Abstract:
When African modern construction system establish with the colonization by
Europe, it starts with soil investigation trend of western sedimentary soil which is
different from African tropical residual soils. Ethiopian soil, as a country located
in tropical region needs to be investigated in a tropical weathered residual soil
concern than simply adopted sedimentary soil investigation trend. Providing the
basic engineering properties of the Tropically Weathered Residual (TWR) soil will
lead the government organizations, consulting and contracting firms to accept and
practicing the contracting, design and construction works with respect to TWR
soils concern.
In this study, After site visit, disturbed sample with its natural moisture content
were taken in to laboratory. Index properties, compaction and CBR tests have been
carried out in the laboratory by altering deterministic parameters and keeping the
necessary standard and requirements which is recommended for investigation of
tropically weathered residual soils. Besides geochemical tests have been done at
Ethiopian geological survey laboratory.
Based on modified testing procedures and treatements, the TWR soils basing
USCS categorized majority of the soils under group name Elastic Silt while the
soils from test pit D only are silty sands; and AASHTO classification system
classified soils from test pit A, B, C and E under group classification A-7-5 with
Group Index > 20, which are referred as poor subgrade material; while soils from
test Pit D-1.5 and D-4.0 are classified as A-7-5 and A-2-7 with group index values
10 and 0 respectively, which are good to be subgrade materials. Using the
geochemical test analysis (silicate analysis) result, the soil are laterites and lateritic
with a Silica-Sesquioxides Ratio less than two and Wesley’s classification system
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classified the soil as Group-C (c) under a group of soils strongly influenced by clay
minerals found only in residual soils.
From the laboratory results it is observed that compaction is greatly affected by
the compactive effort applied and the slightly affected by depth (laterization). CBR
values for fine grained Elastic Silt Soil ranges from 2.49% to 3.80% which is weak
and insufficient to bear loads, and the soaked CBR for coarse grained silty sand,
taken from test Pit D, ranges from 15.92% to 35.87%. The soils from Test Pit D
can be used for subgrade or for embankement construction and can be used as sub
base construction material as a laterites, bear in mind that the results and the
classifications presented here would have been diffirent if we have used the steps
and methods recommended from ERA pavement materials specification manuals.