Laboratory Column Experiments (FORGE)

In soil physics, Time Domain Reflectometry (TDR) is widely used to measure the volumetric water content in the unsaturated zone. Due to the robustness of the in-situ probes and the long-term stability of the measuring system the TDR technique is of particular value for long-term monitoring purposes.

Time Domain Reflectometry: Schematic sketch of the main components

The TDR-measuring principle is based on the propagation of guided electromagnetic waves along an electric transmission line (waveguide). The measuring system consists of two components, a Time-Domain-Reflectometer and an in-situ probe (see above). In its simplest form, the probe is built of two parallel electrodes, which may be terminated by well-defined impedance.

The system operates as follows: the TDR-device produces a voltage signal, which is usually a step function. The signal is sent to the probe by a coaxial cable and then guided as an electromagnetic wave along the electrodes. At the end of the probe the signal is reflected and travels back. The graph below shows the signal or so-called reflectogram. The travel time of the wave to the end of the electrodes and back to the injection point can be determined by calculating the temporal derivative of the reflectogram (see below).

Typical reflectogram of a TDR probe and (b) its derivative

Porosity and permittivities of the solid, liquid and gas phase can be determined by laboratory measurements. Consequently, the permittivity of the composite medium is a measure of the volumetric water content of the porous medium.

Some examples of results from in situ TDR measurement can be seen here - Full-scale High Level Waste Engineered Barriers (FEBEX) in situ results.

Laboratory Column Experiments (FORGE)

During the course of the GMT experiment numerous tests were performed to investigate the small-scale behaviour of the sand-bentonite backfill material (Fig. 1). In these experiments, methods were tested for the visualisation of flow-paths of water and gas, and new concepts developed to describe physically the phenomenology of water and gas transport and, in particular, to up-scale these parameters from the small laboratory scale to the large repository scale.

Figure 1: Columns used to visualise water flow-paths (a) and gas flow-paths (b) through sand/bentonite material

Laboratory Column Experiments (FORGE)

A mock-up experiment was also installed at GTS in June 2010 as part of the FORGE project. Its purpose is to simulate the cavern seal of a L/ILW cavern (i.e. with a cement backfill of the emplacement cavern and a sand-bentonite seal) on an intermediate scale and under well controlled boundary conditions.

A steel cylinder (Fig. 1) is filled with a compacted sand/bentonite (S/B) seal (brown/yellow area in Fig. 1) and two porous mortar disks at both sides (grey areas). To prevent preferential water and gas flow along the interface between the S/B body and the steel cylinder the outer ring space was sealed with pure bentonite powder (yellow area).

As shown in the experiment schedule, the S/B mixture will first be saturated with water, followed by gas injection tests to assess gas migration behaviour at a decimetre scale. At the end of the testing, the experiment will be dismantled in order to analyse in detail the cement-clay interfaces.

Figure 1: Drawing of the mock-up experiment with the steel cylinder confining the experiment, mortar discs at moth ends (grey/brown), the S/B seal (brown/yellow), the pure bentonite (yellow) and the ring-TDRs (black). Instruments are schematically indicated to show the locations of the monitoring levels.

Experiment Schedule

Impressions from Emplacement:

Laboratory Column Experiments (FORGE)

The large column experiment was implemented as part of the GMT experiment. The objective was to provide a medium-scale experiment to calibrate the TDR probes (link to TDR Measurement Principle) applied in the GMT in-situ experiment. The sand-bentonite (S/B) mixture (80% sand and 20% Japanese Kunigel) was emplaced with the same density and with the same initial saturation as the GMT experiment. The experiment is running since 2002 and thus provides one of the longest records of sand-bentonite behaviour during the saturation phase on a decimeter scale.

The experiment now continues within the framework of the FORGE project with the main aim to investigate the gas entry pressure into fully saturated S/B. The main steps of the experiment are listed in the table below.

Laboratory Column Experiments (FORGE)