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Diyafram duvarlarda stabilite probleminin sonlu elemanlar yöntemi kullanılarak irdelenmesi ile ilgili bir inceleme

A study with finite element method for the stability problems of the diaphragm walls

  1. Tez No: 75443
  2. Yazar: ENDER ÇETİN
  3. Danışmanlar: DOÇ. DR. TUĞRUL ÖZKAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1998
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Geoteknik Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 162

Özet

Günümüzde diyafram duvarlar inşaat mühendisliğinin çeşitli branşlarının içinde giderek yerini almaya başlamıştır. Diyafram duvarlar sızdırmazlık yapıları ve/veya istinat yapıları olarak kullanılırlar. Geçirimsizlik perdeleri, aç-kapa yöntemiyle inşa edilen tüneller, yapı temelleri-perde duvarlar, heyelan önleme yapıları, taşıyıcı duvarlar, istinat yapıları ve kopili ayakları diyafram duvarların uygulama alanlarına örnek teşkil etmektedir. Bu çalışma diyafram duvarların uygulamada problem çıkaran stabilite problemlerinin teorik ve LUSAS ile sonlu elemanlar yöntemli incelenmesini kapsamaktadır. Bu iki inceleme ile saha uygulamarında karşılaşılan stabilite problemlerine uygun ve doğru çözümler bulunmasına yardımcı olunmaktadır. Diyafram duvarların genel olarak anlatılması, teorik olarak stabilite problemlerinin değişken zemin türleri için incelenmesi ve LUSAS ile sonuçların karşılaştırılması bu tezin kapsamı içerisindedir.

Özet (Çeviri)

