Denis Jongmans

Ambient seismic noise monitoring of a clay landslide: Toward failure prediction

Given that clay-rich landslides may become mobilized, leading to rapid mass movements (earthflows and debris flows), they pose critical problems in risk management worldwide. The most widely proposed mechanism leading to such flow-like movements is the increase in water pore pressure in the sliding mass, generating partial or complete liquefaction. This solid-to-liquid transition results in a dramatic reduction of mechanical rigidity in the liquefied zones, which could be detected by monitoring shear wave velocity variations. With this purpose in mind, the ambient seismic noise correlation technique has been applied to measure the variation in the seismic surface wave velocity in the Pont Bourquin landslide (Swiss Alps). This small but active composite earthslide-earthflow was equipped with continuously recording seismic sensors during spring and summer 2010. An earthslide of a few thousand cubic meters was triggered in mid-August 2010, after a rainy period. This article shows that the seismic velocity of the sliding material, measured from daily noise correlograms, decreased continuously and rapidly for several days prior to the catastrophic event. From a spectral analysis of the velocity decrease, it was possible to determine the location of the change at the base of the sliding layer. These results demonstrate that ambient seismic noise can be used to detect rigidity variations before failure and could potentially be used to predict landslides.

Spectral Analysis of Prone-to-fall Rock Compartments using Ambient Vibrations

ABSTRACT The dynamic response of four unstable rock compartments in the Alps has been studied using the ambient vibration technique, with the aim of identifying precursors to rockfalls. The test sites present various geological settings (limestone, argillite, and shale-sandstone series), failure mechanisms and volumes. The ambient vibration spectra measured on the unstable compartments systematically showed clear energy peaks at specific frequencies, in contrast with records made on the adjacent stable rock masses. These predominant frequencies were interpreted as resonant frequencies of the unstable compartments, in agreement with 2-D modal analysis. In the horizontal plane, ground motion at the fundamental frequency was found to be systematically parallel to the line of maximum slope gradient, and perpendicular to the main bounding fracture observed at most of the sites. The fundamental frequency of each prone-to-fall compartment shows reversible variations related to temperature fluctuations at differe...

A 2D numerical study of the effect of particle shape and orientation on resistivity in shallow formations

Surficial heterogeneous soils such as till, alluvial fans, or slope deposits are difficult to characterize by geotechnical tests because of the presence of decimeter- to meter-sized pebbles or rocks. The effective resistivity of such two-component media composed of a percentage of resistive particles embedded in a conductive matrix is given by the Bussian’s equation. The application of this equation allows the concentration of resistive particles to be determined if the resistivity values of each component and of the mixture, as well as the cementation exponent m , are known. However, previous theoretical and experimental studies have shown that the effective resistivity is affected by the shape of the particles. The objective of this study is to numerically determine the 2D effects of particle shape and orientation on the resistivity. Two configurations have been considered in the finite element (FE) modeling: laboratory-like measurements and field layout. For circular particles, the numerical results fi...

Application of Ambient Vibration Techniques for Monitoring the Triggering of Rapid Landslides

Ambient vibration techniques are increasingly used for monitoring landslides. Two types of rapid landslides are considered in this study: mudslides in clayey sediments and rockfalls of intermediate size (103–105 m3) in rigid rocks. The change of ambient vibration properties with time allows variations of internal mass characteristics to be obtained. This information is complementary to the surface motion measurements derived from air or ground based techniques and could be included in a monitoring system for landslides exhibiting a rapid mass movement.

Introduction to the JEEG – NSG Geotechnical Assessment and Geo-environmental Engineering Geophysics Special Issue

This Special Joint-Issue of the Journal of Environmental and Engineering Geophysics and Near Surface Geophysics entitled “Geotechnical Assessment and Geo-environmental Engineering” is a selection of original contributions organized under two themes. Near Surface Geophysics presents eight articles on the application of the electrical resistivity techniques to determine the geotechnical properties of the ground, and the integration of geophysical and geotechnical data. The Journal of Environmental and Engineering Geophysics contains seven papers that investigate the stability of structures using seismic techniques. The articles presented in this special joint-issue were selected from a pool of over forty manuscript submissions never published before. The papers are peer-reviewed by a panel of experts from around the world and are at the forefront of the current state-ofknowledge. We hope that both researchers and professionals responsible for site and geomaterials characterization will find the information contained in this collection of practical use.