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Energy Measurement in the Standard Penetration Test (SPT)
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The SPT Test
What is SPT?
"SPT" is an abbreviation for the Standard Penetration Test, which
is the most widely used in-situ soil characterization test in
North America.
The SPT test involves driving a standard cylindrical sampler into
the bottom of a borehole. The total blows required from a hammer,
over the interval 150 to 450 mm (6 to 18 inches) are summed to
give the blow count N, in blows per foot.
The N-value is used as a basis for foundation design and as the
primary index of liquefaction resistance.
Why is the SPT so popular?
The popularity of the Standard Penetration Test can be attributed to
both the ease of the test as well as the inexpensive cost. The continued
popularity of the SPT is due to the many design correlations associated
with the SPT - N value, such as allowable bearing pressure and liquefaction
potential.
Where is the SPT used?
SPT is used world-wide, and in North America, the Standard Penetration
Test is the most widely used in-situ soil characterization test.
Why are we concerned about energy?
Several investigators have measured the hammer energy in various SPT
systems and found considerable variability. Schmertmann and Palacios
(1979) showed experimentally that the measured blow count was inversely
proportional to the energy delivered to the drill rods for blow counts
less than 50. Because of the variable energy input from different SPT
hammer/rod systems, Seed et al. (1985) suggested that measured blow counts
(N-value) be corrected to reference value of 60% of the potential energy
of the SPT hammer. The 60% reference energy was adopted because energy
measurements carried out at that time, mostly on safety hammers, indicated
an average of about 60% of the theoretical potential energy of the SPT.
This generalized approach to dealing with energy variations is questionable
because it does not consider details of the hammer and rod system.
Why is the SPT really a coarse, problematic test?
The range of acceptable dimensions for split spoon samplers given in
ASTM D1586 is shown here.
The standard is much less specific about the required design of the SPT
hammer, stating only that the hammer must have a mass of 140 lbs (63.5
kg) and must drop vertically as freely as possible through 30" (0.76 m)
before hitting the anvil. As a result, different hammer designs have
evolved and they vary considerably. The most commonly used hammer types
in North America are the donut, safety and automatic hammers, as shown
in the following figure.
Different combinations of drill rig, hammer, rods and drillers lead
to wide variations in energy delivered to the rods.
Knowledge of the impact velocity would set an upper limit on the possible
energy delivered to the rods. The following figure
illustrates that a small decrease in the impact velocity due to friction
can result in a considerable reduction in the available kinetic energy
at impact.
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SptSampler.gif','Sampler','width=550,height=240,left=10,top=10')){kind=link}
Hammers.gif','Hammer','width=680,height=480,left=10,top=10')){kind=link}
ImpactVelocity.gif','Velocity','width=475,height=315,left=10,top=10')){kind=link}