Engineering Geological Characterization and Suitability Assessment of Sub-Grade Embankment Fill Material, Sub-Ballast, And Ballast Material of Addis Ababa City Light Railway Transit (AALRT); Case Study Ayat to Mexico Square, Addis Ababa, Ethiopia
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Date
2014-05-05
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Addis Ababa University
Abstract
The present study was mainly focused on engineering geological characterization, assessment
and the suitability analysis of the substructures of the Addis Ababa LRT alignment and to evolve
possible engineering concerns or outlooks for the likely engineering geological gaps along the
proposed alignment. For this field observation, investigation, sampling and laboratory tests for
physical and strength properties assessment were performed on representative samples from subgrade
embankment material (Yerer-2 subgrade) and ballast stone materials (Galan Quarry). In all
LRT project there is no subballast proposed and used by the contractor (CREC). Because instead
of subballast, 5cm thick asphalt concrete is proposed in design specification of the LRT project.
Further, attempts were also made to identify and characterize the topographic classifications and
surface drainage setup of the area by reviewing the relevant literatures and by using the elevation
map of the study area. Based on the laboratory test Soil Classification such as; Grain size
distribution and Atterberg Limits, Strength tests (California Bearing Ratio (CBR)), Moisture/
Density Relationship (modified proctor tests), and direct shear characterization, the sub-grade
was found to be suitable. In addition to this it has been assessed that most of the identified
subgrade fill material is granular in nature. Based on the index properties, with its AASHTO
classification is A-1-a (0), A-1-b (0) and [GW, GP] USCS classification systems. Pertaining to
its consistency limits, the subgrade material falls below A-line in the Cassagrade plasticity chart
indicating that moderately plastic silt to low plastic. The bearing capacity of the subgrade
material is above (ERA, 2002) specifications.
In laboratory gradation, specific gravity, water absorption, impact value, crushing value, abrasion
resistance, soundness, Slake durability test and peterographic analysis were done for the ballast
stone accordingly. Pertaining to the recommended standards and specifications set for ballast
aggregate, all the engineering suitability results indicate that the studied material is suitable for
intended use except for its water absorption values (which is above permissible) AREMA,
(2010), and higher content in plagioclase feldspars which makes an aggregate more sensitive to
weathering.
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Keywords
Sub-grade Embankment Fill Material, Subballast, Ballast Stone Material, Characterization, Index Properties, petrographic thin section