The SPT is a well-established and unsophisticated method, which was developed in the United States around 1925. It has since undergone refinements with respect to equipment and testing procedure. The testing procedure varies in different parts of the world. Therefore, standardisation of SPT was essential in order to facilitate the comparison of results from different investigations. The equipment is simple, relatively inexpensive and rugged. Another advantage is that representative but disturbed soil samples are obtained. The reliability of the method and the accuracy of the result depend largely on the experience and care of the engineer on site.

A split-barrel sampler is driven from the bottom of a pre-bored hole into the soil by means of a 63.5 kg hammer, dropped freely from a height of 0.76 m. The diameter of the pre-bored hole varies normally between 60 and 200 mm. If the hole does not stay open by itself, casing or drilling mud should be used. The sampler is first driven to a depth of 15 cm below the bottom of the pre-bored hole, then the number of blows required to drive the sampler another 30 cm into the soil, the so called N30 count, is recorded. The rods used for driving the sampler should have sufficient stiffness. Normally, when sampling is carried out to depths greater than around 15 m, 54 mm rods are used.

The quality of test results depends on several factors, such as actual energy delivered to the head of the drill rod, the dynamic properties (impedance) of the drill rod, the method of drilling and borehole stabilisation. The actually delivered energy can vary between 50 - 80% of the theoretical free-fall energy. Therefore, correction factors for rod energy (60 %) are commonly used, Seed and De Alba (1986). The SPT can be difficult to perform in loose sands and silts below the ground water level (typical for land reclamation projects), as the borehole can collapse and disturb the soil to be tested. The following factors can affect the test results: nature of the drilling fluid in the borehole, diameter of the borehole, the configuration of the sampling spoon and the frequency of delivery of the hammer blows. Therefore, it should be noted that drilling and stabilisation of the borehole must be carried out with care. The measured N-value (blows/0.3 m) is the so-called standard penetration resistance of the soil. The penetration resistance is influenced by the stress conditions at the depth of the test. Peck et al. (1974) proposed, based on settlement observations of footings, the following relationship for correction of confinement pressure. The measured N-value is to be multiplied by a correction factor C_N to obtain a reference value, N₁, corresponding to an effective overburden stress of 1 t/ft2 (approximately 107 kPa),

where CN is a stress correction factor and p' is the effective vertical overburden pressure.

Seed (1976) proposed a similar correction factor for the assessment of liquefaction problems in loose saturated sands. This relationship was developed for earthquake problems and is based on extensive laboratory tests on mainly loose to medium dense sands,

where s 0' is the effective overburden pressure (in t/ft2) and s 1' is the reference stress (1 t/ft2). The correction of SPT results with respect to the effective overburden pressure is of importance for the evaluation of compaction results. Therefore, consideration should be given to this aspect when compaction criteria are to be based on N-values. Unfortunately, this fact is not always appreciated.

The resistance (N30) has been correlated with the relative density of granular soils. Sand and gravel can be classified as shown in Table 1, Broms (1986).

The Standard Penetration Test is mainly used to estimate the relative stiffness and strength (bearing capacity) of soils. Deformation characteristics of granular soils can be estimated from empirical correlations, Peck et al. (1974). It is also possible to get some indications from SPT of the shear strength in cohesive soils. The SPT used frequently for the evaluation of the liquefaction potential of water-saturated, loose sands and silts in seismic areas, Seed and De Alba (1986).