Metrolarda deformasyon ölçmeleri

Abstract

Metrolar bir uygarlık göstergesi olup, yüzyıllardır insanlığın hizmetindedir. Nüfusun hızlı artışı, teknolojik gelişmelere bağlı olarak araç trafiğinin anormal boyutlara ulaşması, yolların artan araç sayısı na göre çok yetersiz kalışı, metroların artık bir zorunluluk olduğunun kanıtıdır. Dünyanın hemen hemen birçok ülkesinde yapılan çeşitli uzunluktaki metroların içinde çalışanları ile birlikte bir kaza neticesinde nedenli can ve mal kaybı na neden olabilecekleri kolaylıkla tahmin edilebilir. Bu yüzden metroların ve halkın güvenliği için özellikle jeolojik yapısı yumuşak, şehir merkezinin altından ge çen uzun kilometrajlı metroların çevresindeki değişik liklerin ve tünel içi astarlarının davranışlarının sü rekli izlenmesi, bunların projede öngörülmeyen değişim lerinin belirlenerek gerekli önlemlerin zamanında alın ması zorunluluğu ortaya çıkmaktadır. Bu ise metronun yapımı aşamasında başlayan ve hayatı boyunca süren gözlemler ve ölçümler ile sağlanır. Deformasyon ve deplasman ölçümleri ve gözlemleri olarak tanımlanan gözleyerek izleme yöntemiyle met roların görevlerini güven içerisinde yapıp yapmadıkları belirlenir ve elde edilen veriler yardımıyla daha sonra yapılacak metroların tasarımı için yeni görüşler ve ölçütler sağlanır. Bu çalışmada, incelenen objenin mutlak deformas- yonlarını belirleme olanağını veren ölçme tekniği, yöntemi ve aletleri üzerinde durulmuştur.

