The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit.
Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic and liquid. In each state, the consistency and behavior of soil are different, and consequently so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil's behavior. The Atterberg limits can be used to distinguish between silt and clay and to distinguish between different types of silts and clays. The water content at which soil changes from one state to the other is known as consistency limits, or Atterberg's limit.
These limits were created by Albert Atterberg, a Swedish chemist and agronomist, in 1911. [1] They were later refined by Arthur Casagrande, an Austrian geotechnical engineer and a close collaborator of Karl Terzaghi (both pioneers of soil mechanics).
Distinctions in soils are used in assessing soil which is to have a structure built on them. Soils when wet retain water, and some expand in volume (smectite clay). The amount of expansion is related to the ability of the soil to take in water and its structural make-up (the type of minerals present: clay, silt, or sand). These tests are mainly used on clayey or silty soils since these are the soils which expand and shrink when the moisture content varies. Clays and silts interact with water and thus change sizes and have varying shear strengths. Thus these tests are used widely in the preliminary stages of designing any structure to ensure that the soil will have the correct amount of shear strength and not too much change in volume as it expands and shrinks with different moisture contents.
The shrinkage limit (SL) is the water content where further loss of moisture will not result in more volume reduction. [2] The test to determine the shrinkage limit is ASTM International D4943. The shrinkage limit is much less commonly used than the liquid and plastic limits.
The plastic limit (PL) is determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface. The procedure is defined in ASTM Standard D 4318. If the soil is at a moisture content where its behavior is plastic, this thread will retain its shape down to a very narrow diameter. The sample can then be remolded and the test repeated. As the moisture content falls due to evaporation, the thread will begin to break apart at larger diameters.
The plastic limit is defined as the gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm (about 1/8 inch). A soil is considered non-plastic if a thread cannot be rolled out down to 3.2 mm at any moisture possible. [3]
The liquid limit (LL) is conceptually defined as the water content at which the behavior of a clayey soil changes from the plastic state to the liquid state. However, the transition from plastic to liquid behavior is gradual over a range of water contents, and the shear strength of the soil is not actually zero at the liquid limit. The precise definition of the liquid limit is based on standard test procedures described below.
Atterberg's original liquid limit test involved mixing a pat of clay in a round-bottomed porcelain bowl of 10–12 cm diameter. A groove was cut through the pat of clay with a spatula, and the bowl was then struck many times against the palm of one hand. Casagrande subsequently standardized the apparatus (by incorporating a crank-rotated cam mechanism to standardize the dropping action) and the procedures to make the measurement more repeatable. Soil is placed into the metal cup (Casagrande cup) portion of the device and a groove is made down at its center with a standardized tool of 2 millimetres (0.079 in) width. The cup is repeatedly dropped 10 mm onto a hard rubber base at a rate of 120 blows per minute, during which the groove closes up gradually as a result of the impact. The number of blows for the groove to close is recorded. The moisture content at which it takes 25 drops of the cup to cause the groove to close over a distance of 12.7 millimetres (0.50 in) is defined as the liquid limit. The test is normally run at several moisture contents, and the moisture content which requires 25 blows to close the groove is interpolated from the test results. The liquid limit test is defined by ASTM standard test method D 4318. [4] The test method also allows running the test at one moisture content where 20 to 30 blows are required to close the groove; then a correction factor is applied to obtain the liquid limit from the moisture content. [5]
Another method for measuring the liquid limit is the fall cone test, also called the cone penetrometer test. It is based on the measurement of penetration into the soil of a standardized stainless steel cone of specific apex angle, length and mass. Although the Casagrande test is widely used across North America, the fall cone test is much more prevalent in Europe and elsewhere due to being less dependent on the operator in determining the liquid limit. [6]
Advantages over Casagrande Method
The values of these limits are used in several ways. There is also a close relationship between the limits and properties of soil, such as compressibility, permeability, and strength. This is thought to be very useful because as limit determination is relatively simple, it is more difficult to determine these other properties. Thus, the Atterberg limits are used to identify the soil's classification and allow for empirical correlations for some other engineering properties.
