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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3.3
Deformation of Strips during Shear
The plastic strips used in the composite specimen for the
direct shear tests were assessed for physical deformations such
as dents or rupture at the end of each experiment. The nature of
deformations of the plastic strips was examined with respect to
their location in the shear box. Visual inspections revealed that
most of the elements that deformed were within or close to the
shearing zone. The indentations on some of the strips may have
been caused by soil particles as they pressed in to form surface
attachments with the plastic strips (Figure 5a). This was mainly
due to the particle shape and grading of the sandy soils used in
the study that enhanced the frictional bonding between the soil
and the plastic material. Other strips in the specimens were
stretched and compressed due to the shearing action at or near
the shear plane (Figure 5b). The stretched strips were located
parallel to the shearing direction. This indicated that as the
plastic strips were strained relative to the shearing direction,
they improved the soil tensile strength by enabling transfer of
forces arising from the loading conditions. None of the
reinforcements in the composite were severely indented or
ruptured during the shearing since the tensile strength of the
plastic strips was greater than 15MPa compared to the generated
shear forces in the test specimen under for a maximum applied
normal stress of 100kPa.
The laboratory results presented in the study favourably suggest
the possibility of utilizing plastic material as tensile inclusions
in sandy soil to increase the resistance to shear. As
demonstrated in Chebet et al. 2012, the plastic inclusions can
also improve the load bearing capacity and settlement
characteristics of the sand. Additionally, introduction of
perforations on the plastic material further aids in the interaction
between the soil and plastic thereby boosting the soil’s strength
properties. However, a better understanding of the interaction
mechanism in soils reinforced with the plastic material would
be essential to properly document the engineering behaviour of
the soil-plastic composite.
The influence of the soil physical properties, plastic properties
and scale effects would need to be further investigated through
more comprehensive testing in a wider range of stresses using
larger scale tests to eliminate boundary effects. This could in
turn contribute in the development of design methodologies for
projects that may opt to incorporate this type of reinforcement
material resulting in a reduction in project costs. Furthermore,
successful application in the field could permit a reduction of
plastic waste disposed of to landfills bringing along
environmental benefits as a result of more efficient use of
natural resources and reduction of CO
2
emissions.
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4 CONCLUSION
A comprehensive laboratory direct shear testing programme
was undertaken on composite specimens of sandy soils mixed
with random inclusions of plastic strips obtained from high
density polyethylene shopping bags. Two locally sourced soils,
Klipheuwel and Cape Flats sands were selected for the research
and the influence of plastic strips on the shear strength
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