Desteklenmiş derin kazılarda oluşan hareketlerin incelenmesi

Abstract

Bu çalışmanın konusu derin kazı destekleme sistemlerinde oluşan yatay def ormasyonlarm incelenmesidir. îlk olarak derin kazı destekleme sistemleri tanıtılmış ardından toprak basınçları hakkında bilgi verilmiştir. Sonra da def ormasyonlarm belirlenmesinde kullanılan bazı yöntemler hakkında bilgi verilmiş ve deformasyon ölçüm yöntemleri tanıtılmıştır. Daha sonra İstanbul Turotel 3. Bölüm derin kazı inşaatında yapılan deformasyon ölçüm sonuçlarıyla, bu derin kazı destekleme sisteminin bilgisayar programı kullanılarak yapılan çözümünden elde edilen sonuçlar karşılaştırılmıştır.

Is is a legal necessity when new construction is begun in a developed area to provide protection to the adjacent existing buildings when excavation in the new site is to any depth which may cause loss of bearing capacity, settlements, or lateral movements to existing property. New construction may include cut-and-cover work when public transportation or public utility systems are installed below ground and the depth is not sufficient to utilize tunneling operations. The new construction may include excavation from depths of 1 to perhaps İS m or more below existing ground surface for placing a "shallow" foundation or a mat, or to allow placing of one to three more basements of some kind of subbasements. This type of work requires installation of some kind of systems of retaining structure termed braced sheeting, slurry wall, or a cofferdam together with a means of holding the retaining structure in position. The retaining structure may be constructed of one of the following: 1. Sheetpiling C steel, concrete, or wood) 2. Soldier beams Cor piles ) with or without lagging 3. Drilled- in- place concrete piles Cor piers) 4. Concrete poured in a cavity retained with slurry Ca dense liquid) producing a "slurry" wall Systems to hold the retaining walls in place include; 1. Walles and struts or rakers 2. Compression rings Cwhen excavation is relatively small in plan ) 3. Tieback anchorages-cur rent ly most popular, Sheetpiling is commonly used for retaining excavations because it has the highest- strength/weight ratio, and vi much of the piling is reusable and can generally be easily installed either with sheet pile hammers or with vibratory driving devices. It is not usable, however, when the subsoil contains many boulders or is dense and the excavation is deep. Where the soil is rocky or quite dense and where sheet piling will be excessively damaged in driving, a system of soldier beams and lagging is often used. This system consists in a series of H piles C soldier beams) driven on a convenient spacing of 2 to 3 m for using standard- length timber. As excavation proceeds, SO- to 100-mm-thick boards are inserted behind the front flanges, or Cas is becoming common because excavation is simpler) the board placed against the pile and clipped to the front flange using patented fasteners. Where pile driving vibrations using either pile hammers or vibratory drivers may cause damage to adjacent structures or the noise is objectionable, drilled-in-place piles may be used. The piles Cor drilled piers if 760 mm or more diameter) are drilled on as close centers as practical, and filled with concrete. Where earth is retained and water is not factor, the soldier-beam or drilled-in-place pile spacing may be such that lagging or other wall supplement is not required as "arching" or bridging action of the soil from the lateral pressure developed by the pile will retain the soil across the open space. This zone width may be estimated roughly as the intersection of 4S degree lines between soldiers piles. The piles will, of course, have to be adequately braced to provide the necessary lateral soil resistance. In the past the design of the bracing of deep cuts was usually based on the assumption that the earth pressure increased like hydrostatic pressure in simple proportion to depth below the surface. However, both theory and experience have shown that this assumption is rarely justified. The design pressures are different those computed from the methods of classic earth-pressure theories because of the manner in which the pressures are developed, namely the construction procedure of the braced cut. Peck C1943) and later Terzaghi and Peck C1967) proposed empirical pressure diagrams for wall and strut C tie-back) design using measured soil pressures. Pressures reported by Krey in Berlin for sands were incorporated into the pressure diagrams. These pressure diagrams were obtained as the envelope of the maximum pressures found and plotted for the several projects, the pressure envelope was given a maximum ordinate based on a portion of the active pressure using the Coulomb Cor vii Rankine) pressure coefficient. These diagrams are decidedly conservative, as one would expect. Certainly if one designs a strut force based on this pressure diagram and used simply supported beams for the sheeting as proposed by Terzaghi and Peck, the strut force will produce not more than that pressure diagram owing to creep and ground loss; the sheeting will be overdesigned owing to both pressure- diagram discrepancies and sheeting continuity. This was verified by Lambe et al C1970) and by Golder et al C1970) wherein predicted and measured strut loads varied by as much as 100 percent. Swatek et al C1972), however, found reasonable agreement with the Tschebotariof f pressures in designing the bracing system