There are many pitfalls associated with the standard penetration test. If these are not carefully avoided, the N-values collected from SPT are mere numbers rather than being useful indicators of soil parameters. Some of the precautions to be taken are:
Results of standard penetration test are not reproducible in cohesion less soil below water level unless care is taken to maintain the water level inside the borehole always slightly above the natural groundwater level. If the water level in the borehole is lower than natural groundwater level, quick conditions develop and soil becomes loose.
The split spoon sampler must be in good condition with no excessive damage or wear and tear to the cutting shoe.
The drill rods should be the right size and not too heavy or too light. The drill rods also should not be bent.
The fall of the monkey weight should be free. Friction in the pulley or guide rod, or braking action by crew, or interference due to hoist rope can result in higher than actual blow count.
The height of free fall of monkey weight must be 750 mm. It is obvious that the change in the height of fall will result in a value different from the actual value for N.
The bottom of borehole must be properly cleaned before seating the split spoon sampler. Otherwise the test will be carried out in the loose and disturbed soil at the bottom of the bore hole.
If casing is used in borehole it must not be driven ahead of the level at which SPT is being carried out. Otherwise the SPT will be carried out in a soil plug enclosed at the bottom of the casing
The rate of delivery of the blows should not be too fast.
Careless work on the part of drilling crew, improper and incorrect counting of blows and recording must be avoided.