Soil refers to the unconsolidated top layer of the ground surface. It consists of weathered rock fragments, organic matter, minerals, water and air in different proportions. BS 1377-1 1990 defines soil as follows:

An assemblage of discrete particles in the form of deposit, usually of mineral composition but sometimes of organic origin, which can be separated by gentle mechanical means and which includes variable amounts of water and air (and sometimes other gases).

Soil plays a crucial role in the construction industry. It serves not only as a construction material but also as the foundation on which all structural elements are embedded. Some of the uses of soil in the construction industry include earthworks for roadworks, embankments, and native subgrade improvement as well as between retaining walls. Therefore, the physical and mechanical properties of soil must be strictly assessed and controlled to ensure long-term structural integrity. Soil properties determine the stability, bearing capacity, and settlement characteristics of foundations, embankments, and other civil engineering works.

Soil testing is a critical component of construction and civil engineering projects, providing essential data about the physical and mechanical properties of soil. These tests help engineers determine the suitability of soil for supporting structures, ensure stability, and predict potential settlement. This article summarizes key field and laboratory tests conducted on soils in the context of construction and civil engineering.

Soil tests can be broadly divided into two namely:

• Field tests, and
• Laboratory tests

Field Tests

Field tests are tests that are conducted directly on-site or in situ to assess the properties of soil at its natural state and location. Some field tests conducted on soils include:

Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is one of the most common in-situ tests used to determine the geotechnical properties of soil. The test involves driving a split-barrel sampler into the soil at the bottom of a borehole using a hammer of a standard weight dropped from a standard height. The number of blows required to drive the sampler a specific distance (usually 3 m) is recorded as the N-value, which indicates soil density and strength. The SPT provides valuable information for evaluating soil-bearing capacity and identifying soil stratification.

Cone Penetration Test (CPT)

The Cone Penetration Test (CPT) involves pushing a cone-shaped probe into the soil at a constant rate while measuring the resistance to penetration. The CPT provides continuous data on soil stratigraphy, relative density, and strength. It is particularly useful for detecting changes in soil layers and obtaining detailed profiles of subsurface conditions. The test results are used for foundation design, soil classification, and assessing liquefaction potential.

Dynamic Cone Penetrometer (DCP)

The Dynamic Cone Penetrometer (DCP) test involves driving a metal cone into the ground using a standardized weight dropped from a fixed height, typically an 8 kg hammer from 575 mm. After each hammer blow, the depth of penetration is measured, and the rate of penetration per blow—known as the DCP Penetration Index (DPI)—is calculated. This index provides valuable insights into the soil’s bearing capacity, stiffness as well as layer thickness. With the DCP Penetration Index obtained, a correlation to the CBR of the material can be made either using standard graphs or empirical formula.

Plate Load Test

The Plate Load Test is used to determine the bearing capacity and settlement characteristics of soil. It involves placing a rigid plate on the ground and applying a series of incremental loads while measuring the corresponding settlements. The data obtained from this test helps engineers design foundations by providing information on soil compressibility and the expected settlement under structural loads. This test is particularly valuable for assessing the performance of shallow foundations.

Vane Shear Test

The Vane Shear Test is conducted to measure the shear strength of soft clays and silts. A four-bladed vane is inserted into the soil and rotated, and the torque required to shear the soil is measured. This test is simple, quick, and provides a direct measure of undrained shear strength, which is crucial for stability analysis and design of earth structures such as embankments and retaining walls.

Field Density Test

The Field Density Test is used to determine the in-situ density of compacted soils.

Engineers employ different methods to determine the field density of compacted soils, with the sand replacement method being the most common.

In the sand replacement field density test, a small hole is dug in the ground, and the excavated soil is weighed. The hole is then filled with sand of known density, and the volume of sand required to fill the hole is measured. This test helps ensure that soil compaction meets specified requirements, which is essential for the stability and performance of earthworks and pavement layers.

Laboratory Tests

Apart from field tests, engineers also carry out laboratory tests on soils. These tests require that a sample of material is excavated and ferried to the laboratory for the tests to be conducted. Some laboratory tests that are conducted on soils include:

Grain Size Analysis (Sieve Analysis, Grading or Particle Size Distribution)

Grain Size Analysis, also referred to as grading, is used to determine the distribution of particle sizes in a soil sample. The Sieve Analysis involves passing the soil through a series of sieves with different mesh sizes and weighing the retained soil on each sieve. The Hydrometer Analysis is used for finer particles that cannot be effectively sieved, measuring the rate of sedimentation in a suspension. The results of these analyses help classify soils, predict their behavior, and design appropriate construction methods.

