This paper presents a laboratory investigation into the resultant increase in shear strength of sand due to random inclusion of strips of polyethylene (plastic) shopping bag material. Previous tests carried out in a 100 × 100 mm direct shear box revealed relative increases in peak and ultimate friction angles on addition of the plastic strips in the sand specimens. In this study, large direct shear tests using a 305 × 305 mm shear box were undertaken on soil-plastic composite samples to ascertain the improvement in the strength parameters observed and to minimise the scale effects from the smaller specimens in the previous tests. Strips of shredded plastic material were added to sand specimens at various concentrations of up to 0.3% by weight and the effect of varying dimensions of the strips was investigated by using strip lengths from 15 mm to 45 mm and strip widths from 6 mm to 18 mm. Results indicate an improvement of over 20% in the friction angle of the sand on inclusion of the plastic material.
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