Geotechnical and Geophysical Long-term Monitoring at a Levee of Red River in Vietnam

The stability or safety of a levee during flooding depends on both the temporal development of the amplitude of the hydraulic loads in surface water and ground water and the change of soil parameters within the levee and subsoil. Therefore, a levee monitoring system should record the temporal development of the hydraulic loads of surface water and the resulting variations of water flow within the levee and subsoil. Long-term monitoring has been performed at a levee of Red River in Thai Binh province in Vietnam. The monitoring system consists of a permanent electrode array for resistivity imaging and geotechnical tools including tensiometers, frequency-domain reflectometry sensors and an observation well. The components were installed after a detailed inspection of the levee and foundation by drilling, sampling, and a geoelectrical survey. The levee monitoring system provided time series of the sensor data that were related to the variation of pore water pressure and water distribution. Additionally, the water level in the river and the precipitation rate were recorded. A two-dimensional transient finite element modeling of the seepage and groundwater flow was performed using the database of a single flood season. The resulting hydraulic model was used for a stability analysis. It could be shown that the stability calculations taking into account the transient development of the seepage line allows a more favorable safety factor in the stability analysis in comparison with the conventional steady state approach. According to our experience with the prototype of a levee monitoring system (LMS), the water level registration in the river, a tensiometer in the levee body and an observation well behind the levee should constitute the basic configuration of a LMS. The addition of more components improves the reliability of the system even in the case of failures of single sensors.