Dam Monitoring

 Monitoring Dams

Monitoring of dams is a critical component in the long term maintenance and safety of these dynamic structures.  Today, the ability to monitor dams through the use of geophysical techniques, such as electrical resistivity, seismic, and self-potential (SP) are major breakthroughs in maintaining the health and safety of these structures.   HGI has pioneered subsurface geophysical monitoring techniques, with decades of experience in performing subsurface monitoring of hydrodynamic processes.  By repeating geophysical measurements over time, we can monitor changes in soil saturation and soil structural competency to identify and understand current and future problems associated with potential system failures.

 

The ability to monitor dams through the use of geophysical techniques is a major breakthrough in maintaining the health and safety of these structures

Monitoring of dams is a critical component in the long term maintenance and safety of these dynamic structures. The ability to monitor dams through the use of geophysical techniques, such as electrical resistivity, seismic, and self-potential (SP) are major breakthroughs in maintaining the health and safety of these structures. In this image HGI personnel are installing resistivity cables on the crest of a dam.
Our team of geophysicists, geologists, and engineers has developed monitoring systems for deployment on dams of most types, as well as any constructed earthen structures such as levees, embankments, and tailings storage facilities.  This approach is particularly cost effective for earthen dams and embankments, as these methods can be used across the entire dam and the adjacent environment.  Additionally, because the physical application of these technologies is non-invasive, it can provide high-value information safely and cost competitively when compared to drilling, CPT, and other more invasive sampling methods.

 

Monitoring can be set up to occur either continuously or in intervals, providing real-time or periodic changes occurring within the dam.  In either case, an array of non-invasive sensors are installed to make measurements.  In a continuous measurement system, the array is permanently established with a fixed monitoring system installed on site and is operated autonomously or interactively through the internet.  Alternatively, the same array can be established temporarily and data acquired at convenient time intervals.  Changes can be detected when the datasets are differenced (i.e. data from Time 2 minus data from Time 1) showing the total change between datasets.  Such differencing can be done with continuous monitoring and in real-time, particularly if there is grave concern about conditions changing at the dam or embankment site.  Once an array of sensors has been installed, periodic measurements can typically be made in a single day with a small mobile crew.

 

Resistivity profiling is combined with other methods to give the broadest most succinct understanding of subsurface conditions

 

The advantage of the resistivity method, as is the case with most other geophysical methods, is its scalability to meet the desired resolution, coverage, and depth of investigation. In this image, resistivity data is draped over an embankment dam to show scale and conductive targets.
 

Of the geophysical services provided by HGI, electrical resistivity profiling is conducted most often and is combined with other methods to give the broadest most succinct understanding of subsurface conditions.   Additional variations on the electrical resistivity method, such as self-potential, can add additional parameters to constrain investigations and identify problem areas.  For example, the self-potential method is very sensitive to identifying flow-paths in the subsurface and can be very useful for identifying regions of seepage through earthen dams and embankments.

 

Seismic methods can show differences in stiffness and elastic properties of materials within earthen dams and embankments, helping to assess the structural integrity of the dam.  Seismic refraction surveys can provide information on the internal structure of dams and embankments and information on the underlying geology (depth to bedrock for example).  Multi-channel analysis of surface waves (MASW) can identify weak zones in the dams and embankments or underlying geology, which could present failure risks for these structures.  Down-hole and Cross-hole shear-wave seismic surveys provide shear-wave velocity profiles for input into seismic hazard analysis and liquefaction potential studies.

 

Waterborne or streamer electrical resistivity surveys are also particularly useful in investigating the internal structure of dams and levees that are submerged, and also for assessing the geological conditions surrounding dams and levees that are below the water surface.  Bathymetry and sub-bottom profiling provide high resolution information on water and sediment depths within dammed lakes and water bodies and behind dam and embankment structures; additionally these methods are used to monitor sediment loads and deposition over time, and characterize sub-bottom sediments.

Horizontal streamer electrical resistivity slice of survey on Panama Canal taken at 21m below canal water level of electrical resistivity from all data with the Gamboa Sub-reach (turns H–J). Surface geologic information adapted from Stewart et al. (1980) and modified per information in Kirby et al. (2008) and Retallack and Kirby (2007). Boreholes are overlain on the resistivity to discern hard rock from loose sediment.