Recently, the use of plastic products, such as polyethylene (PE) bottles and polypropylene (PP), has been significantly increased, which may lead to many environmental issues. Therefore, it is important to find methods to manage these waste materials without causing any ecological hazards. One of these methods is to use plastic wastes as soil stabiliser materials. In this study, PE and PP have been used in the form of fibres. The effect of the stabilisation was evaluated through carrying out standard laboratory tests. These tests have been conducted on natural and stabilised soils with four fibre contents (1%, 2%, 3%, and 4%) of the soil weight. The tests included the standard compaction test, unconfined compressive strength (UCS) test, California Bearing Ratio (CBR) test, and resilient modulus (Mr) tests. In all these tests, the fibre content was added in two lengths, which were 1.0 cm and 2.0 cm. Laboratory test results revealed that the plastic pieces decrease maximum dry density (MDD) and optimum moisture content (OMC) of the stabilised soils, which are required for the construction of embankments of lightweight materials. In addition, there was a significant improvement in the UCS of soils by 76.4 and 96.6% for both lengths of PE fibres and 57.4% and 73.0% for both lengths of PP fibres, respectively. Results of the CBR tests demonstrated that the inclusion of plastic fibres in clayey soils improves the strength and deformation behaviour of the soil especially with 4% fibre content for both lengths 1.0 cm and 2.0 cm, respectively, to a figure of 185 to 150% for PE and PP, respectively. Furthermore, the results of the Mr tests demonstrated that the mechanical properties improved to an extent. For an increase in fibre content, the resilient modulus increased by about 120% at 4% fibre content for PE. However, for PP, improvement in resilient modulus declined at 3% fibre content. Therefore, for soil stabilisation with fibre material, optimum fibre content shall be sought.

Gravel dust and limestone dust are types of solid waste materials that are obtained from the crusher plant. These materials are dumped-off in high amount causing a negative impact on the environment and creating serious hazards on health. As the construction activities are increased in developing countries, the demand of crushed gravel and crushed limestone for roads, airfields, railway ballast, buildings and concrete work are increased. This study attempted to investigate the effect of gravel dust and limestone dust on geotechnical properties of clayey soil. Testing program including Atterberg limits, compaction, CBR and free swell tests, were performed on clay with the dust at varying amounts (10, 20, 30 and 40%). The results showed that the Atterberg limits of clay decrease in proportion to increases in the amount of dust. The increase in gravel dust contents decreased the compaction characteristics of clay. However, the MDD increased and OMC decreased with increase in limestone dust contents. A series of soaked CBR tests were conducted on the clay-dust mixtures of gravel dust and limestone dust. The dust was mixed with the clay of different weight percentages. The results showed a general increase in the CBR value of clay with the addition of dust. The CBR value increased gradually with the gravel dust content. The maximum CBR was obtained at optimum limestone dust content, approximately 20%. Swelling percentages decreased gradually with the dust content. The tests results revealed that it is possible to use gravel dust and limestone dust for improving the properties of clayey soil.

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