The plasticity index (PI) is a measure of the plasticity of soil. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid and plastic limits (PI = LL-PL). Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay.
Soil descriptions based on PI: [8]
The liquidity index (LI) is used to scale the natural water content of a soil sample to the limit. It can be calculated as a ratio of the difference between natural water content, plastic limit, and liquid limit: LI=(W-PL)/(LL-PL), where W is the natural water content.
The consistency index (Ic) indicates a soil's consistency (firmness). It is calculated as CI = (LL-W)/(LL-PL)
, where W is the existing water content. The soil at the liquid limit will have a consistency index of 0, the soil at the plastic limit will have a consistency index of 1, and if W > LL, Ic is negative. That means the soil is in the liquid state. Moreover, the sum of the Liquidity index and Consistency index is equal to 1 (one)
The curve obtained from the graph of water content against the log of blows while determining the liquid limit is almost straight and is known as the flow curve.
The equation for flow curve is: W = - If Log N + C
Where 'If is the slope of flow curve and is termed as "Flow Index" [9]
The shearing strength of clay at the plastic limit is a measure of its toughness. It is the ratio of the plasticity index to the flow index. It gives us an idea of the shear strength of the soil. [9]
The activity of soil is the ratio of the plasticity index to the clay size fraction. If activity is less than 0.75, the soil is inactive. If activity exceeds 1.4, then the soil is termed active. If activity lies within the above values, then the soil will be moderately active. [10]
NO | Description | Sand | Silt | Clay | LL | PI |
1 | Well graded loamy sand | 88 | 10 | 2 | 16 | NP |
2 | Well graded sandy loam | 72 | 15 | 13 | 16 | NP |
3 | Med graded sandy loam | 73 | 9 | 18 | 22 | 4 |
4 | Lean sandy silty clay | 32 | 33 | 35 | 28 | 9 |
5 | Lean silty clay | 5 | 64 | 31 | 36 | 15 |
6 | Loessial silt | 5 | 85 | 10 | 26 | 2 |
7 | Heavy clay | 6 | 22 | 72 | 67 | 40 |
8 | Poorly graded sand | 94 | 6 | 6 | NP | NP |
Mineral | LL, % | PL, % | SL, % |
Montmorillonite | 100-900 | 50-100 | 8.5-15 |
Nontronite | 37-72 | 19-27 | |
Illite | 60-120 | 35-60 | 15-17 |
Kaolinite | 30-110 | 25-40 | 25-29 |
Hydrated halloysite | 50-70 | 47-60 | |
Dehydrated halloysite | 35-55 | 30-45 | |
Attapulgite | 160-230 | 100-120 | |
Chlorite | 44-47 | 36-40 | |
Allopphane (undried) | 200-250 | 130-140 |
Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. It also relies on knowledge of geology, hydrology, geophysics, and other related sciences.
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.
The cone penetration or cone penetrometer test (CPT) is a method used to determine the geotechnical engineering properties of soils and delineating soil stratigraphy. It was initially developed in the 1950s at the Dutch Laboratory for Soil Mechanics in Delft to investigate soft soils. Based on this history it has also been called the "Dutch cone test". Today, the CPT is one of the most used and accepted soil methods for soil investigation worldwide.
Soil liquefaction occurs when a cohesionless saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in which material that is ordinarily a solid behaves like a liquid. In soil mechanics, the term "liquefied" was first used by Allen Hazen in reference to the 1918 failure of the Calaveras Dam in California. He described the mechanism of flow liquefaction of the embankment dam as:
If the pressure of the water in the pores is great enough to carry all the load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand... the initial movement of some part of the material might result in accumulating pressure, first on one point, and then on another, successively, as the early points of concentration were liquefied.
Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.
Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as geophysical engineering, coastal engineering, agricultural engineering, and hydrology.
