How do you test soil samples for penetration?

1 Standard Penetration Test (SPT)

The standard penetration test (SPT) is one of the most widely used soil penetration tests, especially for shallow foundations and geotechnical investigations. It consists of driving a split-spoon sampler into the soil using a hammer and a drill rig, and counting the number of blows required to achieve a certain depth. The SPT is performed in a borehole that is drilled to the desired depth, and the sampler is advanced in increments of 150 mm. The number of blows for each increment is recorded, and the SPT value (N) is calculated as the sum of the blows for the last two increments. The SPT value reflects the relative density and bearing capacity of the soil.

2 Cone Penetration Test (CPT)

The cone penetration test (CPT) is another common soil penetration test, especially for soft soils and deep foundations. It involves pushing a cone-shaped probe into the soil at a constant rate using a hydraulic or electric system, and measuring the tip resistance and the sleeve friction of the probe. The CPT is performed without drilling a borehole, and the probe is advanced in increments of 20 mm. The tip resistance and the sleeve friction are recorded, and the CPT value (qc) is calculated as the ratio of the tip resistance to the sleeve friction. The CPT value indicates the soil type and strength, as well as the pore pressure and the moisture content of the soil.

3 Dynamic Penetration Test (DPT)

The dynamic penetration test (DPT) is a simple and economical soil penetration test, especially for preliminary site investigations and quality control. It consists of dropping a weight on a rod that is attached to a cone or a plate, and measuring the penetration depth of the rod after each drop. The DPT is performed without drilling a borehole, and the rod is advanced in increments of 100 mm. The number of drops for each increment is recorded, and the DPT value (N) is calculated as the number of drops required to achieve a penetration depth of 300 mm. The DPT value reflects the compaction and stiffness of the soil.

4 Interpretation of Results

The results of soil penetration tests can be used to estimate various soil parameters and properties, such as density, strength, modulus, shear resistance, liquefaction potential, settlement, and bearing capacity. However, the interpretation of results depends on several factors, such as soil type, moisture content, stress history, disturbance, and testing conditions. Therefore, it is important to use empirical correlations and charts that are appropriate for the specific soil and test method, and to compare the results with other sources of information, such as laboratory tests, field observations, and site history.

5 Limitations and Precautions

Soil penetration tests are useful and convenient tools for civil engineering applications, but they also have some limitations and precautions that should be considered. For instance, these tests are not suitable for very hard or very soft soils, or soils with large particles, such as gravel and boulders. Furthermore, they are sensitive to the equipment and the operator, and may vary depending on the type, size, shape, and condition of the probe, the hammer, the rig, and the driving or pushing system. They are also affected by soil heterogeneity, anisotropy, and stratification. Additionally, soil disturbance, consolidation, and drainage can influence test results and may not reflect the in-situ or equilibrium soil state. To ensure accuracy and reliability of results, soil penetration tests should be conducted in accordance with relevant standards and specifications. Test procedures and parameters should be documented and reported. Tests should also be regularly performed at intervals and locations to check for consistency. Moreover, these tests should be supplemented by other methods of soil investigation such as sampling, laboratory testing, geophysical testing, and visual inspection. Finally, results should be interpreted with caution and judgement while being validated with field performance data.

6 Here’s what else to consider

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