EFFECT OF MULTIPLE COMPACTION ON SOME ENGINEERING PROPERTIES OF LATERITE SOILS IN AKURE, SOUTHWESTERN NIGERIA

Abstract

The effect of multiple compaction on the engineering properties of laterite soils in Akure, Southwestern Nigeria was investigated. Mechanical instability of the soils was determined on the addition of mechanical energy through multiple compaction. Fifty one samples developed over charnockite, porphyritic granite and granite gneiss were investigated. The samples were obtained from three different horizons of the laterite soil profiles. Index engineering properties such as natural moisture content, specific gravity of grains, Atterberg limits and particle size distribution were investigated before and after the multiple compaction of the soils. The changes consequence of multiple compaction on the soils was examined by consideration of variation on specific gravity of grains, particle size, atterberg limits and moisture-density characteristics. The results obtained show that specific gravity of grains of uncompacted soils ranged from 2.60 to 2.73 and 2.61 to 2.74 after the fourth compaction. Coarse content for top accumulation varies from 1.5% to 56.1% before compaction and 0.5% to 24.3% after the fourth compaction. The fine content varies from 6.4% to 67.6% before compaction and 30.3% to 71.8% after the fourth compaction. Coarse content for middle accumulation zone varies from 4.6% to 65.3% before compaction and 0.3% to 35.4% after the fourth compaction. The fine content varies from 7.1% to 71.0% before compaction and 23.2% to 85.3% after the fourth compaction. Coarse content for mottled zone varies from 1.12% to 41.2% before compaction and 0.4% to 33.2% after the fourth compaction. The fine content varies from 26.8% to 69.8% before compaction and 38.2 to 77.4% after the fourth compaction. Liquid limit for top, middle and mottled accumulation zones varies from 18% to 39%, 20% to 47% and 24% to 56% before compaction and 23% to 48%, 28% to 65% and 32% to 73% after the fourth compaction. Plasticity index for top, middle and mottled accumulation zones varies from 3% to 8%, 6% to 22% and 8% to 28% before compaction and 18% to 24%, 9% to 35%, 12% to 40% after the fourth compaction. Linear shrinkage for top, middle and mottled accumulation zones varies from 2% to 12%, 3% to 13% and 6% to 16% before compaction and 4% to 14%, 5% to 15% and 8% to 19% after the fourth compaction. The average maximum dry density for top, middle and mottled accumulation zone varies from 1490 kg/m3 to 2053 kg/m3, 1431 kg/m3 to 1981 kg/m3 and 1436 kg/m3 to 1922 kg/m3. The average optimum moisture content for top, middle and mottled accumulation varied from 8.23% to 19.1%, 8.2% to 24% and 8.93% to 24.33%. The results indicate that the engineering properties of the soils can be unstable on the application of excessive mechanical load. The maximum dry density and optimum moisture content assessed by coefficient of variation from compaction curve are independent of the number of compaction. Multiple compaction causes further break-down of larger-size grains (concretion) into fine particles. The larger sizes of the fines break-down under compaction than the concretion despite their similarity in composition. Pedogenesis of the parent rock show that porphyritic granite and granite gneiss derived soils are more suitable than soils derived from charnockite. Middle accumulation zone provides horizon with the best engineering properties than the top accumulation and mottled zone. The parameters obtainable from the grain-size distribution and compaction characteristics were found suitable for assessing mechanical instabilities