Abstract
D.Ing.
Unbound granular material has and is still being used with great success in the construction of road
pavements in South Africa and many other countries around the world. Often this material is used in
the main structural layers of the pavement with very little protection provided against high traffic
induced stresses by way of a surface treatment or thin asphalt concrete layer. The performance of
unbound granular pavement layers depend mainly on the level of densification and degree of
saturation of the material in addition to the stress levels to which the layers are subjected. The main
form of distress of unbound granular layers is the permanent deformation of the layer, either through
the gradual deformation or rapid shear failure of the layer. Design engineers need accurate and
appropriate design procedures to safeguard the road against such rapid shear failure and to ensure
that the road has sufficient structural capacity to support the traffic loading over the structural design
period.
The recent trend in pavement design has been to move away from empirical design methods towards
rational mechanistic-empirical design methods that attempt to relate cause and effect. Although a
mechanistic-empirical pavement design method has been available in South Africa since the midseventies,
increasing criticism has been levelled against the method recently. The models for
characterising the resilient response and shear strength and estimating the structural capacity of
unbound material have been of particular concern. The purpose of the research reported in this thesis
was therefore to develop an improved mechanistic-empirical design model, reflecting the
characteristics and behaviour of unbound granular material.
The new design model consists of three components namely a resilient modulus, yield strength and
plastic deformation damage model with each model including the effects of the density and moisture
content of the material unbound granular where appropriate. The models were calibrated for a range
of unbound materials from fine-grained sand and calcrete mixture to commercial crushed stone
products using the results from static and dynamic tri-axial tests. An approximation of the suction
pressure of partially saturated unbound material was introduced in the yield strength model and was
validated with independent matric suction measurements on the sand and calcrete mixture. The yield
strength model which is a function of the density and moisture conditions as well as the confinement
pressure was calibrated for the individual materials with a high accuracy.
A single plastic strain damage model was calibrated for the combined plastic strain data from all the
crushed stone materials but a single model could not be calibrated for the plastic strain data of the
natural gravels as these materials vary too much in terms of particle size distribution and the
properties of the fines found in these materials. The formulation of the plastic strain damage model
includes the density and degree of saturation of the material. A single resilient modulus model was
calibrated for the combined resilient modulus data from all the materials excluding the data from a
limited number of tests during which large plastic strain occurred. The resilient modulus model again
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incorporates the density, degree of saturation and the stress dependency of unbound granular
material and is on an effective stress formulation for the bulk stress.
Finally, the yield strength, resilient modulus and plastic strain damage models are combined in a
mechanistic-empirical design model for partially saturated unbound granular material. Results from
the proposed design method seem more realistic than results from the current design model and the
model is not as sensitive to variation in the design inputs as the current design model is. In addition to
this, the effects of the density and moisture content of the partially saturated, unbound granular
material on the resilient response and performance of the material is explicitly included in the
formulation of the proposed design model.