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34
УДК 528
А.П. Гук, Йехиа Хассан Мики Хассан СГГ А, Новосибирск
ОСОБЕННОСТИ ВЫПОЛНЕНИЯ ЛЁТНО-СЪЁМОЧНЫХ РАБОТ ДЛЯ МОНИТОРИНГА КРУПНЫХ ТЕХНИЧЕСКИХ СООРУЖЕНИЙ
A.P. Guk, Yehia Khassan Mikky Khassan Siberian State Academy of Geodesy (SSGA)
10 Plakhotnogo U1, Novosibirsk, 630108, Russian Federation
PECULIARITIES OF AERIAL SURVEYS FOR MONITORING LARGE ENGINEERING STRUCTURES
Работы над мониторингом крупных сооружений, которые систематически измеряют основные структурные параметры, предоставляют ценную информацию о текущей оценке целостности, надежности и прочности сооружения. Точность определения позиций фотограмметрических точек значительно улучшилась в последнее десятилетие в связи с новыми разработками в твердом состоянии камеры с ПЗС (прибор с зарядовой связью), которые оправдывают переоценку фотограмметрических методов для измерения деформации на крупных сооружениях. Если обоснованные решения могут быть найдены для сигнализирующих сооружений, фотограмметрический сбор данных может быть завершен в течение нескольких минут с вертолета; в то же самое время возможная точность, в сочетании с экономным использованием аппаратных компонентов и быстрой обработки данных, делает аэрофотограмметрию интересным способом для измерения трехмерных деформаций на больших сооружениях. Одним из крупных инженерных сооружений, которые служат людям в Египте, является Высотная плотина. В данной статье приводятся особенности выполнения воздушной съемки для мониторинга Высотной плотины.
Introduction
It is important that the movements of an engineering structure, which serves the human life of today’s modern world, are exhibiting safe behaviors. So, a lot of deformation monitoring studies for determining and analyzing different kinds of engineering structures such as high-rise buildings, dams, bridges, viaducts, industrial complexes etc., are implemented. During these studies, the used measurement techniques and systems, which could be geodetic or non-geodetic, are determined considering the type of the structure of which deformations will be monitored, its environmental conditions and expected accuracy from the measurements. According to the used monitoring techniques, the deformation measurement equipments are varied. Also according to professions who use the deformation monitoring techniques, these techniques and instrumentation have traditionally been categorized into two groups: geodetic surveys, which include conventional (terrestrial such as precise levelling measurements, angle and distance measurements etc.), and photogrammetric (terrestrial, aerial and digital photogrammetry), satellite (such as Global Positioning System-GPS, InSAR).
Each main measurement technique has its own advantages and drawbacks. Geodetic techniques, through a network of points interconnected by angle and/or
distance measurements, usually supply a sufficient redundancy of observations for the statistical evaluation of their quality and for a detection of errors. They give global information on the behavior of the deformable structure while the non-geodetic techniques give localized and locally disturbed information without any check unless compared with some other independent measurements. On the other hand, the instruments, which are used in non-geodetic measurements, are easier to adapt for automatic and continuous monitoring than conventional instruments of geodetic measurements. Geodetic techniques have traditionally been used mainly for determining the absolute displacements of selected points on the surface of the object with respect to some reference points that are assumed to be stable. Non-geodetic techniques have mainly been used for relative deformation measurements within the deformable object and its surroundings.
Aswan High Dam
The Aswan High Dam is considered a unique structure among all the large irrigation and electric power projects in the world. The dam was chosen to be a rock fill dam provided with an impervious core in its main part. The construction of the dam was completed in 1971 with the financial and technological assistance of the Soviet Union. The aim of this project was to prevent the river's flooding, generate electricity and regulate water for agriculture. The Aswan High Dam is 3830 m in length, 980 m wide at the base, 40 m wide at the crest and 111 m high. It contains 43 million m3 of material. The dam powers twelve generators each rated at 175 megawatts, producing a hydroelectric output of 2100 megawatts (Fig. 1).
Fig. 1. Location of the High Dam and the High Dam Lake (NASA satellite photo)
Aswan High Dam is subject to external loads that cause deformation and permeation of the structure itself, as well as its foundations. Any indication of abnormal behavior may threaten the safety of the dam. Careful monitoring of the loads on a structure and its response to them can aid in determining abnormal behavior of that structure. To help ensuring the safe monitoring of a structure, it
should be permanently equipped with proper instrumentation according to the goals of the observation, structure type and size, and site conditions.
