Taş kolon uygulamasının saha testleri yardımı ile değerlendirilmesi ve iyileştirme optimizasyonu
Evaluation of stone column application with in situ tests and optimization of soil improvement
Yazar: ÖNDER ALTINTAŞ
Danışman: DR. ÖĞR. ÜYESİ BERRAK TEYMÜR
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

Özet:
Giderek artan yapılaşma ve şehirleşme nedeni ile yumuşak-gevşek zeminlerin bulunduğu bölgelerde proje öngörüleri oluşturulmakta, mevcut yapıların standartlarının yükseltilmesi, gerek taşıma gücü ve oturma, gerekse depremli durum davranışı iyileştirmesi amacıyla iyileştirme yöntemleri geliştirilmektedir. Bu yöntemlerden bir tanesi olan taş kolon uygulaması yaklaşık olarak son seksen yıldır bu tip sorunlu zeminlerde bir iyileştirme yöntemi olarak kullanılmaktadır. Yöntem ana amaç olarak zemine sokulan eksantrik hareket yapan bir sondanın gevşek-yumuşak zeminin yerinde sıkıştırılmasını sağlamak ve oluşan boşluğa mukavemet parametreleri yüksek çakıl-kırmataş benzeri granüler malzemenin doldurulması yoluyla yerinde iyileştirilmesini sağlamaktır. Yöntem, hem statik yükler altındaki taşıma kapasitesini artırmak ve zemin özelliklerini iyileştirmek, hem de deprem yükleri altında mukavemet parametrelerinin artırılması ve boşluk suyu basıncının azaltılması yoluyla sıvılaşmaya karşı koyabilecek düzeye çıkarılması amacı ile kullanılmaktadır. 
Türkiye' de ve dünyada tespit edilen önceki çalışmalarda genel olarak laboratuvar ve bazı saha deneyleri baz alınarak taşıma gücü ve oturma davranışındaki iyileştirmelerin genel amaç olarak benimsendiği görülmüştür. Bu çalışma kapsamında, önceki literatür ve saha çalışmaları baz alınarak, proje özelinde taşıma gücü problemi ve sıvılaşma potansiyeli tespit edilmiştir. Projede mevcut literatürde yer alan teorik çalışmaların sahada bir pilot bölgede imalat öncesi ve sonrasında yerinde arazi testleri uygulanmak suretiyle taş kolonların performansı ve teorik yaklaşıma uyumu esas alınarak çalışma yapılmıştır. 
Tez çalışmasına konu olan proje kapsamında taş kolon yapım yöntemlerinden biri olan dipten beslemeli kuru yöntem kullanılmış olup, uygulama öncesi ve sonrasında yapılmış olan koni penetrasyon testi sonuçları konu özelinde değerlendirilmiştir. İyileştirme öncesi ve sonrası zemin mukavemet parametreleri CPT ile yerinde tespit edilip yapılmış olan uygulamanın zeminin mukavemet parametrelerinde önemsenecek düzeyde iyileşmenin olduğu tespit edilmiştir. Diğer çoğu çalışmadan farklı olarak proje optimizasyonu adına sahada üç farklı grid aralığı kullanılarak oluşturulmuş deneme alanlarında testler tekrarlanmış, mevcut problemler ışığında ön testler ile karşılaştırılması yolu ile seçilen grid aralıklarındaki iyileştirme düzeyleri belirlenmiştir. Proje lokasyonu özelinde depremli durum davranışı (sıvılaşma) baskın problem olarak tanımlandığından bu kriter göz önünde bulundurularak optimizasyon kararı verilmiştir. Elde edilen sonuçlar özelinde teorik yaklaşımların yapılan çalışma kapsamında belirli bir miktar konservatif kaldığı değerlendirilmiş olup, özellikle bu tip testlerin yaygınlaşması ve yeni veri sağlanması halinde tez içerisinde öngörülen yaklaşımın daha da ilerletilebileceği düşünülmektedir.

Abstract:
It is necessary to be able to fulfill the requirements of increasing urbanization by project predictions on the unused lands. These suitable lands are generally on soft-loose soils, as in the past mostly lands with good geotechnical conditions are chosen for development and construction in order to avoid problems. Using the advantage of developing technology and new techniques it has become possible in some areas with geotechnical problems to be improved by various soil improvement techniques. With these methods, bearing capacity of soft - loose soils can be increased, settlements will be decreased and the behavior of these soils under earthquakes can improve. One of these methods is stone column application, and it has been used in the last eighty years as a method of improvement in problematic soils. Stone column application is a kind of displacement - replacement method. In this method there is a vibratory probe which penetrates the ground with the help of vibration and its weight. Then the same probe is used for filling the hole formed by itself with stone which has higher strength values and press the stone downwards to penetrate towards sides of the hole. By this way, the soil becomes denser in its place without taking any soil material out, the stone columns work as vertical drainage elements as the spaces between stone particles are ways to drain pore water and an escapeway for excess pore water under dynamic loadings. This method is used for both improving the soil conditions like bearing capacity and other strength parameters under static loads and with improving strength parameters as a mitigation against liquefaction under dynamic conditions.
