Deprem yükleri altındaki kargir ve betonarme istinat duvarlarının risk ve maliyet analizi

Abstract

Bu çalışmada, kütle istinat duvarlarının depreme göre tasarımında güvenlik katsayısına etki eden faktörler araştırılmıştır. Bunlardan en etkin iki tanesi, "zemin içsel sürtünme açısı" ve "sismik katsayı" esas alınarak göçme riski analizi yapılmıştır. Bu analizin amacı, zemin özelliklerinin değişkenliğini gözönüne alarak istatistik ve olasılık hesap yöntemleri yardımıyla güvenlik katsayıları ile göçme riskleri arasındaki bağıntıyı bulmaktır. Böyle bir bağıntı yardımıyla yükseltilmesini istediğimiz duvarların stabilitesini hesaplarken, bir maliyet ve göçme riski analizi yaparak en uygun çözüm bulunur. Göçme riski analizi değişik yükseklikler için tekrarlanarak, duvar yüksekliğinin tasarımdaki yeri ve güvenlik katsayısına etkisi gösterilmiştir. Ayrıca kütle istinat duvarları hakkında günümüze kadar yapılmış olan araştırma ve deneyler sunulmuştur. Etkiyen kuvvetler Mononobe-Okabe denklemleri ile hesaplanmış ve sadece devrilmeye karşı tahkik yapılmıştır. Taban eksantrisitesi tüm hesaplamalarda göz önünde bulundurulmuştur. Risk analizinde değişkenlerin dağılımlarının Normal (Gauss) Dağılıma uyduğu kabul edilmiştir.

A retaining wall is a wall that provides lateral support for a vertical slope of soil. It is a common structure used in many construction projects and design. The most common types of retaining wall can be classified as follows: 1. Gravity retaining walls 2. Semigravity retaining walls 3. Cantilever retaining walls 4. Counterfort retaining walls Gravity retaining walls are the most common types providing slope retention by their weight that consist of a concrete mass, concrete in combination with soil weight, or the weight of an earth alone. They are classified as rigid or flexible, but all are free to deflect at the top, thereby mobilizing active earth pressure. Non-gravity walls are restrained from movement and include basement walls, many bridge abutments and anchored walls. Anchored concrete curtain walls are installed in a manner which provides slope retenrion during construction. Soil reinforcement wall systems include bored piles (bored piers) and "root piles" which have special application. Concrete gravity walls normally provide moderately high capacities, but assurance that the potential failure surface will fall within a free draining granular backfill (the most desirable condition) may require a substantial volume of excavation and backfill. In any event, positive drainage must be provided and there is always the possibility that drain holes will clog, the prevention of which requires periodic maintenance. Types of gravity retaining walls, their common proportions and load diagrams are given in the following sections. Semigravity walls contain a small amount of steel to reduce concrete volume and provide capacity for greater heights. They have constructed to 32 meters. Cantilever walls of reinforced concrete are generally economical for heights in order of about 8 meter because of the necessity to provide adequate strength in the stem-to- base connection. The weight of the earth acting on the heel vii is added to the concrete weight to provide resistance against active thrust. These walls often are designed for Kq earth pressures rather than K, because the amount of bending required to cause K" to act nay cause wall cracking, especially where the wall is supported by strong foundation materials. Counterfort walls are cantilever walls strengthened with counterforts which are generally more economical when heights greater than 8 to 12 m. are required. Proportions are similar to those of the cantilever wall, and the counterforts are spaced at 1/2 to 2/3 H., depending on wall heights. For walls greater than H=10 m., spacing may be in the order of 1/2 H added resistance against over turning abd sliding may be obtained by installing anchors through the base. Buttress walls are often similar to counterfort walls except that the vertical braces are placed on the wall face rather than along the back. Flexible gravity walls are often economical where earth pressures are not great, although reinforced earth walls can be constructed to have high capacity. Flexible walls have the inherent advandages of providing positive slope drainage and a tolerance to settlement. A rock-filled buttress normally is constructed of large peaces of rock rubble, although compacted mass of earth have been used to stabilize moving slopes where space at the toe permits. Gabion walls are constructed by filling wire baskets about 50 cm. on a side, with broken stone in order of 10 to 15 cm across. Retention is obtained from stone weight and its interlocking and frictional strength. The wall face is battered at about 6 degrees from the vertical and the maximum height is in order of 10 meter. They are constructed either with a stepped face or a stepped back. Crib walls, constructed by forming interconnected boxes from timber, precast concrete, or metal members and a filling the boxes with crushed stone or other coarse granular materials, are commonly used with compacted embankments on the level ground. Precast members usually are in the order of 2 m in length and the wall height is limited to an