A study of the hydro-mechanical behaviour of compacted crushed argillite

The argillite extracted from Bure site (France) is proposed, after being crushed and compacted, as a possible sealing and backfill material in the French geological high-level radioactive waste disposal. In this study, the effects of the grain size distribution and the microstructure on the hydro-mechanical behaviour of the compacted crushed argillite have been investigated. The volume change properties were investigated by running one-dimensional compression tests under constant water content (2.4–2.8%) with loading–unloading cycles. Under various vertical stresses, water flooding tests were carried out under constant–volume condition. Depending on the vertical stress level, either swelling or collapse behaviour was observed in the sense that vertical stress increased or decreased upon flooding respectively. A clear effect of grain size distribution has been also identified: finer samples exhibit stiffer compression behaviour and higher swelling potential. To provide a microstructure insight into the macroscopic behaviour feature observed, both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) observations were performed, evidencing that: (i) at the same dry density, the size of inter-aggregate pores is larger for the coarser crushed material; (ii) mechanical compression only reduces the inter-aggregate porosity in the stress range considered; (iii) the micro-mechanisms governing the flooding under constant–volume condition include the swelling of the clay particles, the increase of the intra-aggregate pores and the collapse of the inter-aggregate pores. The results show a strong effect of the grain size distribution on the hydro-mechanical behaviour and thus the close link between the microstructure and the hydro-mechanical behaviour.

Research Highlights
► The compression curve is strongly dependent on the grain size distribution.
► The finer soil exhibited higher swelling potential when water flooded.
► The soil volume changes during compaction was mainly due to the decrease of the sizes of inter-aggregate pores.
► Water flooding under constant–volume conditions also decreased the inter-aggregate pores.
► Water flooding separated the clay particles from the aggregates.