Abstract The influence of inorganic amendments on the structural evolution of a high clay soil used for trench backfill was studied in a simulated trench. The structure of an untreated backfill was compared to the structure of three other backfills treated with inorganic amendments (limestone, gypsum and basic aluminium polychloride). The treated backfills showed only minor structural evolution after the first wetting cycle whereas structure in the untreated backfill continued to evolue over 8 wetting and draining cycles. Among the mechanisms involved to explain the differences in structure, rheological properties appeared as having a high significance. Treated soil had higher void volume and higher air content apparently because the applied chemicals reduced hydration of the clods.