Master's Thesis
Civil Engineering - Earthquake Engineering
March 1392
Abstract
Reinforced soil is a special material that is created from the combination of soil and reinforcing member. Reinforcing components resistant to tensile forces and in the form of wire, tape, mesh, fabric, felt, etc. It can be made of steel, aluminum, plastic, polymer material or any other material. Basically, soil is weak against tension and shear, and the idea of ??reinforced soil is actually a solution to solve this shortcoming. The participation of reinforcements and soil creates special materials that have completely different properties from the properties of its components and provides the possibility of building economic structures that are capable of restraining tensile forces. The use of polymer reinforcements made reinforced soil in a wide range of earthen structures such as retaining walls, earthen roofs, etc. be widely used. Geosynthetics are placed as reinforcing elements and tensile elements in the soil mass and increase the soil load.
Until now, various methods have been presented for the analysis and design of reinforced soil. Based on the conducted studies, most of the design methods of earthen roofs reinforced with geosynthetics are based on limit methods, which limit equilibrium method and finite element method are more useful than others.
On the other hand, in geotechnical issues, inherent uncertainty and variability in soil resistance parameters can be clearly seen. In addition, other errors, such as measurement and approximation errors in the model, cause unavoidable errors in geotechnical issues. Regarding the stability of gables, which is one of the important geotechnical issues, the mentioned items are also included. Considering the unchanging and uncertain nature of the problem of the stability of gables, the discussion of probabilistic analysis makes sense. One of the methods of considering the effect of uncertainties in geotechnical problems and performing probabilistic analysis is the Monte Carlo method. In the Monte Carlo simulation method, which method is probabilistic, for variable input parameters the probability distribution function is considered and with random sampling From these distribution functions, the reliability coefficient is calculated many times and finally a probability distribution function for the reliability coefficient is obtained.
In this thesis, the angle of internal friction, specific weight, tensile strength of the reinforcements and the quasi-static coefficient of the earthquake are considered as uncertain and variable parameters, and the design of earthen roofs has been investigated from the point of view of probabilities, the result of which is to obtain an economic and optimal plan.
Key words: reinforced soil, reliability, confidence factor, Monte Carlo simulation method, first chapter
General
1-1 Introduction
Soil as the oldest and most widely used and at the same time the most complex material used by humans and one of the structural components of all civil structures (the foundation of all structures) has been considered for a long time. On the other hand, due to the relative weakness of soil compared to many existing materials, mankind has been looking for ways to improve the quality and performance of these materials for a long time, and one of these ways is to strengthen the soil by using the large geosynthetic family. Geosynthetics by playing different roles, the weaknesses in the soil such as low resistance, high deformation, the possibility of washing and erosion, etc. (F1). Soils, especially granular soils, are very resistant under compressive stresses. When these soils are armed, the armers They bear tensile stresses and as a result, a composite structure with a wide range of resistance is created.
The stability of gables is one of the important issues in civil engineering. The importance of this issue can be He found out about the damages and casualties caused by landslides and the large amount of large and expensive projects such as dams, roads, etc. that deal with the slope. One of the measures taken to stabilize soil slopes is soil reinforcement technique. In this technique, by placing the reinforcement in the soil, the engineering properties of the soil are improved (F2). The idea of ??building a reinforced earthen wall or roof has actually been a solution to avoid building retaining walls in the traditional way.The construction of reinforced earth walls and roofs is also possible with various forms, in this case, reinforced walls with concrete facade and earth roofs reinforced with geogrid or geotextile can be mentioned.
In recent years, the use of these types of structures in the country has made great progress. The cost of building a wall or The reinforced earth roof is less compared to the traditional retaining wall. From a technical point of view, these structures perform better than others. Due to the large amount of materials used in the construction of these structures, it is clear that providing a suitable design will save a lot of costs. In the process of designing reinforced earth roofs, it is common to use reliability coefficients in various parts of the design, including checking the internal stability of the roof. Usually, reliability coefficients are applied to compensate for the shortcomings caused by loading information, material properties and how the structure is implemented. The reliability coefficients used in the design of roofs are usually chosen based on experience, implementation records and also based on the importance of the project. It is obvious that if there are measurement errors in determining the properties of materials used in the roofs or operational errors, the reliability coefficients alone will not be responsible for the situation. In such situations, if the mechanical properties of the roof components are assumed to be less than their actual value, the design is done in a high-handed way and the project implementation cost increases, and if these properties are assumed to be more than the actual value, then the correct design has not been done. Therefore, it is better to use other methods, at least in order to control this method, in addition to the design method using the confidence factor method (E1). In this chapter, the general concepts of reinforced soil and the method of confidence are expressed.
1-2 General definitions and concepts of reinforced soil and reliability
In geotechnical engineering, different methods are used to improve mechanical properties. Among these methods, soil stabilization by adding lime and cement, dynamic compaction, preloading and reinforcement can be mentioned. Soil reinforcement technique has been used as a useful and economical method to solve many scientific problems in geotechnical engineering, such as improving loose land, stabilizing slopes, reducing soil pressure, and many other things. Reinforced soil is formed from the combination of porous soil resistant to tensile forces. The use of geosynthetics in soil reinforcement is a relatively new technique in geotechnical engineering that has become popular in the last two decades. Among the advantages and disadvantages of roofs reinforced with geotextile or geogrid, the following can be mentioned: a) Advantages This method is very economical. and does not have special equipment. On the other hand, the presence of many prefabricated parts allows for relatively fast construction.
Regardless of the height and length of the gables, the gables do not need to maintain the structure during construction (molding, etc.) like what is common in the case of retaining walls.
Reinforced gables are relatively flexible (formable) and can tolerate large lateral deformations and accept large vertical settlement differences (heterogeneous settlements). The plasticity of roofs reinforced with geosynthetics leads to the use of a lower safety factor in the design of load capacity compared to conventional rigid structures.
Reinforced roofs have a good potential against earthquake loading because they are malleable and this cohesive soil mass also has inherent energy absorption properties.
The resistance of geosynthetics decreases over time, in contact with soil loads and heat.
Construction of reinforced roofs in excavation areas requires a large volume of operations.
Excavation from behind reinforced roofs faces limitations (F2)
However, since the use of geosynthetics is relatively new, there are many effects of them such as creep, Aging and durability are not known based on practical experience. It should also be noted that geosynthetics exposed to ultraviolet radiation may degrade quickly, which should be added at the end of the construction of a suitable cover to the external surfaces of the roof that is exposed to sunlight.