TDR Volumetric water content calculations of bentonite section M2

Heater no. 2 remained in operation during the dismantling of heater 1.

In section M2, nine out of the original ten Time Domain Reflectometry (TDR) devices are still working. After determining the water content and density parameters of the excavated bentonite in the laboratory, the remaining in-situ TDR's could be re-calibrated by this data.

In the graph below, the water saturation at the different TDR locations within the bentonite are shown for the 8 years of this experiment.

Buffer Saturation 1997-2005

The main results agree with the laboratory measurements made after the excavation of bentonite from section M1. Close to the heater, the volumetric water content stayed constant at about 20%. This corresponds to about 12% gravimetric water content at a density of 1.65 g/cm3. The same water content was determined in the bentonite blocks before emplacement.

Close to the rock/bentonite interface, up to 44% volumetric water content can be observed. This corresponds to a gravimetric water content of about 28% at a dry density of 1.50 g/cm3 and suggests almost full water saturation at this density. Clearly, the closer to the rock, the more rapid the water uptake by the bentonite.

The in-situ wet density and gravimetric water content of the bentonite blocks close to the TDR locations in section M1 were determined via laboratory analysis of the excavated bentonite. From this laboratory data, the volumetric water content and the dry density was calculated.

The results of these measurements in a horizontal profile are displayed below. The data from the laboratory analysis of the excavated bentonite yields similar data to those measured in-situ by the TDRs, which gives confidence in the evolution of volumetric water content determined by the TDRs.

Gravimetric Water content and density determined by laboratory analysis

The temperature profiles for the Febex bentonite show a uniform decrease from the sensors closest to the heater outwards towards the tunnel wall. The heater controls are set to provide a constant temperature of 100 ° C at the surface of the heater, this explains the rapid rise in temperatures at the beginning of the monitoring period.

This graph also highlights that the excavation of the first heater had a minimal effect on temperature conditions around heater no. 2. The large drop in tempearure was a result of failure of the electrical heater during the early part of 2003.

Evolution of temperature within the FEBEX bentonite and surrounding rock