Direct shear tests had been carried out using fully automatic large shearbox apparatus SHEARMATIC
300 from CONTROLS s. r. l.. Specimens size was 0.3 m x 0.3 m x 0.15 mm (specimens were compacted at the
maximum dry density 2091 kg.m-3 and optimal moisture content), consolidation time 20 min and shear speed
0.5 mm/min. Specimens without polyester fibres and with polyester fibres in amount of 0.5%, 1.0% and 1.5% of soil
dry mass had been tested at normal stresses 50 kPa, 100 kPa and 200 kPa. The tests were terminated when the
horizontal displacement has reached 20% of the width of the specimen. Calculation of maximum shear stress had
been carried out for two cases (see also [3]):