Advancements in geophysics on dams1 July 2022
Dr Jo Hamlyn, Senior Project Geophysicist, and Christian Bird, Technical Manager at TerrDat, explain how integrated geophysical surveys can help answer important questions about dam and spillway condition.
Thirty years ago, geophysical survey company TerraDat helped pioneer the early use of geophysics for site investigation by applying modern technological advancements in geophysics to environmental and engineering problems.
Today, TerraDat remains a dynamic and innovative company providing high-quality, comprehensive and detailed geophysical surveys to the geoengineering sector. Drawing on extensive experience in engineering and environmental geophysics, the company has developed an industry-specific suite of geophysical techniques to investigate the composition and conditions of critical water supply infrastructure such as dams and spillways. These surveys have aided operators across the UK to maintain these critical structures well into and beyond their design life, a challenge that the pressures of climate change will intensify.
The appropriate use of geophysics as an investigative tool provides detailed subsurface information at high spatial resolution at minimal environmental impact and cost. It is vital that suitable techniques are selected and that the resulting data interpretation is well-informed and detailed, as inappropriate techniques and vague interpretations may lead to false assurances about a structure's condition. The correct choice of method(s) will rely on knowledge of several factors such as the structure's design, construction materials, condition, buried structures/utilities and surrounding geology.
Reservoirs and associated structures vary significantly in their design and construction, and it is unwise to propose or rely on a single geophysical methodology to fit all dam and spillway investigations. An integrated geophysical survey measuring several different physical properties will allow for a holistic understanding of reservoir structures. This article presents two case studies that illustrate how integrated geophysical surveys helped answer important questions about dam and spillway condition.
Case study 1: Do leaks exist across the dam, and if so, where are they located?
A geophysical survey was conducted to investigate seepage through a Victorian embankment dam with a clay core faced with local stone. A side-channel spillway is located at the southern end of the embankment, and this comprises a granite masonry weir and water ladder that flows into the original watercourse. There is an engineered bank adjacent to the spillway.
Geophysically, identifying leaks and seepages at embankments relies on deducing zones of comparably wetter material, either at the surface or depth. Electromagnetic ground conductivity (EM) surveys map variations in the bulk electrical conductivity of near-surface materials. When applied to embankments, a localised increase in conductivity values will typically represent a relative increase in clay content or saturation.
Electrical Resistivity Tomography (ERT) generates subsurface geoelectrical cross-sections by mapping the apparent resistivity of the ground and forward modelling (inverting) a scenario likely to cause the measured differences in resistivity. The resulting sections can indicate subsurface geological or engineered layers; localised changes within these layers relate to localised changes that can be analysed and interpreted to provide information on the structure's condition. Localised zones of reduced resistivity will typically highlight moisture ingress or seepage, as water will typically reduce the subsurface's resistivity. Localised increased resistivity generally indicates the presence of dry granular material, possibly associated with loss of fines.
EM data were acquired along 2m spaced parallel survey profiles across the downstream shoulder, as far as the spillway's edge. Six ERT profiles were collected across the downstream shoulder and alongside the spillway. The example (Plate 1) shows the results of the EM survey and the ERT profile adjacent to the northeast side of the spillway.
The EM plot shows two characteristic response types: across the dam, the ground is highly conductive, representing shallow clay-rich materials used in the shallow construction of the embankment. There does not appear to be any localised anomalous zones across the front of the dam, suggesting there are no leakages in this area. The engineered bank adjacent to the spillway comprises less conductive material indicating the presence of dry/granular material. This material is assumed to be relatively homogeneous, and therefore the localised reductions in resistivity observed are considered to be caused by localised increases in moisture.
The ERT profile presented in Plate 1 was located along the northern edge of the spillway to investigate the shallow area of increased electrical conductivity identified by the EM survey. The ERT has mapped resistive values at the surface derived from the granular material forming the engineered bank. This resistive upper layer contains two significant incursions of decreased resistivity, which likely show moisture ingress. Feature A is a narrow, sub-vertical zone of low resistivity that dips towards the dam; this is thought to represent moisture ingress from the spillway into the adjacent bank and is likely hydrologically connected with the broad, shallow zone of low resistivity identified at the surface. Feature B is a zone of very low resistivity related to an observed seepage identified during a dam inspection. At depth, the ERT section indicates the presence of a broad area of decreased resistivity, likely to be associated with an increased clay content within the dam or bedrock beneath the spillway.
