EFFECT OF POLYPROPYLENE FIBER ON CBR VALUE AND DIRECT SHEAR STRENGTH OF SILTY SAND

Abstract

Silty sand is a frequent cause of failure for roads, embankments and foundations. It remains uncohesive, highly liable to change when wet and only moderately strong to support loads when used in construction, unless this is upgraded. This dissertation analyzes the feasibility of achieving these improvements by using a lightweight, chemically resistant, synthetic fiber called polypropylene (PP) at different mixtures in silty sand. Samples of soil were taken from the campus of DTU and subjected to conventional characterization – specific gravity, 2.60; natural water content, 10.20%; Atterberg limits, 0.28 (liquid limit) and 0.09 (plasticity index). Based on the grain size distribution and fines content of 13.8%, nature of the soil is classified as SM (Silty Sand) as per IS-1498. Each of these separate batches were then dosed with fiber at the various dosages (0%, 0.25%, 0.50%, 0.75% and 1.00% by dry soil weight) and Standard Proctor compaction, CBR, and direct shear testing was conducted on each. The trend was clear for all results.. The story was even more dramatic when it came to bearing capacity(CBR value): that rose from 5.69% for untreated soil to more than double (11.68%) at 0.75% fibre before falling slightly as fibres started to clump together instead of spreading out evenly, at 1.00%. Shear behavior also followed a similar path: the friction angle increased in value from 29.3° to 33.6° at 0.75% fiber, and apparent cohesion (which effectively was negligible in the control) gradually increased to achieve a value of 14.2 kPa at the highest dosage.Comprising all three test series the result was unmistakable; 0.75% PP fibre content provided the greatest overall improvement in terms of having a strong bearing capacity, favourable shear parameters and an acceptable density loss. The results indicate that polypropylene fiber is a viable, cost effective and environmentally friendly material for the stabilization of silty sand, applicable to the subgrade, embankment and shallow foundation.