Properties of Granitic Residual Soil

Properties

• Granitic residual soil sourced at or near the surface has normal to low clay content, with a correspondingly low presence of mica minerals. This results in both low liquid limits and a low plasticity index. Natural clay content, and therefore moisture content, increases in correspondence with the depth from which the soil is sourced.

Importance of Analysis

• All soils have unique chemical compositions, microstructure and mineralogy. It is vital that designers, architects and structural engineers have a full knowledge of the predicted characteristics of soils so that their projects can take into account that likely behavior. The presence of contaminants, including chemicals, affects the properties of soil and therefore affect how it behaves under stress. Researchers can identify soil’s compressibility, its coefficient of permeability, its grain size and distribution, its natural water content, its shear strength, its specific gravity and its Atterberg limits. The latter is primarily a measurement of settlement characteristics.

Mineralogy and Microstructure

• Granitic residual soil consists primarily of quartz and kaolinite -- a low-expansion clay -- with a mineral presence of illite, montmorillonite and muscovite. Where quartz-based soils are typically granular, kaolinite minerals encourage a flaky shape to form. The clay’s pH value measures between 4.34 and 6.93, meaning granitic residual soils are acidic. Sulfate concentrations typically measure between 2.21 parts per million, or ppm, and 17.58 ppm; chloride concentrations typically measure between 6 ppm and 75 ppm.

Contaminated Soils

• The increasing rate of development has led to a corresponding decrease in the availability of virgin soil. The reuse of soil that has a degree of contamination inherent from a previous use is becoming more common. Such soils are commonly used in projects such as foundation work below embankments and roads. In such applications, a finite knowledge of the soil’s properties is less vital than, for instance, in the creation of substrates beneath buildings.

  Soil that has, for instance, seen previous use around the holding tanks of petrochemical plants has a presence of both lime and oil. This makes the soil less compressible; increased stiffness and resistance to compression is an asset.