Atterberg Limits Tests

The Atterberg Limits Tests measures the plastic and liquid limits of soil, which define the boundaries between different soil consistency states. The Liquid Limit (LL) is the moisture content at which soil changes from a plastic to a liquid state, while the Plastic Limit (PL) is the moisture content at which soil changes from a semi-solid to a plastic state. The Plasticity Index (PI), calculated as the difference between LL and PL, indicates the plasticity characteristics of the soil. These limits are essential for soil classification and predicting soil behavior under varying moisture conditions.

Proctor Compaction Test

The Proctor Compaction Test determines the optimal moisture content and maximum dry density of soil required for effective compaction. Soil samples are compacted in a mold at different moisture contents, and the dry density is measured for each sample. The relationship between moisture content and dry density is plotted, and the optimum moisture content is identified as the point where the maximum dry density is achieved. This test ensures that soil compaction meets design specifications, which is crucial for the stability and performance of embankments, subgrades, and other earth structures.

California Bearing Ratio (CBR) Test

The California Bearing Ratio of soil is one of the most widely encountered soil properties in any earthworks. The California Bearing Ratio (CBR) Test measures the strength of soil layers by comparing the penetration resistance of soil to that of a standard crushed rock. A cylindrical plunger is pressed into the soil at a constant rate, and the resistance is measured. The CBR value is expressed as a percentage of the resistance of the soil compared to the standard material. This test is widely used in pavement design to assess the load-bearing capacity of subgrade soils and determine the thickness of pavement layers.

Unconfined Compressive Strength Test

The Unconfined Compressive Strength (UCS) Test measures the compressive strength of cohesive soils. A cylindrical soil specimen is placed under axial load until failure occurs, and the maximum load is recorded. The UCS provides a direct measure of the soil’s ability to withstand compressive forces, which is essential for foundation design and stability analysis of earth structures.

Triaxial Compression Test

The Triaxial Compression Test is a comprehensive laboratory test used to determine the mechanical properties of soil under controlled conditions. A cylindrical soil specimen is enclosed in a rubber membrane and subjected to confining pressure while an axial load is applied. The test can simulate different loading conditions, such as drained and undrained, and provides valuable data on soil strength, deformation, and pore pressure behavior. The results are used for advanced geotechnical analysis and design of foundations, slopes, and retaining structures.

Consolidation Test

The Consolidation Test measures the rate and amount of settlement that soil undergoes under a given load over time. A soil specimen is placed in a consolidation cell and subjected to incremental loading while the corresponding settlements are measured. The data obtained from this test helps predict the long-term settlement behavior of soil and is essential for designing foundations and earth structures to minimize settlement-related issues.

Permeability Test

The Permeability Test measures the rate at which water flows through soil, providing information on the soil’s drainage and seepage characteristics. Various methods, such as constant head and falling head tests, are used to determine the coefficient of permeability. This data is crucial for designing drainage systems, assessing the potential for groundwater contamination, and evaluating the stability of slopes and retaining walls.

Specific Gravity Test

The Specific Gravity Test determines the specific gravity of soil particles, which is the ratio of the density of soil particles to the density of water. This test helps in soil classification and provides essential data for calculating other soil properties, such as void ratio, porosity, and degree of saturation. The specific gravity is used in various geotechnical analyses and design calculations.

Direct Shear Test

The Direct Shear Test measures the shear strength of soil by applying a horizontal shear force to a soil sample until failure occurs. The test provides valuable data on the soil’s shear strength parameters, such as cohesion and angle of internal friction, which are essential for stability analysis and design of foundations, slopes, and retaining structures.

Conclusion

Soil testing is a fundamental aspect of construction and civil engineering, providing crucial data on the properties and behavior of soil. Field tests like the Standard Penetration Test (SPT), Cone Penetration Test (CPT), Plate Load Test, Vane Shear Test, and Field Density Test offer valuable insights into soil conditions on-site. Laboratory tests, including Grain Size Analysis, Atterberg Limits Test, Proctor Compaction Test, California Bearing Ratio (CBR) Test, Unconfined Compressive Strength Test, Triaxial Compression Test, Consolidation Test, Permeability Test, Specific Gravity Test, and Direct Shear Test, provide detailed information on soil characteristics. Together, these tests ensure that soil meets the necessary standards and specifications, contributing to the safety, stability, and success of construction projects.

In the subsequent articles, we will be highlighting some of these tests in detail.