TABLE 13 (continued)
Procedures Which May Affect the Measured "N" Values
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* Driller frequently supplies drive hammers with
*
* Not using correct
* weight
* weights varying from the standard by as much as
*
*
* 10 lbs.
*
*
*
*
* Weight does not strike * Impact energy is reduced, increasing "N" values.
*
* the drive cap
*
*
* concentrically
*
*
*
*
*
* Not using a guide rod
* Incorrect "N" value obtained.
*
*
*
*
* Not using a good tip
* If the tip is damaged and reduces the opening or
*
* on the sampling spoon
* increases the end area the" N" value can be
*
*
* increased.
*
*
*
*
* Use of drill rods
* With heavier rods more energy is absorbed by the
*
* heavier than standard
* rods causing an increase in the blow count.
*
*
*
*
* Not recording blow
* Incorrect "N" values obtained.
*
* counts and penetration *
*
* accurately
*
*
*
*
*
* Incorrect drilling
* The SPT was originally developed from wash boring
*
* procedures
* techniques. Drilling procedures which seriously
*
*
* disturb the soil will affect the "N" value, e.g.
*
*
* drilling with cable tool equipment.
*
*
*
*
* Using drill holes that * Holes greater than 10 cm (4 in) in diameter are
*
* are too large
* not recommended. Use of larger diameters may
*
*
* result in decreases in the blow count.
*
*
*
*
* 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.)
*
*
*
*
* Improper logging of
* Not describing the sample correctly.
*
* soils
*
*
*
*
*
* 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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7.1-91
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