Basal reinforcement, where high tensile geogrids are employed beneath structures, is becoming an
increasingly accepted construction technique along the eastern coast of southern Africa. The presence
of compressible, soft, thin and shallow clay horizons usually associated with complex estuarine or
lagoonal deposits are a major consideration when using basal reinforcement as a founding technique.
Basal reinforcement involves the use of high tensile strength geogrids beneath a structure to form a
reinforced sand foundation. Deformation behaviour under loading is an important component of
stability analysis of earth structures. If reinforcement is used, the mechanisms become altered.
Geotechnical centrifuge modelling is a unique physical modelling technique, as it allows replication
of in situ stresses, which is most important because soil behaviour is a function of stress. This is
achieved by placing the model at the end of the centrifuge arm, and subjecting it to an increased
gravitational field, which creates the correct stress distribution in the model. Centrifuge modelling
provides an appropriate technique to observe the behaviour of compressible, soft, thin and shallow
clay horizons when basal reinforcement is utilized. An appropriate centrifuge model was constructed
and compared the behaviour of the clay horizon under unreinforced and reinforced conditions.
Reinforcement configurations were adjusted to observe the influence of additional geogrid layers, and
extension of the width of the reinforcement. It was found that deformation behaviour is distinctly
different between unreinforced and reinforced tests. Vertical deformation in the unreinforced test
localised to the region directly beneath the platform, with little lateral disturbance to the clay horizon
beyond the platform edge. As such, the sand directly beneath the platform acts as a deeper rigid
platform. The deformation behaviour of the clay horizon changes with the inclusion of reinforcement.
When reinforcement is included a wider portion of clay is deformed. The lateral width of this
deformation zone is controlled by the width of the reinforcement, as the applied load is spread. A
‘wide-slab’ effect is evident with an increase in the geogrid width, as the tensioned membrane-effect
is mobilised to increase the capacity of the reinforced foundation sand. This results in a wider portion
of the clay deforming. Addition of geogrid reinforcement to the sand foundation under a wide
platform load enhances deformation of the clay, but has the advantage of an increased load-bearing
capacity of the system. Furthermore, the addition of multiple layers of reinforcement contributes to
this increase in load-bearing capacity. Additionally, increasing the installation width of the
reinforcement contributes to an increased vertical load-bearing capacity. However, this resultant
increase is only mobilised after a certain amount of vertical displacement. This is likely due to the
reinforcement requiring a certain amount of vertical displacement to mobilise tension in order to
support the applied load. The behaviour of a thin compressible clay horizon changes with the
inclusion of reinforcement under a wide platform load. The deformation behaviour of the clay is
increased by additional layers of reinforcement as well as an increase in the width of the
reinforcement. However, the increase in deformation comes at the benefit of an increased vertical
load-bearing capacity of the reinforced foundation sand.