Soil testing and analysis

Understanding soil properties is crucial to a large variety of agricultural, geological, and soil science applications as well as to geotechnical, environmental, and foundation engineering projects. Soil testing can be conducted in situ or at the laboratory.

Background

Soil is the top part of the Earth’s crust; it comprises loose, unconsolidated materials that have been formed by physicochemical and disintegration of rocks. Soils are usually composed of mixtures of inorganic minerals, organic matter (humus), water, and air. Inorganic soils are mainly classified as coarse (granular) soils and fine (cohesive) soils. Coarse soils include boulders, cobbles, gravel, and sand. Fine soils include silts and clays that have a mean particle diameter of less than 0.062 millimeter. The properties of coarse soils are affected mainly by mechanical forces such as gravity, buoyancy, drag, and inertia. In addition to the mechanical forces, fine soils are affected by electrochemical phenomena such as Van der Waals forces (weak molecular forces based on electric polarization) and electric surface forces.

Depending on the purpose of the study, there are a number of in situ and laboratory soil tests. In general, soil testing and analysis can be categorized as physical or chemical. Some of the tests are focused directly on soil properties, while others investigate the behavior of the soil relative to the water content.

Physical Testing and Analysis

Physical testing investigates the physical properties and behavior of soils. The most fundamental level of physical testing includes estimation of specific gravity, determination of moisture content, sieve analysis, and hydrometer analysis.

Specific gravity is defined as the ratio of “unit weight of soil solids only” to “unit weight of water.” For sandy soils the specific gravity ranges from 2.63 to 2.67, while for clays it is between 2.67 and 2.90. The specific gravity of organic soils is less than 2.0. Moisture content is defined as the ratio of “the weight of water present in a soil sample” to “the weight of dry soil.” Moisture content is about 15 to 20 percent for coarse soils and 80 to 100 percent for clays. Organic soils may have moisture content in excess of 500 percent.

In order to determine the grain-size distribution of coarse soils, the soil sample is subjected to sieve analysis. This analysis is conducted by using a stack of sieves that have a decreasing sieve diameter from the top to the bottom. The sieves are usually made of woven wires. After the sieves are shaken for a specified period of time, the amount of sediment retained at each sieve is weighted. Based on these data, each particle diameter is plotted in terms of the percentage of material that is finer than this particular diameter. For the plot, a semilogarithmic paper is used. For the determination of particle size distribution of fine soils, the hydrometer analysis is applied. The lower limit of the particle diameter that can be detected by this analysis is 0.001 millimeter. The hydrometer measures the particle diameter indirectly. This analysis is based on the principle that the hydrometer will be subject to higher buoyancy forces in a well-mixed water-sediment system. However, as the suspended solid particles settle, the of the water-sediment mixture decreases, and the hydrometer tends to sink. The grain-size distribution is integral to studies involving sediment transport, protection against erosion, and soil contamination.

Engineering Testing and Analysis

Tests that are most useful in geotechnical and foundation engineering are those used to define the so-called Atterberg limits for soils: liquid limit, plastic limit, and shrinkage limit. The liquid limit is the moisture content that causes clay soils to behave as viscous liquids. Liquid limit is estimated by counting the number of “blows” required (using a Casagrande device) to close a groove made in the soil sample. The plastic limit of moisture content is the point at which the soil sample will turn from a plastic state to a semisolid state. The plastic limit is determined by the ability of the sample to roll and form threads 3.18 millimeters in diameter without crumbling. When clay soils are losing moisture, their volume generally decreases. The moisture content, expressed as a percentage, at which the soil volume ceases to decrease is called the shrinkage limit.

To determine the performance of soils under various loading conditions, a number of soil tests are necessary. These tests include the Proctor compaction test, the unconfined compaction test, the shear test on sands, the consolidation test, and the triaxial tests in clay. The Proctor compaction test identifies the optimum moisture content that would result in a maximum dry unit weight of soil. This information is applied to the design of airports, highways, and structural foundations. The unconfined compaction test determines the stress-strain relation of a soil specimen and is useful in studies regarding retaining walls and landslides.

The shear test in sands is used to estimate the ability of sands to sustain shear loading. It is directly related to its angle of internal friction. Quantification of the time-dependent settling of saturated clays subject to increased loading is achieved by the consolidation test. Finally, the triaxial tests in clays define the general stress relationships for unconsolidated undrained, consolidated drained, or consolidated undrained specimens.

Permeability tests are integral to hydrogeological studies. They define the flow of water through a soil sample under either a constant hydraulic head or a falling hydraulic head. Permeability tests are indicative not of soil porosity but of the connectivity among the pores and their ability to form conduits that allow the water to flow freely through the soil.

Environmental Testing and Analysis

Soils and sediment can be subjected to contamination by a variety of pollutants. Fine sediments and organic soils, because of their large specific surface, show a particular affinity to adsorb or absorb chemicals in dissolved or particulate form. The most common chemical testing of soils involves estimation of the pH value; carbonate, chloride, sulfate, and organic content; and total dissolved solids. The organic content can be defined easily through a loss-on-ignition test. The total dissolved solids can be estimated by evaporation.

Bibliography

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Das, Braja M. Soil Mechanics Laboratory Manual. 5th ed. Austin, Tex.: Engineering Press, 1997.

Day, Robert W. Soil Testing Manual: Procedures, Classification Data, and Sampling Practices. New York: McGraw-Hill, 2001.

Head, K. H. Soil Technicians’ Handbook. New York: Halsted Press, 1989.

"Is Soil Testing Worth It?" AgAmerica, 21 Dec. 2023, agamerica.com/blog/is-soil-testing-worth-it/. Accessed 6 Jan. 2025.

Kézdi, Árpád. Soil Testing. Vol. 2 in Handbook of Soil Mechanics. New York: Elsevier Scientific, 1974-1990.

Liu, Cheng, and Jack B. Evett. Soil Properties: Testing, Measurement, and Evaluation. 6th ed. Upper Saddle River, N.J.: Prentice Hall, 2008.

Mudroch, Alena, and José M. Azcue. Manual of Aquatic Sediment Sampling. Boca Raton, Fla.: Lewis, 1995.