Effects of fines on mechanical behaviour of sandy soils

In dredging works, fines are encountered in different ways. Fines can be produced during the cutting of rocks or can be found in natural deposited sand. The presence of fines in sand soil is generally recognized as problem in geotechnical engineering. It is believed that fines in a sand increases soil compressibility and tendency to creep. Also it is often assumed that the strength, stiffness, hydraulic conductivity and liquefaction potential of a soil increases with increasing fines content. Therefore, during tender phase of reclamation works, it is always recommended that the soil should contain minimum amount of fines 10% of fines. It has been recognized that the availability of soils which have small amount of 10% of fines is very rare. In most of projects soil with maximum amount of fines above 10% are encountered. The objective of this thesis is to investigate the mechanical behavior of sand soil with different amount of fines. The main interest is on the compressibility of sandy soils. The compressibility of sandy soil is investigated and their results are shown. A series of oedometer test for saturated silt sand mixtures is presented. During testing the following material were used River Maas sand and crushed sand, artificial silt (crushed quartz) and Belgium silt. The mixture were created by mixing River Maas sand and crushed sand with either artificial silt or Belgium silt as percentage of dry mass. Sand silt mixtures with fines contents of 7%, 10%, 15%, 20%, 25%, 30%, and 50%.The samples were prepared in different initial relative densities using slurry deposition methods. In oedometer the mixtures were loaded in interval of 3.5, 6.9, 13.9, 34.6, 69.8, 138.5, 277.1, 554.2 and 1108.3 kPa. Result shows that, the coefficient of consolidation of sand silt mixture increases with fines content from 7% and reaches the peak value at 15% then the coefficient of consolidation decreases with increase of fines contents. The compression index of silt sand mixtures decreases with increase on fines contents from 7% to 30% and then increases with increase of fines contents. The value of compression index for fines contents of 15 to 30% is low due to its highest packing density. The secondary compression on artificial silt sand mixture is very low it is between 0.001-0.004 but for Belgium silt sand the secondary compression is significantly higher. The difference in secondary compression is due to their difference in mineralogical composition and grain size and shape. It can be conclude that the presence of fines on sand does not increase the compression and secondary compression of the sand but it is improving it. Therefore for mixtures with fines content above 10% between 15 to 35% are useful for reclamation activities because of their low primary compression and low secondary compression.GeotechnologyCivil Engineering and Geoscience