Detecting and Monitoring Internal Erosion Using Acoustic Emission Technology
Internal erosion presents a significant hazard to Reclamation's embankment dams, dikes, levees, abutments, spillways, and foundations. Signs of active internal erosion are often discovered by local residents or during visual inspection (i.e., during Comprehensive Facility Reviews [CFRs]), but a much better alternative would be to continuously and remotely monitor for internal erosion. One potential means to continuously monitor for internal erosion is passive acoustic emission (AE) monitoring. AE monitoring involves using acoustic sensors (e.g., geophones or accelerometers) to listen for acoustic energy released through internal sources. While many types of acoustic emission are beyond the audible range, one common example in the audible range is the crackling sounds that are heard as a wood pencil is slightly bent. No cracks are visible and the pencil has not experienced permanent deformation, yet due to internal stress relaxations and grain realignments, audible acoustic energy is released. Preliminary research by the USDA and Ole Miss University has shown that AE exists due to internal erosion.
AE is preferred as a long-term, continuous monitoring technique as it relies solely on internal energy sources and is easily accomplished remotely. This is in contrast to many other techniques that require an external energy source and personnel onsite. The only requirements for passive AE monitoring are a robust, self-powered piezoelectric sensor and a means to transmit the data to a remote monitoring computer.
This research will incorporate AE monitoring into an existing Reclamation research project related to internal erosion to investigate the efficacy of AE and the methodology with which Reclamation could use AE on its facilities. In addition, a local university has agreed to partner with Reclamation on the research to answer basic questions regarding best practices for monitoring the AE signals to detect the onset and progress of internal erosion.
Need and Benefit
Reclamation has a need to take advantage of new cost-effective technologies to provide continuous monitoring for many different failure modes on many types of structures. Passive AE monitoring is one such technology and holds great potential as a continuous and remote monitoring tool for internal erosion at hundreds of Reclamation built and managed earthen structures.
Internal erosion (often called piping) has resulted in several near failures of Reclamation facilities and in costly emergency responses and repairs. Recent examples include A V Watkins Dam in Utah and Red Willow Dam in Nebraska. Complete failure of the dam was avoided in both cases seemingly by chance. In the case of A V Watkins Dam, a local farmer happened to notice cloudy discharge near the toe of the dam and alerted the authorities. If this discharge had been noticed even one day later, it is possible the dam would not have been saved. In the case of Red Willow Dam, Reclamation geologists and drillers from the Technical Service Center (TSC) and the GP Region discovered sinkholes on the downstream shell of the dam (by falling into them!) during reconnaissance to locate survey points. Continuous, remote monitoring via passive AE could have potentially alerted personnel to the onset of these internal erosion failure modes months or years in advance and could have saved significant amounts of taxpayer money.
Internal erosion can occur in embankments, dikes, abutments, foundations, and under spillways and is one of the most significant risks to our structures and the people that live downstream of them. This research is an important step in Reclamation's evolution towards continuous and remote monitoring of this failure mode. By involving Reclamation personnel and local universities, the research efficiently addresses both applied and basic questions. Reclamation personnel will have the opportunity to work through the logistics of passive AE monitoring on a lab scale. Conceptual Site Models (CSM) work towards answering the basic questions discussed above represents a direct and tangible advancement of the state of the art of AE monitoring of earthen structures. All this is accomplished in the framework of an existing research project funded by the U.S. Army Corps of Engineers (Corps), which results in a high return on a relatively small investment made by the Science & Technology Program.
This research will result in a full report made to the Science & Technology program. It is envisioned that this report will be used by TSC and regional personnel as both an educational tool and a guide when considering installing passive AE on the structures they manage. Further, the results of the research will be prepared for publication in the peer-reviewed literature and for presentation at technical conferences. This wide dissemination will promote rapid acceptance of the technology and spur further advances in the state of the art of continuous remote monitoring.
Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Implementation of Geophysical Techniques to Monitor Embankment Dam Filter Cracking (final, PDF,
By Robert Rinehart
Publication completed on May 22, 2014