Abstract:
The shear strength of soils is one of the important aspects to be considered in any geotechnical activity. The problems concerned with the bearing capacity of road subgrade, slope stability of embankments and the design of retaining structures are all dependent on the shear strength characteristics of soils. Depending on the application in hand and the requirements, one needs to carry out modification of the soil properties. There is ample scope for the bulk utilization of fly ash in soil mixes for geotechnical applications such as in the construction of roads, embankments and backfilling behind retaining structures. In this research work, the characteristics of two soils, a fine-grained residual lateritic soil (red soil) and granular riverbank sand (Brahmaputra sand) blended with a low-calcium fly ash and ordinary Portland cement was studied through a systematic series of compaction tests, direct shear tests, CBR tests, unconfined compression tests and triaxial consolidated drained tests. Depending on the type of test, the maximum amount of soil replaced with fly ash by weight ranged from 50% to 90%. The amount of cement added was up to a maximum of 3 % to 5% by weight of the soil-fly ash mixes. This provided a wide range of gradation and texture of the mixes. Compacted specimens of the mixes were cured up to 28 days. Based on the maximum dry unit weight obtained from the compaction tests, the specimens for the remaining tests were prepared. The strength tests were conducted on the as-compacted specimens without saturating them. Direct shear tests were carried out only on specimens of sand and sand-fly ash mixes. The results from the tests were analysed to examine the compaction and strength characteristics of the modified soils due to the application of fly ash and cement. The obtained values will also be useful in judging the suitability of those proportions of the soil mixes for their use in diverse geotechnical applications that include the construction of subgrade and subbase of pavements. As the fly ashes produced in the same plant at different times can be different, and as locally available soils can vary widely in their properties, characterization of their mixes is a must for their field usage. It is to be noted that fly ash can be used to stabilize soil, and even soil can be used to stabilize fly ash as well..
