Section 7.
PENETRATION RESISTANCE TESTS
1. GENERAL. The most common test is the Standard Penetration Test (SPT)
which measures resistance to the penetration of a standard sampler in
borings. The method is rapid, and when tests are properly conducted in the
field, they yield useful data, although there are many factors which can
affect the results. A more controlled test is the cone penetrometer test in
which a cone shaped tip is jacked from the surface of the ground to provide
a continuous resistance record.
a.
Standard Penetration Test (SPT).
(1) Definition. The number of blows required to drive a split spoon
sampler a distance of 12 inches after an initial penetration of 6 inches is
referred to as an "N" value or SPT "N" value.
(2) Procedure. The test is covered under ASTM Standard D1586 which
requires the use of a standard 2-inch (O.D.) split barrel sampler, driven by
a 140 pound hammer dropping 30 inches in free fall. The procedure is
generalized as follows:
(a) Clean the boring of all loose material, and material
disturbed by drilling.
(b) Insert sampler, verifying the sampler reaches the same depth
as was drilled.
(c) Obtain a consistent 30-inch free-fall drop of the hammer
with two wraps of a rope around the cathead on the drill rig. (Cables
attached to the hoisting drum should not be used because it is difficult to
obtain free fall.)
(d) Drive the sampler 18 inches, or until normal maximum
resistance (refusal) is reached, using the standard hammer and drop.
(Refusal is defined as a penetration of less than 6 inches for 100 hammer
blows.)
(e) Count and record the number of blows required to drive each
6 inches of penetration.
(3) Correlations. See Figure 1 and Table 4, Chapter 1 for
approximate correlations between the "N" values from the standard
penetration test and the compactness of granular soils and the consistency
of fine grained soils.
(a) Relative Density of Granular (but fine grained) Deposits.
Assuming that the test is a true standard test, the "N" value is influenced
by the effective vertical stress at the level where "N" is measured, density
of the soil, stress history, gradation and other factors. The work reported
in Reference 10, SPT and Relative Density in Coarse Sands, by Marcuson and
Bieganouski, establishes statistical relationships between relative density
(D+r,) in percent, "N" (blows/ft), effective vertical stress (pounds per
7.1-85
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