Engineering Geological Assessment of Granular Material from Weathered Adigrat Sandstone as Filter Material - Special Consideration to Shumbrit Dam, Western Amhara

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Addis Ababauniversity


Granular filters are used in embankment dams to prevent impermeable materials from internal erosion, while draining seepage water to prevent saturation of the downstream embankment. Such materials are usually assessed from courses of rivers or streams that contain alluvial sand and gravel materials and their suitability or performance has been evaluated using existing filter design criteria based on particle size ratios. During this work same assessment of a granular material which derived from weathering of the Lower Sandstone unit of the Mesozoic Rock Formation of Ethiopia in protecting red tropical clayey silt soil has been done. Its overall characteristics, especially those relevant to filter design, have been studied. It is also compared with respect to an alluvial source, found at relatively distant place from research site. The geometrical criteria have been mainly checked using the formulas and recommendations proposed by many scholars and standards. In addition to this, the final gradation or grain size distribution of a suitable filter material has been prepared based on the gradation of the base material following the design procedures prepared by United States Soil Conservation Service (USSCS). The residual granular material, which was sampled and analyzed for grain size distribution, nearly satisfies most of the filter criteria, but it is a little bit finer. Of the various criteria available, the USSCS method and Sherard recommendations have been given more emphasis as they consider the type of base material and accordingly proposed different approaches and ratios for design of the filter. The gradation curve for the average residual filter (averaging three samples from different localities) prepared and also checked with the empirical criteria and design rules together with one alluvial soil from the distant source. The assessment result (especially USSCS method) indicates that the residual material almost satisfies the geometrical criteria, except it becomes a little bit finer. Whereas, the average residual soil and the alluvial sediment satisfies completely both requirements of filtration and draining. As to the other characteristics of the filter, all materials (residual and alluvial) have satisfactory quality; the grains of the residual sand are dominated with quartz, while the alluvial is fragments of basaltic rocks. The behaviour of the base soil, which is related to its dispersive nature, also examined in a laboratory test. The result indicates that the base soil can be categorized as non-dispersive



Western Amhara