Overview
It is highly desirable to localise and quantify water inflow zones in front of the tunnel face using indirect methods without any interruption of the excavation process. The spatial temperature field in the subsurface is influenced by water circulating in extended zones with a natural velocity of more than 10-8 m/s (~1 m/year). This allows to characterise the hydrogeological conditions by rock temperatures measured along the advancing tunnel. Such temperature represent a one-dimensional data set only, their interpretation cannot reveal the spatial characteristics of water bearing zones. Since for tunnelling only the water-bearing zones ahead of the advancing face are relevant, the following procedure is foreseen: First the rock temperature prediction along the planned tunnel trace is calculated with a 3D FE model based on heat conduction.
The accuracy of localising and specifying water bearing zones in front of the tunnel face by interpretation of temperature data will be evaluated under this project. The range of application and the limits of this method will be determined with respect to the needs of the tunnel excavation. The required sensitivity analysis will be carried out with standardized models using the three-dimensional hydraulic-thermal finite element method.
The evaluation methodology is made by three-dimensional finite element modelling with the program FRACTURE the GEOWATT AG.
Funding
Results
In tunnelling water bearing zones represent a high risk and often imply weak mechanical properties of the rock. Experiences during the excavation of the Lötschberg base tunnel showed clear indications of major water inflow zones through significant deviations of the measured rock temperatures from forecasted values.
During the advance of the investigation system for the Piora syncline of the Gotthard base tunnel even temperature deviations up to 20°C have been measured. Therefore comparisons between measured and forecasted rock temperatures in deep tunnelling may provide indications of water circulation systems in the rock.
In the frame of this research project it has been evaluated to what extend hydrogeological scenarios can be concluded from measured rock temperatures. In tunnelling, especially scenarios of water bearing zones intersecting the tunnel axis are of primary interest.
Therefore the method of prediction ahead with temperature data always assumes the measured rock temperatures being influenced by a water bearing zone ahead of the advancing face. Since the spatial structure and the permeability of a water bearing zone is not unique for a given rock temperature in a tunnel a significant number of zone models must be calculated for the interpretation of the measured temperature data. All zone models with a calculated temperature in the tunnel similar to the measured values represent possible hydrogeological scenarios ahead.
The evaluation has been carried out with the Finite Element Method (FEM) on the basis of an artificial mountain model with properties typically observed in reality (topography, pressure distribution, rock characteristics) including water bearing zones. This model was used to calculate the rock temperature along the tunnel axis which would been measured during a real advance. The hydrogeological properties of possible water bearing zones ahead have been forecasted on the basis of the rock temperature for three positions of a simulated advance. For each position the evaluation of the results led to a likelihood for initial inflow rates, initial pressures and water volumes to be expected ahead of the advancing face. The initial inflow rate corresponds to the value after the storage effect has decayed but before the initial water table has started to decrease.
Based on the theoretical evaluation provided in the project it can be concluded that the method “Indirect prediction ahead of water bearing zones with temperature data" may b