R. Roncella

Surface and subsurface non-invasive investigations to improve the characterization of a fractured rock mass

Three-dimensional assessment and modelling of fractured rock slopes is a challenging task. The reliability of the fracture network definition is of paramount importance for several engineering and geotechnical applications, and so far, different approaches have been proposed to improve the assessment procedure. A thorough knowledge of the actual fracture system is necessary to construct an accurate geometrical model of the rock mass and to determine block size distribution within the rock body. This paper describes the integration of diverse techniques used to define the rock mass fracture pattern, focusing on the most important fracture features, which are joint orientation, spacing, and persistence. A case study in the north of Italy was selected in order to show the potential of an integrated approach where surface and subsurface investigations are coupled. The rock surface was analysed by means of both standard geological mapping and terrestrial laser scanning. Ground penetrating radar surveys were conducted to image and map the discontinuity planes inside the rock mass and to estimate fracture persistence. The results obtained from the various investigation methodologies were employed to construct a model of the rock mass. This approach may lead to a better understanding of fracture network features, usually observed only on the rock surface. A careful analysis of block size distribution in a rock body can be of valuable help in several engineering and risk mitigation applications.

Photogrammetry for geological applications: automatic retrieval of discontinuity orientation in rock slopes

The stability of a rock slope depends on the rock mass geo-structure and its discontinuities. Discontinuities show up at the rock surface as smooth and often plane surfaces. From their location and orientation the main families of fractures can be inferred and a stability analysis performed. To gather information on their distribution, surveys are typically carried out with geological compass and tape along scan lines, with obvious limitations and drawbacks. Here an highly automated image-based approach is presented to compute the required rock parameters: an accurate high resolution Digital Surface Model of the area of interest is generated from an image sequence and segmented in plane surfaces within a multi resolution RANSAC search, which returns location and orientation of each plane. To avoid measuring ground control points, the camera may be interfaced to a GPS receiver. Multiple overlapping and convergent images are captured to achieve good accuracy over the whole network, minimize occlusions and avoid poor object-camera relative geometry. The method is applied to the rock face of Corma di Machaby (Italy): the results are compared to those of a traditional survey with compass and to those of a laser scanner survey.

Moisture Susceptibility and Stripping Resistance of Asphalt Mixtures Modified with Different Synthetic Waxes

Wax modification of asphalt binders represents an actual resource in warm-mix asphalt technology. Despite its benefits in reducing plant temperatures and atmospheric emissions, the presence of wax can be associated with side effects on the quality and durability of asphalt pavements. A simple procedure to evaluate the influence of wax on asphalt stripping resistance and moisture susceptibility is proposed. An unmodified asphalt binder was blended with waxes having different chemical structures (paraffinic and polyamidic); wax-modified asphalt mixtures were then prepared with various aggregate types. Moisture sensitivity was initially studied using conventional methods, such as indirect tensile strength measured after water conditioning. The stripping resistance of bitumen-coated aggregates was then quantitatively evaluated by identifying the stripped surface with a digital image analysis method. In contrast with expectations, results indicated that enhanced performances can be obtained in the presence of wax-modified asphalts. However, the type of wax was found to be decisive. In the same way, results of contact angle and adhesion measurements performed on bitumen–wax blends indicated that changes in asphalt–water affinity occur according to the type and content of wax.

Change Detection and Deformation Analysis in Point Clouds: Application to Rock Face Monitoring

The paper outlines a method to compare two digital surfaces of the same rock face to detect major changes resulting from detached rocks and deformations. A terrestrial laser scanning survey is used for data gathering. After georeferencing, if the cliff has a complex morphology, a 3D segmentation algorithm is applied to split the whole rock surface into more subregions with an almost planar structure. In each subregion the raw point cloud is resampled on a regular grid and multitemporal differences are analyzed. Anomalies in differences, which should be very close to zero if no geometric variations have occurred, are identified with the following purposes: (a) localizing gross changes due to rock detachments, (b) removing global rigid-body displacements, and (c) understanding local cliff deformations. In the case where the rock face is covered by vegetation, this has to be filtered out, e.g., by visual inspection of RGB images co-registered to the point cloud. This paper also describes a procedure to carry out vegetation filtering in automatic way from the analysis of near-infrared images captured by a camera integrated to laser scanner. The application of the full processing pipeline has been tested on a real case study located in the Italian pre-alpine area. Here, after filtering some vegetation, a total rock fall volume of 0.15 m3 was detected on a cliff of about 375 m2 and within a period of six months.

Micromechanical Analyses for Measurement and Prediction of Hot-Mix Asphalt Fracture Energy

A verification of fracture energy density is presented as a fundamental fracture threshold in hot-mix asphalt. Fracture energy density was evaluated with the semicircular bending (SCB) test. Experimental analyses were enhanced by a digital image correlation system capable of providing a dense and accurate displacement-strain field of composite materials at the microstructural level and suitable for describing the cracking behavior of materials at crack initiation. The resulting fracture behavior in the SCB was predicted with a displacement discontinuity method to explicitly model the microstructure of asphalt mixtures and to predict their fracture energy density. The input parameters for the displacement discontinuity micromechanical model of the SCB were obtained from the Superpave® indirect tensile test. The predicted crack initiation and crack propagation patterns are consistent with observed cracking behavior. The results also imply that fracture in mixtures can be modeled effectively with a micromech...

