contents of the mixtures within a ±0.5% error.
For the pressure plate and filter paper method, the soil was statically compressed into a metal container with a certain volume at predetermined void ratios (e1=0.65, e2=0.70, and e3=0.75) in three stages.  Samples were taken from the compacted soil with metal molds with a diameter of 50 mm and a height of 20 mm. The samples were kept in deionized and deaerated water for seven days to reach saturation level. To prevent the samples from swelling, filter paper and porous stones were placed on them during the saturation stage.
The soils utilized for the shear box test were prepared in pairs of 2 kg the day before the test, utilizing the same procedure as the soil samples prepared for the SWCC test. The cured samples, prepared at the water content determined based on the research scenario, were statically compressed in a single step into the 60 mm x 60 mm x 25 mm shear box test cell.
To maintain the water content of the sample during the experiment, stainless steel plates were used instead of porous stones and perforated metals in the cutting cell. The plates were of the same thickness and placed in the upper and lower parts of the cell. Additionally, a thick layer of silicone grease was applied outside the test cell to prevent contact of the sample with air. The tests were conducted in a laboratory environment at a fixed room temperature of 24°C.
Soil samples required for the model test were prepared using the same procedure used for the SWCC and shear box test samples. Each scenario required a sample with 3 different void ratios, 5 different saturation degrees, and 4 different model foundations. The prepared samples were left to cure for 24 hours at room temperature in airtight containers to obtain a homogeneous soil-water mixture.
After the 24-hour curing time was completed, three different samples were taken from different regions to control the water content. It was verified that the water content of the mixtures in all the containers remained within ±0.5% evaporation/moisture change margin of error. Samples with controlled water content were placed in the model tank after rapid mixing to avoid the possibility of leaching due to the effect of gravity.
With weight/volume calculations, the samples with the desired void ratio and saturation degree were statically compressed into the model tank as two layers. Compaction was carried out by means of the compaction frame loaded with a 30 ton capacity hydraulic hand pump. Surface combing was done to prevent stratification that may occur between layers.
The study aimed to determine the shear resistance of unsaturated Adapazarı silt by utilizing a conventional direct shear box test device for practicality reasons. Matric suction values of unsaturated samples were measured through pressure plate and filter paper methods. The fully automated direct shearing device from Wykeham Farrance with a square shear box measuring 6x6 cm in diameter and 2.5 cm in height was used for the shear box experiments. Shear box tests were conducted following the ASTM standard D3080-98 at normal stress levels of 59, 98, and 147 kPa. The normal loads were determined by considering the cover load of the silt soil sample in natural conditions. To verify that there were no changes in the moisture content of the samples during the cutting box experiment, the wet and dry weights of the cut samples were determined after the test, and their water content was checked. The values of b of silt soil were calculated using shear box test results and matric suction values.
Model foundation loading tests were conducted as deformation-controlled tests. If soil samples are loaded at a velocity at a speed between 1.00 mm/min and 1.20 mm/min, undrained conditions will be achieved based on previous research on compacted samples. The capillary stretching effect can be measured under these conditions. However, the present investigation was simulated under undrained loading conditions with very fast loading speed of 1.15 mm/min. To confirm that the water content value of the samples did not change after the model loading tests, samples were taken from 1-3 cm and 5-7 cm below the sample surface using sampling rings of 25 mm height and 48 mm diameter. Filter paper was placed on the top and bottom of the samples without allowing the water content to change. After being wrapped with stretch film and aluminum foil, the samples were kept in a heat-insulated box for 9 days to equilibrate. The matric suction values of the samples were determined by using filter paper calibration curves and being controlled. Additionally, soil samples were taken from the model cell at 3 different depths (0-20 mm, 50-70 mm and 100-125 mm depth) for their water content was also checked and samples were taken for free compression tests (UC).
In this research, the shear strength of unsaturated silt soil was assessed using the conventional shear box test as recommended in the literature, along with the matric suction values determined from the soil-water characteristic curve. The results indicate that this method is a practical alternative to the costly and complicated DOZ soil testing devices, as it can be carried out in almost any soil laboratory and provides accurate outcomes.
The shear resistance and strength parameters of silt samples may vary depending on the matric suction and structure of the soil (void ratio), while the shear behavior has shown very little variation.
For all silt samples with three different void ratios, a cohesive value increase in accordance with matric suction rise  value was observed, consistent with the literature. However, in terms of apparent shear resistance (), an increase was observed with an increase in matric suction rise value, contrary to the literature.
In the literature, a discrepancy has been observed between the values obtained through bearing capacity equations for shallow foundations in unsaturated soils and the outcomes of conducted model tests. Within the scope of this study, a novel equation is proposed based on the total stress approach, aiming to provide accurate results for shallow foundations in unsaturated silty soils. This  equation that provides appropriate results for shallow foundations on unsaturated silty soils is proposed according to the total stress approach.