The study of the behavior of fine-grained soils is one of the most important to a geotechnical engineer because any soil that contains more than 50% of fine-grained soils (soils passing sieve size 425 μm sieve), the behavior of the soil is controlled by these fines and they abundantly occur in nature.
Field classification of fine-grained soils is often required in practice and the common methods to do this would be shown in this post. A natural heap of soil usually contains coarse-grained particles and fine-grained particles and sometimes organic soils. Highly organic soils often classified as peat can be identified by their colour, odour, spongy feel and fibrous texture. To know whether a soil should be classified as coarse-grained or fine-grained, spread the heap on a flat surface and observe the soils visually; if more than 50% of the particles are visible to the naked eye, then the sample is classified as fine-grained soil, otherwise it is coarse-grained soil. If the soil falls into the class of fine-grained soils, the following tests can be used to classify it further in the field;
Dilatancy Test
Procedure
i. Remove particles larger than 425 μm from the soil sample.
ii. Prepare a small pat of the moist soil from this sample adding enough water to make it soft but not sticky
iii. Put the pat on one palm of the hand and shake horizontally by striking vigorously against the other hand several times.
iv. Observe a positive reaction that occurs when a film of water appears on the surface of the pat which appears glossy.
v. Squeeze the pat between the fingers of the hand and observe how quickly the water and glossiness disappear from the face of the pat and the pat becomes stiff and then cracks or crumbles.
vi. According to this observation, plastic clays give no reaction, very fine clean sands give the quickest and most positive reaction while inorganic silts and rock flour give moderately quick reaction.
Dry Strength Test
Procedure
i. Remove particles larger than 425 μm from the soil sample.
ii. Mould a small specimen to the consistency of a putty (ensure that the required consistency is achieved before test).
iii. Dry the moulded soil completely by oven, sun or air drying.
iv. Access the strength of the soil by breaking and crumbling it between the fingers.
Note: The dry strength depends on the nature and the proportion of colloidal size particles in the soil; also the dry strength decreases with decreasing plasticity
v. High dry strength indicates that the soil is a highly plastic clay soil; light dry strength and a smooth powdery feel when dry shows a sample of inorganic silts while slight dry strength with a gritty powdery feel indicates silty sands.
Toughness Test
Procedure
i. Remove particles larger than 425 μm from the soil sample.
ii. Mould a small specimen to the consistency of a putty (ensure that the required consistency is achieved before test).
iii. Roll the specimen on a smooth surface into a tread of approximately 3 mm in diameter.
iv. Repeatedly mould and roll this tread until it crumbles.
v. When crumbled, gather the crumbles pieces and subject them to kneading until the lump crumbles.
Note: The tougher the tread and stiffer the kneaded lump just before crumbling indicate greater activity of the colloidal size particles.
vi. Inorganic clay of high plasticity always exhibit tougher tread and stiffer kneaded lump before crumbling while inorganic clay of low plasticity or organic clay exhibit weakness of tread and quick loss of coherence of kneaded lump before crumbling.
