Humid air as a driver of compaction creep in granular salt backfill: How a little water makes a big difference

Understanding how granular salt compacts is essential for estimating the timescales required to seal backfilled galleries and shafts in radioactive waste repositories hosted in rock salt. It is well established that the presence of brine, even as thin liquid films on grain surfaces, strongly accelerates compaction by enabling fluid-assisted grain boundary diffusion, or pressure solution. For this reason, the addition of liquid brine to salt backfill in radioactive waste repositories hosted in rock salt has been considered as a means to accelerate compaction and reduce sealing timescales. However, the presence of liquid brine also enhances corrosion of the waste canister and promotes gas generation, which has therefore been used as an argument against the intentional addition of brine.

Previous studies on salt-moisture interaction show that adsorbed fluid films retain liquid-like properties down to relative humidities of at least 40%, suggesting that pressure solution can operate well below full saturation. However, compaction behavior at relative humidities below the deliquescence point of salt (about 75% RH) remains poorly constrained. 

We present compaction experiments on fine-grained sodium chloride conducted under controlled relative humidities between 53% and 73%. Under these conditions, pressure solution is the dominant compaction mechanism, although compaction rates are 2-4 orders of magnitude lower than when pore spaces are fully saturated with brine. Nevertheless, our results demonstrate that humid air alone significantly accelerates the compaction of granular salt compared to dry conditions, with pressure-solution creep rates increasing systematically with relative humidity. These findings suggest that, in repository backfill, moisture from the surrounding host rock or from ventilation systems may be sufficient to induce pressure-solution-controlled compaction, even in the absence of intentionally added liquid brine or brine inflow from the host rock. Interaction between granular salt backfill and humid air therefore plays a key role in backfill evolution, with important implications for compaction rates, sealing timescales and the long-term containment of radioactive waste.