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Engineered barrier emplacement experiment in opalinus clay

Deliverables

Manufactured GBM will be emplaced remotely in any waste tunnel. Emplacement procedures are needed that result in the desired packing density without major gaps due to bridging between the canisters, rails and tunnel walls. Gaps should also not exist at the tunnel top. The technique should provide consistent emplaced density quality control. The feasibility of different GBM transport and emplacement methods (pneumatic, conveyor belt, spiral or screw conveyor) will be tested in a to-scale model using mixtures with different properties (grain size, water content or others). The model will include as an inset, forms that represent the underlying bentonite blocks and the dummy canister. Sampling results from each emplacement methods and material will provide guidelines as to how to refine the technical parameters of the machinery used for the different emplacement techniques (e.g. dimensions of blow pipe, auger size, air pressure and flow rate for pneumatics). As well, the sampling characterisation will identify the achievable emplacement density and help determine the reliability and consistency of each technique. Based on the backfill sample characterisation, the equipment performance and handling, the preferred emplacement technique and material properties will be identified for the emplacement to be undertaken at Mt. Terri in the EB tunnel.
The experiment will be monitored during the whole operational phase, collecting the information provided by the different instrumentation systems: standard geomechanical (pore pressure, total pressure, humidity, displacement, etc.), seismic sensors and geo-electric measurements. All these measurements will provide very valuable information about the evolution of HM processes in different parts of the buffer and the surrounding rock. The obtained data base will be a key point both to analyse and understand the complex interaction of the bentonite buffer and the host rock in response to the artificial hydration considering the salinity of the formation water, and to validate numerical models of coupled HM processes.
The preparation of the detailed design of the experiment, also called "Test Plan", requires carrying out several technical discussions, basic engineering studies and calculations. In principle it has only a direct application to the present project, but the provided information as well as the gained experience/knowledge for this document development could be an input or background to the project partners for other similar experiments.
The most significant expected result is a validated model of coupled HM processes in an engineered barrier system composed of compacted bentonite blocks at the bottom, and pellets-based backfill at the top, in a real scale horizontal drift excavated in the Opalinus clay formation, with adjusted parameters and including the EDZ effect. Some new developments and modifications of the numerical model are necessary in order to take into account the special characteristics of the backfill material. It is interesting to note that they affect mainly the details of the model but not its central core. This basic model provides a robust nucleus with the sufficient flexibility to incorporate changes as required by the contrast with reality. It can be stated that, at present, a THM model of the experiment with a very high degree of demonstrated reliability is available. Nevertheless, the following areas are envisaged as worthy of attention in the future: Incorporation of the information coming from the new laboratory testing programme in the mechanical and hydraulic constitutive laws; Modification of the constitutive laws approach to incorporate the effects of the double structure of the pellets-based backfill. At mid term the "Scoping calculations" report has been issued.
A report on the geophysical characterisation of the excavation-damaged zone (EDZ) around the test section will state: -The applied methods. - The effort (financial and technical) for each method. - The achieved information from each method. - The relevance of the results for the characterisation of the EDZ in the Opalinus clay, which may be very important in the overall performance of the engineered barrier system. Hydrogeological characteristics and geophysical characteristics will be jointly analysed and compared to increase the confidence into geophysical measurements as effective methods to determine the hydraulically important EDZ. The result will enable better design and optimisation of necessary future measurements to characterise the EDZ based on the experience summarised in this report. Current status: Projected summarising publication after the termination of the project. One Diploma-theses on geo-electrical measurements will be finished in the first half of 2002 at the Technical University of Berlin.
The design and successful emplacement of the GBM in the EB tunnel at Mt. Terri is a principal objective of this project. Once the compacted bentonite blocks and dummy canister have been placed in the drift, the remaining drift volume will be filled with the GBM produced (T22) using the preferred backfill technique identified in the mocked up scale model (T23). A preliminary installation document for the emplacement will be prepared based on the results of T22 and T23. This will be followed by the equipment and material mobilisation to the site. Five working days are estimated as the time needed for emplacement. During this time there will be extensive onsite sampling and characterisation as will as complete records made of the activity with extensive photo documentation. The final volumes and emplacement densities will be determined all within the context of a previously defined QA procedure. The interfaces needed to minimise the time between the final filling and the completion of the tunnel outer concrete plug completion will be established in advance. The final product of this activity will be a complete record of the emplacement with sampling results and site activity documentation. If possible, a video of the emplacement may be prepared.
