Geophysical applications for levee assessment

Levees are important engineering structures that build along the rivers to protect the human lives and shield the communities as well as agriculture lands from the high water level events. Animal burrows, subsurface cavities, and low density (high permeability) zones are weakness features within the levee body that increase its risk of failure. To prevent such failure, continuous monitoring of the structure integrity and early detection of the weakness features must be conducted.;Application of Ground Penetrating Radar (GPR) and Capacitively Coupled Resistivity (CCR) methods were found to be very effective in assessing the levees and detect zones of weakness within the levee body. GPR was implemented using multi-frequency antennas (200, 400, and 900 MHz) with survey cart/wheel and survey vehicle. The (CCR) method was applied by using a single transmitter and three receivers.;Studying the capability and the effectiveness of these methods in levee monitoring, subsurface weakness feature detection, and studying the structure integrity of levees were the main tasks of this dissertation. A set of laboratory experiments was conducted at the Geophysics Laboratory of the University of Arkansas at Little Rock (UALR) to analyze the polarity change in GPR signals in the presence of subsurface voids and water-filled cavities. Also three full scale field expeditions at the Big Dam Bridge (BDB) Levee, Lollie Levee, and Helena Levee in Arkansas were conducted using the GPR technique. This technique was effective in detecting empty, water, and clay filled cavities as well as small scale animal burrows (small rodents). The geophysical work at BDB and Lollie Levees expressed intensive subsurface anomalies which might decrease their integrity while the Helena Levee shows less subsurface anomalies.;The compaction of levee material is a key factor affecting piping phenomenon. The structural integrity of the levee partially depends on the density/compaction of the soil layers. A reduction in density or compaction of part of the levee body may create a zone of weakness during a flood. The Energy Variation Method (EVM) was used to detect the changes in electromagnetic (EM) energy reflected from the layers as a measure of the variation in density. Synthetic modeling was conducted to simulate a cross-section of a levee body to study the ground penetrating radar (GPR) signal behavior as it propagates in the model. EVM was calculated from the reflected signal to predict the changes in density (compaction). Comprehensive fieldwork was conducted to collect a 2,500 meter profile along part of the Helena Levee, Helena-West Helena, AR, for possible density variations in the soil using EVM of GPR signals. EVM is clearly capable of detecting changes in the dielectric constant. The association of these changes to the variation in density/compaction allows the EVM method to directly delineate these variations. Using such a robust method will definitely improve the assessment process as applied to levee structure.;CCR data was collected to study the electrical resistivity distribution within the levee and its usefulness in levee's assessment. CCR was less effective to detect the animal burrows and other small anomalies due to nature of the method in its limitation of detecting very small features.