Other Factors Affecting SPT Results

Table of Contents:

A number of investigations have summarized factors other than energy that can affect the SPT N-value (Fletcher 1965; Schmertmann 1978; Decourt 1989; Kulhawy 8c Trautmann 1996). Nearly all of these factors can be placed in two categories: (1) equipment variations and (2) operator or procedural variations. If a calibrated automatic hammer is used, the remaining factors are summarized in Table 2.3.

Diameter of Drill Rods

ASTM D1586 specifies that the rods must have a stiffness equal to or greater than that of parallel wall “A” size rods. In the U.S. and elsewhere in the world, the SPT is most often performed using either “A” size rods or “N” size rods. Different styles of rod threads (square and tapered) may be used, but typically, a single rod size will be used in the string. Most reported comparisons between SPTs conducted with both A and N rods at the same site do not show significant difference in N values (Gibbs 8c Holtz 1957; Palmer 8c Stuart 1957; Brown 1977; DeGodoy 1971; Yokel 1982; Matsumoto 8c Matsubara 1982). It appears that up to depths of about 30 m (100 ft), the use of drill rod sizes in the range most often used in the field and recommended by ASTM D1586 produces a negligible effect on N-values. ASTM D6066 suggests that for depths greater than 15m (50ft), BW and NW rods are preferred to avoid whipping or buckling. Figure 2.6 show a side-by-side comparison of N-values obtained using AWJ and NWJ rods in sand.

Drill Rod Length

The total length of drill rods may influence the energy reaching the spoon from the hammer drop (e.g., Schmertmann 8c Palacois 1979; Yokel 1982; Decourt 1989; Morgano & Liang 1992; Odebrecht et al. 2004). A secondary effect of longer rods is the tenancy for increased rod whip and loss of energy from multiple rod connections, both of which tend to reduce the energy reaching the sampler. A correction factor is often used to adjust N-values for the total length of rods used, i.e., depth (e.g., Skempton 1986).

Equipment variations	Sampler dimensions	Variations in exact sampler dimensions vary around the world. Sampler should conform to the latest ASTM standard and should be measured before use
	Liners/no liners	Use of liners vs. no liner but spoon with internal relief increases blow counts
	Use of damaged or deformed tip on sample spoon	Damaged shoe may change blow counts
	Using damaged drill rods	Drill rods that are slightly bent or otherwise damaged tend to reduce energy transfer giving artificially high N-values
Procedural variations	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
	Failure to maintain sufficient hydrostatic head in the 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
	Using a too large pump	Too high a pump capacity will loosen the soil at the base of the hole causing a decrease in blow count
	Over-washing ahead of casing	Low blow count may result in dense sand since sand may be loosened by over-washing
	Drilling method	Drilling technique (e.g., cased holes vs. mud stabilized holes) may result in different N-values for the same soil.The SPT was originally developed from wash boring techniques. Drilling procedures which seriously disturbs the soil will affect the N-value, e.g., drilling with cable tool equipment
	Rate of testing	In saturated soils, a fast rate of testing may increase pore water pressures.
	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
	Loose drill rod connections	Energy losses can occur from loose rod connection giving artificially high N-values
	Marking drive increments	Drive marks should be made after the spoon and drill string have been just set on the bottom of the borehole but before the hammer is attached or rods are released
	“Seating” the spoon before marking the rods	There is no such thing as “seating” of the spoon before marking the three 0.15m (6in.) incremental drive lengths
	Sampler plugged by gravel	Artificially high blow counts result when gravel plugs sampler; resistance of loose sand could be highly overestimated
	Carelessness in counting the blows and measuring penetration	Poor observations of incremental blow counts may produce errors in N-values.
	Using drill holes that are too large	Holes greater than 100 mm (4 in.) in diameter are not recommended. Use of larger diameters may result in decreases in the blow count from stress relief at bottom of hole
	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 day

Figure 2.6 Comparison of N-values from AWJ and NWJ drill rods.