Integration of Geophysical and Geotechnical Methods for Road Route Characterization: the Addis Ababa-Awash New Detour Road at Lake Beseka: Main Ethiopian Rift, Metehara Area.
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Date
2014-06-06
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Addis Ababa University
Abstract
Integration of Geophysical and Geotechnical Methods for Road Route Characterization: the Addis
Ababa-Awash New Detour Road at Lake Beseka: Main Ethiopian Rift, Metehara Area.
Tibebu G/Michael
Addis Ababa University, 2014
Adequate knowledge supported by proper investigation of the sub-surface structure condition is a
vital task for effective assessment and control of weakening of road morphology. The present
research study was carried out to characterize the general suitability of the proposed route corridor
by an integrated geophysical and geotechnical survey.
This research mainly focused on carry out detailed mapping of geological structures, provide
information on the subsurface sequence, competence and with a view to extracting geo-engineering
information in order to understand the basic impact and interaction of the cracks and its implication
on the newly constructed detour road and to evolve the possible remedial measures for the likely
engineering geological problems. The survey was aimed at imaging the shallow subsurface with a
view to evaluate the inferred geological Structures and competency of the shallow formation as
foundation materials.
Geotechnically, in order to assess the general suitability of the construction material for selected
segments of the subgrade and embankment, samples were collected and tested for grain size
distribution, Atterburg limits, swelling potential, California bearing ratio and in-situ density
determination at the site. In general the construction material for the new road is slightly non plastic
in nature, gap graded but good CBR. Thus, it is anticipated that this may result in to high
deformability, segregation which may lead to instability. Also, the construction materials contain
more fine materials than the desired percentage.
Geophysical technique involving Electrical imaging and magnetic methods were applied and the
data were processed, analyzed and interpreted. Two to three subsurface layers were delineated
within the study area. The first of these is a topsoil of loose sand, peat and/or clay. This layer is in
good correlation with the soil layers in geotechnical test pit logs. The existence of loose sand and
silty clay at the top surface is capable of being inimical to road structures. The subsurface layers up
to the depth of 2m are mechanically unstable owing to weak, saturated horizons with low resistance
value, which may not serve as good foundation materials. In all survey lines, three to four major
subsurface geological structures possibly of fissures/fractures are delineated up to the depth of 65m
which aids the saline Lake water easily invade the road so that the susceptible to deformation .
Hence, Geophysical and geotechnical tests showed good agreement.
From overall assessment, the problem of geologic structures, unsuitable subgrade materials and
poor grain size distributions of the subsoil, the engineering structures which may be crossed the
section, are not prolong for required designed life owing to high traffic load. Therefore, in addition
to tectonic activity of the area, the presence of low penetrative resistance, low resistivity saturated
sandy clay and weathered ignimbrites in the study area is identified as the cause of cracking and
sinking of road in this area. On the basis of this, shallow subgrade foundation may not be possible
in the study area unless adequate soil treatment is done; otherwise shifting to a competent soil and
rock section area to depth of pronounced thickness of subgrade materials with less frequency of
geological structures is recommended to northern part from the current route line near the periphery
of the lake. This would transfer loads of such highway to a stratum of high bearing capacity even if
the cost of excavation to the geometric design level is too high.
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Keywords
Main Ethiopian Rift, Lake Beseka, Ground Cracks, Suitability Analysis, In-Situ Density, and Subgrade Soil Evaluation