SITE INVESTIGATION FOR GEOTECHNICAL EARTHQUAKE ENGINEERING - Geotechnical Earthquake Engineering - page 131

TABLE 5.1 Factors That Can Affect the Standard Penetration Test Results

Factors that can affect
the standard penetration

test results Comments

Inadequate cleaning of SPT is only partially made in original soil. Sludge may be trapped in

the borehole the sampler and compressed as the sampler is driven, increasing the

blow count. This may also prevent sample recovery.

Not seating the sampler Incorrect N value is obtained.

spoon on undisturbed
material

Driving of the sample The N value is increased in sands and reduced in cohesive soil.

spoon above the bottom
of the casing

Failure to maintain The water table in the borehole must be at least equal to the piezometric

sufficient hydrostatic level in the sand; otherwise the sand at the bottom of the borehole may

head in boring be transformed to a loose state.

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.

Overdriven sample Higher blow counts usually result from overdriven sampler.

Sampler plugged by Higher blow counts result when gravel plugs the sampler. The

gravel resistance of loose sand could be highly overestimated.

Plugged casing High N values may be recorded for loose sand when sampling below

the groundwater table. Hydrostatic pressure causes sand to rise and
plug the casing.

Overwashing ahead Low blow count may result for dense sand since sand is loosened by

of casing overwashing.

Drilling method Drilling technique (e.g., cased holes versus mud-stabilized holes) may

result in different N values for the same soil.

Not using the standard Energy delivered per blow is not uniform. European countries have

hammer drop adopted an automatic trip hammer not currently in use in North

America.

Free fall of the drive Using more than 1.5 turns of rope around the drum and or using wire

weight is not attained cable will restrict the fall of the drive weight.

Not using the correct Driller frequently supplies drive hammers with weights varying from

weight the standard by as much as 10 lb.

Weight does not strike Impact energy is reduced, increasing the N value.

the drive cap
concentrically

Not using a guide rod Incorrect N value is obtained.

Not using a good tip If the tip is damaged and reduces the opening or increases the end area,

on the sampling spoon the N value can be increased.