amount equal to twice the member length. Wall heights are increased by doubling box sections in depth, but high walls are very sensitive to transfers differential settlements and the weakness of cross members precludes support of high surcharge loads. The section resisting overturning is taken as a rectangle of dimension H x B. Reinforced earth walls are constructed of a compacted backfill into which strips or ties, usually of galvanized steel are embedded to absorb tensile forces developing in the fill. The strips are attached to a thin outer skin to vm retain the face, which often made of precast concrete panels for durability aesthetic reasons, and must have sufficient strength at all points, including connections, to resist lateral earth pressures. Ties must also be of adequate length and width to develop sufficient soil-tie friction. Lenghts are usually 0.8 to 1.2 times the structure heights, and to mobilize adequate friction requires backfill materials with a minimum internal friction angle of 25 degrees and a maximum of 25% passing the no. 200 sieve. Heights to 15 m or more are not uncommon. Anchored concrete curtain walls consist of a thin "curtain" of reinforced concrete in the order of 20 to 30 cm. thick, which is constructed from the top down as the slope is cut in benches, and tied back with anchors penetrating into strong materials. They may also be constructed in the same manner as a normal wall and backfilled. They are useful for deep cuts where high retention capacity is required, open excavations for structures, retentions of sidehill fills, and retention of embankments. Construction from the top down is major advantage since the slope is always provided with support, whereas with gravity walls excavation into a slope for construction from the bottom up leaves the slope unsupported unless a bracing system or some other means of support is provided. Another advantage is that capacity can be increased even after the wall is completed by the installation of additional anchors. Anchored precast concrete panels are a recent innovation. They are installed along benches in a slope cut steeply and they are readily conformed to curved slopes. Steel sheet-pile walls are formed by driving steel sheet into a slope or excavation and tying the sheets back with anchors. They are seldom used for sidehills slope retention because of their tendency to deflect and corrode and their relatively high costs. The satisfactory performance of a retaining structure requires adequate resistance to lateral forces and, in many cases, limited deformations. At times such structures must also serve as water barriers. In all categories of retaining structures there are a number of choices, and the selection is based on economic comparisons. In the selection process, evaluations are made of geologic conditions, the nature of the overall construction program, and the propose of retaining structure. The last two factors influence the height of the retaining structure and whether it must function as a water barrier. They also effect the lateral forces to be contained; the desired lifespan, which affects the type of materials to be selected for the retention system; and the deflection tolerances, which affect the system's rigidity. The following is general review of some of the more important evaluation factors. IX As with all planned construction, a thorough and comprehensive geotechnical investigation is required to define soil, rock, and ground water conditions. Detailed ground topography is very important for soil-slope retention design, and information on conditions at the bottom of the body of water is necessary for the design of waterfront structures. Slope retention is addressed in a manner similar to that employed in any slope stability problem with particular attention given to the base support of the structure, to seepage forces, and in some cases to the stability of the immediate area behind the wall as well as to the overall stability of the slope. In most cases of gravity wall, however, a portion of the slope is excavated and replaced with a free-draining granular material which significantly reduces the problem of determinig the lateral forces to be resisted. For the case of open excavations in particular, detailed information on soil stratification is required to permit accurate assessment of potential seepage flows into the excavation. In addition to the lateral forces to be contained, assessment of the stability of the excavation bottom is required, particularly where the soils are soft clays. Open excavations in rock masses may encounter high residual stresses, and these need to be defined. With waterfront structures, the more difficult problems to be evaluated arise from the common condition of soft organic or clay soils. The strength and compressibility of these materials require close definition. The forces to be resisted by a retaining structure are imposed by lateral earth pressures for the most part, although many retaining structures support a vertical load in addition to their own weight. The lateral forces result not only from the earth mass or water body to be contained, but in case of earth support the forces may be increased by some form of surcharge loading behind the