Water content in fine-grained sedimentary rocks is an important consideration in many engineering projects. It has been well establish that water content of these rocks can significantly changes most aspects of physical and mechanical properties. To investigate the effect of water content on the mechanical behaviour of fine-grained sedimentary rocks three different types of shales; clayshale, mudshale and mudstone, from different locations. The samples were tested at different water content ranging from oven-dried to saturated condition. Preparing samples of standard size for testing was a troublesome task. Many samples break during coring, cutting, grinding and storing because of disintegration of these rocks when subject to change in water content. This nondurable behaviour of these rocks is responsible for numerous slope instability problem, underground excavation problems and embankment failure. Water content has been demonstrated to have a marked influence on the strength and deformation properties. The uniaxial compressive strength and modulus of elasticity decrease significantly as the water content increases. The results show a reduction of more than 90% in uniaxial compressive strength from oven-dried to saturated condition. This strength reduction due to moisture content for the rocks studied is significantly higher than the values reported by other researchers. A general equation was developed for this type of fine-grained sedimentary rocks that may be used for predicting uniaxial compressive strength from the available information on water content. Modulus of elasticity was found to be influenced by anisotropy, but the most significant influence, regarding reduction of the modulus of elasticity, was the change from dry to saturated condition. Test results show a significant reduction in the static modulus of elasticity from oven-dried condition to saturated condition. The results of these tests show an average reduction in modulus of elasticity, from that in dry state, of about 84% for the shales studied.

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