What does the Standard Penetration Test (SPT) measure?

The Standard Penetration Test (SPT) measures the number of blows required to drive a sampler into the ground to a specified depth, typically 300 mm (1 foot) in undisturbed soil conditions. This test provides valuable information about the soil's resistance to penetration, which correlates with various engineering properties, such as density, consistency, and bearing capacity. It is commonly used in geotechnical engineering for site investigations, where understanding the subsurface conditions is crucial for the design and construction of foundations and other structures.

The number of blows recorded during the test as the sampler is pushed into the ground gives insights into the soil's characteristics. A higher number of blows indicates denser or more compacted soil, while a lower number suggests a softer or less compacted stratum. This relationship is vital for engineers to assess the suitability of the soil for construction projects.

The other options provided do not accurately represent what the SPT measures. The test does not directly measure soil compaction, water content, or density, although the results can be used indirectly to infer these properties.