MEASUREMENTS OF WATER POTENTIAL AND WATER-CONTENT IN UNSATURATED CRYSTALLINE ROCK

A water desaturation zone develops around a tunnel in water-saturated rock when the evaporative water loss at the rock surface is larger tha n the water flow from the surrounding saturated region of restricted p ermeability. We describe the methods with which such water desaturatio n processes in rock materials can be quantified. The water retention c haracteristic theta(psi) of crystalline rock samples was determined wi th a pressure membrane apparatus. The negative water potential, identi cal to the capillary pressure, psi, below the tensiometric range (psi < -0.1 MPa) can be measured with thermocouple psychrometers (TP), and the volumetric water contents, theta, by means of time domain reflecto metry (TDR). These standard methods were adapted for measuring the wat er status in a macroscopically unfissured granodiorite with a total po rosity of approximately 0.01. The measured water retention curve of gr anodiorite samples from the Grimsel test site (central Switzerland) ex hibits a shape which is typical for bimodal pore size distributions. T he measured bimodality is probably an artifact of a large surface rati o of solid/voids. The thermocouples were installed without a metallic screen using the cavity drilled into the granodiorite as a measuring c hamber. The water potentials observed in a cylindrical granodiorite mo nolith ranged between -0.1 and -3.0 MPa; those near the wall in a vent ilated tunnel between -0.1 and -2.2 MPa. Two types of three-rod TDR Pr obes were used, one as a depth probe inserted into the rock, the other as a surface probe using three copper stripes attached to the surface for detecting water content changes in the rock-to-air boundary. The TDR signal was smoothed with a low-pass filter, and the signal length determined based on the first derivative of the trace. Despite the low porosity of crystalline rock these standard methods are applicable to describe the unsaturated zone in solid rock and may also be used in o ther consolidated materials such as concrete.