TABLE 13
Procedures Which May Affect the Measured "N" Values
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* Inadequate
* SPT is only partially made in original soil.
*
* cleaning of
* Sludge may be trapped in the sampler and compressed
*
* the borehole
* as the sampler is driven, increasing the blow count.
*
* (This may also prevent sample recovery.)
*
*
*
*
*
* Not seating the
* Incorrect "N" values obtained.
*
* sampler spoon
*
*
* on undisturbed
*
*
* material
*
*
*
*
*
* Driving of
* "N" values are increased in sands and reduced in
*
* sthe sample
* cohesive soils.
*
* spoon above
*
*
* the bottom of
*
*
*
*
* the casing
*
*
*
* Failure to main-
* The water table in the borehole must be at least
*
* tain sufficient
* equal to the piezometric level in the sand, otherwise
*
* hydrostatic
* the sand at the bottom of the borehole may be
*
* head in boring
* transformed into a loose state.
*
*
*
*
* Attitude of
* Blow counts for the same soil using the same rig can
*
* operators
* vary, depending on who is operating the rig, and
*
*
* perhaps the mood of operator and time of drilling.
*
*
*
*
* Overdrive sampler * Higher blow counts usually result from overdriven
*
* sampler
* sampler.
*
*
*
*
* Sampler plugged
* Higher blow counts result when gravel plugs sampler,
*
* by gravel
* resistance of loose sand could be highly overestimated.
*
*
*
*
* Plugged casing
* High "N" values may be recorded for loose sand when
*
*
* sampling below groundwater table. Hydrostatic
*
*
* pressure causes sand to rise and plug casing.
*
*
*
*
* Overwashing ahead * Low blow count may result for dense sand since sand
*
* of casing
* is loosened by overwashing.
*
*
*
*
* Drilling method
* Drilling technique (e.g., cased holes vs. mud
*
*
* stabilized holes) may result in different "N" values
*
*
* for the same soil.
*
*
*
*
* Not using the
* Energy delivered per blow is not uniform. European
*
* standard
* countries have adopted an automatic trip hammer not
*
* hammer drop
* currently in use in North America.
*
*
*
*
* Free fall of the
* Using more than 1-1/2 turns of rope around the drum
*
* drive weight is
* and/or using wire cable will restrict the fall of
*
* not attained
* the drive weight.
*
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7.1-90
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