Geological sounding and hydroelectricity
The solidity of the infrastructure is one of the sine qua non technical conditions for making it safe. Since 2017, geotechnical and geological studies have been carried out by specialist firms, enabling the project’s APS and APD to be developed. The technical team is continuing to build up its knowledge to ensure that it has all the scientific data it needs prior to construction.
Our Senior Project Engineer Felix B will be talking to you about the details of this highly specialised exercise, in particular the geological survey.
What is the relationship between the hydroelectric facility and the geological survey?
Geological boreholes enable underground investigations to be carried out (analysis of extracted soil, special soil resistance tests once the borehole has been drilled, etc.), providing greater knowledge of the composition of the soil and its overall resistance. This knowledge is key for hydroelectric installations, enabling validation of the positioning and design of (i) civil engineering structures, first and foremost the dam, whose stability depends to a large extent on the quality of the soil on which it will be built, and (ii) the underground components of hydroelectric projects, where appropriate.
Precise knowledge of the soil is therefore essential to ensure the feasibility of hydroelectric projects under satisfactory safety conditions: good quality rock (rock with few fractures, homogeneous, etc.) guarantees the durability of civil engineering structures (bridge, dam) and underground galleries; conversely, poor quality soil would make it difficult to install structures, or even threaten the feasibility of the project.
What are the technical steps involved in carrying out a geological survey?
The first step is to find the right location for the geological boreholes, including, for example, the banks and bed of the river at the level of civil engineering structures, and the strategic points of underground galleries (entrances, middle of tunnels and cavern intended to house the plant).
The second stage involves positioning the drill rig correctly on the surface, so that the borehole reaches the desired location in the ground depending on the characteristics of the borehole (desired depth, inclination, etc.). It is also necessary to guarantee access to clear water to allow clean and efficient drilling.
The third stage consists of drilling the soil in accordance with the desired characteristics of the borehole. The speed of a borehole changes according to the nature of the soil, being inversely proportional to its hardness. For example, in the case of Volobe, the last borehole drilled had an average rate of 10 metres per day.
In core drilling, the soil extracted during the drilling process is then preserved in the form of rock cylinders, known as cores, enabling direct analysis of the composition of the soil investigated. Once the borehole has been completed, special calibrated tests can be carried out to determine the precise strength of the rock mass in which the borehole is inserted.
The second stage involves positioning the drill rig correctly on the surface, so that the borehole reaches the desired location in the ground depending on the characteristics of the borehole (desired depth, inclination, etc.). It is also necessary to guarantee access to clear water to allow clean and efficient drilling.
The third stage consists of drilling the soil in accordance with the desired characteristics of the borehole. The speed of a borehole changes according to the nature of the soil, being inversely proportional to its hardness. For example, in the case of Volobe, the last borehole drilled had an average rate of 10 metres per day.
In core drilling, the soil extracted during the drilling process is then preserved in the form of rock cylinders, known as cores, enabling direct analysis of the composition of the soil investigated. Once the borehole has been completed, special calibrated tests can be carried out to determine the precise strength of the rock mass in which the borehole is inserted.
In the case of the volobe project, are the results of the survey conclusive?
As a reminder, the Volobe Amont project comprises six kilometres of underground galleries that will (i) convey water to the hydroelectric plant located in an underground cavern, and (ii) return the water to the natural riverbed. The final borehole was drilled in the second half of 2024 to identify the precise location of the cavern that will house the hydroelectric plant, to ensure that it can withstand the stresses associated with hydroelectric power generation. A 200m-long borehole was drilled, inclined at 20° in order to capture any vertical faults in the rock mass and reaching a depth of 33 metres below sea level, or 187m below ground level. The results of the survey were conclusive, confirming the very good quality of the rock mass that will house the hydroelectric plant, with healthy rock that has very few fractures and is fairly homogeneous.