Objectives of the monitoring
The preservation of water resources is a key issue in the construction of the new Lyon–Turin railway link. From the very beginning of the project, in the mid-1990s, Alpetunnel—the predecessor of TELT—carried out a comprehensive inventory of springs within a 2 km corridor around the base tunnel and subsequently initiated monitoring of some of them. This demonstrates the importance and seriousness with which this issue has always been addressed. Long-term spring monitoring makes it possible to detect any flow variations considered abnormal in light of the understanding of the spring’s functioning and the prevailing conditions at the time (rainfall, seasonality, etc.). A detailed interpretation, based on advanced hydrogeological expertise and supported by other monitoring parameters, makes it possible to determine whether underground works may or may not be responsible for an observed reduction in flow.
If TELT’s underground structures are proven to have an impact on a spring, TELT implements compensatory measures to restore the water use concerned (for example, connecting a new spring unaffected by the works to the drinking water supply network).
The key indicator
Flow rate
Although it is not the only parameter measured at springs, it is naturally the main indicator used in this monitoring programme. It is expressed in litres per minute, a unit better suited than litres per second for low-flow springs. Depending on the configuration of the water source, flow is measured using various methods: by bucket measurements, with a drainage basin, by pumping, and other techniques.
Partners
The consultancy firm commissioned by TELT: Géotec
Previously carried out directly by an employee of Alpetunnel, then LTF, and later TELT, this monitoring is now performed by technicians from the company commissioned by TELT.
Municipal staff
They accompany Géotec in the field, and the measurement results are recorded at the town hall.
Characteristics of the monitoring
Flow rate Temperature Conductivity
Temperature and conductivity provide indications about the origin of spring water and the pathways through which the springs are supplied.
Discover…
DHI, or the Index of the Probability of Interference/Impact of Works on Springs
Hydrogeological studies, initially conducted by the University of Savoie and subsequently continued and refined throughout the project, have made it possible to determine, for each of the springs originally inventoried, the probability that underground works could interfere with their functioning. This is reflected by the DHI index, which synthesises eight variables, including the permeability of the rock mass, the thickness of the rock cover, and the distance from the tunnel, among others.
TELT has incorporated into its monitoring network all water points identified as “at risk” according to these studies, namely all drinking water supply abstractions (AEP) considered in the study regardless of their DHI value, given their sensitive nature, as well as all other springs with a DHI ranging from low to high. The monitoring programme also includes several reference points for which the risk of influence is considered nil, but which allow for a better assessment of the natural evolution of groundwater resources.