Confinement as a key but overlooked factor controlling erosion rate in loess pipes and tunnels

Approximately 10 % of the Earth's land surface is covered by loess. The ability of loess to maintain vertical cliffs indicates its considerable strength, but at the same time loess is susceptible to rapid erosion. The erosion mechanisms of undisturbed loess in pipes, tunnels, or gullies are poorly understood due to the rapid development of these features during heavy rains. We developed and tested a confinement technique to simulate erosion in the deeper loess profile to explain erosion and stabilization mechanisms in loess pipes, tunnels, and deep gullies. Our loess blocks encased in concrete exhibit erosion rates and progressions similar to those in natural loess under field conditions. This approach thus allows simulation of pipe erosion on decametric blocks in controlled laboratory conditions, providing insights that can be directly applied to understanding erosion mechanisms in the field. The experiments revealed a dual behavior of loess controlled by confinement: unconfined loess blocks disintegrate rapidly by air slaking when exposed to even a small trickle of water, while loess confined by the surrounding soil mass maintains its original structure and resists erosion by flowing water. Thus, confinement serves as an important stabilizing mechanism that significantly increases the erosion resistance of loess by restricting its expansion, a prerequisite for loess disintegration. Pre-existing holes <8 cm in diameter tend to resist expansion, while larger holes or fractures induce roof instability, leading to collapse and rapid expansion. This mechanism explains the frequent development of loess tunnels from animal burrows, along fractures or other inhomogeneities. • unconfined loess erodes orders of magnitude faster than loess confined by overburden • confinement is a key mechanism reducing erosion in loess pipes and tunnels • aboratory concrete confinement effectively simulates field loess erosion • openings <8 cm in diameter expand very slowly in deep loess • preexisting holes or fractures expand rapidly into pipes by tensile failures