Atterberg Limits and the Unified Soils Classification System.
some value of water content, the solids become so tightly packed that the volume cannot decrease further and the volume remains constant. The shrinkage limit is defined as the water content below which the soil cannot shrink.
Figure 3.2 Volume versus water content of saturated clay.
Professor Arthur Casagrande of Harvard University developed a testing device to standardize the test for liquid limit. The original device was not mounted on rubber feet, as are modern standardized devices, and the results varied, depending on whether the machine was operated directly over a table leg or not. Casagrande found that consistency was improved if the machine was placed on a copy of a telephone directory. The rubber feet were added to simulate the directory, and the consistency of readings from the liquid limit
machines was improved to a satisfactory point. The modern version of the liquid limit testing machine, as standardized by the American Society for Testing and Materials (ASTM), is shown in Figure 3.3.
machines was improved to a satisfactory point. The modern version of the liquid limit testing machine, as standardized by the American Society for Testing and Materials (ASTM), is shown in Figure 3.3.
Atterberg limits are used to classify and describe soils in several useful ways. One widely used and extremely useful soil descriptor is the plasticity index (PI ), which is defined as
Figure 3.3 Liquid limit machine.
In addition, the liquidity index (LI ), is used to indicate the level of the water content relative to the liquid limit (LL) and plastic limit (PL).
If the water content of a soil is higher than the LL, then LI is greater than 1. If a soil is drier than the PL, then LI is negative. A negative value of the LI usually indicates a problem condition where the potential for expansion on wetting is severe.
Unified Soil Classification System This system was developed by the U.S.
Army Corps of Engineers and the Bureau of Reclamation in 1952 and was subsequently adopted and standardized by the ASTM. Today, the Unified Soils Classification System is widely used in the English-speaking world.
The system uses a two-letter classification. The first letter is used to classify the grain size.
If more than 50% by weight is larger than the #200 sieve (0.074-mm openings), then a coarse-grained classification is used. If more than 50% is larger than a #4 sieve, then the letter G is used; otherwise, the letter S is used.
The system uses a two-letter classification. The first letter is used to classify the grain size.
If more than 50% by weight is larger than the #200 sieve (0.074-mm openings), then a coarse-grained classification is used. If more than 50% is larger than a #4 sieve, then the letter G is used; otherwise, the letter S is used.
For sands and gravels, the second letter describes the gradation:
W= well graded
P = poorly graded
M = contains silt
C= contains clay
M = contains silt
C= contains clay
Examples of common classifications are GP, GW, GC, GM, SP, SW, SM,
and SC.
If more than 50% by weight passes the #200 sieve, then the soil is finegrained and the first letter is M for silt, C for clay, or O for organic soil.
The second letter for fine-grained soils describes their plasticity. The letter L is used for low plasticity (LL < 50), and the letter H for high plasticity (LL > 50). The Atterberg limits are tested on the fraction of soil passing the #40 sieve, not the full fraction of soil. Examples of common classifications of fine-grained soils are ML, CL, MH, CH, OL, and OH. If a soil is highly organic, with noticeable organic fibers, it is given a peat designation, PT.
and SC.
If more than 50% by weight passes the #200 sieve, then the soil is finegrained and the first letter is M for silt, C for clay, or O for organic soil.
The second letter for fine-grained soils describes their plasticity. The letter L is used for low plasticity (LL < 50), and the letter H for high plasticity (LL > 50). The Atterberg limits are tested on the fraction of soil passing the #40 sieve, not the full fraction of soil. Examples of common classifications of fine-grained soils are ML, CL, MH, CH, OL, and OH. If a soil is highly organic, with noticeable organic fibers, it is given a peat designation, PT.
Relative Density The degree of compaction of granular soils is measured as relative density (Table 3.2), usually expressed as a percentage:
Grain Size Distribution The uniformity coefficient Cu and the coefficient of curvature (called the coefficient of gradation) Cc were developed to help engineers evaluate and classify the gradations of granular materials. These coefficients are used to determine when different materials can be used effectively as filter materials in earth dams. Their definitions are
Well-graded soils have uniformity coefficients greater than 4 for gravels and 6 for sands, and a coefficient of curvature between 1 and 3. If the soil is not well graded, it is poorly graded.
A soil with a gap-graded distribution has ranges of soil particle sizes missing. In many applications, such as filter zones in earth dams used for drainage, gap-graded soils are avoided because they can be susceptible to subsurface erosion or piping.Examples of gradation curves for well-graded and gap-graded soils are shown in Figure 3.4.
The results of the classification of the soils encountered in a borehole are of great importance to the foundation engineer because of the difference in the behavior of various soils on the imposition of loadings. The classification will guide the specification of laboratory tests, and possibly in situ tests, for acquisition of the data necessary for design.
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