The Cone Penetrometer
Test (CPT) yields in situ measurements of soil characteristics allowing detailed assessment of site-wide liquefaction
potential. CPT measurements are converted to equivalent
Standard Penetration Test (SPT) blowcounts and then used
in the blowcount method of liquefaction potential assessment
(16). The CPT measurements are treated as equivalent SPT
blowcounts for several reasons: the SPT method was developed
based on observed cases of liquefaction (14); the method
is in routine use, making the expansion of the method's
data base likely; and most importantly, the SPT and CPT
are both similarly influenced by most soil compositional
and environmental variables (8). Reasons for use of the
CPT-SPT-liquefaction potential method instead of the SPT
method are that the CPT is faster and less expensive (2),
is not subject to the number of equipment and procedural
error sources as is the SPT (3,9), and provides better
resolution of overall site characteristics.
The use of the CPT method requires that the CPT field measurements
yield identification of material types and that ground
water elevations are known or determined through use of
a CPT tip equipped with a pore-fluid-pressure sensing
element. In addition, CPT data must be correlateable to
SPT data either before or after measurements are normalized
for the effects of overburden pressure. The applicability
of CPT data to these determinations and, therefore, to
assessment of liquefaction potential using the simplified
blowcount method is analyzed in this paper through use
of a set of field measurements performed using both the
CPT and SPT at liquefaction susceptible sites.
Field investigations included electric CPT measurements
together with SPT measurements where both a standard rope-around-the-cathead
"donut" hammer and a mechanical "free-fall"
trip donut hammer were used. Eight CPT soundings and four
standard hammer and four trip hammer borings were performed
at each of two sites. Soundings were performed prior to
SPT borings which were placed at horizontal separations
of less than ten feet.
The standard electric friction cone penetrometer described
in ASTM D3441-75T (7) was used for all measurements. Data
recordings were taken on a two-channel strip-chart recorder
having the chart drive locked to the advance of the sounding
rods. All penetrations were at 2 cm/sec and instrument
inclination was continuously monitored.
The standard donut hammer and trip donut hammer were
both used on the same drill rig with the same crew. The
standard hammer has a total weight of 225 pounds including
anvil, guide, and coupling, while the trip hammer has
a total weight of 210 pounds. Drop height was controlled
by the driller with the standard hammer but was pre-set
for the trip hammer. Rotary wash drilling was used with
a baffled fish-tail bit for all borings. Blows were applied
at a rate of between 30 to 40 per minute for both hammers.
All procedures and equipment followed ASTM D-1586 (1)
except that liner samplers were used without liners as
is common practice (8).
Data obtained during field investigations and
from subsequent laboratory soil classifications (ASTM
D421-58; D422-63) were used to evaluate the adequacy of
CPT measurements for determination of soil type and prediction
of SPT measurements. Although correlations between CPT
and SPT were made for soils ranging from sands to clays,
it is assumed that the SPT method (16) of liquefaction
potential assessment is valid only for cohesionless soils
with low fines content; no fines – content adjustments
(l'7, 19) were used.
FIGURE 1: CPT BEHAVIOR TYPE CLASSIFICATION CHART
The assessment of soil types using
electric CPT measurements has been fully described elsewhere
(2). A summary of the assessment method is given in Figure
1, where a chart defining boundaries between soil-behavior
types determined from CPT data is shown. During initial
computer processing of the CPT logs the data is checked
against those soil-behavior type boundaries and then tabulated.
Likewise, prior to calculation of liquefaction potential,
expressed as Factors of Safety, the data is scanned for
material susceptibility, which is assumed as low-fines
content cohesionless sands represented by materials
falling in Zone 1 of Figure 1. Higher-fines content sands
and cohesionless silts are represented by Zone 2, and
cohesive soils by Zones 3 and 4.