Testing Accuracy and Repeatability of UAV Blocks Oriented with GNSS-Supported Aerial Triangulation

UAV Photogrammetry today already enjoys a largely automated and efficient data processing pipeline. However, the goal of dispensing with Ground Control Points looks closer, as dual-frequency GNSS receivers are put on board. This paper reports on the accuracy in object space obtained by GNSS-supported orientation of four photogrammetric blocks, acquired by a senseFly eBee RTK and all flown according to the same flight plan at 80 m above ground over a test field. Differential corrections were sent to the eBee from a nearby ground station. Block orientation has been performed with three software packages: PhotoScan, Pix4D and MicMac. The influence on the checkpoint errors of the precision given to the projection centers has been studied: in most cases, values in Z are critical. Without GCP, the RTK solution consistently achieves a RMSE of about 2–3 cm on the horizontal coordinates of checkpoints. In elevation, the RMSE varies from flight to flight, from 2 to 10 cm. Using at least one GCP, with all packages and all test flights, the geocoding accuracy of GNSS-supported orientation is almost as good as that of a traditional GCP orientation in XY and only slightly worse in Z.

Terrestrial Photogrammetry and Numerical Modelling for the Stability Analysis of Rock Slopes in High Mountain Areas: Aiguilles Marbrées case

Several high-altitude slope instability phenomena, involving rock blocks of different volumes, have been observed in recent years. The increase in these phenomena could be correlated to climatic variations and to a general increase in temperature that has induced both ice melting with consequent water seepage and glacial lowering, with a consequent loss of support of the rock face. The degradation of the high-altitude thermal layer, which is known as “permafrost”, can determine the formation of highly fractured rock slopes where instabilities can concentrate. The present research has developed a methodology to improve the understanding and assessment of rock slope stability conditions in high mountain environments where access is difficult. The observed instabilities are controlled by the presence of discontinuities that can determine block detachments. Consequently, a detailed survey of the rock faces is necessary, both in terms of topography and geological structure, and in order to locate the discontinuities on the slope to obtain a better geometric reconstruction and subsequent stability analysis of the blocky rock mass. Photogrammetric surveys performed at different times allow the geostructure of the rock mass to be determined and the rock block volumes and detachment mechanisms to be estimated, in order to assess the stability conditions and potential triggering mechanisms. Photogrammetric surveys facilitate both the characterisation of the rock mass and the monitoring of slope instabilities over time. The methodology has been applied in a case study pertaining to the North Face of Aiguilles Marbrées in the Mont Blanc massif, which suffers from frequent instability phenomena. A slope failure that occurred in 2007 has been back-analysed using both the limit equilibrium method (LEM) and 3D distinct element modelling (DEM). The method has been supported and validated with traditional in situ surveys and measurements of the discontinuity orientation and other rock mass features.

Photogrammetric bridging of GPS outages in mobile mapping

A photogrammetric strategy for the orientation of image sequences acquired by Mobile Mapping Vehicles (MMV) is presented. The motivations for this are twofold: to allow image georeferencing in short GPS outages for a MMV under development at the University of Parma, currently without an IMU; to improve the consistency of image georeferencing between asynchronous frames. The method may also contribute to limit the drift errors of low-cost integrated IMU/GPS systems in GPS outages. Drawing on techniques developed for Structure and Motion (S&M) reconstruction from image sequences and accounting for the specific conditions of the MMV imaging geometry, highly reliable multi-image matches are found, refining image orientation with a final bundle adjustment. Dealing with scenes containing poor image texture, the automation of the convergence of the bundle to the solution is still a problem. After a successful orientation of an image sequences of ca 70 m, the accuracy of the orientation and reconstruction process was checked in a test field by means of GPS observations and check points. Although not all constraints between synchronous image pairs are yet enforced, the accuracy degradation along the sequence was found to be still well within the specifications for the MMV.

Network Design and Quality Checks in Automatic Orientation of Close-Range Photogrammetric Blocks

Due to the recent improvements of automatic measurement procedures in photogrammetry, multi-view 3D reconstruction technologies are becoming a favourite survey tool. Rapidly widening structure-from-motion (SfM) software packages offer significantly easier image processing workflows than traditional photogrammetry packages. However, while most orientation and surface reconstruction strategies will almost always succeed in any given task, estimating the quality of the result is, to some extent, still an open issue. An assessment of the precision and reliability of block orientation is necessary and should be included in every processing pipeline. Such a need was clearly felt from the results of close-range photogrammetric surveys of in situ full-scale and laboratory-scale experiments. In order to study the impact of the block control and the camera network design on the block orientation accuracy, a series of Monte Carlo simulations was performed. Two image block configurations were investigated: a single pseudo-normal strip and a circular highly-convergent block. The influence of surveying and data processing choices, such as the number and accuracy of the ground control points, autofocus and camera calibration was investigated. The research highlights the most significant aspects and processes to be taken into account for adequate in situ and laboratory surveys, when modern SfM software packages are used, and evaluates their effect on the quality of the results of the surface reconstruction.

Where is photogrammetry heading to? State of the art and trends

The objective of this paper is to highlight current trends in photogrammetry, trying to foresee where they will lead the discipline in the next years. To this aim, first some remarks on the challenges brought to photogrammetry by other sensors and a brief historical survey of some research topics, where an increasing convergence between photogrammetry and computer vision is apparent, will be presented. Then, a necessarily concise review of the advances in automation in three basic photogrammetric tasks (namely image orientation, surface reconstruction and object restitution) will be illustrated. The purpose of the review is to highlight how the fruitful dialog between photogrammetry and computer vision led to today’s achievements and to point out what kind of approaches seem to be winning in the search for viable and robust solutions in the automation of processes. Finally, the conclusions will look at this convergence in the perspective of academic career.