Efektif gerilmelerle şev stabilitesi analizi

Abstract

Doğal şevlerin stabilitelerinin incelenmesi, stabiliteyi artırıcı önlemlerin alınması, dolgu ve yarma şevlerinin güvenli ve ekonomik olarak projelendirilmesi konuları, sayısal bir anal i al e sonuçlanan detaylı bir arazi ve laboratuvar çalışması gerektirmektedir. Gerçekçi bir anal is, atabil i teyi etkileyebilecek tüm parametrelerin belirli saman ve sınır şartları içinde değerlendirilmesi ile mümkün olmaktadır. Efektif gerilmelerle şev stabilitesi analisi, stabilitenin toplam gerilmelerle çalışılması durumunda f arkedi lemeyen eğilimini gösterdiğinden, boşluk suyu basıncının bilindiği veya yeter doğrulukta tahmin edildiği tüm problemler için geçerlidir. Bu çalışmada, efektif gerilmelerle şev stabilitesi analizinde olayı etkileyen değişkenler incelenmiş, efektif mukavemet parametrelerinin ve boşluk suyu basıncının elde edilmesi anlatılmıştır. Daha sonra, basit ve dairesel kayma yüseyleri için kullanılan bası anal is metotları tanıtılmış ve boşluk suyu bası ncı -güveni i k sayısı ilişkisi incelenmiştir. Tesin son bölümünde uygulamadan bir örnek üserinde üç ayrı yöntem kullanılarak usun dönem stabilitesi incelenmiş ve elde edilen güvenlik sayıları karşılaştı rılmıştı r.

