and overconsolidation ratio (OCR). The Gibbs & Holtz correlation of Figure
3 reported in Reference 11, Direct Determination and Indirect Evaluation of
Relative Density and Earthwork Construction Projects, by Lacroix and Horn is
commonly used to estimate the relative density from SPT.
(b) Undrained Shear Strength. A crude estimate for the
undrained shear strength can be made using Figure 4. Correlations are not
meaningful for medium to soft clays where effects of disturbance are
excessive.
(c) Shear Modulus at Very Small Strains. A crude estimate of
the shear modulus at small strains for sandy and cohesive soils can be
obtained from the statistical relationships in Figure 5 (Reference 12, On
Dynamic Shear Moduli and Poisson's Ratios of Soil Deposits, by Ohsaki and
Iwasaki).
(d) Limitations. Except where confirmed by specific structural
property tests, these relationships are suitable for estimates only. Blow
counts are affected by operational procedures, by the presence of gravel, or
cementation. They do not reflect fractures or slickensides in clay, which
may be very important to strength characteristics. The standard penetration
test results (N values) are influenced by operational procedures as
illustrated in Table 13 (modified from Reference 13, Properties of Soil and
Rock, by the Canadian Geotechnical Society).
b. Cone Penetrometer Tests (CPT). This test involves forcing a cone
into the ground and measuring the rate of pressure needed for each increment
of penetration. (See Figure 6). The most commonly used cone test is the
Dutch Cone Test (DCT).
(1) Resistance. For the Dutch Cone, resistance to penetration is
the sum of point resistance and frictional resistance on the sides of the
shaft. The more sophisticated systems can differentiate between the point
and frictional components of the resistance, and the ratio between
frictional and point resistance (Friction Ratio) is one aid in
differentiating between various soil types. Clean sands generally exhibit
very low ratios (low friction component in comparison to point resistance),
while an increase in clay content will usually result in a higher ratio,
more often the result of a reduction in point resistance rather than an
increase in frictional component.
(2) Correlations. Correlations have been developed for the cone
penetration test with bearing capacity, relative density of sands, strength
and sensitivity of clays and overconsolidation, as well as with SPT values
and pile design parameters. Procedures and limitations of the cone
penetration test and its correlations are described in Reference 14,
Guidelines for Cone Penetration Tests Performance and Design, Federal
Highway Administration.
(3) Advantages and Limitations. The static cone test can be used as
a partial replacement for conventional borings. The speed of operation
allows considerable data to be obtained in a short period of time. The
major drawbacks of static cone tests are the non-recoverability of samples
for identification, difficulty in advancing the cone in dense or hard
deposits, and need for stable and fairly strong working surface to jack the
rig against.
7.1-86
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