Soil consistency limits
In its natural state, soil appears in various forms. All soil characteristics, from its natural humidity to the distribution of particle sizes, provide us with a background for understanding its behavior. Each piece of information that brings us closer to understanding its character helps us predict future situations or remedy existing ones at a particular location with greater confidence and certainty. One of the essential tests in this regard is the soil consistency limits test.
Given the large number of different types of soil, engineering practice is facilitated by soil classification – a procedure of organizing different soils into groups of similar properties. This enables engineers to study, understand and compare their test results. By classifying the soil, it is possible to roughly determine the expected movement range of soil properties.
In geotechnical practice, we describe soil in order to characterize its physical and mechanical properties. For coherent soils, information on their consistency is especially important.
Soil consistency refers to the level of soil stiffness in the state of natural humidity. Moist fine-grained soils show a distinct plasticity, depending on their humidity content. If the humidity is very low, the soil is in a solid consistency state, it is brittle and can easily break into pieces. By gradually increasing the humidity, the soil changes to a semi-solid consistency state, then to a plastic consistency state, and finally, at high humidity, to a liquid or granular consistency state. At the limits between these consistency states, the corresponding humidity values are called the compression limit wS, the plasticity limit wP and the yield limit wL. These consistency limits are also called Atterberg limits. Fine-grained soil has the property of plasticity at all humidity values between the plasticity limit and the liquid limit.
The liquid limit wL is the humidity at which the soil changes from a liquid to a plastic state. To determine the liquid limit, Arthur Casagrande developed a special device in which a wet soil sample is spread around an oval bowl, then an 11 mm wide groove is cut in the soil sample with a special knife, and finally the bowl is dropped rhythmically from a certain height onto a base. The number of blows of the bowl onto the base necessary to close the groove in the soil sample to a length of about 1 cm is counted. This procedure is repeated at different sample humidities. The humidity of the sample is recorded on the ordinate of the corresponding diagram, and the number of blows required to close the groove is recorded on the abscissa in a logarithmic scale. The point corresponding to 25 blows is marked on the resulting curve. The humidity content of the sample corresponding to that point is the liquid limit wL.
Alternatively, the liquid limit may be determined using the falling cone method, where a cone – either with a mass of 80 g and tip angle of 300, or with a mass of 60 g and tip angle of 600 – is placed against a bowl with moist soil and allowed to penetrate into the soil sample under its own weight. The liquid limit is reached when the cone penetrates 20 mm into the soil for a cone with an angle of 300, or 10 mm for a cone with an angle of 600.
The plasticity limit wP is the humidity at which the soil becomes too dry and loses plasticity, i.e., it cannot be shaped without breaking its continuity. The procedure for determining the plasticity limit includes manually rolling small cylinders of wet soil on a suitable surface (Figure 3). If the cylinders break at a thickness of about 3 mm, they are at the limit of plasticity.
In soil mechanics, the plasticity limit and the liquid limit are the most important parameters which define the range of soil humidity at which the soil behaves plastically, so the plasticity index is defined as:
It is important to know how close the natural soil humidity is to the liquid limit, so the consistency index is defined as:
Based on the limits of soil consistency conditions, specifically, based on the liquid limit and soil plasticity index, a plasticity diagram is created which helps us classify fine-grained soils.
As simple as they may seem, both methods of determining the liquid limit, as well as the method of determining the plasticity limit, have been repeatedly proven as appropriate and reliable methods of soil testing for soil classification purposes and for defining the physical and mechanical properties of soil.
*References:
Kovačiević, M.S., Jurić-Kaćunić, D. (2014): Europska klasifikacija tla za inženjerske potrebe, Građevinar 66 (2014) 9, 801-810.
Mulabdić, M., Glavaš, T. (2000): Određivanje konzistentnog stanja tla, Građevinar 52 (2000) 12, 719-725.
Szavits-Nossan, V. (2012): Mehanika tla i stijena, 3. Predavanje, Građevinski fakultet Sveučilišta u Zagrebu
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