Several methods were employed to investigate the effect
of increasing overburden stress upon CPT data and CPT-SPT
correlations (5). Although trends were found relating
the CPT overburden-pressure normalization factor, Cp,
and the SPT factor, CN, the lack of CPT data
showing the effect of overburden alone, and the difference
in CN and Cp relations proposed by various
authors (4, 6, 10, 13, l5) prevents assignment of a high
degree of certainty to any one relation. Therefore, CPT-SPT
correlations are made prior to overburden stress adjustments.
The predicted SPT values are then adjusted using
the relation suggested by Seed (15):
CN = 1-1.25 log
where rv' is given in tsf.
The conversion of electric CPT measurements
into equivalent SPT measurements has been investigated
by several researchers (5, 7, 8). The method described
herein builds upon the results of those investigations
from which it appears that the most rational approach
is the detailed modeling of SPT measurements in terms
of components of resistance; these components can be estimated
from CPT measurements. Schmertmann (11, 12) provides detailed
information regarding the entire SPT process, from energy
contained in the falling hammer through to the energy
used in static or dynamic advance of the SPT sampler.
The CPT-SPT conversions presented herein draw heavily
upon that information.
FIGURE 2: ELEMENTS OF ENERGY DISSIPATION
The geometries of the SPT sampler and the electric
friction-cone penetrometer are shown in Figure 2. The
stresses qci and fsj developed on
the cone penetrometer tip and sleeve at each depth increment
di and dj during continuous penetration
are imposed upon the SPT sampler end and outside and inside
areas at the same depth increments, yielding quasi-static
penetration forces. The sampler is then considered held
stationary while the soil moves past it, and the forces
acting at each location on the sampler are converted,
by multiplication of distance moved, to energy expenditures
required for that quasistatic penetration of the sampler.
Adjustments need be made to those CPT-based energies
to account for the difference between static and dynamic
soil resisting stresses, the influence of a borehole and
initial six inches of SPT sampler embedment, the difference
between static and dynamic energy expenditures during
penetration, and the difference between total energy available
for sampling and energy provided by the theoretical thirty-inch
free fall of a 140 pound SPT hammer. Finally, the potential
energy lost by the SPT hammer-rod system during the sampling
interval needs be considered. The concept is summarized
as forcing equivalence between available energy, Ei,
and dissipated energy, Ed.
where E is energy; Wh and Whr
are buoyant weights of hammer and hammer-rod system; q
is the fraction of theoretical hammer energy actually
delivered to the sampler; dh, di,
di and dj, are distances of hammer
fall, (30 inches) sampler total penetration (12 inches),
sampler incremental tip penetration, and incremental penetration
of each area of outside sleeve; qci, fsi,
and fsj are incremental stresses measured
by cone tip and sleeve and applied to SPT sampler tip,
outside sleeve, and inside of cutting shoe; Ae,
Asj, and As are areas of SPT sampler
projected end area, outside sleeve element, and inside
of cutting shoe; C1, C2, C3
are conversions between cone stresses and SPT sampler
stresses on tip, outside sleeve, and inside cutting shoe,
and Ci is an adjustment for the difference
between static- and dynamic-penetration energy losses
resulting from rate, damping, and energy transfer differences
between the CPT and SPT.
The adjustment factors, C1, C2, C3,
Ci, have been selected based upon limited research
data. Schmertmann (11) suggests that C1, and
C2, the constants relating quasi-static CPT
and SPT end bearing and local side friction stresses,
are equal to 1.0. For the studies presented herein, C1
is weighted by decreasing percentages of end bearing measured
beyond the current elevation of the tip to account for
the dependence of end bearing on soil resistances ahead
of the tip (18). The present use of C2 is also
somewhat modified to account for the observed rapid increase
in CPT sleeve friction stresses immediately beneath a
soil surface. Thus the sleeve stress applied to the SPT
sampler side is decreased beneath the value measured with
the CPT in the zone immediately beneath the boring. This
term also accounts for the indeterminate loss of side
friction at borehole bottom due to disturbance during
the SPT boring. C3 is an adjustment of the
measured CPT side friction to account for the difference
between soil-sampler friction stresses on the inside of
the cutting shoe as compared to the outside. Without any
positive information available defining C3, it
has been assumed at 1.0. In addition, the inside friction
force has been limited to the end-bearing force of a soil-plugged
sampler, although no adjustment of end bearing stress
is made to account for the increased area of the plugged
sampler. Ci is an adjustment to account for
the difference between static- and dynamic-penetration
energy dissipation, and has been directly linked, based
upon the static to dynamic penetration ratios summarized
by Schmertmann (11, 12), to the ratio of friction to end-bearing
resistance contributions during equivalent quasi-static
SPT sampler penetration. Finally, assuming a constant
for each hammer, the ratio of calculated energy expenditures
to theoretically available energy should equal the energy
transfer efficiency, g.
