P. Sentenac

An approach for the geophysical assessment of fissuring of estuary and river flood embankments: validation against two case studies in England and Scotland

This paper proposes a two-stage geophysical approach to map the vertical cracking and the structural integrity of flood embankments made up of clay geomaterials susceptible to fissuring. The first stage is based on a ‘coarse-resolution’ investigation using conventional electrical resistivity tomography (ERT) equipment to identify the fissured zones in the embankment. This step is complemented by an additional geophysical technique, electromagnetic, to verify the ERT measurements. The second stage is based on a ‘high-resolution’ investigation using a miniature ERT system previously developed at the laboratory scale for detailed mapping of the fissure patterns. The ‘coarse-resolution’ stage is the major focus of this paper and was validated against two case studies in England and Scotland. Longitudinal ERT survey provided a tomographic picture of the upper desiccated zones of the embankments and fissured areas in 2-D, validating the range of resistivity results obtained previously on a fissured clay model in the laboratory. A transversal embankment resistivity tomography was also completed to show the positions of fissured zones in detail in the field. The electromagnetic technique as a fast screening tool allowed cross checking the ERT results and was also efficient in detecting high and low conductivity zones, indicating areas of potential weakness during flash floods and heavy rain. The southern embankment in England showed more fluctuations in the conductivity and resistivity than the north embankment in Scotland, likely to be due to the differences in climate, vegetation and location characteristics between the two sites. Conclusions were also drawn on the potential weaknesses for both embankments and the effect of vegetation on conductivity measurements.

Novel Approach for Health Monitoring of Earthen Embankments

AbstractThis paper introduces a novel modular approach for the monitoring of desiccation-induced deterioration in earthen embankments (levees), which are typically used as flood-defense structures. The approach is based on the use of a combination of geotechnical and noninvasive geophysical probes for the continuous monitoring of the water content in the ground. The level of accuracy of the monitoring is adaptable to the available financial resources. The proposed methodology was used and validated on a recently built, 2-km-long river embankment in Galston (Scotland, United Kingdom). A suite of geotechnical probes was installed to monitor the seasonal variation of water content over a 2-year period. Most devices were calibrated in situ. A novel procedure to extrapolate the value of water content from the geotechnical and geophysical probes at any point of the embankment is shown. Desiccation fissuring degrades the resistance of embankments against several failure mechanisms. An index of susceptibility is ...

Water retention behaviour of an embankment model

The depth of the unsaturated zone is strongly influenced by environmental factors. Approximately two-thirds of the world has sufficiently hot or arid climatic conditions so that the unsaturated zone forms more than a very shallow surface layer. Moreover, even in regions where unsaturated conditions are limited to a shallow surface layer; many practical problems in geotechnical engineering (i.e. slope stability, pollutant migrations) are significantly affected by the hydro-mechanical processes occurring in the unsaturated zone. Soil is used extensively as a fill material (i.e. earth-works such as road, railway and flood embankments, earth dams or behind retaining structures) and the unsaturated condition of the fill will crucially affect its behaviour. Seasonal variations in climatic conditions affect the water retention in soils; as a result they swell and shrink. During the dry spells the shrinkage of soils is commonly associated with the formation of cracks. This work presents preliminary results related to the analysis of a scaled flood protection embankment subjected to drying and wetting processes. The evolution of the unsaturated state of the fill under changes in the environmental conditions is explored and the potential influence of desiccation cracks in the soil mass is analyzed in this work.