Challenge

The Comoé dam in south-western Burkina Faso serves the major population centre of Banfora. It also provides water for irrigation and a large sugar factory, and controls flooding on the River Comoé. With concerns about the impact of leaks within the dam basement on structural longevity, the Ministry of Water and AGETEER (Water and Rural Infrastructure Implementation Agency) commissioned Fugro to investigate.

We combined a number of sophisticated geophysical techniques to efficiently survey all areas of interest without disturbing the sensitive structure. Using existing Geo-data to help corroborate findings, we accurately located and characterised leaks inside the dam to support timely decision-making and implementation of remedial works to safeguard the dam.

Solution

At 1164 m long, 24 m high and 6 m wide at its crest, the Comoé dam is classified as ‘large’ by the International Commission on Large Dams. Some leakage through its embankment had been evident since the reservoir was first filled in 1991, due to uncontrolled water percolation through porous laterite in the reservoir’s spillway foundations. Over time, pools began to appear downstream and progressive erosion was observed at the base of the dam.

Concerned about long-term viability, and given the dam’s size and regional importance, the Comoé dam safety study was commissioned, which showed that the best and most cost-effective solution would be a steel pile wall extending down from the crest of the embankment to the solid substratum, where it would be anchored. As well as creating a leak-tight seal up the entire height of the dam, it would prevent further erosion inside the body of the embankment and foundations. With a need for highly accurate data to guide critical repairs, AGETEER then engaged Fugro to map the leaks which had appeared at the base of the embankment.

Our team conducted an overall survey of the dam, recording high-efficiency spontaneous polarisation (SP) and other electrical measurements. Once we had acquired all the necessary data, we then focused our attention on the left bank, which showed marked signs of leakage. Here, our field investigation included brine tracing and resistivity tomography, and the use of SP to detect anomalous flows.

We also cross-checked our temperature measurements from piezometers and relief wells against geophysical survey results. Analysis of our geophysical measurements met the challenge of identifying and delineating the circulation of upstream-downstream flows on the left and right banks of the dam.

Innovative highlight

Our phased and integrated geophysical investigation allowed us to add project value while meeting our client’s programme budget and schedule. We used advanced, non-intrusive, light-footprint electrical sensing techniques to detect and define zones of leakage, and then applied our interpretative skills to integrate existing survey data, deriving additional insight and detail.

The result was a highly accurate risk map of leakage paths and direction of flow within the affected areas of the dam embankment and base. This map helped our client to verify and optimise the proposed remediation strategy using a steel pile wall identified by the Comoé dam safety study.

Impact

Our geophysical survey results, combined with hydrogeological, geological and geotechnical data from previous surveys, made it possible to assess the precise extent of remedial works required for the Comoé dam. The use of light-footprint techniques overcame access issues and eliminated the physical impacts of investigating potentially weakened structures.

Our high-definition leak-mapping maximised opportunities for the client to optimise the design, cost and timescale of effective remediation works. Our Geo-data also supported accurate decisions in the design and planning of the remedial works, helping to ensure the long-term safety and operational resilience of the Comoé dam, an asset critical to the welfare and livelihoods of a large population.