Josep A. Gili

Using Global Positioning System techniques in landslide monitoring

The precise determination of point coordinates with conventional Global Positioning System (GPS) techniques often required observation times of one to several hours. In the last few years, new GPS methods have been developed (among them, the fast-static and real time kinematic), with higher productivity and good theoretical precision. The main objective of this paper is to ascertain the performance of these methods in landslide monitoring practice. We present, first of all, the basic principles of the GPS, the equipment and working procedures. We discuss afterwards the applicability of the GPS to the monitoring of landslide surface displacements. Compared with the classical surveying methods, the GPS allows a larger coverage and productivity with similar accuracy. Furthermore, it can work in all kinds of weather conditions and a direct line of sight between stations is not required. Finally, we present an example of the performance of the GPS equipment in the landslide of Vallcebre, Eastern Pyrenees (Spain). This landslide has been periodically monitored since 1987 with terrestrial photogrammetry and geodetic measurements [theodolite, electronic distance metres ( EDM )]. The movement extends over an area of 0.8 km2 and has experienced displacements as large as 1.6 m during the period 1996‐1997. 14 campaigns, over a period of 26 months, using both static and kinematic GPS methods have been carried out. The GPS measurements have been compared with the results obtained with the EDM, inclinometers and wire extensometers, and checked against fixed stable points. The precision achieved with the GPS measurements is 12 to 16 mm in the horizontal plane and 18 to 24 mm in elevation.

Measurement of landslide displacements using a wire extensometer

A continuous recording of landslide displacements is often required in order to better understand the complex relationship between the triggering factors and the dynamics of the movement. In this paper, we discuss the performance of the borehole wire extensometer and the interpretation of its results. The analysis for the case of a translational slide shows that the displacements measured with the wire extensometer are systematically smaller than the movements observed at the ground surface. A relationship between the wire readings and the horizontal component of the landslide movement has been established by means of three equations representing different stages of the wire displacement within the borehole. The applicability of these equations and the interpretation of the wire extensometer readings have been successfully checked at two landslide sites: Vallcebre in the eastern Pyrenees and Alvera in the Dolomites.

Atmospheric Phase Screen Compensation in Ground-Based SAR With a Multiple-Regression Model Over Mountainous Regions

In this paper, a new model-based technique for the compensation of severe height-dependent atmospheric artifacts, using ground-based synthetic aperture radar (SAR) data over mountainous regions, is proposed. The method presented represents an extension of already existing techniques, but now taking into account the effect of steep topography in the atmospheric phase screen compensation process. In addition, the technique is adapted to work with polarimetric SAR data, showing, in that case, a noticeable improvement in the compensation process. The method is validated in the mountainous environment of El Forn de Canillo, located in the Andorran Pyrenees, where there is a slow-moving landslide that nowadays is being reactivated coinciding with strong rain episodes. In this framework, ten zero-baseline fully polarimetric data sets have been acquired at X-band during a one-year measurement campaign (October 2010-October 2011) with the GB-SAR sensor developed at the Universitat Politècnica de Catalunya. First, the impact of the severe atmospheric fluctuations among multitemporal GB-SAR measurements is carefully studied and analyzed. Hence, the need to correctly estimate and compensate the resulting phase differences when retrieving interferometric information is put forward in the frame of differential-SAR-interferometry applications.

Using several monitoring techniques to measure the rock mass deformation in the Montserrat Massif

Montserrat Mountain is located near Barcelona in Catalonia, at the north-east corner of Spain, and its massif is formed by conglomerate interleaved by siltstone/sandstone with steep slopes very prone to rock falls. The increasing visitors number in the monastery area, reaching 2.4 million per year, has pointed out the risk derived from rock falls for this building area and also for the terrestrial accesses, both roads and rack railway. A risk mitigation plan is currently been applied for 2014-2016 that contains monitoring testing and implementation as a key point. The preliminary results of the pilot tests carried out during 2014 are presented, also profiting from previous sparse experiences and data, and combining 4 monitoring techniques under different conditions of continuity in space and time domains, which are: displacement monitoring with Ground-based Synthetic Aperture Radar and characterization at slope scale, with an extremely non uniform atmospheric phase screen because of the stepped topography and atmosphere stratification; Terrestrial Laser Scanner surveys quantifying frequency for unnoticed activity of small rock falls, and monitoring rock block displacements over 1cm; monitoring of rock joints with a wireless net of sensors; and tentative surveying for singular rocky needles with Total Station.

Comparing Satellite Based and Ground Based Radar Interferometry and Field Observations at the Canillo Landslide (Pyrenees)

The landslide of El Forn de Canillo in the Principality of Andorra is one of the largest in the Pyrenees. In 2007 a monitoring system (boreholes equipped with inclinometers, extensometers and piezometers) was set up. Between 2010 and 2011, surface displacements were measured using differential interferometry techniques (GB-SAR and DInSAR). The interferograms of both radars have yielded mutually consistent results, compatible with the inclinometric measures. Furthermore, the observations with TerraSAR-X, with greater spatial coverage, have shown that the displacements are significantly higher in the upper part of the slope (up to 4 cm/year). Field surveys evidenced the presence of activity indicators (open tension cracks, ground disturbance and structural damages) that confirm the existence of these movements. In El Forn landslide, the combined use of radar techniques with conventional instrumentation allows for a more complete and representative interpretation of the behavior of the slope.

Simulation of rockfall fragmentation mechanism in a GIS-based tool

Rockfalls are frequent instability processes in road cuts, open pit mines and quarries, steep slopes and cliffs. The attitude and persistency of joints within the rock mass define the size of kinematically unstable rock volumes. Furthermore, the rock block will eventually split in several fragments during its propagation downhill due its impact with the ground surface. Knowledge of the size, energy and trajectory of each block resulting from fragmentation is critical in determining the hazard of buildings and protection structures. The objective of this study is to simulate stochastically the fragmentation mechanism in rockfall propagation trajectories and in the calculation of impact energies in a GIS-Based tool which includes common modes of motion for falling boulders. A stochastic fragmentation model is proposed and tested to determine if it can simulate the fragmentation phenomena properly.

Landslide monitoring with staring-spotlight data: Canillo case study

One of the main applications of SAR interferometry is the monitoring of hazard risks related with urban subsidence and landslide instabilities. The different sensors available offer different carrier frequencies, image swaths and resolutions. Depending on the extension of the area and phenomena to monitor, one particular mode can be more suitable than others. In this paper the benefits of high-resolution X-band for landslide monitoring is addressed. The selected mode is the experimental Staring-Spotlight of TerraSAR/TanDEM-X and the area under study the landslide of El Forn de Canillo (Andorra) that is perfectly oriented for orbital DInSAR processing. The paper demonstrates the benefits of high resolution data and the processing challenges that have to be addressed.