The Standard Penetration Test (SPT) is a combined sampling and in-situ test technique applied within a borehole. The test method provides a soil sample for identification purposes and for laboratory tests that allow the use of disturbed samples. In addition, there are many geotechnical correlations which relate SPT blow count, or N-value, and geotechnical behavior. .
To get reliable results the SPT should be carried out under controlled conditions. The procedures followed during the test effect the test results. These are, method of releasing the hammer, rod energy ratio, dynamic efficiency, standard rod energy ratio, effect of rod length, using sampler with liners, and size of borehole diameter.
Although standard penetration tests (SPT) are commonly used for evaluating subsurface conditions, there are many problems with using them to numerically characterize the static properties of residual soils. The dynamic penetration of the sampler severely remolds the soil and destroys the important latent rock structure of residual soils. Low N-values are often recorded and may indicate that the soil is much more compressible than it actually is. With SPT, the engineer cannot separate the parasitic test variability from true site variability. .
The real dilemma for the engineer is evaluating whether the low N-value is indicative of a soft zone or whether it is a result of destroying the latent rock structure. This uncertainty forces the engineer to use very conservative values for soil properties and assume the site has high variability. The engineer must use low allowable bearing pressures for foundation design, whether they are needed or not. When the foundation design is overly conservative, the engineer has wasted the owner's money. .
The SPT is commonly performed at 1.5 meter depth intervals. Often, a soft or loose zone can be missed between sampling intervals or identified as being thicker than it actually is due to an error in estimating the location of strata changes.