It is commonly seen on the past studies done that the improvement on the bearing capacity and settlement was adopted as the general objective of the soil improvement based on data from some in situ tests and experiments at laboratory. Within the scope of this study, the change in liquefaction potential is determined in the light of previous literature and field studies. Moreover, the theoretical studies in the present literature were conducted based on the adaptation of the stone columns to the performance and theoretical approach by performing field tests on site before and after soil improvement application in three pilot areas and with the assessment of these tests an optimization is advised according to the results.
In this thesis, there is general information about the definition and the properties of stone columns, types of stone column application techniques according to feeding point and the presence of a fluid in the application. After general information about stone columns is given, a case study of a stone column application as a ground improvement method for reducing the liquefaction potential is introduced. The project studied as the case study is located on coastline of Çanakkale, as there is mainly sandy and non-plastic silty soil which is loose and uncemented. That is why, the site is expected to have high liquefaction potential from the results of the initial in situ tests and boreholes done on site. During the site investigations ten boreholes, six CPT's, pressuremeter tests at seven different locations and some laboratory work are performed to define the geotechnical properties of the site. Soil investigation data is processed and is decided that the major problem is the liquefaction potential of the site. The soil below foundation is mostly a combination of sand, silty sand, sandy silt with some clayey interlayers which are not site spread. Thus, the soil is mostly non plastic and very susceptible for liquefaction and it is decided to improve the foundation soil to reduce liquefaction potential of the site. The replacement stone columns is chosen as the improvement method because of its advantages about liquefaction on sites where groundwater level is high and its densifying effect on the surrounding soil. The project does not have serious settlement and bearing capacity problems because the loads from building to be constructed is not very high, however the improvement about liquefaction would also improve the soil by means of bearing capacity and settlements.
In the case study presented here, stone column construction method used was bottom-up method. In this method, a vibratory probe is let to penetrate by its own weight and with the help of vibration, after penetration is completed (in this method no material is taken out, probe only penetrates in the soil by pushing the material sideways and densifying the surrounding soil) stone column material is put to the bunker on the very top of the probe with a free outlet into the probe. Then the probe moves up and let the material fill the open space then moves down to push the material down to sides and densify the stones. This process is done from bottom of the column to the surface at every 1-1.5m. These columns with a diameter of 0.80m are located 1.6m from center to center as a square grid form in the design according to the theoretical approaches in the literature, but it is decided to check different column spacings for project optimization. Three different trial improvement zones were prepared for this analysis and optimization procedure. One improvement zone was the ongoing project area with column spacing of 1.6m and area ratio of 0.20, second trial improvement zone had a column spacing of 1.8m and area ratio of 0.16, the last trial improvement zone had a column spacing of 2.0m and area ratio of 0.13. They were all equally spaced on the vertex of a square and the spacings were measured from center to center. 
Before doing any soil improvement, cone penetration tests (preCPT's) were performed to be compared with the cone penetration tests (postCPT's) to be done after the production of stone columns in the trial improvement zones. As a result of the comparisons while improvement was observed at the middle point of the square grid which is the farthermost point from the stone columns at the trial areas with 1.6m column spacing and 1.8m column spacing, no improvement was observed in the centerpoint of the square grid with 2.0m column spacing. 
After the comparisons of the trial zones with the CPT data collected before and after the soil improvement it was also tested via the plate loading tests at the trial zones having 1.6m and 1.8m column spacings. The loading test is performed on an area of 1.40x1.40m centered on a stone column of 0.8m diameter. As in the project there was a 50cm layer of gravel under the plate and above the stone column top level. Loadings were performed until they reached 300 kPa pressure value then unloaded, maximum settlements were 7.5 and 5 mm and elastic settlements turned back to 2.5 and 5 mm as a result of unloading. The tested area could bear the pressure which was six times of the design value. The difference between two settlement values was explained due to the spreading of gravel layer sideways.
Before and after the soil improvement, the soil strength parameters were assessed with the CPTs done before and after improvement. Unlike many other studies, project optimization was done and three different grid intervals were used in the field, and the tests were repeated. Due to the location of the study, liquefaction due to earthquakes is defined as the dominant problem for the project. Optimization decision has been taken by considering these criteria and other conditions. The results obtained for this study showed that theoretical approaches that were considered gave slightly conservative values compared with the obtained values in the field.