Standard penetration test

After the borehole has been advanced to a desired depth and excess cuttings have been removed, testing and sampling can be conducted.

The sampling tube is driven into the ground at the bottom of a borehole by blows from a hammer with a mass of 63.5 kg (140 lb) falling a distance of 75 cm (30 in).

The soil strength parameters which can be inferred from SPT results are approximate, but may give a useful guide in ground conditions where more advanced laboratory testing is not practical or possible.

Another benefit of the test is the collection of a disturbed soil sample for moisture content determination, as well identification and classification purposes.

While sample quality is generally not suitable for laboratory testing for engineering properties because of the disturbance of the soil causing changes in engineering properties, the use of a thin-walled tube sampler may result in less disturbance in soft soils.

Although this is not a standard penetration test, and should not be regarded as such, it may at least give an indication as to whether the deposit is really as loose as the standard test may indicate SPT can also be used for empirical determination of a sand layer's susceptibility to soil liquefaction, based on research performed by Harry Seed, T. Leslie Youd, and others.

Despite its many flaws, it is usual practice to correlate SPT results with soil properties relevant for geotechnical engineering design.