The results can be used for the design of foundations. SPT is widely used for measuring the undisturbed strength of the soil and for assessing its resistance to liquefaction due to ground vibrations caused by earthquakes or other dynamic forces. The Standard Penetration Resistance is measured as the number of blows ‘N’ required to drive a split spoon sampler to a depth of 300 mm using a 63.5 kg weight falling freely through a height of 750 mm.
The standard penetration test recovers a highly disturbed sample, which is generally not suitable for tests that measure properties of the in-situ soil structure, such as density, strength, and consolidation characteristics. To overcome this limitation, the test is often run with a larger sampler with a slightly different tip shape, so the disturbance of the sample is minimized, and testing of structural properties is meaningful for all but soft soils. However, this results in blow counts which are not easily converted to SPT N-values – many conversions have been proposed, some of which depend on the type of soil sampled, making reliance on blow counts with non-standard samplers problematic.
Standard penetration test blow counts do not represent a simple physical property of the soil, and thus must be correlated to soil properties of interest, such as strength or density. There exist multiple correlations, none of which are of very high quality. The use of SPT data for direct prediction of liquefaction potential suffers from the roughness of correlations and from the need to “normalize” SPT data to account for overburden pressure, sampling technique, and other factors. Additionally, the method cannot collect accurate data for weak soil layers for several reasons
The results are limited to whole numbers for a specific driving interval, but with very low blow counts, the granularity of the results, and the possibility of a zero result, makes handling the data cumbersome.
In loose sands and very soft clays, the act of driving the sampler will significantly disturb the soil, including by soil liquefaction of loose sands, giving results based on the disturbed soil properties rather than the intact soil properties.