IMPROVING THE STRENGTH PROPERTIES OF SUBGRADE SOILS WITH FLY ASH

Subgrade soils encountered during road construction are not always good materials to respond to the imposed stresses which has become a dominating factor for the failure of pavements in Nigeria. The subgrade materials were sourced from three locations within South Africa namely, Heanertsburg Village (A), Laudium, South West of Pretoria (B), and Eskia Mphahele drive to Francis Baard Street, Pretoria (C). Fly Ash (FA) was added to samples A, B, and C at 3-12%, 5-15%, and 9-12% respectively. Sieve analysis, compaction, Unconfined Compressive Strength (UCS) and Indirect Tensile Strength (ITS) tests were conducted on the virgin and stabilized soil samples for a curing time of 1, 7 and 28 days. The soils were classified as A-7-6, A-6 and A-2-6 according to AASHTO for samples A, B and C respectively. UCS and ITS was improved with the addition of FA to all the soil samples. The UCS results for sample A (406 kN/m2) and B (625 kN/m2) falls short of 1710 kN/m2 specified for cement stabilized base materials but 12% and 10% FA treated soils may be used as improved subgrade for flexible pavement construction. However, sample C result meets the requirement of 687-1373 kN/m2 for sub-base at 100% relative compaction. All the stabilized samples increase in ITS with respect to increasing curing days and FA. Only sample C attained to the specified ITS value. Therefore, FA can be used to improve the subgrade in order to withstand the imposed stress