The "EB Experiment" aims to the "in situ" demonstration of an emplacement technique in horizontal drifts in consolidated clay formations, using pellets as backfill material in the upper part of the clay barrier, and bentonite blocks at the bottom. The full-scale test will be carried out in a drift specially excavated in the Opalinus clay formation at the Mont Terri underground laboratory. A dummy canister, with the same dimensions and weight as the reference canister, will be placed on top of a bed of bentonite blocks, the upper part of the drift being backfilled with a material made basically of bentonite pellets. The drift will be sealed with a concrete plug. The obtained experience/knowledge during these operations will serve to asses or to modify actual concepts: one of the options being considered for repositories in horizontal drifts is to perform the backfill operation (or at least a part of it) using bentonite pellets. This has in principle many advantages in terms of easier handling and emplacement when compared to the use of large bentonite blocks as it makes also easier an automated or teleoperated backfill operation.
A report will present and analyse the results of repetitive measurements to identify the changes in the geophysical and hydrogeological characteristics of the EDZ due to the interaction of the buffer material, saturation fluid and the host rock. The results will serve as a basis for the development of future seal and backfill systems. This information is important especially for waste disposals in argillaceous rocks but also of interest for other host rocks. Especially the development of the EDZ with time will be of interest for comparison of different host rocks. Current status: Projected summarising publication after the termination of the project.
In order to improve the knowledge of HM behaviour of pellet-based backfill, a laboratory test programme has been included in the project. The objectives of the tests are: - Definition of the most relevant properties in the hydraulic behaviour of bentonite barrier: retention curves and permeability to water of the backfill under different densities, and saturation degrees. - Definition of the mechanical behaviour of the pellet-based backfill. Soaking under load, swelling pressure and suction controlled oedometer tests will be carried out. - Study of the coupling between the hydraulic and the mechanical behaviour. New laboratory techniques (new devices and new test procedures) will be developed in order to obtain the water retention curve at controlled dry density and to evaluate the swelling pressure of the pellet-based backfill.
The entire buffer should be saturated in about six months. This demonstration is too short for natural water flow from the Opalinus Clay to do this so artificial hydration is needed. However, it has to be done so that it does not provide 'noise' or artefacts in the processes being monitored. Given the buffer dimensions and to simulate conditions of natural saturation, this system must have as its basis, injection from the tunnel walls along the buffer/rock contact. Injection along the canister surface and from within the buffer will also be evaluated. Scoping calculations will help evaluate the hydration's alternative design and operational concepts and conditions. The water used for the injection will have to be synthetic as it should have a chemical composition similar/equivalent to the Opalinus clay formation water. The high salinity of this water, and its high corrosion potential, must also be taken into account in the selection of the component materials used for the hydration system. In addition to being corrosion resistant, the materials used in the system must be strong enough to continue to provide pathways for water to enter the buffer even after it has begun to swell in the injection point areas. The results are needed to further understand how to be able to reduce the time scaling of processes in these low permeability environments without creating artefacts tied directly to the process being accelerated.
The proposed engineered barrier consists of pre-compacted bentonite blocks in the bottom 20 to 25 percent of the tunnel with GBM filling the remaining voids. This bentonite mixture (pellets, granular and powder) should have high dry densities (>2g/cm3) in the production stage for the pellet sized material in order to arrive at emplacement densities approaching 1.5g/cm3. Defining how to achieve a product grain size distribution for emplacement that is close to the Fuller curve (defines best packing and density) is a main objective of this work along with a complete characterisation of the produced product. The bentonite raw material needs pre-processing (e.g. drying to a specific water content) so that pellets, when prepared can have as high as possible a dry density. Next, the dried material itself is fed into a specially designed roll press. This process ensures that the desired material density is achieved and maximises the amount of useable product material. Tests will be carried out to optimise the initial water content, specific compaction pressure for the roll press and final densities. After the pellets are produced, they will be ground, milled and sorted with screens to receive the desired grain size distribution to allow as dense as possible packing for emplacement.
The "EB Experiment" will be instrumented and monitored for a period of about one-year. A large variety of sensing systems is planned to be used for the monitoring of the H-M processes in the backfill and the rock. Instruments measuring the most relevant parameters, such as humidity, swelling pressure, pore pressure, and displacements will be installed within the buffer and in the surrounding rock. The good performance of those instruments will depend on their adequate selection and construction given the expected environmental conditions: high corrosive media because of the salty water that will be used for the saturation (similar to the Opalinus clay formation water) and the swelling pressure of the backfill. The final result will be the evaluation of the sensor and data acquisition systems reliability in a quasi-realistic environment after a period of at least one-year under these aggressive conditions. The gained experience and acknowledge could be used to suggest modifications and improvements for future tests and monitoring in real repositories.