The standard penetration test (SPT) is an in-situ dynamic penetration test designed to provide information on the geotechnical engineering properties of soil. This test is the most frequently used subsurface exploration drilling test performed worldwide. The test procedure is described in ISO 22476-3, ASTM D1586 and Australian Standards AS 1289.6.3.1. The test provides samples for identification purposes and provides a measure of penetration resistance which can be used for geotechnical design purposes. Various local and widely published international correlations that relate blow count, or N-value, to the engineering properties of soils are available for geotechnical engineering purposes.
Test procedure
A borehole shall be advanced incrementally to permit intermittent or continuous sampling. Intervals are typically 1.5 m (5 ft) or less in homogeneous strata. Tests and sampling should be done at every change in strata.
After the borehole has been advanced to a desired depth and excess cuttings have been removed, testing and sampling can be conducted.
The test uses a thick-walled sampling tube, with an outside diameter of 5.08 cm and an inside diameter of 3.5 cm, and a length of around 65 cm. 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 76 cm (30 in). The sample tube is driven a total of 45 cm into the ground and the number of blows needed for the tube to penetrate each 15 cm (6 in) interval up to a depth of 45 cm (18 in) is recorded. The sum of the number of blows required for the second and third 15 cm (6 in) intervals of penetration is termed the "standard penetration resistance" or the "N-value". N-value provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae.
The test may be stopped if any of the following conditions are met: (1) a total of 50 blows have been applied in any one of the 15 cm (6 in) intervals, (2) a total of 100 blows have been applied over the total 45 cm (18 in) depth, or (3) there is no observed advance of the sampling tube during 10 successive blows.
Uses
The great merit of the test is that it is simple, inexpensive, and widely used. 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.
When the test is carried out in granular soils below groundwater level, the soil may become loosened. In certain circumstances, it can be useful to continue driving the sampler beyond the distance specified, adding further drilling rods as necessary. 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. When used for this purpose, the N-value should be normalized to a standard overburden stress level.
Correlation with soil mechanical properties
Despite its many flaws, it is usual practice to correlate SPT results with soil properties relevant for geotechnical engineering design. SPT results are in-situ field measurements and are often the only test results available. As such, the use of correlations has become common practice in many countries.
One approximate relationship between SPT N-value, relative density, and bulk density for coarse-grained material can be seen in the table below. This is cited in the US Army Corps of Engineers engineering manual publication on sheet pile design developed after Terzaghi and Peck (1948) and Teng (1962).