1362
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1 INTRODUCTION
The Millennium Challenge Account (MCA) Compact is an
intervention muted by the United States of America to
support the alleviation of poverty stricken areas in the
Ghanaian economy. The intervention focused mainly on
agriculture infrastructure, which included some road
construction. The Agogo
–
Dome trunk road, 75.21km, was
one of such that received attention to the Afram Plains, a
major agriculture area. The project spanned between October
2009 and October 2011.
The geology of the area is a well-defined regional
sedimentary terrain of the Voltaian System of Ghana, which
abounds in consolidated and unconsolidated sediments or
materials with other low degree metamorphism associations
[Ghana Geological Survey Department]. Locally the main
lithological units underlying the area are the Upper to Lower
Paleozoic Voltaian Afram shale and sandstone. Lateritic
depositions (residual in nature) that are commonly used for
road fill, sub-base and base layers including embankment for
this project were scarce within the geological environment
during the feasibility studies. Also earlier geotechnical
appraisal of materials within the project area was silent about
the shale.
The project special specification requested the use of
geogrid membrane to provide reinforcement for soft and
weak in situ materials, and lateritic materials of class G30 for
fill and embankment, G40 for sub-base and G80 for base
courses. During the construction stages shale, which abounds
in the area was also evaluated in the laboratory with
conventional test protocols such grading, Atterberg’s limits
and soak California Bearing Ratio (96hr-CBR). However
laboratory test results classified the shale as marginal material
to be utilized as a fill for road and embankment structures.
Due to this marginal condition, there was the need to evaluate
its performance from a trial section. A 100m length, 1.0m
thick, trial section was constructed as part of the length of the
road under normal environmental and traffic conditions for
two months. The field evaluation processes included dynamic
cone penetrometer (DCP) test for CBR determination and
plate loading for settlement and shear strength coupled with
petrographic and X-ray fluorescence laboratory studies. Field
test results were convincing enough for the shale to be used as
intended.
This paper presents the laboratory and field results
obtained. The observed field test results merited the use of the
shale as fill and embankment structures for about 30.0km
stretch of the road.
1.1
Shale
Shale is generally a clastic water depositional material
composed chiefly of silt and clay. There are varying
classification of shale depending on the mineral content,
fossil content and depositional history.
The use of shale as road construction material is not very
common. Sethi and Schieber (1998) have commented on the
use of the Ordovician Martinsburg Shale and Devonian
Brailler Shale, in West Virginia, as lightweight (expanded)
aggregate for concrete, brick, asphalt, railroad, ballast, road
base and fill. Okogbue and Aghamelu (2010) also compared
the geotechnical properties of three shale Formations from
southeastern Nigeria. In summary they concluded that they
are likely to be satisfactory as fill and embankment materials.
Afram Shale in the Eastern part of Ghana has not been
utilized as construction material probably due to perceived
geotechnical challenges associated with Shale in general. The
Shale within the project corridor is mostly moderately
weathered and deep seated; physically observed to be friable
when expose to the atmosphere.
1.1.1
Design Method for test section
The 100m long test section spans between, chainage,
ch29+675 and ch29+775. The intended pavement design for
soft and weak zones of the road was to place a biaxial geogrid
reinforcement followed by a 200mm thick layer of selected
granular material complying with the requirements for
material class G30 of the standard specification. However the
geogrid was eliminated at the test section and instead
substituted with the 1.00m thick moderately weathered shale.
Conventional procedure included clearing the vegetative
cover and topsoil followed by 200mm thick lifts of the shale.
Each lifts was compacted using the vibratory roller for twenty
(20) passes and, visual indication that, close contact was
being obtained between aggregates. In order to check on
adequate compaction proof-rolling was implemented. A
twenty ton fully loaded truck was engaged to move slowly on
the compacted surface and concurrently observing any
movements made by wheels (tyres) for the proof-rolling. Any
observed movement was corrected by further compaction.
This test section provided the platform as formation level for
sub-base, base and bituminous layers at the reference
chainage. The compacted section is presented at Figure 2.
1.1.2
Laboratory test results
Tests were carried out in five laboratories in Ghana and one
laboratory in Nairobi, Kenya while the construction was in
progress. The soil and aggregate results obtained from the
laboratories is shown in Table 1. Table 2 shows the
petrography of the Afram Shale and their major oxides
composition obtained from X-ray fluorescence analysis is
shown in Table 3.
Table 1. Summary of soil and aggregate laboratory results on Afram
Shale.
Test
Lab
MDD
g/cm
3
LL
%
PL
%
PI
%
CBR
%
<425μm
%
PM
1.88 30.9 19.9 11.0 11
16
176
1
2.11 27.7 11.0 16.7 12
21
350.7
1.77 45.0 20.7 24.3 10
19
461.7
2
1.79 44.2 19.6 24.6 10
1.2
24.0
1.78 33.8 20.1 13.7
1
13.7
3
1.85 40.5 19.4 21.1
1
21.1
4 1.73 45.0 21.0 24.0 8
10
240.0
5 1.93 38.8 21.4 17.4
1
17.4
6 1.90 36.0 19.0 17.0 10
1
17.0
Reference laboratories: 1
–
Ghana Highway Authority (GHA) Kumasi Lab,
Ghana; 2
–
Building and Road Research Institute, Kumasi, Ghana; 3
–
Nairobi
Lab, Kenya; 4
–
GHA, Accra Lab, Ghana; 5
–
China WE Suhum Lab, Ghana; 6
–
China Jiansu Jianda Corporation Lab, Agogo, Ghana.