Subways: are indicators of civilisation and have been in use for hundreds of years. Rapid Increase of the population, traffic jams due to the technological progress and insufficiency of the roads in comparison with the vehicle amount increased the importance of subways. At the subway construction sites accidents may occur, causing casualties and damages of equipments. There fore, in order to take necessary precautions in any case of collapse, especially at soft grounds in city centers, it is necessary to monitor tunnel linings and surface. Points to have extra measures which are not indicated in the project should also be taken into account. This can be done by making readings; starting from the beginning stage of the construction till the end of the site. Monitoring methods of deformation and displacement measurements and observations help to see wheather or not tunnels run safely and later these data help to design other subways. In this work, the measurement techniques and tools that define absolute deformation of the object have been studied. The New Austrian Tunnelling Method (NATM) requires deformation measurements at the excavation stages (top-heading, bench, invert, etc....) as an essential means for monitoring and controlling the tunnel support system. Tunnel surveying is one of the most difficult concept in engineering surveying and differs from surveying on the surface in many respects. Today, such measurements are considered indispensable tunnelling, to be safe and economic. The problem is to achieve a high degree of accuracy while measuring in underground areas, impeded by construction work, impaired visibility and limited access conditions. IX It is necessary to achieve a high degree of accuracy because the layout of tunnels is always» based on the least favourable geometric configuration of the open traverse orientated at one end. Measurement process becomes even more difficult if.the azimuths have to be carried from air locks into the area under pressure. Another problem with low-lying tunnels, is that the azimuth has to be transfered to the dept of 30-40 m (100-130 ft) with optical plumbs, or with mechanical plumb lines and thus even the fixed points move at least for a period of time. Marking of the stations, is also different from that on the surface. The transit thus has to be centred below rather than above the stations. All these factors hav» an adverse effect on the closure error and require the surface triangulation, serving as a basis for the tunnel layout. More recently, geodetic measurerno-nts are gaining; an increased acceptance in tunnel deformation measurements. This is particularly relevant for deep tunneling where accuracies of +/-- 1 mm are often desired for the facts stated below.. Minimal interference with the tunnelling operations.. Absolute rather than relative deformation measurements.. Uninterrupted i.e. non uianipuiable data stream and automatic plotting of the measurements for rapid and secure evaluation of the tunnel deformation state.. Convergence measurements as part of the general geodetic measurement programme. Geodetic tunnel convergence measurements use luminuous diodes. In general, these diodes are inserted in standard convergence bolts. All diodes of a measuring section have to oe connected via electric cables to a battery. In tunnel practice, tnis has been proven to be cumbersome. The deformation of the tunnel lining is determined by the opto-trigonometrical measurement of the marked convergence bolts in an absolute zero based coordinate system. The accuracy which is obtained, is better than ± 2 mm. Every cross-section that is measured, is allowed to have a user defined number for convergence bolts. The distance between the measuring sections is based on geological conditions. This principle permits free stationing of the surveying instrument which is an electronic theodolite with integrated coaxial distance meter. Maximum reliability of measured results is guaranteed by a direct data flow from the theodolite to the plotter. Years of experience in handling of NATM-measurement programme (mainly inner-city subway construction) already led to the completion or substitution of NATM- convergence measurements using tape extensometers with trigonometrical measurement devices. The use of conventional theodolites has provided accurate results with the measurement consoles. The examination and analysis of this measuring system have shown that the influence of errors of the measurement can be three times higher than expected accuracy. These errors are based on the geometry of the set-up of the measuring instruments, the optical conditions such as dust, light, darkness, vertical deformation, settlement, etc. Sometime targets could not be measured continuously because of a visual obstruction. Another reason for error readings is the target which is demaged by several effects. All of these disadvantages could be eliminated by the method introduced in this thesis. The key point to the solution of the problems is the use of self- recording theodolites and highly precise electronical distance meters. Along with the technical changes, a change of consideration was made from a purely static to a dynamic point of view in order to detect the deformation process. Movements of points are determined relative to the moving trihedral of the tunnel axis in any absolute coordinate system. The points to be observed are marked by targets or reflectors mounted on standard convergence bolts. There is no longer a need for fixed measuring consoles. Only one observer is required to do the entire survey, this person needs only to know the position of three to six preset points. Because XI the order of observation is not bound to a strict rule, it is easy to cope with a variety of optical conditions that can exist inside a tunnel. All measuring data are registered in a data recording module which is linked to the survey instrument. The position of the theodolite should be chosen to obtain the best optical conditions for measuring the targets and fixed points as well as to guarantee the least interference with the advance work. The accuracy of the coordinates of the target depends on the accuracy of the coordinates of the instrument station. The three dimensional coordinates of the location of the observer will be calculated and averaged by measuring three to six fixed points. If necessary, all of the points ever measured could be used for this calculation with respect to their movements or their geotechnical stability. For example, points with a short-time high speed of deformation caused, for instance, by nearby excavations will be excluded. Under certain circumstances unscheduled assumptions such as crown heave can be introduced as control functions to eliminate errors. Convergence is the measurement of length changes between fixed points in underground cavities and tunnels. It has become common practice to use convergence tapes and levelling tunnel deformation measurements. Supervision of convergence movements is caused by, for example:. Loosening or stress-relieving of underground cavities, tunnels adits, quarries, shafts.. Inward movement of open foundation pits.. Distortion, tilting or displacement of constructions,. Sliding of slopes and excavations. One measuring length has two convergence bolts where the relative length changes have to be determined. For this purpose the tape extensometer is attached to the bolts and the perforated tape is tensioned in a well- defined and reproducible way by means of a tensioning element. Change of length or distance compared to the zero measurement can be read off by means of a dial gauge. The tape extensometer can be calibrated on its corresponding frame before and after each seasurement. XII A convergence measuring section has to be installed immediately close to the face when the top heading has moved forward. The first reading has even to be taken after installation. All these measurements are essential for the economy and safety of the tunnels which are the most important principles of NATM during and after construction. If the measurement data are evaluated properly, the cost of the tunnel can be reduced.