Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 223

4.4.1 Cone Penetration Test (CPT)
The CPT (Dutch cone) is modified to piezocone penetration depth. The piezo-
cone test (CPT testing that also gathers piezometer data, called CPTU testing) is
a CPT with additional measurement of the pore water pressure at one or more
locations (U 1 , U 2 and U 3 ) on the penetrometer surface, as shown in Figure 4.1 .
The CPT is an in-situ testing method for determining the geotechnical engi-
neering properties of soils and for delineating soil stratigraphy. It was initially
developed in the 1950s at the Dutch Laboratory for Soil Mechanics in Delft to
investigate soft soils (which is why it has also been called the Dutch cone test).
Today, the CPT is one of the most used and accepted in-situ test methods for
soil investigation world-wide.
The test method consists of pushing an instrumented cone, with the tip
facing down, into the ground at a controlled rate, usually 20 mm/s. The resolu-
tion of the CPT in delineating stratigraphic layers is related to the size of the
cone tip, with typical cone tips having a cross-sectional area of either 1000
or 1500 mm

, corresponding to diameters of 36 and 44 mm.
American Society of Testing Materials (ASTM) presents the apparatus and
test procedure for the CPT and the measurement of q c . In particular, ISO (2005)
prescribes cones with a base area in the range of 500 mm 2 to 2000 mm 2 and a
penetration rate of 20

²

5 mm/s.
It is noted that the CPT-based design methods were established for cone
resistance values up to 100 MPa. Caution should be used when applying the
methods to sands with higher resistances.
The early applications of CPT mainly determined soil bearing capacity. The
original cone penetrometers involved simple mechanical measurements of the
total penetration resistance to pushing a tool with a conical tip into the soil. Dif-
ferent methods were employed to separate the total measured resistance into
components generated by the conical tip due to the tip friction and friction gen-
erated by the rod string.
In the 1960s, a friction sleeve was added to quantify the friction component
and to aid in determining soil cohesive strength ( Begemann, 1965 ). Electronic

±

U 3

Pore pressure
filter location

Friction
sleeve

U 2

U 1

Cone tip

FIGURE 4.1 Sketch for cone penetration test (CPT).