The project builds on a master’s project from the winter semester of 2015 and is intended to review the knowledge gained therein. It was about the deformation analysis of a dam with the help of terrestrial laser scanning and a block-by-block evaluation method. The project work revealed that the stationing accuracy of laser scanning must be improved in order to be able to significantly detect deformations. In this project, this should be improved by some modifications to the measurement setup. The first interim results already show an increase in accuracy.
For the deformation analysis of an object, discrete points are to be compared for each measuring epoch. In a network measurement, which has been the usual geodetic monitoring method up to now, such points are realized by prisms on the object. Deformations can then be determined for these points. In comparison, laser scanning offers the advantage of being able to carry out a complete, area-based analysis. However, since the measurement accuracy of a single point does not meet the requirements, a method was developed as part of the project work at the university with which this can be compensated. This procedure was examined in detail in laboratory tests and at the Ennepetalsperre and assessed as practicable. One result of the work was that the stationing accuracy (transformation of the scanning into the superordinate coordinate system) has to be improved in order to meet the accuracy requirements. In two new epochs (summer 2016 and winter 2017) an improved stationing procedure will be tested at the Ennepetalsperre. With these results, the proposed deformation analysis is then re-evaluated.
During the new epoch on July 25, 2016, a network measurement was carried out at the Ennepetalsperre. In the course of the project work mentioned above, rod tripods were used for the targets of the laser scanner. Since these do not offer the necessary stability and accuracy compared to wooden tripods in combination with tripods, only wooden tripods were used for the targets in the new era. Since the laser scanner used offers the possibility of carrying out the target measurement in two positions, the laser scanner could be stationed with greater accuracy.
The following epoch was carried out in spring 2017 with the same measurement strategy. The deformation analysis was then carried out on the basis of the two epochs according to the scheme proposed in the project work.
With the improvements mentioned above, an increase in stationing accuracy can be expected. This could also be proven by the new epochs. The root mean square of the residual gaps is 1.7 mm and 1.6 mm, respectively. In the zero epoch (January 2015) the result was 3.5 times worse. For the deformation analysis, the part of the wall was divided into 45 blocks and one level was estimated for each block per epoch. The displacement of the individual blocks can then be expressed in terms of the displacement of these planes. This results in a comprehensive deformation analysis for this area.
These results were evaluated by a tachymetric network measurement. For this purpose, there are reflectors on the dam wall, which are aimed at by measuring pillars. For a large area of the blocks, the displacements are too small to be significantly detected using laser scanning. At the points where larger deformations (> 4 mm) occur, these were also recognized. In order to test the method further, blocks were manually formed in the upper area of the wall, since larger displacements are to be expected here. Then, using the same principle, the shift between two levels was determined and checked for significance. The result shows agreement with the values from the tachymeter measurement and thus confirms the selected method.