A testing procedure was proposed to study water movement in compacted bentonite and the development of swelling pressure (ps) when compacted bentonite specimens were wetted. In this procedure, a multi-ring mold was introduced for ps measurements, after which the specimen was sliced for X-ray diffraction to find movement of water in the interlayer space of montmorillonite. Results revealed a relation between four phases of ps development and evolution of four states of interlayer water molecule arrangement of montmorillonite (L): when ps reached its first peak in phase I, L moved from 1 row water arrangement (1w) to at least 2w; when ps decreased and re-increased in phases II or III, L moved from 2w to at least 3w; and when ps reached a steady state in phase IV, L = 3w. The w distribution in the compacted bentonite was also measured as water absorption time increased. Based on those results, the global water movement was estimated in terms of diffusivity (D) following a method employing Boltzmann transform. Results of comparisons implied that D calculated using this method matched experimental data well and the method was rather easily handled.

This study starts from a question: how would initial water content (wi) affect equilibrium swelling pressure (peq) of compacted bentonites in swelling pressure tests (ps tests)? However, discussions, based on experiments of a bentonite, Kunigel V1 (K_V1), are extended to issues: 1) wi effect on peq, 2) pore water density (ρpw), 3) co-existence of crystalline and osmotic swelling, and 4) relation between peq and compaction tests (pc tests). It is revealed that wi has insignificant effect on peq in general, though significant effect may appear for relatively high wi and dry density (ρd) conditions. Two swelling types, crystalline swelling with basal spacing of montmorillonite (d001= 1.0 nm - 1.9 nm) and osmotic swelling (d001 ≥ ∼4.0 nm), co-exist in a w range from 36 to 65%. With the fact that ρpw of K_V1 may range from 1.1 to 1.2 Mg/m3, it is expected that crystalline swelling may mainly govern peq in ps tests if the final dry density of a specimen (ρdf) is larger than 1.46 Mg/m3, while osmotic swelling may mainly govern if ρdf <1.05 Mg/m3. Together with results of pc tests, the effect of wi on peq are explained by the proposed conceptual model.

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