Aşırı Yüklemeler Altında Toprakarme İstinat Yapılarının Tasarımı

Abstract

Toprakarme istinat duvarları, konvansiyonel dayanma yapılarına alternatif olarak son 40 yılda yaygın bir şekilde uygulama alanı bulmaktadır. Esnek yapıları nedeniyle büyük deformasyonları karşılayabilmesi, sismik kuvvetler etkisi altında yüksek performans göstermesi, imalat kolaylığı sağlaması ve ekonomik olması yönü ile karayolu, demiryolu, fabrika inşaatlarında, su ve liman yapılarında bu sistem tercih edilmektedir. Gelişen dünya ile birlikte zorlayıcı topoğrafik ve jeolojik koşullar altında mühendislik yapılarının inşa edilmesi bir zorunluluk olarak ortaya çıkmıştır. Geleneksel sistemlerin ekonomik çözümler ortaya koyamadığı bölgelerde, toprakarme istinat sistemleri önemli bir alternatif olmuştur. İnşa edilecek yapıların ekonomik, işlevsel ve güvenli olması, toprakarme istinat sistemlerinin doğru tasarımı ile mümkün olmaktadır. Taşıma gücü kontrolü, oturma analizleri, sistemde meydana gelebilecek devrilme ve kayma gibi geoteknik problemler, toprakarme istinat yapılarının analiz edilmesinde göz önüne alınması gereken başlıca kritik durumlardır. Ortaya çıkan kompleks geometriler ve bu geometrilerin bir sonucu olarak meydana gelmesi beklenen yüksek sürşarj yüklemeleri, istinat yapısının inşa edileceği bölgedeki zemin koşulları ile bir arada düşünüldüğünde, bu etkiler altındaki toprakarme istinat yapılarının tasarımının bir geoteknik problemi olduğu, bu problemin çözümünün geoteknik mühendisliğinin temel yaklaşımları ile mümkün olabileceği anlaşılmaktadır. Bu yaklaşımlar kapsamında, imalat ve tasarım süresince, sistemin temel bileşenlerinden, donatı, dolgu ve yüzey kaplama elemanları gibi yapısal elemanların ve çevresel koşulların doğru analiz edilmesi problemin doğru bir şekilde anlaşılması ve çözümlenmesi açısından gereklidir. Söz konusu durum, bu tez çalışması kapsamında, toprakarme istinat yapılarının tipleri ve genel hesap yöntemleri ile ortaya konmuştur. Sonrasında bu yöntemler ışığında, palyeli geometriler için hesap yaklaşımları araştırılmıştır. Derlenen veriler göz önüne alınarak bir proje vaka analizi üzerinden, aşırı yüklemelere maruz kalacağı öngörülen palyeli bir toprakarme istinat yapısının tasarımı araştırılmıştır. Tasarım kapsamında ilk olarak, mevcut geoteknik koşullar için yapı davranışı incelenmiştir. Sistemde meydana gelebilecek stabilite problemlerine karşı alınacak önlemler araştırılmıştır. Sistemin, tasarıma uygun bir şekilde imalatının yapılması, analiz sonuçlarının gerçeği yansıtması açısından son derece önemlidir. Bu amaçla, imalat sırasında, dolgunun her bir tabakası üzerinde sıkışma kontrolleri yapılarak, elde edilen veriler, imalat öncesinde yapılan deney sonuçları ile karşılaştırılmıştır. Karşılaştırmalı sonuçlarla, dolgunun şartnamelerce belirlenen kriterlere uygunluğu doğrulanmıştır.