Plots showing measured N value against calculated loss
of energy (including the potential energy of the hammer-rod
system) and theoretical maximum available energy during
the SPT sampler penetration were prepared for each boring-sounding
comparison. Typical plots are shown in Figures 3 and 4
for two sets of standard and trip hammer borings where
one set is from each of two different sites.
FIGURE 3: CPT ENERGY DISSIPATION (FT-TON) VS SPT N VALUES
Examination of these plots reveals
nonlinearity between calculated maximum available energy
and calculated dissipated energy, even for the essentially
constant material type represented in Figure 3. As g is assumed independent of depth, material type,
and blowcount (12), and because the transfer ratio between
hammer kinetic energy and incident rod energy is constant
for the trip hammer, then Figures 3 and 4 show that with
increasing blowcount more energy is being lost than is
accounted for. A possible cause of this behavior, in addition
to the falseness of any of the previous assumptions is
that as the site penetration resistance increases, the
ratio of average dynamic to average static penetration
forces decreases, resulting in a higher "quake"
threshold to be overcome by each pulse of the SPT rod
energy train. Thus fewer pulses produce "set"
of the sampler, resulting in the decrease in time to a
given percentage of total set as blowcount increases (12).
This reduction in number of pulses containing sufficient
force to induce sampler penetration results in the apparent
loss of calculated dissipated energy with increasing blowcount.
It also appears that the deficit in energy dissipated
during sampling increases with the decreasing ratio of
energy transmitted into rods divided by site penetration
FIGURE 4: CPT ENERGY DISSIPATION (FT-TON) VS SPT N VALUES
The estimation of some energy efficiency
factor becomes complicated by this phenomenon. However,
the data in Figures 3 and 4 reveal very similar behavior
for the trip hammer measurements at the two sites. Thus
comparing at equal depths the slopes of the best-fit relations
for the different hammers should yield a ratio reflecting
the ratio of the two hammer efficiencies. Once the controlled
trip hammer energy is actually measured, then the energy
dissipation relation for any particular hammer can be
scaled to any desired ratio of the calibrated energy efficiency.
For example, a comparison of the standard and trip hammer
energy ratios at one location reveals a ratio of hammer
efficiencies of 0.48 at one site and 0.72 at the other.
If it was known that the trip hammer efficiency was 70
percent at that location, then the standard hammer efficiencies
would be calculated as 33.6 and 50.4 percent, respectively.
Considering the number of variables that can affect energy
delivered by the standard hammer (3), such a variance
is not surprising, and is supported by the average blowcount
ratios determined for the two sites (5).
FIGURE 5: PREDICTED VERSUS MEASURED BLOWCOUNT
Regardless of actual hammer energy, the CPT method
may be used to calculate blowcounts representative of
any particular SPT hammer through hammer – specific calibration.
For example, a comparison of a predicted continuous versus
measured N value profile is given for one boring-sounding
combination in Figure 5. Although the predicted SPT values
are shown continuous, the value at any depth rep-resents
the equivalent SPT blowcount for a depth interval starting
12 inches above that depth. In Figures 6 and 7, the range
of predicted and measured blowcounts for each hammer at
each site are shown. Several features are
of interest in these figures.