structure such as from fills, railways, crane loads, or existing building foundations. In addition to backfill loads, waterfront structures are subjected to current, wave, and ice forces. Since the distribution of the lateral forces relates to a number of factors, such as wall flexibility, the forces are not determined untill a retaining system is selected. Prior to analysis of lateral earth pressures, a tentative retaining system is selected and dimensioned based on consideration of such factors as the purpose and general capacity of the wall and whether it is to be perminent or temporary, impervious or pervious, flexible or rigid, and deflectable or nondeflectable. Required structure capacity is determined primarily by wall height and the magnitude of the forces to be retained. Wall def lectibility relates primarily to the problem of potential backslope subsidence and the subsequent settlement of structures located in the area behind the wall. This problem is common during the construction of many open excavations in urban areas. The magnitude of the earth pressures imposed against the structure relates to the soil type to be retained, groundwater conditions, and whether or not the wall will be subject to displacement. Basement walls, for example, undergo essentially no movement upon completion of construction, and design is based on at-rest pressures. Most other walls will undergo at least a slight deflection and design is based on active earth pressures or in some cases, such as anchored bulkheads, on active and passive earth pressures. Wall flexibility controls the distribution of the earth pressures and determines whether evaluations may be based on classical theory or on emprical relationships. Gravity retaining walls also require evaluation against sliding, overturning, and base bearing capacity failure and settlement. Open excavations require evaluation of potential bottom failures by heave or piping, or possible foundation failure, as well as overturning. Waterfront structures may require evaluation of foundation support, sliding, or overturning, depending on the type of structure purpose. Once earth pressures are determined, wall to retain soil slopes may be designed with adequate weight to resist the lateral forces or they may employ anchors. The wall itself will require a section of adequate thickness or reinforcing to provide resistance against bending and shear, walls for open excavations and some waterfront structures will require a section moduluds adequate to resist the bending that results from the lateral earth pressures. All supports are designed to resist the necessary reactions. Pile foundations may be provided beneath some slope retaining walls and bridge abutments, for example, and are an integral part of relieving platforms. In general, walls to retain sidehill cuts are designed with positive drainage; temporary wall, such as those for open excavations, may permit some drainage, but it is controlled to within tolerable limits. Permanent walls for building basement are designed to be impermeable. Costruction of a retaining system requires particularly close attention to details. If a wall retains a sidehill slope, slope stability during construction must be maintained, and the installation of an adequate system to provide permanent wall drainage is an important consideration. In open excavations, the construction method used and the sequence of operations play a major role in the distribution of earth pressures, which can vary substantially from those calculated for the design. They xi also are a major cause of excessive backs lope subsidence resulting commonly from loss of ground due to inadequate seepage control or excessive wall deflection. Monitoring of wall performance during construction is an important consideration an permits economical contigency designs. With waterfront structures, the sequence of construction operations plays a significant role. For example, the removal of soft organic soils prior to wall construction reduces substantially the forces to be retained, as does backfilling from the seaward side toward the land, rather than the reverse. In this study, the factors affecting the safety factor in the seismic design of gravity retaining walls are investigated. Failure risk analysis is carried out based on the most effective two parameters; "angle of shear strength" and "seismic coefficient". The scope öf this analysis is to find the relationship between safety factor and failure risk considering the variation of soil properties by statistical and probabilistic calculation methods. While investigating the stability of the wall with respect to increasing height based on such relationship, the optimum solution is derived by performing a cost and failure risk analysis. Repeating the risk analysis for various wall heights, the role of wall height in design and its effect on safety factor is shown. The research and experiments conducted so far about gravity walls are presented. The acting forces are determined by the Mononobe-Okabe equation and the investigation is carried out only against overturning. Base eccentricity is considered in all cases. The distribution of those independent variables is assumed to be Normal (Gauss) Distribution.