on a Chicago, III., excavation 21.3m deep. Swatek, however, used a "staged-construction" concept along with the Tschebotariof f pressure diagram. In general, the Tschebotariof f method may be more correct when the excavation depth exceeds about 16 m. As part of the design for a deep excavation the ground movements likely to occur must be assessed with respect to their wider effects. This is particularly important in the proximity of other buildings and buried services. There are two options available to the designer. First, he may perform an approximate empirical prediction based upon the increasing body of case-history data. This will tend to yield order of magnitude displacements and influence fields. Such an approach may be acceptable in some cases where the workmanship and ground control are known to be of high standards, where the construction procedure is well-tried and proven, and where the location is reasonably free from sensitive adjacent structures and services. The second option involves the use of numerical modelling techniques, such as the finite element method. Since deep excavation generates horizontal and vertical soil displacements behind the supporting walls, and heave at the base, it is useful to consider the empirical evidence that is available predicting these movements. Observational data were classified in terms of four board soil types: cohesionless sand, cohesive granular soil, soft to medium clay, and stiff clay, and the main factors controlling inward movements of supported deep cuts as follows; Ca) Horizontal and vertical brace spacing Cb) Depth of excavation below brace level before brace is installed viii Cc) Length of excavation parallel to wall made at any one level prior to installing braces at that level Cd) Elapsed time between excavation and brace installation Ce) Details of prestressing and wedging braces Cf) Details of excavating and placing lagging between soldier beams. Based on field data and finite element studies Mana and Clough C1981) produced a method of estimating movements within braced cuts in clay soils. It involved the determination of several factors including the wall stiffness, strut stiffness, excavation depth and the factor of safety against basal heave. Such a prediction - involving coefficients and multipliers based on the above factors- can be particularly valuable for design engineers needing to make rapid assessments of the influence of design changes. Deep excavations usually provide ideal modelling scenarios for application of the finite element method. Stratified deposits having differing geotechnical properties easily handled as is the process of staged excavation, but there remains the general problem of defining ground deformation moduli. Choice of values CE,v) should be based on engineering judgment stemming from a distillation of evidence derived from laboratory tests on large soil samples, perhaps down-the-hole tests Cfor example screw plate), perhaps field tests C f or example plate bearing) and on case history knowledge of the performance of buildings adjacent to excavations in similar ground. The finite element method is most generally applicable to stiff clays. Recent investigations suggest that these soils may reasonably be approximated as porous elastic materials, particularly when modelled two-dimensional ly in plain strain. The finite element method can be used to analyze a braced excavation. Both the finite element of the elastic continuum and the methods of the sheet pile/beam on elastic foundation computer program can be used. Both these methods will be discussed with some of the limitations and disadvantages of each presented. Either method can be used construction and work best in an interactive computer environment. The methods can be used for either for braced (struts and/or rakers) or tieback construction. Both methods have best application for making rough prediction of expected field performance in ix terms of wall deformations and ground loss, Neither of the finite element methods is likely to predict wall movements accurately except as a happy coincidence for several reasons: Ca) The forces are distributed through a flexible system; the earth quantities involved are large. Cb> Methods of wall and bracing installation tend to make the wall pressure model indeterminate; soil creep tends to produce transitory lateral pressures. Cc) Soil properties are not accurately known and response to this type of loading is uncertain. Cd) Accuracy and care in wall construction varies widely. Ce) Perimeter loads are often unknown. Cf) Installation and measurement accuracy of monitoring system. The finite element method of the elastic continuum has been reported by Clough and Tsui Cİ974), by Clough et al C1972), and by others. It was in a preliminary analysis of the wall used in the following example, but with questionable success CLambe, 1970)- The finite element method requires interaction with the computer program and at the present state of art has no particular advantage over either the sheet pile finite element analysis or the somewhat crude approximations using the lateral pressure diagrams. This method can be used for staged excavations and it is necessary to incorporate into the model some means to ensure when the brace forces are applied that the wall does not move too far into the soi 1 except perhaps at the top one or two nodes where the overburden does not confine the soil such that this movement is possible.