Analysis of slope stability is the principal geotechnical desing task for both natural and permenant or temporary excavated slopes. There are several important problems involving slope stability. Such problems most often arise in connection with the construction of highways, canals and basements. Slope stability is an extremely important consideration in the desing and construction of earth dams. There are also important problems involving the stability of natural slopes. The results of a slope failure can often be catastrophic, involving the loss of considerable property and many lives. The cost of flattening of a slope to achieve greater stability can be tremendous. Hence, although safety must be assured, undue conservatism must be avoided. There are a few terms to define slope movements according to the type of soil and movements. The term "landlide" is most used as general term to define down-slope movements of soil or rock masses as a result of shear failure at the boundaries of moving mass. Landslide classification is very important in terms of definition of problem. There are far too many different classifications of landslides. Rigorous classification of landslides seems impossible given the complexity of slope movements. Such a classification would probably be of limited utility anyway, because of its likely complexity Landslide classifications have been based on landslide morphology, the way in which movement occur, the rate of movement, the size of material, the age of failure and various combinations of those. The most important point is that a classification should reflect user needs. There are two main approaches in carrying out slope stability analysis. By far the most popular methods of slope stability analysis in use today are VII limiting eguilibrium methods; this popularity is due to the simplicity and ease of the methods and the profession has built up great experience in their use. It is considered the failure is on the point of accuring along an assumed or a known failure surface. The shear strength reguired to maintain a condition of limiting equilibrium is compared with the available shear strength of the soil, giving the average factor of safety along the failure surface. In addition to methods which utilise limiting equilibrium methods» many other analytic methods exist. These include methods which involve the theory of plasticity, finite difference solutions and finite element techniques. However they are limited by fact that they are generally more difficult to use, and require much more computation time and data preparation time on the part of the user. Slope stability analysis can be carried out in terms of total stress or effective stress. Total stress analysis covers the case of a fully saturated clay under undrained conditions, i.e. for the condition immediatelly after constructions, Whereas, effective stress analysis is valid for every condition. Because, the changes of pore water pressure can be taken into account in analysis. It is nesessary to determine effective shear strength parameters for slope stability analysis. Tri axial tests are the most common means of obtaining the peak shear strenght parameters. Direct shear tests are used to obtain both residual and peak strength values. Selection of strength for desing is important due to the fact that reliable results are found out. Whether peak, softened or residual strength are used in the analysis of slope stability depends on presence of existing slide planes and fissuring. In slope stability analysis, it is necessary to work with curved failure surfaces or compound surfaces made up of several straight lines. These assumptions will suffice for most practical problems. In some problems, the location of the failure surface and the shape of the sliding mass are influenced by weak strata within the soil. Although a full calculation of safety factor for a given slope requires many trial failure surfaces, the mechanics of calculation will be demonstrated by considering a single failure surface. Slopes in overconsol i dated fissured clays require special consideration. A number of cases are recorded in which failures in this type of clay have occured long after dissipation of excess pore water had been VIII completed. These slopes often failed even though conventional analysis indicated that they should be stable. It is called "progressive failure" Analysis of these failures showed that the average shear strength at failure was well below the peak value. It is probable that large strains occur locally due to the presence of fissures, resulting in the peak strength being reached, followed by a gradual decrease towards the critical state value. The strength of an overconsol idated clay at the critical state, for use in the analysis of a potential first-time slip, is difficult to determine accurately. Tension cracks generally occur near the crest of a slope and reduce the overall stability of a slope by decreasing the cohesion which can be mobilized along the upper part of a potential failure surface. Omitting the tension crack don't lead to great difference. Thus, they can be neglected. In practise, slope stability problem is usually considered in two dimensions and conditions of plane strain are assumend. There is no rigorous method for treating three-dimensional effects. It has been shown that a two-dimensional analysis gives a conservative results for failure on a three-dimensional surface. The measurement of water pressure in the saturated zone is most commonly carried out using piezometers. They can be grouped into those that have a diaphram between the transducer and pore water and those that do not. The most reliable way in determining actual pore water pressure or its seasonal fluctuations is usage of piezometers. A further consideration is selection of method in inputting pore water pressure data for stability analysis. Three principle methods are usually adopted for representing pore water pressure data: 1- Piezometric lines 2- Pore pressure grids 3- Pore pressure ratio (ru) In practice, for purposes of analysis it is most convenient to express the pore water pressure at any point in terms of the pore water pressure ratio (ru). IX The general solutions are based on the assumption that ru is constant throughout the cross-section. In most problems the value of the pore pressure ratio is not constant over the whole surface but, unless there are isolated regions of high pore pressure, an average value (weighted on an area basis) is normally used in desing. For many slope failures, the observation that the surface along which sliding took place was not planar but curved, led to the idea of using curved failure surfaces for the analysis of slope stability. Particularly, methods of analysis based on circular surface were developed due to the fact that a circle is merely a special and simple type of curved failure surface. Early solutions for circular surfaces were obtained by Fell en i us (1927), who used a method of slices, and Taylor (1937, 1948) who used a friction circle method to produce charts of stability numbers which could be used to determine factors of safety against slope failure. Friction circle method can only be used for uniform materials. If the slope is composed of more than one soil, or if unusual patterns of seepage exist methods of slices must be used. In methods of slices, the failure mass is broken up into a series of vertical slices and the equilibrium of each of the slices is considered. The major differences of these methods involve tha way in which the unknown quantities that arise in the analysis are dealt with. All of the methods of slices are particularly suitable for solution by computer. More complex slope geometry and different soil strata can be introduced. The common property of methods of slices is that the problem is statically indeterminate. Breaking the mass up into a series of vertical slices does not remove the problem of statical indeterminacy. Hence a unique solution is impossible without doing some assumptions. With Fellenius's method, it is assumed that the interslice forces are equal and opposite for each slice and so cancel. Thus the safety factor computed by this method will be in error. Despite the errors, this method is widely used in practice. X because of its errors on the safe side. Hand calculations are possible. Bishop (19BB) presented a method in which the i ntersl ice forces were taken into account in the analysis. Bishop's original method contains too many unknowns -to enable a direct solution to be found. Thus, simplified version of Bishop's method have been developed. In this method» it is assumed that the forces acting on the sides of any slice have sero resultant in the vertical direction. This type of analysis is accurate enough for most practical purposes and most widely used method of slope stability analysis. Spencer proposed a method of analysis in which the resultant i ntersl ice forces are parallel and in which both force and moment equilibrium are satisfied. Spencer showed that the accuracy of the Bishop's simplified method, in which only moment equilibrium is satisfied, is due to the i nsensi ti vi ty of the moment equation to the slope of interslice forces. The purpose of this study is to examine the stability of slopes in terms of effective stress. For this reason, in the initial chapters the general information has been gi%'en related to the subjects of classifications of landslides, the main approaches of slope stability analysis, the evaluation of effective shear strength parameters and pore water pressure. Then the most common methods of slope stability analysis have been explained. Finally, an analysis programme has been carried out on the model which was selected from the section of Bursa-Yalova Highway. In the analysis, the possibility of variations pore water pressure ratio and cohesion have been considered. Long-term stability analysis have been carried out by using Fellenius's method. Bishop's Simplified method and Spencer's method. The results of the analysis have been evaluated in terms of factors of safety, locations and radius of slip circles and number of slices. For the present case, although the analised slope seemed quite stable, long-term stability would be critical. The effect of reduction of the residual cohesi on seemed more effective than increasing of the pore water pressure. XI Fellenius's method gives conservative results regarding to the other accurate methods. But, the error increases because of the combined effect of increasing pore water pressure and decreasing residual cohesion. The number of slices doesn't influence the results considerably. This is why they can be neglected. Bishop's simplified method is the moat -common one used in the analysis because it gives quite accurate results when compared with Spencer's method.