Dam and Embankment Investigations
HGI has extensive experience in applying a variety of geophysical methods for Dam and Embankment Investigations. A number of geophysical measurements are highly sensitive and can provide forewarning to potential issues related to concrete dam surrounds, earthen dams, and embankments.
Using geophysics for earthen dam and embankment investigations has the distinct advantage of imaging through an entire earthen structure to understand structural integrity, seepage, and core strength.
Every earthen containment system (dam, levee, or impoundment) is unique in design, construction, and use, and these properties present challenges in assessing reliability. Getting a holistic view of the structure in the context of its design and construction is the ultimate goal of understanding its integrity. The use of geophysical characterization methods offers this complete view by using electrical resistivity profiling and streaming potential (SP) for example to create an internal picture of these complex structures.
HGI’s methods provide high resolution, continuous cross-sections for dam and embankment investigations to assess the health of these retaining structures and identify potential issues before they become problems.
The electrical resistivity method is particularly sensitive to moisture content, saturation of subsurface materials, and especially the movement of fluids through structures. These surveys can be used as a complementary tool to normal monitoring methods, such as piezometers and visual inspections, by filling in the gaps between measurement locations, expanding the monitoring coverage, and/or detecting issues before they become apparent on the surface. In addition, the electrical resistivity method can be used to characterize and assess the construction materials within earthen dams and embankments, survey the underlying geology, and identify potential geohazards such as faults and fracture zones.
Dam and Embankment Investigations Case Study
Patagonia Lake State Park
Patagonia Lake State Park, AZ
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The Patagonia Lake Dam has been rated as a high hazard facility by the Arizona Department of Water Resources (ADWR) and Arizona State Parks has monitored this dam periodically using piezometers and land surveys. The dam’s design is based on climate and flood models and engineering practices of the late 1960s. Failure of this dam could cause 4.6 billion gallons of water and sediments to be transported down Sonoita Creek towards the Santa Cruz River devastating the surrounding riparian environment and infrastructure, and the growing community of Rio Rico.
In 2009, with cooperation from Arizona State Parks, HGI instrumented the face of the dam with 533 resistivity sensors, placed along four electrical resistivity transects. The dam was characterized during the baseline survey, from which all subsequent surveys will be compared. Decreases in resistivity indicate an increase in moisture content / saturation and it is anticipated that problems deep in the dam’s core can be detected long before they manifest at the surface.
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Baseline characterization resistivity survey results.
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The electrical resistivity method can also be deployed using a towed array behind an ATV or person to rapidly and cost effectively survey large areas. This makes it a very cost-efficient method for assessing the internal structure and identifying any potential issues in levees and embankments. In addition, HGI has a wide range of experience with conducting waterborne or streamer electrical resistivity surveys on lakes, rivers, and waterways. The streamer electrical resistivity method can be used to rapidly assess the submerged portions of dams and embankments or to investigate the submerged underlying geological and geotechnical conditions.
Additional variations on the electrical resistivity method, including induced polarization and self-potential, can add additional parameters to improve 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 be very sensitive to the 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.
Dam and Embankment Monitoring
HGI has developed an effective monitoring system for deployment on and around dams and embankments called “Safety Net”. The Safety Net approach is particularly cost effective for earthen dams and embankments as the “net” can be spread over the entire dam as well as the adjacent environment. Various complications arising from the presence of conductive infrastructure (pipelines and other metallic materials) are eliminated by continuous monitoring. An example demonstrating potential moisture accumulation at the toe of the dam is presented below.
Note that either survey; i.e. Time 1 or Time 2, the moisture change at the toe of the dam is not easily detected. Only when the datasets are differenced (Data from Time 2 minus Data from Time 1) does the problem become apparent. 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. Continuous monitoring will provide a real-time image of changes occurring within the portions of the dam covered by Safety Net. Alternatively, an array of sensors can be installed and measurements made quarterly, or any other convenient time interval. Once an array of sensors has been installed, measurements can be made in a single day with a mobile crew. Continuous measurements can be made with a temporary or fixed monitoring system installed on site and operated autonomously or interactively through the internet.
Safety Net can be used exclusive of any piezometer wells, but, ideally, calibration against piezometer wells provides superior results. Safety Net affordably fills in the gaps between monitoring wells and provides temporal data between sampling times at the wells. No other system provides as comprehensive a dataset for dam site monitoring as Safety Net.
Bathymetry and Sub-bottom Profiling Surveys
HGI has experience conducting a variety of waterborne surveys, including electrical resistivity, bathymetry, and sub-bottom profiling. Waterborne or streamer electrical resistivity surveys are 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, monitor sediment loads and deposition over time, and characterize sub-bottom sediments.
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The example below shows the bathymetry results from a survey conducted in Patagonia Lake, AZ. The data can be collected rapidly and a number of instruments can be mounted and operated concurrently in a typical waterborne survey.
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Patagonia Lake Bathymetry
