What is Driving Energy? Its Significance in Standard Penetration Test (SPT)
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Center-hole or Donut hammer for
standard penetration test
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This sampling procedure and sample collected in this test method are not our concern in this article; we will mainly discuss about driving resistance. Driving resistance is related to driving energy which is exerted by falling a driving mass on drilling head. For convenient of discussion, at first, it is required to include some terminology about drilling equipment and test procedure.
Drilling rods-Rods through which downward force is exerted to advance drill bit with necessary torque into the borehole.
Drive weight assemblage-This include a hammer, anvil, a guide system for falling hammer, drill rod and necessary hoisting attachments.
Drill head or anvil-This is an essential part of drive-weight assemblage on which falling hammer strikes. It is a media to transmit hammer energy into drill rods.
Hammer-This is the important part of drive weight assemblage which exert impact weight of 623±9N (140±2 lbf) by falling from specified height. This energy is used to penetrate hole and to make sample for field or laboratory.
Hammer fall guide-This term is self-explanatory i.e. guide the falling of the
hammer.
and forth. This is a lifting system around which a rope is wrapped by operator
to lift and successive dropping of hammer by tightening and loosing of the rope
with counterclockwise and clockwise rotation of drum.
of soil to be tested against penetration. The N-value is blow counted to penetrate
each foot on the ground which is a summation of blows counted to drive the
split-barrel sampler up to 150 mm to 450 mm (6 to 18 inches) depth from where
the sampler is seated.
Driving energy
Standard penetration test has been widely used all
over the world since 1927. But some modification in recoding N value was made
in 1967. The previous testing required only seating of the sampler and driving
300 mm. The procedure can reduce N count up to 50% as the driving of sampler
first 150 mm to seat the sampler on undisturbed sample presents significant
frictional resistance on the sampler to penetrate next 300 mm. Interestingly many
corrections are still exist depending on N counting obtained this previous
procedure.
Reproduction of N-values
After ASTM included standard penetration test, it was often reported that N-values taken from adjacent boreholes with same test arrangement or from nearby holes but using different equipment can not be reproduced. This becomes an important issue for the credibility of this test or making correlation between penetration resistant recorded using different equipment. Due to the wide popularity of this test, geotechnical engineers paid attention to this problem and Gibbs & Holtz in 1957 figured out key points why reproducibility is failed.
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Length of drill rod
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Overburden pressure
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Worn or warped driving shoe
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A rock encounteted during penetration which is generally detectef by an proffesional operator
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Quick condition in the drilled hole
By following ASTM D1586 formation of quick condition can be avoided. The regular inspection of drive shoe is essential, especially, after finding refusal i. e.during hard stratum.
effective stress of the soil becomes zero. This condition in the bottom of the
hole is occurred when
- Bit or auger plug is withdrawn rapidly
- Difference in water level is arisen
between ground water table and in the hole or at stem of hollow auger.
Why is driving energy in SPT significant?
N-values revealed that the difference between the input energy of driving and
actual energy exerted on the sampler hinders the reproducibility. The input energy
gets dissipated around the sampler into the adjacent soil. The amount of
dissipation makes the difference.
would have direct relation with driving energy. The theoretical value of
driving energy can be computed as follows
Substituting v from the
equation (2) into equation (1) will produce
Riggs et al. found the actual input value of driving energy Ea exerted on the sampler to penetrate through ground ranging from about 70~100 percent. Kovacs & Salomone obtained in the previous years (1982) a further lower value of about 30~80 percent. The possible causes of discrepancies are as follows:
- Equipment from different suppliers- many manufacturers are producing a wide variety of equipment; Different types of drilling rigs with different types of hammer are used, The combination two equipment may results wide
variations in results. But the usual practice in North America is to use rotary auger and sampler is driven by safety hammer. Now-a-days automatic hammer are becoming popular and many papers are available in the web about co-relation between safety hammer and automatic hammer. - Configuration of drive hammer is important as the anvil seems to influence amount of energy exerted on the sampler too.
- The system may have hammer with automatic trip or having rope catheaded operated by operator. The
difference in actual driving energy, Ea occurred with variation in falling height.
turns and has wide use.
result smaller N value than that have liner. It is common not to have liner in sampler.
of it is not significant when length is more than 10 m however when length is shorter
and N value less than 30 it is significant. Gibbs and Holtz in 1957 at first
noticed this effect which was later quantified by Mclean et al. by developing computer
modeling to analyze effect of drill rod length and other factors like sampler resistance.
than that for normally consolidated soils.
· Overburden pressure; for a same density N values are found smaller in soil for smaller P0’ as it is nearer the ground surface.
on producing higher N values is also observed in cemented zone with little
overburden pressure.
So it is required to standardize SPT based on the energy
ratio, Er. Er can be computed as
velocity while impact occurred with anvil or energy measured in drill rod just
below drive head (anvil). However, when we use equation (3) it would provide a
fair value as it is not dependent on equipment used to drive.
What is standard energy ratio in SPT?
energy ratio of a particular system of equipment. Now we have wide scattered
values of Er and blow count N resulted based on Er which
should be a unique value for a soil at certain depth. Thus we require a
standard energy ratio Erb which will be a reference for Er
as an example when an estimated Er for particular drill rig is 45,
if Erb=70, then we need an adjustment of N value to represent value
for drill rig of Er=70.
Different researchers suggested several values for the
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Erb
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Proposed
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50~55 (taken 55)
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Schmertmann (1983)
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60
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Seed et al.
,1985 (later supported by Skempton, 1986) |
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70~80 (taken 70)
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Riggs (1986)
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