CHARACTERIZATION OF SUBGRADE MATERIALS FROM LOCAL SOURCES FOR USE IN THE NIGERIAN EMPIRICAL-MECHANISTIC PAVEMENT ANALYSIS AND DESIGN SYSTEM
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Date
2016-01-26
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Ahmadu Bello University Zaria
Abstract
Subgrade materials from different locations in Nigeria were characterized for use in the
Mechanistic – Empirical Pavement Design. The basic soil index properties of the
subgrade soil materials from Master Test Section (MTS) 1 that identified the material
response to external stimuli of traffic loading and environmental conditions were
obtained in the laboratory.Three samples each from the MTSs making a total eighteen
(18) samples were obtained and subjected to laboratory test to determine their basic
physical properties. Testing include particle size distribution, Atterberg limits, specific
gravity, compaction characteristics, Unconfined Compression test and the California
Bearing Ratio tests. The samples were classified according to American Association of
State Highway and Transportation Officials (AASHTO) and the Unified Soil
Classification System (USCS).The AASHTO soil classification shows that the subgrade
soil samples obtained from the MTS 1 were eitherclayey soil (A-6 and A-7-6) or silty
soils (A-4, A-5 and A-2-4). The AASHTO soil classification generally showed that the
subgrade samples were ―Fair to Poor‖ in subgrade properties for use as construction
materials.The USCS soil classification indicated that most of the samples were lean clay
soil with gravel (CL) except few that were either clayey gravel (GC) or clayey sand
(SC).This showed that the subgrade samples were mostly clay soil. The California
bearing ratio (CBR) values were generally less than 3%. This implies that the strength
of the subgrades were poor for engineering construction purposes.Resilient modulus
constitutive equation for estimating the resilient modulus of Nigerian subgrade soils was
adopted through evaluation of existing resilient modulus constitutive equations using
the repeated load triaxial test result conducted on Nigerian subgrade soils. From the
evaluation, the National Cooperative Highway Research Programresilient modulus
constitutive equation was adopted for purposes of predicting resilient modulus of
Nigerian subgrade soils.Comprehensive statistical analysis using multiple linear
regression used to develop correlations between basic soil properties and the resilient
modulus model parameters.The correlation showed that the resilient modulus model
parameters (ki) can be estimated from basic soil properties.These correlations developed
can be used to estimate the resilient modulus of the compacted subgrade soils with
reasonable accuracy and can be utilized to estimate level 2 resilient modulus input for
Mechanistic – Empirical Pavement Design.