Stability of dams constructed on problematic substrates

Abstract

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Dissolution of soluble substrates such as gypsum presents a major hazard to dams in many parts of the world. This research simulates hypothesised conditions beneath the Mosul Dam, northwest Iraq, where collapse of a karstic system associated with continuous fresh water supply from its massive reservoir water is a recognised problem. The gypsum substrates at Mosul Dam vary in purity and thickness. Experimental work used gypsum rocks and gypseous soils. Gypsum rocks from northern Iraq and similar rocks from Bantycock gypsum mine, UK, were analysed for short-term mechanical response following immersion (5 to 50 weeks) and long-term loading during immersion (maximum 50 weeks). New experimental devices were developed from a conventional oedometer. Cylinder samples (NX, standard diamond drill core size = 54mm diameter, length/diameter ratio equal to 2.5) provided a proxy for massive gypsum strata, while thin samples (NX = 54mm in diameter, 20mm thickness) represented thin layers and lenses. Rectangular bar samples (240 x 40 x 20 mm and 140 x 40 x 20 mm) were tested for short-and long-term mechanical four-point bending behaviour. Samples were permanently submerged at a variety of water pressures, 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. Small increases in strain were recorded by the end of each test but no failures occurred within 60 days of tests. However, notable failure due to water pressure and axial stress over long time periods of 166 and 238 days occurred. Visible physical changes were observed, notably a decrease in sample mass and volume. Similar change was recorded in ultrasonic velocities. These indicate that gypsum collapse risk beneath dams requires prolonged exposure to dissolution. Gypseous soils from Iraq and similar artificially-prepared soils were also tested. Gypseous soil samples (diameter = 50mm and length = 20mm) and box model strata results showed that gypseous soils are significantly weakened by dissolution over 15 weeks and 50 weeks respectively. Dams built on gypsum substrates are likely to experience ongoing weakening of their foundations, with a progressively increasing risk of failure. This is expected to be enhanced for dams with a large and deep reservoir that induces high ground water pressure.This study is funded by the Ministry of Higher Education and Scientific Research, Scholarship & Cultural Relations Directorate, Iraq