First, the range of predicted blowcounts agrees well with
the range of measured blowcounts at any depth. Next, the
difference in blow-counts predicted, or measured, for
the same site but using different hammer energies is very
significant. For corrected blowcounts less than about
35, the Factor of Safety against liquefaction calculated
using the simplified method (16) changes in direct proportion
to a change in measured blowcount. Thus the routine investigation
using blowcounts taken at five foot intervals can provide
very misleading results, depending primarily upon variability
of the profile. In addition, examination of the ratio
of friction to end bearing components of the SPT blowcount
reveals that in all but clean sands, the friction component
dominates the total resistance. As has been shown (3),
borehole disturbance can cause significant reduction of
effective stresses at the borehole bottom, thus greatly
reducing the measured blowcount. It appears then, that
any but the most careful of field operations may produce
a wide range in measured blowcounts.
FIGURE 6: PREDICTED VERSUS MEASURED BLOWCOUNT
FIGURE 7: PREDICTED VERSUS MEASURED BLOWCOUNT
The calculation of liquefaction potentials from CPT
data is easily accomplished once the CPT data are converted
to equivalent SPT blowcounts. The equivalent SPT blowcounts
are first normalized for overburden effects using equation
(1). The normalized equivalent blowcounts, N1'
are then converted to cyclic strengths and induced stresses
are calculated following the procedures of the simplified
method (16). Dividing the cyclic strength ratio by the
induced stress ratio yields a factor of safety against
liquefaction. This procedure is followed for each CPT
sounding and the vertical profiles of factors of safety
versus depth are compiled into horizontal cross sections
of the site. Examination of the cross sections reveals
if there is continuity of susceptible materials with low
factors of safety. Statistical processing of any continuous
vertical and horizontal zones of weak and susceptible
materials can be performed to allow calculation of average
factors of safety. In addition, zones of questionable
susceptibility are readily identified from the cross sections.
These zones can then be sampled for subsequent laboratory
In summary, as the range comparisons of Figures 7 and
8 show, although a very extensive and carefully controlled
sampling program can define site blowcount variability,
the continuous CPT profiles give a more complete description.
The increased description of site variability coupled
with the greater degree of measurement repeatability afforded
by the CPT method as compared with the SPT method allows
increased confidence in the results of some subsequent
deterministic or probabilistic evaluation of site liquefaction
The following conclusions are drawn based upon the
blowcounts predicted using CPT measurements agree well
with measured blowcounts, indicating that both test methods
are similarly influenced by the same soil characteristics
and that CPT measurement may therefore be directly used
in SPT type liquefaction potential assessments.
blowcount profile predicted from CPT measurements provides
much clearer resolution of stratigraphy in terms
of both material type and material penetration resistance
than is normally obtained with SPT results;
blowcounts determined from CPT measurements may be
more reliable than those actually measured using
uncalibrated equipment or poorly controlled procedures
during the SPT;
between any one CPT-predicted and SPT-measured set
of blowcounts are less than potential differences in
blowcounts themselves due to changes of equipment,
procedure, and, in certain cases, sampling interval;
predicted from CPT measurements can be adjusted, once
the CPT is calibrated against actual blowcount hammer
energy measurements, to account for the dependence of
any particular SPT-based design procedure upon a specific
Most of the field investigations referenced in this
paper were performed under USGS Contract No. 14-08-0001-17790.
In addition, data analyses were supported by Ertec Research
and Development grants. Finally, the interpretation of
the volumes of field data was facilitated by the conscientious
efforts of Messrs. George Edmonds, Mike Moore, Mike Leue,
and Richard Miller.
APPENDIX I. – REFERENCES
1981 Annual Book of ASTN Standards, Part 19, Soil
and Rock; Building Stones, American Society for
Testing and Materials, Philadelphia, PA, 1981.
B. J., and Olsen, R. S., "Soil Classification Using
Electric Cone Penetrometer"" ASCE Geotechnical Engineering
STP – October, 1981.
Kovacs, W. D., Evans, J. C.,
and Griffith, A. H., "A Comparative Investigation
of the Mobile Drilling Company's Safe-T-Driver with
the Standard Cathead with Manila Rope for the Performance
of the Standard Penetration Test," School of Civil Engineering,
Purdue University, Nay, 1979.
