Engineering-geological properties of carbonates and shale: their implications for dam construction in Mekelle, Northern Ethiopia

Abstract

Growing water demand poses severe problems to the population in the Mekelle Outlier, Northern Ethiopia. Hence, storing of rain water for water supply becomes one of the top agenda in the area. Several earth-fill dams are constructed for irrigation and drinking water supply purposes over the last 15-20 years. However, as collected data indicated more than 60% of these earth-fill dams have excessive leakage due to the problematic engineering geological nature of the carbonates and shale rocks of the study area. Giba dam is one of the currently proposed largest dams to alleviate the water supply problem of the Mekelle city. In the current study, engineering-geological mapping, core drilling, geophysical surveys and laboratory works have been conducted for the dam project to evaluate the engineering-geological nature of rocks of the area. Qualitative and quantitative rock masses properties such as permeability, strength and deformation are analyzed using Packer test, Rock Quality Designation (RQD), and Rock Mass Rating (RMR) systems. Analyzed results displayed that: (i) the RQD values are highly variable for all the rock masses. For example, 60% of limestone (Lst), 50% marly limestone (MLst) and 72% shale (Sh) are categorized as poor /very poor RQD values. RMR values also imply that Lst, MLst and gypsum are classified class-III while Sh is classified in class-IV (ii) considering the rock mass shear strength parameters (C, ϕ), the Lst, MLst, and gypsum have a moderate strength while Sh as low strength. More than 92% the Lst and 84% of the MLst falls in the 5-50 and >50 Lugeon Value classes. Thus, area covered by both the Lst and MLst needs treatment (e.g. grouting). Similarly, 50% and 20% of the packer test values of shale falls in the <1 and 1-5 Lugeon value classes respectively. The studied rock properties implies that the limestone layer is not suitable for the construction of the earth-fill dams in terms of water tightness while that of the calcareous shale and/or mud rock is good site for reservoir area as it is water tight. Keywords : Carbonate rocks, Shale, Rock mass, Packer test, Giba dam, Mekelle, Ethiopia.