Retaining structures are indispensible elements of every highway and railway design. Retaining structures are used not only for bridge abutments and wing walls but also for slope stabilization and to minimize the amounth of embankments in construction of different type of structures such as factories, dams and, railways. For many years, retaining structures were almost exclusively designed as a conventional solutiols which is made of reinforced concrete and with the gravity or cantilever type walls which are essentially rigid structures and cannot perform well against significant differential settlements unless founded on deep foundations. With increasing height of the filling soil to be retained and poor foundation conditions, the cost of reinforced concrete retaining walls increases rapidly. Mechanically Stabilized Earth Walls (MSE) is costeffective reinforced-soil structures that can accommodate much larger settlements than conventional retaining structure systems. Using reinforcing elements in the soil mass, the strength of the soil can be improved. Facing system is generally essential to prevent soil raveling between the reinforcing elements. MSE system allows very steep slopes and vertical walls to be constructed safely. Different type of methods have been used since ancient times to improve soil. The use of straw to improve the quality of adobe bricks had been observed since the earliest human history. Many primitive people used wooden sticks to support mud dwellings. French natives along the Bay of Fundy in Canada used sticks to reinforce mud dikes for 200 years approximately during the 17th and 18th century. Some other early examples of basic soil reinforcement methods include dikes of earth and tree branches, which have been used in China and along the Mississippi River. Other examples include wooden pegs for preventing of erosion and landslide control in England, and bamboo or wire mesh, used for revetment erosion control. Soil reinforcing can also be achieved by using live plant roots. The modern methods of soil reinforcement for retaining wall construction were developed by the French architect and engineer Henri Vidal in the early 1960s. His research led to the invention and engineering phenomena of Reinforced Earth, a system in which steel strips are used for reinforcing the soil mass. Today, MSE walls are the wall of choice in most fill situations, and MSE walls are used extensively in worldwide. MSE wall systems has many advantages compared to the classical systems by means of their flexibility, high seismic performance, easy construction facilities and cost saving parameters. Main application areas can be classified as, highways, railways, factory constructions, hydraulic and shore structures. Cost saving advantage of MSE walls are the main reason of choosing this system in engineering applications. Site specific costs of a MSE structure depend on many parameters, such as cutfill amount, wall-slope size and type, soil type, backfill material quality, facing type, temporary or permanent applications. It has been found that MSE walls with precast concrete facings are usually less expensive than conventional type of retaining structures. Construction in extreme environmental and geological conditions has appeared as an obligation with the development of industry and urban cities. MSE wall has been preferred as an alternative of the conventional systems where the classical methods can not be applied properly. The main properties of a structure, listed as economy, safety and functionality, can be provided by the correct design of MSE walls. MSE walls are usually designed according to Load and Resistance Factor Design (LRFD) method. In LRFD, the external and internal stability of the MSE wall is evaluated at all appropriate strength limit states.Besides, overall stability and lateral/vertical wall movement are evaluated at the service limit state, as well. Extreme case load combinations are used to design and analyze for conditions such as vehicle impact, excessive surcharge loadings and seismic effects. For the internal stability analysis, tensile and pullout resistance of the reinforcement is checked. The structural resistance of facing elements and connection parts should be checked, too. Limiting eccentricity, sliding and bearing resistance cases should be considered for the external stability check, according to LRFD design of the MSE walls. Overal stability and compound stability cases are mostly related to global stability. The external stability of an MSE wall is maintained assuming that the reinforced soil mass acts as a rigid body. This is because, the wall facing and the reinforced soil act as a coherent block with lateral earth pressures acting on the back side of that block in the proper design. This is called coherent gravity method. Internal stability analysis depend on the soil-reinforcement interaction, tensile resistance of the reinforcement and durability of the reinforcing strips. Calculations are listed respectively as determination of the maximum factored load for each reinforcement, comparison of this maximum factored load to the factored pullout resistance and to the factored tensile resistance of the reinforcement for all applicable strength, service, and extreme event limit states. The major geotechnical engineering problems such as bearing capacity, differential or total settlement, overturning and sliding should be taken into consideration during the design of a retaining wall. These conditions are valid for MSE walls as well. The design of MSE wall should be examined as a geotechnical problem when this issue meets with the environmental effects with the complex geometries and excessive surcharge loadings. The solution of this problem is possible with the logical determination of construction components and environmental conditions during the design and application on site. This master thesis is prepared to explain the typical systems of MSE types with general calculation hypothesis. The design methods for tiered MSE walls are researched. After that, a case study has been investigated including a tiered MSE wall faces with excessive surcharge loadings. The design of MSE wall is explained with the consideration of geotechnical conditions. Structural behavior is questioned. The precautions against possible stability problems are explained in detail. Mechanical and electrochemical properties of backfill material is also explained. The results of necessary tests are compared with the limit values in the related codes and standarts. Suitable fill material affects the wall performance by the durability of the structure in the long term. The general terms of typical design of Mechanically Stabilized Earth Walls are valid for this project. On the other hand, assumptions for the analysis should be reconsidered because of the complex geometry of the structure. The excessive loadings caused by trucks, cranes and storage area require special solution that consider the situation as a geotechnical engineering problem. Therefore, analysis carried on by this point of view. Soil improvement is an important topic for the structures constructed on poor foundation soils. This project has been examined for soil improvement. It is clear that, MSE wall system can not be designed properly without any soil stabilization application under excessive loadings with poor subsoil conditions. Therefore, suitable improvement system was chosen for this application. Project details were explained. The stability analysis before and after the improvement were compared in order to show the efficiency of stabilization method. It is important that, construction works and site applications should be compitable with numerical analysis of the design. For this purpose, each layer of the fill has been tested by its compaction behavior. Compaction test results have been compared with the tests carried out before the construction. The validity of test results has been shown by comparing the results with the interval values noted in the standarts. MSE wall design is a complete geotechnical engineering problem if the structure is expected to resist excessive surcharge loads with the poor subsoil conditions. Therefore, the design of the MSE wall is a complex task that requires detailed site investigation and a series of analysis. All of the stability conditions for static and seismic cases should be confirmed. The proper design of the structure should be based on this methodology.