Dissolution of soluble substrates such as gypseous soils presents a major hazard to their geotechnical properties and holding/adjusting structures. This research simulates hypothesised conditions at Mosul Dam, northwest Iraq, where collapse of a karstic system associated with continuous fresh water supply from the massive reservoir is a recognised problem. Naturally-occurring gypseous soils from Iraq and similar laboratory-prepared artificial soils were used. Samples were analysed for long-term mechanical response during immersion (50 weeks). A conventional oedometer was used for the long-term testing. Samples (54mm diameter, 19mm thickness) provided a proxy for gypseous soil strata. Samples were permanently submerged at atmospheric pressure, with the influence of groundwater recharge and flow on dissolution simulated by regular changes of water. Stress on each sample was progressively increased to a maximum of 2688 kPa. Visible physical changes were observed in all samples, notably a decrease in mass and volume. Step-wise increases in strain were recorded as loading and dissolution progressed, with differences occurring between samples reflecting sample characteristics. Experiments show that the recorded settlement is from the compaction of soil particles, in addition to the compressibility resulting from the dissolution of gypsum. The soil grains associated with gypsum in the samples have an effective role in the behaviour of the samples and their settlement. The soil grains may exhibit immediate settlement and/or consolidation settlement, the amount of which is controlled by particles size. These indicate that gypsum collapse risk beneath dams, and in areas adjacent to a dam, requires prolonged exposure to dissolution.
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