SUMMARY In recent years slurry trenching and construction of slurry trench cut-off walls and cast-in-place diaphragm walls have found wide-spread applications. Even though many investigators have attempted to understand the fundamentals of the stability of slurry trenches and developed certain theoretical approaches for the problem, there are still many aspects which are not completely understood. The state-of-the-art in slurry trenching is mainly based on field experience and practial applications. Slurry trenching or the utilization of slurries or“drilling muds”for drilling and trenching is relatively recent development in civil engineering projects. The slurry trenching technique used to support the vertical sides of an open excavation dates back to the 1940's, although drilling muds were suggested to oil engineers for well drilling as early as 1887. However, significant progress in the preparation and control of slurries occurred in the early 1940' s and post-war years in Europe, and has spread steadily all over the world since then. Its present applications in civil engineering work nainly include: 1. Drilled Concrete Piers 2. Contiguous Drilled-Pier Walls (Bored Piles) 3. Continuous Diaphragm Wall 4. Slurry-Trench-Cut-off Walls The last two applications mentioned above utilize the slurry trenching technique. The use of slurries is also evident in special applications. These include (I) The sinking of caisson. (2). Slurry trenches intercepting vibrations and shock wawes. (3) Slurry face sheilds, which enable the cutterhead of a tunneling machine to operate ina sluury medium, and; (4) the horizontal penetration of large pipes and tubes, where the slurry reduces skin friction thereby acting acting as a lubricant, in addition to many other applications. At the present, slurry trenching with its two basic systems : Slurry trench cut-off wall and cast-in-place diaphragm wall are used to solve numerous foundation, sheeting and dewatering problems, which seemed difficult prior to the introduction of this method. Both techniques are initiated with a common process, which consists of excavating a narrow trench without the use of significant lateral support other than that provided by a bentonite-water-slurry which is pumped into the trench, so that the slurry is maintained at or near the top of the trench throughout the excavation process. Geotechnical issues relevant to the construction and design of slurry walls usually are articulated with reference to there distinct phases. The first is trench excavation under slurry protection, and the associated stability requirements. The second is the general excavation when the wall acts as ground support, and the associated limitations on ground movement. The third and last stage is when the wall interacts with other structural components as part of the permanent structure. The geotechnical requirements for each successive stage differ considerably. During trench excavation under slurry, concern is with practical matters such as stability of the open trench, nature of soil and the presence of obstacles, fluid loss potential through highly pervious zones, sloughing and peeling off in unstable layers, and effects of constructions procedures. During the excavation period the wall is subjected to lateral earth stresses, water pressure, and effects of surcharge loads. Vertical loads may be induced from superimposed dead weight, interaction with the superstructure, and other conditions such as inclined anchorages. Where a diaphragm wall is designed and built to act as permanent component of the final structure, final loading conditions, load effects, and distribution may differ considerably from the loads that existed during the construction period. In addition, the soil behavior may revert to the long-termdrained condition, markedly different from the short-term construction period, particularly for cohesive soils. Where tribution and magnitude of horizontal earth stresses a distinction must be made between the construction period and long-term service. Typically, under long-term conditions the initial earth pressure at rest is expected to be restored. In this case the relevant parameter is the at rest earth coefficient K". The bentonite slurry' s main function is to xert a stabilizing pressure on the permeable face of excavation, and for such purpose to be accomolished the density of the slurry plays the major role in such requirement. However, the density cannot exceed certain limit since this would produce a very thick and heavy fluid that could not be pumped into the trench or displaced by the backfill material easily. In order to exert a stabilizing pressure on permeable walls of an excavation, the bentonite slurry must form a seal on the surface which it contacts, thus avoiding both loss of slurry in to the soil with consequent reduction in angle of friction and the increase of bentonite concentration, an increase of pore water pressure. In additionto sealing of the face, the filter cake formed on the walls of the trench will improve the slurry' s ability to exrt its maximum stabilizing effect. During the excavation process, soil from the trench material will become mixed with the slurry resulting in the raising of the density the suspension, and consequently settling at the base of the excavation and forming a sludgy layer at the buttom. This sludgy layer will cause difficulty in displacement by the backfill material and the kaying in of the slurry wall in to the miprevious layer beneath. It is therefore necessary to minimize the settling of cuttings in the slurry in order to avoid this problem. Hence, a good trenching sluury should have enough gelation to suspend sands uniformly from top to bottom, since a low gelation fluid will allow for excess settling of sand particles at bottom. An idealized trenching slurry should have : Minimum Allowable Density -To stabilize the trench walls.Low viscosity -This would assure easy pumping and later displacement by the backfill material High Gelation Enough to maintain cuttings in suspension, eliminating the need for extra cleaning of the excavation bottom, and easy pumping into the trench. The main properties of bentonite slurries that must be considered have been discussed. Although it is customary to indicate the slurry properties, it is hardly feasible to specify and proportion a slurry prior to field activities because of the wide variations in the properties of diffrent bentonites and controlling agents factors. However, some investigatiors had attemped to draw some limits or specifications for bentonite slurry trenching. The slurry trenching techniques have developed rapidly during the past few years. However, the mechanism by the which the clay slurry stabilize such excavations is not completely understood. In long, rectangular excavations, the hydrostatic pressure of the slurry is insufficient to stabilize the trench accordind to classical earth pressure theories. Therefore, other stabilizing mechanism were investigated and proposed to account for this discrepancy. One of the most important things for diaphragm walls is the monitoring. There are several reasons of slurry walls and adjacent structures during and after construction. During construction of a slurry wall (trench excavation to concrete placement), the following performance criteria must be ensured and quantified : 1. Trench stability 2. Verticalitiy of trench 3. Guide wall stability During and after excavvation alongside a finished diaphragm wall, the performance criteria are extended to include the following : 1. Sufficient preventation and control of ground movement behind the wall.2. Adequacy of lateral support systems ( usually cross-lot bracing or ground anchors). 3. Preventation and control of bottom heave (base stability). 4. Control of ground water level to avoid distress to nearby structure. Instrumentation is therefore a routine phase of the construction program. In general, instrumentation methods are available for three main activities : (1) deformation measurements, (2) load and stress measurements, and (3) water and earth pressure measurements. The primary purpose of this study is to investigate stability control of trenches, the performance of trenches excavated in different type of soils. This is done in order to establish general guidelines for the overall behaviour and stability of such trenches. After, the results of these investigations are compared with Finite Element Method (LUSAS). In the preliminary stages of this study, the identity and nature of the diaphragm wall is invetigated. This included studying its typical types, advantages and disadvantages. In the second stages of this study, the stability problems of different types of soils are searched. This included studying its properties, behaviours, movements and like these. The last stages of this study, Finite Element Method is used for understanding soil movements and behaviours. Result of these analysis are given in a comperative way

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