Doygun olmayan zeminlerde şev stabilite analizi
Slope stability analysis for unsaturated soils
Yazar: ÖZGÜN GÖKOĞLU
Danışman: DOÇ. DR. AYKUT ŞENOL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Bu çalışmada, kumlu kil, düşük plastisiteli kil ve kum zeminlerin klasik zemin mekaniği ve doygun olmayan zemin mekaniği ilkelerine göre yeraltı su seviyesi değişimleriyle beraber şev stabilite analizleri sonucu elde edilen güvenlik sayıları incelenmiştir. Kumlu kil numunesi İstanbul Güneşli bölgesinden elde edilmiş olup, diğer numuneler daha önce yapılmış çalışmalardan elde edilmiştir. Laboratuvar çalışmaları Şubat 2014 – Temmuz 2014 tarihleri arasında İstanbul Teknik Üniversitesi, Ord. Prof Dr. Hamdi Peynircioğlu Zemin Mekaniği Laboratuvarı'nda gerçekleştirilmiştir. Deneyler iki aşamalı olarak yapılmıştır. İlk aşama zeminin mühendislik ve fiziksel özelliklerinin belirlendiği; elek analizi, hidrometre, Atterberg limitleri analizi, piknometre, standart kompaksiyon ve kesme kutusu deneyleridir. İkinci aşamada ise zeminin doygun olmayan zemin mekaniği parametrelerini belirleyebilmek için basınç plakası deneyi uygulanmıştır. Kesme kutusu ve basınç plakasında kullanılan numuneler standart proktor deneyi sonrasında belirlenmiş olan optimum koşullara yakın koşullarda hazırlanmıştır. Şev stabilite analizlerinde kullanılan diğer numuneler için de farklı araştırmacılar tarafından benzer deneyler yapılmış olup, numunelerin doygun ve doygun olmayan parametreleri belirlenmiştir. Deneysel çalışmalar sonucunda elde edilmiş olan parametrelerle doygun ve doygun olmayan koşullarda şev stabilite analizleri yapılmış ve doygun olmayan parametrelerin elde edilen güvenlik sayısına etkisi incelenmiştir. Analizler, sıkıştırılarak hazırlanmış numunelerin zemin su karakteristik eğrisinden elde edilen uniform matrik emme basıncına göre, yeraltı su seviyesinin değişken olduğu ve zeminin tamamen suya doygun olduğu durumlar için yapılmış ve yeraltı su seviyesinin değişimiyle güvenlik sayısının değişimi araştırılmıştır. 
Elde edilen sonuçlara göre; doygun olmayan zemin parametrelerinin şevlerin güvenlik sayısına etkisinin en çok gözlemlendiği numune kil oranının en yüksek olduğu numunedir. Numunelerde kum oranı arttıkça; doygun ve doygun olmayan zemin mekaniği ilkelerine göre yapılan analizlerin sonuçları arasındaki fark azalmaktadır. Yeraltı su seviyesinin yükselmesiyle her üç numunede de doygun ve doygun olmayan kabuller için elde edilen güvenlik sayıları birbirlerine yaklaşmakta; zeminin suya tamamen doygun olduğu durumda elde edilen güvenlik sayıları ise aynı olmaktadır. Şevler doygun duruma gelmeden de göçmeler meydana gelebildiğinden, doygun olmayan zemin mekaniğinin önemi artmaktadır.

Abstract:
Analysis of slope stability is one of the main issues of geotechnical engineering. Natural or human made slopes' problems are focus area, which geotechnical engineers have to bring a solution. There are main acceptances, which base on saturated soil mechanic principles to analyze slope stability.  However, this approach sometimes causes the uneconomical solutions. According to researches that were made in the last years, unsaturated soil mechanichs approach has been started to use in the slope stability analyses and this approach creates the economical and reliable results for slope stability analyses. 
Depending of the developments in the technology, the laboratory devices has been used widely and the quality of devices has increased. Thanks to this development, soil mechanich acceptances has been started to change. The unsaturated soil mechanichs principles are one result of this change. In the scope of this thesis, the effects of properties of unsaturated soil mechanic's parameters to factor of safety of slopes were searched. Effects of changing of ground water table level on the factor of safety of slopes and the different impacts of unsaturated soil mechanic principles on the various soil types were the main topics in the thesis.
Although the unsaturated soil mechanic approach has been appeared since early of 1900's, the development of acceptances has begun for last twenty years because of reasons, which was mentioned above paragraph. Especially for the dry conditions and the conditions where the ground water table level is far from the surface, the unsaturated soil mechanic parameters affect the stability of slopes, bearing capacity of foundations, consolidation and infiltration of soils. Unsaturated soil mechanic researches remark that the unsaturated soil parameters should not be ignored especially for clay soils.
In this thesis, factor of safety that obtained from the results of slope stability analysis of sandy clay, low plasticity clay and poor graded sand soil are examined with the change of ground water table level for saturated and unsaturated soil mechanics principles.  The sandy clay sample is from Günesli vicinity of Istanbul and the other samples are got from the other researches. The thesis has two parts. In the first part, laboratory tests are available. The laboratory tests were performed in the Ord. Prof. Dr. Hamdi Peynircioğlu Soil Mechanics Laboratory, Istanbul Technical University between the dates February of 2014 and July of 2014.  The tests were performed by two stages. In the first stage, sieve analyses, hydrometer, Atterberg limits, pycnometer, standard compaction and direct shear tests are available. The second stage consists of pressure plate test, which gives the unsaturated soil properties. In the light of sieve analysis and Atterbeg limits, the soil sample that was obtained from the field was described as low plasticity clay according to Unified Soil Classification System. The samples, which were used for direct shear test and pressure plate test, were preapeared as near the optimum conditions these are defined from standart proctor test. The other samples, which were used for slope stability analyses have experienced to same tests from other researchers with sample obtained from field. 
