Experimental investigations of piping phenomena in bentonite based buffer material

Document available in extended abstract form only. Formation of channels in a clay based buffer material is often referred to as 'piping'. Piping is likely to occur in bentonite based buffer materials in a fractured host rock during the early evolution of the repository when strong hydraulic gradients are present. After water saturation of the repository and reestablishment of the hydraulic gradients piping will not be an issue. However, piping in the early phase may still have implications for long-term performance: 1. if the pipes fail to close there may be remaining conductive pathways in the engineered barrier, and 2. piping may lead to erosion or redistribution of material which needs to be taken into account in the long-term performance assessment. This means that the piping process may affect requirements on rock characterization, water inflow and water management during the installation phase, buffer material properties and buffer installation methodology. As a part of the 'Bentonite re-saturation' program, RWMC has initiated and performed studies of the piping process. The main objectives of the studies are to answer: 1. Under what conditions can pipes form? 2. How do pipes evolve with time? 3. When and how do pipes close/reseal? 4. How does piping affect the buffer properties? 5. How much mass can be lost by erosion? The answers will be used in the development of the requirements stated above as well as input to long term performance assessments. overview of the experiment Test apparatuses were manufactured for investigation of the piping phenomena, see Figure 1. The apparatuses have drainage gutter to prevent clogging to take place with eroded material, and to keep an advection field around specimens. There is also a storage chamber for eroded material on the apparatuses. In the investigation, specimens of bentonite block and pellets were used. The block specimen consisted of a mixture of Japanese Na type bentonite, termed Kunigel V1, and 30 wt% silica sand. In this test, there was a gap between the specimen and the acrylic cell. The water or saline water (0.5 M) was supplied from the bottom and the flow was along the gap between the specimen and the cell wall. The initial bulk dry density of the block was 1.936 Mg/m3. After swelling and sealing of the gap the final dry density of the specimen was 1.6 Mg/m3.The individual pellets were made from pure Kunigel V1. In the tests, the test cell was filled with pellets, either in the form of large pellets only or a mixture of large and small pellets. The bulk dry density of large pellets only was approximately 1.1 Mg/m3, and that of the mixture of large and small pellets approximately 1.5 and 1.57 Mg/m3. Initially, there are large voids in the pellets specimens. The water or saline water was supplied from the bottom, and the inflow passes through the large voids selectively. The maximum inflow rate was set 0.1 l/min based on the requirements on the water inflow to a disposal hole from SKB. During the supply of water, the formation of water channels was observed and injection pressure was measured. Moreover, after the tests, the total weight of eroded material was measured. As results, in the both cases of block and pellets, the formation and convergence of water channels was observed. In the early stage of the test, several small water channels were generated between the specimen and cell. However, during the test, the small water channels converged into one single channel. The inflow rate when the pipes converged increased with decreasing of the number of pipes. Therefore, as the pipes converge, the erosion becomes larger than swelling and the thickness of the pipe becomes larger. This tendency was the same in both the pellets and the block specimen cases. However, in case of the block specimen, accumulation of sand in the pipes was observed. The example of the transition of the water channel due to swelling and erosion is shown in Figure 2. In this case, the specimen constituted of a mixture of large and small pellets. The bulk dry density was 1.57 Mg/m3. The inflow rate was initially 0.1 l/min. In this case, after the convergence of water channels, inflow rate was stepwise decreased to 0.001 l/min. It was found that the width of the water channel became smaller due to the swelling behavior with a decreasing inflow rate. In the case of 0.001 l/min of inflow rate, supply pressure was increased with time and water channel was finally completely sealed by swelling. The project is still ongoing and more tests will be performed. However, some preliminary conclusions can still be drawn: - Piping will occur if there is an inflow of water and voids in the system. This has to be considered in the design and engineering as well as in the long-term assessment. - If several pipes are formed initially, they seem to converge with time and form one single channel, at least for the scale that has been studied. - Pipes can be self-sealed, but only if the inflow rate is very low. - There are indications of accumulation of sand in the pipes for sand bentonite mixtures. This may have impact on the resealing ability. (authors)