Environmental Engineering Reference
In-Depth Information
weights (static force), or pushing hydraulically. Initially, only penetration resistance at the
cone tip was measured (
q
c
or
q
t
). Later, a cone was developed with a sleeve to measure
shaft (side) friction
f
s
(Begemann cone) in addition to tip resistance. The Begemann cone
was termed a subtraction cone. It measures the total sleeve plus tip force on the cone and
the tip resistance when pushed into the ground. Sleeve friction is calculated by subtract-
ing the tip resistance from the total resistance.
Fugro, ca. 1965, developed an electric cone (the compression cone) that measured and
recorded both tip resistance and shaft friction separately. Some electric cones have a max-
imum value for sleeve friction of the order of 20 tons. The subtraction cone has no sleeve
friction limit; the only limit is the total penetrometer force. Subtraction cones can be used
where sleeve friction is high, such as in very stiff clay, and the limit of the electric cone is
exceeded.
CPT Operations
Modern cones are pushed continuously into the ground by a hydraulic-force apparatus
reacting against a machine. The apparatus can be mounted on a variety of platforms,
including truck or track mounts, small portable units, and barges or drill ships. The inte-
rior of a truck-mounted CPT is shown in
Figure 2.38a.
Large modern rigs have capacities
of up to 30 metric tons. CPT rigs are often mounted with test boring drill rigs, but the reac-
tion force is limited.
Advanced by hydraulic thrust, the electric cones employ load cells and strain gages that
measure electronically both tip resistance and local sleeve friction simultaneously. The
results are recorded digitally at the surface with an accuracy of measurement of usually
better than 1%. Readings are usually taken at 5 cm intervals. Cones vary in size with areas
of 10 and 15 cm
2
the most common because ASTM criteria apply. The 15 cm
2
cones can
push well in loose gravels, cemented sands, and very stiff fine-grained soils and weath-
ered rock. Various cone sizes are shown in Figure 2.38b.
The CPT method permits rapid and economical exploration of thick deposits of weak to
moderately strong soils and provides detailed information on soil stratification. There
have been many modifications to cone penetrometers in the past 20 years. The test can
measure
in situ
many important soil properties applicable to geotechnical and environ-
mental studies as summarized below. The interpretation of strength and compressibility
properties are covered in
Section 3.4.5.
Although soil sampling is possible with a special tool, soil samples are normally not
obtained. CPT data are usually confirmed with test borings and soil sampling, but the
number of borings is significantly reduced.
See also ASTM D5778, Sanglerat (1972), Schmertmann (1977), and Robertson et al. (1998).
Engineering Applications
Standard CPT
. The common application of the CPT is to obtain measurements of engi-
neering strength properties. In relatively permeable soils, such as fine and coarser sands,
pore pressure effects during penetration at standard rates often have negligible influence,
and the CPT measures approximately fully drained behavior. In homogeneous, plastic
clays, the CPT measures approximately fully undrained behavior. Mixed soils produce in-
between behavior.
Piezocones
are currently in common use (CPTU). They have a porous element near the tip
and a built-in electric transducer to measure pore water pressure in addition to tip resist-
ance and shaft friction. Information on stratification and soil type is more reliable than the
standard CPT. The interpretation of material strength properties is improved, and data are
obtained on deformation characteristics. To obtain pore-pressure data, penetration is
