Geotechnical engineers classify the soil particle types by performing
tests on disturbed (dried, passed through sieves, and remolded) samples
of the soil. This provides information about the characteristics of the
soil grains themselves. It should be noted that classification of the
types of grains present in a soil does not account for important effects
of the structure or fabric of the soil, terms that
describe compactness of the particles and patterns in the arrangement of
particles in a load carrying framework as well as the pore size and
pore fluid distributions.
The liquid limit and plastic limit are arbitrary limits determined by tradition and convention. The liquid limit is determined by measuring the water content for which a groove closes after 25 blows in a standard test Alternatively, a fall cone test apparatus may be use to measure the liquid Limit. The undrained shear strength of remolded soil at the liquid limit is approximately 2 kPa. The plastic limit is the water content below which it is not possible to roll by hand the soil into 3 mm diameter cylinders. The soil cracks or breaks up as it is rolled down to this diameter. Remolded soil at the plastic limit is quite stiff, having an undrained shear strength of the order of about 200 kPa
The Plasticity index of a particular soil specimen is defined as the difference between the Liquid limit and the Plastic limit of the specimen; it is an indicator of how much water the soil particles in the specimen can absorb. The plasticity index is the difference in water contents between states when the soil is relatively soft and the soil is relatively brittle when molded by hand.
Thus if Dr = 100% the sand or gravel is very dense, and if Dr = 0% the soil is extremely loose and unstable.
Classification of soil grains
In the US and other countries, the Unified Soil Classification System (USCS)is often used for soil classification. Other classification systems include the British Standard BS5390 and the AASHTO soil classification system.Classification of sands and gravels
In the USCS, gravels (given the symbol G) and sands (given the symbol S) are classified according to their grain size distribution. For the USCS, gravels may be given the classification symbol GW (well-graded gravel), GP (poorly graded gravel), GM (gravel with a large amount of silt), or GC (gravel with a large amount of clay). Likewise sands may be classified as being SW, SP, SM or SC. Sands and gravels with a small but non-negligible amount of fines (5 % - 12 %) may be given a dual classification such as SW-SC.Atterberg Limits
Clays and Silts, often called 'fine-grained soils', are classified according to their Atterberg limits; the most commonly used Atterberg limits are the Liquid limit (denoted by LL or wl), Plastic Limit (denoted by PL or wp), and Shrinkage limit (denoted by SL). The shrinkage limit corresponds to a water content below which the soil will not shrink as it dries.The liquid limit and plastic limit are arbitrary limits determined by tradition and convention. The liquid limit is determined by measuring the water content for which a groove closes after 25 blows in a standard test Alternatively, a fall cone test apparatus may be use to measure the liquid Limit. The undrained shear strength of remolded soil at the liquid limit is approximately 2 kPa. The plastic limit is the water content below which it is not possible to roll by hand the soil into 3 mm diameter cylinders. The soil cracks or breaks up as it is rolled down to this diameter. Remolded soil at the plastic limit is quite stiff, having an undrained shear strength of the order of about 200 kPa
The Plasticity index of a particular soil specimen is defined as the difference between the Liquid limit and the Plastic limit of the specimen; it is an indicator of how much water the soil particles in the specimen can absorb. The plasticity index is the difference in water contents between states when the soil is relatively soft and the soil is relatively brittle when molded by hand.
Classification of silts and clays
According to the Unified Soil Classification System (USCS), silts and clays are classified by plotting the values of their plasticity index and liquid limit on a plasticity chart. The A-Line on the chart separates clays (given the USCS symbol C) from silts (given the symbol M). LL=50% separates high plasticity soils (given the modifier symbol H) from low plasticity soils (given the modifier symbol L). A soil that plots above the A-line and has LL>50% would, for example, be classified as CH. Other possible classifications of silts and clays are ML, CL and MH. If the Atterberg limits plot in the"hatched" region on the graph near the origin, the soils are given the dual classification 'CL-ML'.Liquidity Index
The effects of the water content on the strength of saturated remolded soils can be quantified by the use of the liquidity index, LI:
Relative density
The density of sands (cohesionless soils) is often characterized by the relative density, Dr
Thus if Dr = 100% the sand or gravel is very dense, and if Dr = 0% the soil is extremely loose and unstable.
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