Geotechnical Engineering Online Library

Internal erosion through or beneath levees frequently occurs during flood events, resulting in sand boils. Determining the extent of damage from the seepage channels that caused the sand boils remains difficult, and many sand boils are simply monitored in subsequent floods. There is a need for a nondestructive methodology to map the voids caused by seepage channels so that needed repairs can be identified prior to the next flood. The objective of this study is to identify the electrical response of internal erosion pathways with electrical resistivity tomography (ERT). A case study using ERT on the Hager Slough Levee, which has experienced sand boils since the 1990s, is presented. Three ERT surveys were conducted along the levee in an area of apparent damage (i.e., crown settlement and sand boils) and where nearby boring logs were available. Laboratory resistivity tests provide support for the field ERT interpretations. The results indicate that ERT may be used to identify zones of damage within and beneath a levee. If used in practice, ERT has the potential to improve remediation of levees by identifying needed repairs.

The paper describes five different dike failures that occurred in recent decades. The case histories were located on different rivers and involved different loading conditions. The observed failure mechanisms involved erosion and stability problems. The types of erosion observed were both internal erosion due to extreme groundwater flow and external erosion caused by the river flow and wave action. The case involving instability was caused by uplift, i.e., increased hydraulic head led to a sharp reduction in maximum shear strength between subsoil layers. Two cases demonstrate the importance of dike management and maintenance. Despite the variations in the loading conditions and failure mechanisms, all the cases show that the strength of a dike depends not only on the material used to build the dike but also on the strength of the subsoil.

Bridge scour is the number one cause of bridge collapse in the USA yet the existing guidelines are felt to be excessively conservative. These apparently conflicting statements are due to the fact that most bridge foundations designed before 1987 did not consider scour as part of the design. The Observation Method for Scour (OMS) was developed to address the conservatism inherent in the current procedures by relying significantly on past observations at the bridge. The OMS works in four steps. Step 1 consists of collecting the maximum observed scour depth at the bridge, Zmo. Step 2 consists of finding out what is the biggest flood velocity Vmo that the bridge has been subjected to since its construction. Step 3 answers, by using an extrapolation function, the question: what will be the scour depth Zfut if the bridge is subjected to a major flood velocity Vfut. Step 4 is a comparison between Zfut and the allowable scour depth Zall for the foundation. Eleven bridge scour case histories in Texas and in Massachusetts are presented where the OMS was applied and the results are used to compare predicted and measured values of Zfut for both the OMS and the current FHWA guidelines. The advantages and drawbacks of the OMS are outlined in a final section.

During the spring and summer of 2008, record rainfall in the Midwest United States led to severe flooding as water overtopped the levees bordering the Mississippi River and its tributaries. The erosion associated with the overtopping resulted in levee breaches in many places. After the flooding, a field reconnaissance team was sent to collect time-sensitive data and provide a comprehensive overview of the performance of the levees during the flooding. Two locations where levee overtopping occurred are particularly interesting because of their differing site conditions and performance. This paper presents the levee overtopping case histories of the Winfield-Pin Oak site which was overtopped and severe erosion led to failure, and the Brevator site which was also overtopped but did not fail. Included are a hydrological investigation, documented site conditions, geotechnical soil properties, a soil erodibility analysis, and the documented levee vegetative cover. Levee performance is influenced by the flood conditions, the site conditions, and the soil properties. Both sites in this study experienced large levels and durations of overtopping water, but it is proposed that the Brevator site survived because of its vegetative cover and more erosion resistant soils. Erosion is a very complicated phenomenon that cannot be described by any one parameter, but in all cases, dense and consistent native vegetative cover can greatly improve the overall levee performance.