Besides the difference in grain size, the chemical composition of soil can also be helpful in distinguishing between various types of soils. Sand and gravel usually consist of the same minerals as the original rock from which they were created by the erosion process. This can be quartz, feldspar or glimmer. In Western Europe sand mostly consist of quartz. The chemical formula of this mineral is SiO2.
Fine-grained soils may contain the same minerals, but they also contain the so-called clay minerals, which have been created by chemical erosion. The main clay minerals are kaolinite, montmorillonite and illite. In the Netherlands the most frequent clay mineral is elite. These minerals consist of compounds of aluminum with hydrogen, oxygen and silicates. They differ from each other in chemical composiition, but also in geometrical structure, at the microscopic level. The microstructure of clay usually resembles thin plates. On the microscale there are forces between these very small elements, and ions of water may be bonded. Because of the small magnitude of the elements and their distances, these forces include electrical forces and the Van der Waals forces.
Although the interaction of clay particles is of a different nature than the interaction between the much larger grains of sand or gravel, there are many similarities in the global behaviour of these soils. There are some essential differences, however. The deformations of clay are time.
Consistency limits
For very fine soils, such as silt and clay, the consistency is an important property. It determines whether the soil can easily be handled, by soil moving equipment, or by hand. The consistency is often very much dependent on the amount of water in the soil. This is expressed by the water content wL. (see also chapter 3). It is defined as the weight of the water per unit weight of solid material,
When the water content is very low (as in a very dry clay) the soil can be very stiff, almost like a stone. It is then said to be in the solid state. Adding water, for instance if the clay is flooded by rain, may make the clay plastic, and for higher water contents the clay may even become almost liquid. In order to distinguish between these states (solid, plastic and liquid) two standard tests have been agreed upon, that indicate the consistency limits. They are sometimes denoted as the Atterberg limits, after the Swedish engineer who introduced them.
The transition from the liquid state to the plastic state is denoted as the liquid limit, wL. It represents the lowest water content at which the soil behaviour is still largely liquid. As this limit is not absolute, it has been defined as the value determined in a certain test, due to Casagrande, see Figure 2.2. In the test a hollow container with a soil sample may be raised and dropped by rotating an axis. The liquid limit is the value of the water content for which a standard V-shaped groove cut in the soil, will just close after 25 drops. When the groove closes after less than 25 drops, the soil is too wet, and some water must be allowed to evaporate. By waiting for some time, and perhaps mixing the clay some more, the water content will have decreased, and the test may be repeated, until the groove is closed after precisely 25 drops. Then the water content must immediately be determined, before any more water evaporates, of course.
An alternative for Casagrande’s test is the fall cone, see Figure 2.3. In this test a steel cone, of 60 grams weight, and having a point angle of 60◦ , is placed upon a clay sample, with the point just at the surface of the clay. The cone is then dropped and its penetration depth is measured. The liquid limit has been defined as the water content corresponding to a penetration of exactly 10 mm. Again the liquid limit can be determined by doing the test at various water contents. It has also been observed, however, that the penetration depth, when plotted on a logarithmic scale, is an approximately linear function of the water content. This means that the liquid limit may be determined from a single test, which is much faster, although less accurate .
The transition from the plastic state to the solid state is called the plastic limit, and denoted as wP . It is defined as the water content at which the clay can just be rolled to threads of 3 mm diameter. Very wet clay can be rolled into very thin threads, but dry clay will break when rolling thick threads. The (arbitrary) limit of 3 mm is supposed to indicate the plastic limit. In the laboratory the test is performed by starting with a rather wet clay sample, from which it is simple to roll threads of 3 mm. By continuous rolling the clay will gradually become drier, by evaporation of the water, until the threads start to break. For many applications (potteries, dike construction) it is especially important that the range of the plastic state is large. This is described by the plasticity index PI. It is defined as the difference of the liquid limit and the plastic limit,
The plasticity index is a useful measure for the possibility to process the clay. It is important for potteries, for the construction of the clay core in a high dam, and for the construction of a layer of low permeability covering a deposit of polluted material. In all these cases a high plasticity index indicates that the clay can easily be used without too much fear of it turning into a liquid or a solid. In countries with very thick clay deposits (England, Japan, Scandinavia) it is often useful to determine a profile of the plastic limit and the liquid limit as a function of depth, see Figure 2.4. In this diagram the natural water content, as determined by taking samples and immediately determining the water content, can also be indicated.
An international classification system
The large variability of soil types, even in small countries such as the Netherlands, leads to large variations in soil properties in soils that may resemble each other very much at first sight. This is enhanced by confusion between terms such as sandy clay and clayey sand that may be used by local firms. In some areas tradition may have also lead to the use of terms such as blue clay or brown clay, that may be very clear to experienced local engineers, but have little meaning to others.
Uniform criteria for the classification of soils do not exist, especially because of local variations and characteristics. The soil in a plane of Tibet may be quite different from the soil in Bolivia or Canada, as their geological history may be quite different. The engineer should be aware of such differences and remain open to characterizations that are used in other countries. Nevertheless, a classification system that has been developed by the United States Bureau of Reclamation, is widely used all over the world. This system consists of two characters to indicate a soil type, see Table 2.2. A soil of type SM, for instance, is a sility sand, which indicates that it is a sand, but containing considerable amounts of
non-organic fine silty particles. This type of soil is found in the Eastern Scheldt in the Netherlands. The sand on the beaches of the Netherlands usually is of the type SW. A clay of very low plasticity, that is a clay with a relatively small plasticity index is denoted as CL. The clay in a polder in Holland will often be of the type CH. It has a reasonably large range of plastic behaviour. The characterization well graded indicates that a granular material consists of particles that together form a good framework for stress transfer. It usually is relatively stiff and strong, because the smaller particles fill well in the pores between the larger particles. A material consisting of large gravel particles and fine sand is called poorly graded, because it has little coherence. A well graded material is suitable for creating a road foundation, and is also suitable for the production of concrete. Global classifications as described above usually have only little meaning for the determination of mechanical properties of soils, such as stiffness and strength. There may be some correlation between the classification and the strength, but this is merely indicative. For engineering calculations mechanical tests should be performed, in which stresses and deformations are measured.
