Shear strength and microstructure characteristics of soil reinforced with lignocellulosic fibers-Sustainable materials for construction

• Adding 1 percent of lignocellulosic fibers, as eco-friendly materials, can lead to high ultimate deviatoric stress (up to 67.25% increase) and low peak pore water pressure (up to 51.76% decrease) in one day-cured specimens. • Curing time improves soil strength, resilient modulus, secant modulus and energy absorption capacity of the reinforced specimens. • Effective cohesion is more influential in enhancing the soil strength than effective internal friction angle. • The improvement in soil strength of lignocellulosic fibers-reinforced sandy soil is the result of a physical process. Increasing environmental concerns and depleting non–renewable resources have prompted many researchers to evaluate the effectiveness of eco-friendly materials, such as lignocellulosic fibers, on the geotechnical properties of soils. In this research, the effects of fiber content (0.5, 1 and 2 percent by dry weight of soil), fiber length (0.5, 1 and 1.5 mm) and curing time (one and seven days) on the strength behavior of sandy soil reinforced with three types of randomly distributed lignocellulosic fiber pulps, namely soft woods bleached kraft pulp (S.B.), old corrugated containers (OCC) and high yield wheat straw soda pulp (W.S.) were investigated. To this end, a series of consolidated undrained (CU) triaxial compression tests were conducted. Furthermore, the environmental and microstructure characteristics of the lignocellulosic fibers-treated soil were evaluated using pH test, X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. At identical conditions, those specimens reinforced with 1 percent of fibers demonstrated the highest ultimate deviatoric stress and lowest pore water pressure. Furthermore, longer fibers led to more strength and pore water pressure while S.B. fibers (pure cellulose) were more effective to enhance the shear strength of the investigated soil. Curing time also led to the higher strength but lower pore water pressure. Besides, the increased shear strength of the reinforced specimens was mainly resulted from the effective cohesion rather than the effective internal friction angle. The effective cohesion of seven day-cured specimens reinforced with 1 percent of 1.5 mm-long S.B., OCC and W.S. fibers increased up to 36.6, 31.1 and 29.6 kPa, respectively. Moreover, no considerable pH change or trace of heavy metals was observed in the treated soil specimens. The results of SEM and XRD analyses also demonstrated that the change in the soil strength behavior was merely the result of a physical process.