This thesis addresses different aspects of spatial data handling in connection with investigations of large rockslides. As such, most of the research was carried out in a cross disciplinary and highly applied context. The focus of the thesis is on spatial data handling methodology which directly or indirectly can be used to support in rockslide investigations. Rockslide investigation is a comprehensive term covering all aspects of the evaluation process; from the initial planning of field investigations to data analysis and communication of final results. Central topics addressed in this thesis are; a) How data reduction affect the geometrical accuracy of digital terrain models b) How interactive geometric modelling and geovisualization can be used in complex rockslide investigations and c) How statistical analyses can be used to evaluate displacement measurements of unstable rock slopes. Digital terrain modelling forms an important component of the methodology used for rockslide investigations. The first subtopic addressed in this thesis is related to the construction of Triangulated Irregular Networks (TINs) from Light Detection and Ranging (LIDAR) data. As the LIDAR technology tends to generate large data volumes, the resulting terrain models are generally too large to be efficiently handled by ordinary workstations. Therefore, comparisons of various data reduction (decimation) methods were conducted. Their performances were evaluated by means of deviations from terrain models constructed from full datasets. Evaluation criteria included deviations in volume, surface area and elevation. The results showed that the method using a vertical point selection threshold combined with a data dependent triangulation had the overall best performance when tested on 30 different test datasets. The main objective of the geovisualization part of this thesis was to determine the geometric shapes and locations of potential basal sliding surfaces, for the Åknes rockslide in western Norway, along with the volumes of unstable rock associated with different sliding scenarios. The Åknes rockslide is one of the world's most investigated rockslides due to its potentially catastrophic consequences. A custom written geovisualization application for the Åknes investigation provided the visual context needed for data interpretation and interactive geometric modelling of sliding surfaces. This geovisualization approach enabled geoscientists to develop different sliding scenarios. A scenario putting the basal sliding surface at a depth of 105m to 115m below the topographic surface, delineating an unstable rock volume of 43 million m3, was considered as the most realistic. Statistical approaches for analyzing displacement measurements were also addressed in this thesis. Several methods including regression analysis, spectral analysis and hypothesis testing were demonstrated to measurements obtained from Global Positioning System (GPS), total stations and extensometers at the Åknes rockslide. Displacement measurements obtained from lasers and crackmeters at the Nordnes rockslide in Northern Norway were also analysed. As with the Åknes rockslide, the Nordnes rockslide has the potential for devastating consequences in terms tsunami generation. Consequently, thorough statistical analyses of the available displacement data are crucial in order to obtain accurate estimates for the displacement rates as well as for gaining insight into the sliding processes. Displacement data from both sites clearly showed seasonal variations but the overall long term displacements were regarded constant. Prediction intervals were derived from the current monitoring data from the Nordnes site. These prediction intervals are considered useful for evaluation of future displacement measurements.

This study investigates the February 17th, 2006 massive rockslide that occurred in the island of Leyte, Philippines following heavy rainfall and four minor earthquakes. The rockslide is considered one of the largest and most catastrophic slides in the last few decades as it completely inundated the village of Guinsaugon, taking the lives of approximately 1,400 of the 1,800 residents of the village. The distinct element simulation of the rockslide is performed using 3DEC (Three-Dimensional Distinct Element Code) in order to investigate the underlying triggering mechanism of the slide as well as the behavior of the debris flow. The 3DEC models were established based on field observations from the U.S. Reconnaissance team and material and joint properties based on in-situ and laboratory test results. The possible triggering mechanisms considered in the distinct element analyses were the rainfall-induced hydraulic pressurization of the fault forming part of the main scarp, as well as the seismic acceleration due to the minor earthquakes that occurred the morning of the slide. The results of the analyses and simulations indicate that the rainfall-induced hydraulic pressurization of the fault was potentially the main trigger for the initiation of the slide. The minor earthquakes, which occurred before and around the time of the slide initiation, appeared to have very little effect on the triggering mechanism and the debris flow are comparable to witness accounts and field observations. The results presented in this study are expected to provide better understanding of rockslides such as the one that occurred in the Philippines on February 17, 2006. With further improvements in computational capabilities in the future, distinct element simulations can have the potential to reliably predict the initiation and behavior of slides, and help mitigate their impact. / Master of Science