The Sample I is the sample which the experimentals applied in the labarotory by author of this thesis. The Sample II was taken from Gülsen's research and the classifition of the soil is poor graded sand. The Sample II consists of 100 percent sand. The sample III is named as Indian Head Till is from Kutlu's research. According to Unified Soil Classification System, the soil is low plasticity clay. For each soil samples, the parameters that were used in slope stability analyses were defined. These parameters were effective cohesion (c') and effective internal friction angle (ɸ') for saturated and unsaturated soil acceptances and soil water characteristic curve (SWCC) and matric suction for unsaturated soil acceptance. SWCC and matric suction was determined with pressure plate test for Sample I and Sample III and with tempe-pressure cell for Sample II. Both of the test devices use the axis translation technique. The axes translation technique was mentioned in the thesis with details. 
Soil water characteristic curve is one of the main parameter, which specifies the behavior of unsaturated soils. Soil water characteristic curve is relationship of matric suction and degree or saturation or water content or volumetric water content. Soil water characteristic curve can be obtained from experimental, estimation or fitting methods. In the light of researches for the unsaturated soil mechanics, properties of soils as the clay or sand content, plasticity or coefficient of compressibility has significiant effects on the soil water characteristic curve. Suction pressure of soils is the other main parameter for unsaturated soil mechanic approaches. It includes both osmotic suction and matric suction parameters. Osmotic suction is related with chemical contents, which takes part in pores of soil. Because of difficulties to gauge the osmotic suction, it is usually neglected. Therefore, the matric suction becomes the main parameter for the suction pressure. The matric suction is difference between pore air pressure and pore water pressure. İt can be described as negative pore water pressure. Pore water pressure inceares under the ground water table level with depth and matric suction increases above the ground water table level. Researchs about unsaturated soil mechanics proove the positive effect of matric suction to the shear strength of soils. Because of ıts positive effects to the shear strength of soils, the calculations become more economical ıf we compare with saturated solution. 
In the second part of the thesis, slope stability for saturated and unsaturated conditions was analyzed using parameters, which were obtained from experimental studies and the effect of unsaturated soil parameters on the factor of safety was defined. The change of factor of safety was examined with change of ground water table level. For each soil samples, same geometry of cross section was used for the models. Each stability of slopes was analyzed with the SLOPE/W, which takes part in GeoStudio packet programme. SLOPE/W programme benefits from Limit Eqiulibrium Methods. In the thesis, principles of Limit Equilibrium Methods and the usage of SLOPE/W are shared with details.  The results were obtained with Bishop Limit Equilibrium Method for each soil type. Mohr-Coulomb failure hpyothesis was accepted for the soil behavior.
Three ground water conditions were examined for the solutions. For three soil samples, soil parameters for saturated and unsaturated acceptances were determined.   
The first condition was the initial condition of the soils. There was no ground water level for initial conditions. For saturated soil acceptances, the parameters that were defined for the analysis were effective cohesion and effective internal friction, which were obtained from direct shear tests and unit weight of the soils. For unsaturated soil acceptances, the matric suction value was acted to effective cohesion. Defined cohesion for dry condition according to unsaturated acceptances was greater value than defined effective cohesion according to saturated acceptances. For unsaturated soil acceptances, soil water characteristic curve was necessary to account the cohesion value. Beside the soil water characteristic curve, to describe the cohesion of unsaturated soil acceptances for dry conditions, effective cohesion that was obtained from direct shear test and matric suction of the soil must be contributed to calculation of cohesion. The calculation of cohesion according to unsaturated soil principles for dry condition was told with details in the next chapters. 
In the second condition for the soils, the ground water table level was defined and systematically it was assessed nearer positions to surface of ground. For saturated soil acceptances, the parameters which has determined for the stability analysis were effective cohesion, effective internal friction angle and unit weight of the soils. To get the factor of safety according to unsaturated soil acceptances, effective cohesion, effective internal friction angle, unit weight of soil and soil water characteristic curve were necessary. Soil water characteristic curve that was determined from experimental ways was defined in the programme related with volumetric water content and matric suction values. 
For the third condition of the soil, soil pores was completely full of pore water. The ground water table level was fifty centimeter above of surface. The parameters were same with the conditions that ground water table level was defined. After the geometry of cross section, soil parameters and ground water conditions were defined, the factor of safety for the slope stability analyses was obtained.  SLOPE/W gave the crititical slip surfaces of circle and factor of safety for critical slip circle's surface was the factor of safety value for the slope stability analyses. 
The greatest factor of safety was obtained for the initial conditions according to unsaturated soil acceptance. The difference between saturated soil and unsaturated soil analyses was the most apparent value for Sample III. However, the difference was so little for the Sample II. That demonstrates, while the clay content was raising, unsaturated soil parameters had more significiant role in the shear strength theory. 
While the ground water table level was increasing, factor of safety difference between saturated and unsaturated slope stability analyses decreased. This remarks that, while the soil was becoming more saturated, the effect of unsaturated soil parameters declined. With the increasing of ground water table level, factor of safety for unsaturated soil acceptances has significiantly declined for especially Sample III. 
When the ground water table level was above of ground surface, there was no factor of safety difference between analyses with unsaturated soil principles and saturated soil principles. This is because there was no pore air pressure in the soil when the ground water table level above of ground surface. Because of this reason, unsaturated soil parameters had no effect on the shear strength of soils, when the soil pores were full of pore water.
For the thesis, both of experimental and analytical studies were done. The results were presented with details. In the scope of the research, three soil samples were used. For a detailed study, the number of soil samples may increase. To define the ground water level, the rain infiltration case can be considered.