At this site, the combination of EM and ERT techniques proved to be a very effective way of non-intrusively investigating the shallow subsurface of the dam for increased moisture content. Seepages were observed in both plan and cross-section, allowing engineers to design appropriate intrusive investigations and remediation strategies.
Case study 2: What is the current condition of the spillway?
A geophysical survey was commissioned by Hafren Dyfrdwy at Pen-y-Gwely reservoir to investigate the condition of the spillway with respect to the presence of voiding and seepages. The spillway comprises a stoned lined curved weir, water ladder and byewash channel that joins the original watercourse at the toe of the dam.
Ground Penetrating Radar (GPR) was used to provide detailed cross-sectional images, and depth plans to identify anomalous areas of high-amplitude GPR response (that may be related to voiding), the presence of engineered structures or sub-surface boundaries. The flat areas of the spillway were surveyed using an Utsi TriVue radar system at a 0.5 m spacing, whereas each step of the water ladder was surveyed using an 800MHz Mala system. Electrical Resistivity Tomography (ERT) was used to investigate the existence of seepages on either side of the spillway. ERT profiles were acquired using a Wenner-Schlumberger array with 1 m spaced electrodes. Data examples are presented in Plate 2.
Based on historical drawings provided by the client, the water ladder steps are thought to comprise two masonry blocks vertically stacked on top of each other. These are positioned against the next step with a slight vertical offset. The masonry is embedded into a layer of Lias Lime concrete with a variable thickness. GPR data exhibitng anomalous responses within or below the masonry blocks (such as high amplitude zones, diffraction hyperbola or broken reflectors) may suggest damage to the structure and the potential for voiding.
The example radargrams, shown in Plate 2, present data collected on the water ladder. The joints between the masonry blocks are observed as shallow diffractions, and the underlying concrete causes the planar high amplitude reflector. Deep diffraction hyperbolae, increased signal amplitude and short or broken reflectors may be indicative of structural defects within the underlying concrete. These are highlighted in blue in radargrams R1 and R2.
At about 2m BGL, the ERT profile has measured resistivities between 100 and 400 Ohm.m; these values are typical of mudstones, which are known to underly the dam and spillway. Above this layer, the ground is more resistive. The increased resistivity values are likely to represent granular engineered ground adjacent to the spillway. However, this material contains two anomalous zones of decreased resistivity. The engineered material is assumed to be relatively homogenous; therefore, decreased resistivity is likely to result from increased moisture.
The combination of GPR and ERT techniques has proven to be an effective way of non-intrusively investigating the spillway at a high spatial resolution. Accurate locations of possible voids, structural defects/features and areas of seepage not visible from the surface were provided to the engineers assisting them in designing appropriate intrusive investigations and remediation strategies.
Static geophysical surveys have repeatedly proved advantageous in assessing and identifying structural defects across dams and spillways. However, there is growing interest within the water supply industry to provide innovative, near-real-time monitoring of this critical infrastructure. In response, SPiVolt has been developed by TerraDat to provide a reliable leakage monitoring system by combining a detailed understanding of Self-Potential (SP) signals with novel processing steps and modern telemetry. The SP technique is widely used in academia to study groundwater movement, and the SPiVolt system transfers this extensive academic foundation to real-world, commercial applications.
The equipment required to measure SP signals is lightweight and installed just below the ground surface; therefore, the system can be installed manually with minimal environmental impact and at a low cost. The physical system uploads data in near-real-time to the SPiVolt data portal, which also displays the static geophysical survey data for the site (acquired as standard before SpiVolt installation). In order to help understand the condition of the structures and how they change through time, the SPiVolt portal also displays satellite-based ground deformation data supplied by SatSense Ltd. Therefore; it is possible to not only observe seepages and changes in seepage rate related to weather events and water level changes but also any deformation of the surface as well.
Previously, seepages/leakages would have to be investigated intrusively or monitored by borehole instrumentation requiring extensive excavation and minimal site coverage. The SPiVolt system reduces the need for intrusive investigation as leaks can be quantified and monitored over time. SPiVolt is also useful in environments where the movement of subsurface water is of concern, such as on unstable slopes prone to landslides and within flood embankments. Plate 3 shows a schematic of the SPiVolt system and the generation of the SP phenomenon within the subsurface. The figure also shows a dam following installation and an example of data within the data portal.