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, a soil may appear in one of four states: solid, semi-solid, plastic and liquid. In each state, the consistency and behavior of a soil is 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 the soils change from one state to the other are 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-born American geotechnical engineer and close collaborator of Karl Terzaghi (both pioneers of soil mechanics).
Distinctions in soil are used in assessing the soils that are to have structures 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 that 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.
Moisture content is defined as quantity of water that exists in the soil mass. It can represent either the naturally present or water which is manually added. The term moisture content is otherwise known as water content. It is expressed in the percentage. The moisture content in the soil is to be determined using simple test. Take the small quantity of soil from the sample for which the moisture content is to be found.
Take weight of the soil sample after oven drying the sample. Take the weight of the oven dried soil sample. The moisture content is calculated using the expression below.
Here, the term w is moisture content in soil sample, wm is the moist weight of the sample and wd is oven dried weight of the soil.
The moisture content is important for the compaction of soil. It also affects the permeability of soil. When the moisture content increases, permeability of soil gets decreases. The shear strength is greatly affected by the moisture content. When water content in soil is low, then shear strength is high. When the moisture content is higher in soil, the bearing capacity of the soil also decreases.
The moisture will partially fill the voids present inside the soil. Thus, percentage of water present in the clay soil, with same voids volume as the sandy soil will be higher. This is because of the voids in the clayey soil, which will not interconnected, compared to the sandy soils or granular soils. This is because of the lower permeability of the clayey soil.
A triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil (e.g., sand, clay) 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 (usually vertical) direction, and applying fluid pressure to the specimen to apply stress in the perpendicular directions. (Testing apparatus which allows application of different levels of stress in each of three orthogonal directions are discussed below, under “True Triaxial test”.)
The application of different compressive stresses in the test apparatus causes shear stress to develop in the sample; the loads can be increased and deflections monitored until failure of the sample. During the test, the surrounding fluid is pressurized, and the stress on the platens is increased until the material in the cylinder fails and forms sliding regions within itself, known as shear bands. The geometry of the shearing in a triaxial test typically causes the sample to become shorter while bulging out along the sides. The stress on the platen is then reduced and the water pressure pushes the sides back in, causing the sample to grow taller again. This cycle is usually repeated several times while collecting stress and strain data about the sample. During the test the pore pressures of fluids (e.g., water, oil) or gasses in the sample may be measured using Bishop’s pore pressure apparatus.
From the triaxial test data, it is possible to extract fundamental material parameters about the sample, including its angle of shearing resistance, apparent cohesion, and dilatancy angle. These parameters are then used in computer models to predict how the material will behave in a larger-scale engineering application. An example would be to predict the stability of the soil on a slope, whether the slope will collapse or whether the soil will support the shear stresses of the slope and remain in place. Triaxial tests are used along with other tests to make such engineering predictions.