This is an index of articles relating to soil.
The standard penetration test (SPT) is an in-situ dynamic penetration test designed to provide information on the geotechnical engineering properties of soil. This test is the most frequently used subsurface exploration drilling test performed worldwide. The test procedure is described in ISO 22476-3, ASTM D1586 and Australian Standards AS 1289.6.3.1. The test provides samples for identification purposes and provides a measure of penetration resistance which can be used for geotechnical design purposes. Various local and widely published international correlations that relate blow count, or N-value, to the engineering properties of soils are available for geotechnical engineering purposes.
The Fall cone test, also called the cone penetrometer test or the Vasiljev cone test, is an alternative method to the Casagrande method for measuring the Liquid Limit of a soil sample proposed in 1942 by the Russian researcher Piotr Vasiljev and first mentioned in the Russian standard GOST 5184 from 1949. It is often preferred to the Casagrande method because it is more repeatable and less variable with different operators. Other advantages of the fall cone test include the alternative to estimate the undrained shear strength of a soil based on the fall cone factor K.
A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuities in soil or rock masses.
In geotechnical engineering, soil compaction is the process in which stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water being displaced from between the soil grains, then consolidation, not compaction, has occurred. Normally, compaction is the result of heavy machinery compressing the soil, but it can also occur due to the passage of, for example, animal feet.
Geotechnical investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions; this type of investigation is called a site investigation. Geotechnical investigations are also used to measure the thermal resistance of soils or backfill materials required for underground transmission lines, oil and gas pipelines, radioactive waste disposal, and solar thermal storage facilities. A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves soil sampling and laboratory tests of the soil samples retrieved.
In materials science, a triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil and rock, and other granular materials or powders. There are several variations on the test. In a triaxial shear test, stress is applied to a sample of the material being tested in a way which results in stresses along one axis being different from the stresses in perpendicular directions. This is typically achieved by placing the sample between two parallel platens which apply stress in one direction, and applying fluid pressure to the specimen to apply stress in the perpendicular directions.
In soil science, soil gradation is a classification of a coarse-grained soil that ranks the soil based on the different particle sizes contained in the soil. Soil gradation is an important aspect of soil mechanics and geotechnical engineering because it is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity. In a design, the gradation of the in situ soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil, if it is not high in clay quality.
Albert Mauritz Atterberg was a Swedish chemist and agricultural scientist who created the Atterberg limits, which are commonly referred to by geotechnical engineers and engineering geologists today. In Sweden he is equally known for creating the Atterberg grainsize scale, which remains the one in use.
Arthur Casagrande was an American civil engineer born in Austria-Hungary who made important contributions to the fields of engineering geology and geotechnical engineering during its infancy. Renowned for his ingenious designs of soil testing apparatus and fundamental research on seepage and soil liquefaction, he is also credited for developing the soil mechanics teaching programme at Harvard University during the early 1930s that has since been modelled in many universities around the world.
Preconsolidation pressure is the maximum effective vertical overburden stress that a particular soil sample has sustained in the past. This quantity is important in geotechnical engineering, particularly for finding the expected settlement of foundations and embankments. Alternative names for the preconsolidation pressure are preconsolidation stress, pre-compression stress, pre-compaction stress, and preload stress. A soil is called overconsolidated if the current effective stress acting on the soil is less than the historical maximum.
An oedometer test is a kind of geotechnical investigation performed in geotechnical engineering that measures a soil's consolidation properties. Oedometer tests are performed by applying different loads to a soil sample and measuring the deformation response. The results from these tests are used to predict how a soil in the field will deform in response to a change in effective stress.
In soil mechanics, dilatancy or shear dilatancy is the volume change observed in granular materials when they are subjected to shear deformations. This effect was first described scientifically by Osborne Reynolds in 1885/1886 and is also known as Reynolds dilatancy. It was brought into the field of geotechnical engineering by Peter Walter Rowe.