One of the ways to classify the soil apart from granulometry is thanks to the Atterberg limits, which are basically indexes derived from the branch of agronomy, which define the consistency of a soil in relation to its water content, by determining from humidity.
Moisture is defined as the weight of the water in the soil, divided by the weight of the dry soil, the weight of water is measured by making the difference between the weight of the moist soil minus the weight of the dry soil.
To do this, the soil is dried in an oven and the sample is measured before and after drying.
Taking the above into account, Atterberg defined three limits: the shrinkage or consistency limit that separates the dry solid state and the semi-solid state, the plastic limit (Wp) that separates the semi-solid state from plastic and the liquid limit (WL), that separates the plastic state from the semi-liquid; These last two limits are the ones that are most used in practice, they are determined with the fraction of soil that passes through the sieve No. 40 A.S.T.M (0.1 mm).
The plastic limit
To determine the plastic limit of Atterberg, with the palm of the hand begin to knead a little dry soil until obtaining elongated ellipsoids of about 3mm in diameter for a length of 25 to 30mm, if at that moment it has been achieved that the masses have these approximate measurements, the ellipsoids begin to crack in fractions of about 6mm, it is said that that humidity is the plastic limit of the soil.
This limit is determined by kiln-drying various materials made ellipsoids that have been made under similar conditions.
If they do not crack, the ellipsoids are made again so that they lose moisture and then crack.
The liquid limit
The liquid limit is estimated by taking a mass of soil mixed with enough water to a mold called Cuchara de Casagrande.
Then proceed to open a groove in the center of the mass, forming a channel approximately 2mm wide in the lower area.
Subsequently, the mold is placed on top of a rigid base and controlled blows begin. The Atterberg liquid limit corresponds to the humidity of the sample such that when 25 blows are given the channel closes about 12mm. As it is difficult to achieve this condition, the humidity is determined by interpolation, from two samples, where the closing of the channel must be achieved with more or less blows than 25.
Shrinkage limit or consistency
The Atterberg shrinkage or consistency limit is the one that is determined by the minimum amount of water that is necessary to fill only the pores of the dry soil sample, therefore, it corresponds to a smaller volume of the soil mass.
This limit is also characterized in that it determines the border limit between the semi-solid state and the solid state.
Despite the Shrinkage limit being reached, the sample volume does not decrease, even when the moisture content is reduced.
This test consists of placing a sample of moist soil inside a shallow cylinder, which has a known volume, from which the air is then extracted and dried in an oven at a temperature of 110°C until its weight is constant..
Next, the dry soil is submerged in mercury, measuring the displaced volume.
Thus, the reduction of the volume in the soil sample allows determining the Shrinkage Limit of that soil.
The main component of a soil, whose influence is decisive in the magnitude of the Shrinkage Limit, is clay, since the shrinkage limit decreases as the clay content increases.
On the other hand, in sands, this limit is located very close to the liquid limit, and in sandy soils with clay, depending on the clay and silt content, the Shrinkage limit can be of the order of 12 to 20%.
Also, in clays, this limit generally reaches values between 5 and 10%. In this type of soil, the bearing capacity increases when the percentage of humidity decreases.
On the contrary, in confined sands, the bearing capacity is generally high, even with considerable percentages of contained water.
Plasticity Index and Large House Plasticity Chart
When the liquid and plastic limit are determined, a representative point of each soil sample represented on the Casagrande plasticity chart can be calculated, which basically represent the relationship of the liquid limit with the plasticity index.
The plasticity index is: IP=LL-LP (liquid limit minus the plastic limit)
Thanks to various studies, Casagrande was able to define that soils with a liquid limit greater than 50 are considered to be of high plasticity, that is, they admit a large amount of water and generally experience significant plastic deformations.
Below this value, soils are considered to be of low plasticity.
A line called Line A was also defined, which is parallel to the direction with which it orders the sample of the same land.