Blow counts are affected by operational procedures, by the presence of gravel, or cementation. They do not reflect fractures or slickensides in clay, which may be very important to strength characteristics. The standard penetration test results (N values) are influenced by operational procedures as given in the following table.

Standard Penetration Test
Standard Penetration Test



Its Affect

1. Inadequate cleaning of the borehole SPT is only partially made in original soil. Sludge may be trapped in the sampler and compressed as the sampler is driven, increasing the blow count. This may also prevent sample recovery.
2. Not seating the sampler spoon on undisturbed material Incorrect “N” value obtained
3. Driving of the sample spoon above the bottom of the casing “N” values are increased in sands and reduced in cohesive soils.
4. Failure to maintain sufficient hydrostatic head in boring The water table in the borehole must be at least equal to the piezometric level in the sand, otherwise the sand at the bottom of the borehole may be transformed into a loose state.
5. Attitude of operators Blow counts for the same soil using the same rig can vary, depending on who is operating the rig, and perhaps the mood of operator and time of drilling.
6. Overdriving the sampler Higher blow counts usually result from overdriven sampler
7. Sampler plugged by gravel Higher blow counts usually result when gravel plugs sampler, resistance of loose sand could be highly overestimated.
8. Plugged casing High “N” values may be recorded for loose sand when sampling below ground water table. Hydrostatic pressure causes sand to rise and plug casing.
9. Over washing ahead of casing Low blow count may result for dense sand since sand is loosened by over washing.
10. Drilling method Drilling technique (e.g. cased holes vs mud stabilized hole) may result in different “N” values for the same soil.
11. Not using the standard hammer drop Energy delivered per blow is not uniform. European countries have adopted an automatic trip hammer not currently in use in North America.
12. Free fall of the drive weight is not attained Using more than 1-1/2 turns of rope around the drum and/or using wire cable will restrict the fall of the drive weight.
13. Not using correct weight Driller frequently supplies drive hammers with weight varying from the standard by as much as 5kg.
14. Weight does not strike the drive cap concentrically Impact energy is reduced, increasing “N” values.
15. Not using a guide rod Incorrect “N” value obtained
16. Incorrect drilling procedures The SPT was originally developed from wash boring techniques. Drilling procedures which seriously disturb the soil will affect the “N” value, e.g. drilling with cable tool equipment.
17. Using drill holes that are too large Holes greater than 10 cm in diameter are not recommended. Use of larger diameters may result in decreases in the blow count.
18. Inadequate supervision Frequently a sampler will be impeded by gravel or cobbles causing a sudden increase in blow count; this is not recognized by an inexperienced observer. Accurate recording of drilling, sampling and depth is always required.
19. Improper logging of soils Not describing the sample correctly
20. Using too large a pump Too high a pump capacity will loosen the soil at the base of the hole causing a decrease in blow count.


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