Combination of geodetic techniques in monitoring of engineering structures

Abstract

It has a considerable importance to have the movements of an engineering structure within certain limits for the safety of the community depending on it. To determine whether an engineering structure is safe to use or not, their movements are monitored and possible deformations are detected from the analysis of observations. An appropriate observation technique, which can be geodetic or non– geodetic (geotechnical–structural) according to classification in Chrzanowski and Chrzanowski (1995), is chosen with considering the physical conditions of the observed structure (its shape, size, location and so on), environmental conditions (the geologic properties of the based ground, tectonic activities of the region, common atmospheric phenomena around the structure and so on), the type of monitoring (continuous or static) and the required measuring accuracy for being able to recognize the significant movements. Until the beginning of the 1980’s, conventional measurement techniques have been used for detecting the deformations in large engineering structures. After that the advances in space technologies and their geodetic applications provided impetus for their use in deformation measurements. GPS positioning technique has the biggest benefit of high accuracy 3D positioning; however, the vertical position is the least accurately determined component due to inherent geometric weakness of the system and atmospheric errors. Therefore, using GPS measurement technique in deformation measurements at millimeter level accuracy requires some special precautions, such as using forced centering equipment, applying special measuring techniques like the rapid static method for short baselines and designing special equipment for precise antenna height readings. In some cases, even these special precautions remain insufficient and hence, the GPS measurements need to be combined with another measurement technique to improve its accuracy in height component. In geodetic evaluation of deformations, static observations obtained by terrestrial and/or GPS technique are subject to a two–epoch analysis. The two–epoch analysis basically consists of independent Least Squares Estimation (LSE) of the single epochs and geometrical detection of deformations between epochs. Here, the aim is analysing 1D and 3D deformations of an engineering structure using GPS and levelling measurements data. During the 1D deformation analysis, three different approaches were performed separately. In the first and second approaches, height differences from precise levelling measurements and GPS measurements respectively were input in the analysing algorithm. In the third approach the combination of height differences from both techniques were evaluated for vertical deformation. While combining the two measurement sets, Helmert Variance Component Estimation (HVCE) and Minimum Norm Quadratic Unbiased Estimation (MINQUE) techniques were used. 3D deformation analysis only with GPS measurements was accomplished using S-transformation technique. The theories behind the used deformation analysis and variance component estimation methods are summarized in the chapters. Thereafter the optimal solution for combining the GPS and precise levelling data to improve the GPS derived heights and hence to provide reliable inputs via the optimal solution for the deformation investigations are discussed. The highway viaduct of which deformations were inspected in this study is 2160 meter long and crosses over a lake on 110 piers. It is located in active tectonic region very close to the North Anatolian Fault (NAF). With the aim of monitoring its deformations, four measurement campaigns including GPS sessions and precise levelling measurements were carried out with six–month intervals. The session plans were prepared appropriately for each campaign on a pre–positioned deformation network. The results of this study, experienced with measurements of the viaduct, are thought to be important remarks for deformation analysis studies using GPS measurements. As the first remark, GPS measurement technique can be used for determining deformations with some special precautions like using forced centering mechanisms to avoid centering errors, using special equipments for precision antenna height readings, using special antenna types to avoid multipath effects etc. However, even though these precautions are taken to provide better results in 1D and 3D deformation analysis, GPS measurements have to be supported with Precise Levelling measurements.

Özet

Bu çalışmanın amacı, İstanbul’un güneybatısında, Büyükçekmece Gölü üzerinde yer alan ve TEM (Transit Avrupa Otoyolu − Transit European Motorway) otobanının bir bölümünü oluşturan Karasu viyadüğünün deformasyonlarının GPS ve nivelman ölçmeleri ile araştırılmasıdır. Bu amaçla altı aylık periyotlarla iki yılda gerçekleştirilen ölçme kampanyalarından elde edilen veriler değerlendirilmiştir. Deformasyonların bir boyutlu (1B) (düşey) analizlerinde, öncelikli olarak GPS ve nivelman ölçülerinden türetilen yükseklik farkları ayrı ayrı değerlendirilmiştir. Yükseklik farklarının birlikte değerlendirildiği 1B deformasyon analizinin sonraki bölümünde, GPS ve nivelman yükseklik farklarının birleştirilmesinde Helmert’in HELMERT ve Rao’nun MINQUE Varyans Bileşen Tahmini (VBT) yaklaşımları kullanılmıştır. Üç adımda gerçekleştirilen 1B deformasyon analizi sonrasında, yalnızca GPS ölçüleri kullanılarak S-transformasyonu uygulanmış ve viyadüğün olası deformasyonları bir kez de üç boyutlu (3B) olarak irdelenmiştir. Makalede, bu araştırmada uygulanan her bir deformasyon analizi yaklaşımının teorik yapısı da özet olarak verilmektedir. Çalışmanın sonuçlarında: Transit Avrupa Otobanının önemli bir bağlantı noktasında yer alan ve 2160 metre uzunluğundaki viyadüğün deformasyonlarının değerlendirilmesinin ve ortaya konmasının yanı sıra, büyük mühendislik yapılarındaki deformasyonlarının araştırılmasında uygulanacak ölçme ve analiz yöntemleri konusunda gelecek çalışmalara katkı sağlanması da amaçlanmıştır.