F., III, and Bieganousky, W. A., "SPT and Relative Density
in Coarse Sands:" Journal of the Geotechnical Engineering
Division vii, ASCE, Vol. 103, No. GT ll. November, 1977.
Martin, G. R., and Douglas,
B. J., "Evaluation of the Cone Penetrometer for
Liquefaction Hazard Assessment," U.S. Geological Survey
Open File Report 81-234, 1981.
R. B., Hanson, W. G., and Thornburn, T. H., Foundation
Engineering, 2nd Ed. John Wiley & Sons, Inc.,
New York, 514 p., 1973.
G., The Penetrometer and Soil Exploration, Elsevier
Publishing Company, New York, N.Y., 1972.
Schmertmann, J. H., "Predicting the qc/N
Ratio," Final Report D-636, Engineering and Industrial
Experiment Station, Department of Civil Engineering,
University of Florida, Gainesville, October, 1976.
Schmertmann, J. H., "Use the SPT to Measure
Dynamic Soil Properties? – Yes, But...!," Dynamic
Geotechnical Testing, ASTM STP 654, American Society
for Testing and Materials, 1978, pp. 341-355.
Schmertmann, J. H., Guidelines
for Cone Penetration Test, Performance and Design,
U.S. Department of Transportation, FHNA-TS-78-209, 1978.
J. H., "Statics of the SPT," Journal of the
Geotechnical Engineering Division, ASCE, Vol. 105, No.
GT5, May 1979, pp. 655-670.
J. H., "Energy Dynamics of SPT," Journal of the
Geotechnical Engineering Division, ASCE, Vol. 105, No.
GT 8, August 1979, pp. 09-926.
I. W., "The Interpretation of Begemann Friction
Jacket Cone Results to Give Soil Types and Design Parameters,"
Proceedings of 7th ECSMFE, Brighton, Vol. 2, pp. 265-270,
H. B., and Idriss, I. M., "Analysis of Soil Liquefaction:
Niigata Earthquakes," Journal of the Soil Mechanics
and Foundations Division, ASCE, Vol. 93, No. S5I3, May,
1967, pp. 83-1QS.
H. B., "Evaluation of Soil Liquefaction Effects
on Level Ground During Earthquakes," Liquefaction
Problems in Geotechnical Engineering, ASCE Preprint
2752, Philadelphia, PA, 1976.
H. B., "Soil Liquefaction and Cyclic Mobility Evaluation
for Level Ground During Earthquakes," Journal of
the Geotechnical Engineering Division, ASCE, Vol. 105,
No. GT2, February, 1979, pp. 201-255.
K., and Yoshimi, Y., "Field Correlation of Soil
Liquefaction with SPT and Grain Size," International
Conference on Recent Advances in Geotechnical Earthquake
Engineering and Soil Dynamics, Vol. 1, pp. 203-208.
D. D., "The Influence of Gravity, Pre-stress, Compressibility,
and Layering on Soil Resistance to static Penetration,"
Ph. D. Dissertation, University of California at Berkeley,
S. G., "Influence of Fines on Evaluating Liquefaction
of Sand by CPT," International Conference on Recent
Advances in Geotechnical Earthquake Engineering and
Soil Dynamics, Vol. 1, St. Louis, Mo., 1981, pp. 167-172.
APPENDIX II – NOTATION
symbols are used in this paper.
= cross-section area of CPT tip;
= cross-section area of SPT sampler;
= incremental surface areas of SPT sampler;
= factor relating end bearing stresses from CPT to SPT;
= factors relating friction stresses from CPT to SPT;
= ratio of dynamic to static penetration energy dissipation;
= SPT overburden correction factor;
= distance of SPT hammer fall;
= depth of SPT sampler penetration;
= total SPT sampler penetration depth;
= energy dissipated during SPT sampler penetration;
= energy available for SPT sampler penetration;
= CPT and SPT quasi-static end bearing stresses;
g = SPT hammer energy transfer efficiency;
rv' = effective vertical overburden stress