Engineering Geological Assessment of Granular Material from Weathered Adigrat Sandstone as Filter Material - Special Consideration to Shumbrit Dam, Western Amhara
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Date
2009-07
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Addis Ababauniversity
Abstract
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
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Western Amhara