In earthen or embankment dams (like Aswan High Dam) obviously will deform altogether differently than concrete ones. With earthen dams, the deformation is largely characterized as more permanent. The self weight of the embankment and the hydrostatic pressure of the reservoir water largely force the fill material to settle, resulting in a vertical deflection of the structure. The reservoir water pressure also causes permanent horizontal deformation perpendicular to the embankment centerline, while in the earthen dams elastic behavior is slight.
Location of Monitoring Points
Measurements of horizontal and vertical displacements are required at the dam crest, and upstream and downstream toe locations. Surface displacement monuments should be located to provide coverage across the length of the dam extending to the adjacent stable areas. Provisions should be made to detect relative and absolute movement of armor on the dam face. Typically, the spacing of points near abutments and appurtenant structures are closer by about 50 percent than for the points at the midsection of the crest to provide denser movement data with respect to the surrounding sides, spillways, and foundation areas. New or temporary monitoring points may be concentrated in areas where significant movement is detected or repairs are underway. The location of monitoring points with the different monitoring conditions may be as following:
a. Normal conditions. Monitoring schemes include survey stations at the points where maximum deformations have been predicted plus a few observables at the points which, depending on previous experience, could signal any potential unpredictable behavior, particularly at the interface between the monitored structure and the surrounding material.
b. Unusual conditions. Once any abnormal deformations are noticed, then additional observables are added at the locations indicated by the preliminary analysis of the monitoring surveys as being the most sensitive to identification of causative factors.
c. Long-term monitoring. The spatial distribution of survey monuments should provide complete coverage of the structure, extending to stable areas of the project if possible. A minimum of four monitoring points are recommended to model behavior in a plane section (tilts, subsidence, etc.). For linear structures, monuments are placed at intervals that provide coverage along the structure's total length.
Methodology
Depending on the type and condition of the Aswan High Dam, Long-term measurements are far more common and somewhat more complex given their external nature. Long-term monitoring of a structure’s movement typically requires observations for monitoring points on the structure from external reference points. These external reference points are established on stable ground well removed from
the structure or its construction influence. They inter-connected and are termed the «Reference Network». The reference network must also be monitored at less-frequent intervals to ensure these reference points have not themselves moved. Traditional geodetic survey instruments and techniques may be employed to establish and monitor the reference network points. After choosing monitoring points on the dam as discussed, photogrammetric data capture can be achieved within minutes from a helicopter, and data processing must be done to calculate accurate 3D deformation measurements on this High Dam.
Photogrammetric Techniques
If an object is photographed from two or more survey points of known relative positions (known coordinates) with a known relative orientation of the camera(s), relative positions of any identifiable object points can be determined from the geometrical relationship between the intersecting optical rays which connect the image and object points. If the relative positions and orientation of the camera are unknown, some control points on the object must be first positioned using other surveying techniques. The main advantages of using photogrammetry are the reduced time of field work; simultaneous three dimensional coordinates; and in principle an unlimited number of points can be monitored. The accuracy of photogrammetric point position determination has been much improved in the past decade with the new developments in solid state cameras with CCD sensors, which justifies a re-evaluation of photogrammetric techniques for deformation measurements on large structures. Solid state cameras with CCD sensors have become available in static as well as in dynamic applications. Continuous monitoring with real time photogrammetry becomes possible with the new developments and digital image processing techniques.
Photogrammetry operations
When performing photogrammetric based deformation surveys and using an airplane or helicopter for the platform, a camera with an image motion compensator must be used. Typically, 5 to 20 exposure stations are necessary to insure sufficient precision for the object point coordinates are determined. To ensure the whole photo taking portion of the survey is performed correctly, it is highly recommended that only experienced personnel be used for this phase of the survey.
The photogrammetric reduction process also should be done by experienced personnel trained in image coordinate measurement with the appropriate equipment. If practicable, it is recommended that this process be automated in order to eliminate potential gross errors possible with self-calibration.
Pre-processing photo control survey data
Pre-processing of conventional survey data consists of applying statistical tests at the time the observations are made in order to reject probable outliers, and applying atmospheric, instrument calibration, standardization, and geometric
corrections so data can be imported to subsequent network adjustment software. Pre-processing of conventional survey observations can either be done manually or by appropriate verified and validated PC based programs.
Pre-processing photogrammetric survey data
Pre-processing of photogrammetric based survey data will include the screening of measured image coordinates in order to reject observation which are outliers and determination of 3D object coordinates and associated variance-covariance matrix in the local coordinate system. Determination of the 3D object coordinates should be accomplished by a computer based bundle adjustment program with self-calibration. Also, in the bundle adjustment, the focal length, position of the principal point, coefficients of radial and asymmetric lens distortion must be treated as weighted unknowns. Atmospheric refraction can be neglected if the exposure distance is kept to what is recommended.
© А.П. Гук, Йехиа Хассан Мики Хасан, 2008
