Cone Penetration Tests (CPT)

Cone penetration tests (CPT)

An Electric Cone Penetration Test (CPT) is a geomechanical probing technique for shallow subsurface exploration. CPT combines rapid and cheap insight in the mechanical composition of the subsurface in the upper tens of meters. The widest application is currently found in geomechanical applications, i.e. surveys for road and railway constructions and the foundation of buildings and houses in areas with weak subsurface. The principles of CPT are published in Lunne et al. (1997) and Coerts (1996).

Principles of cone penetration tests

Cone resistance, sleeve friction and friction ratio CPT surveying involves the penetration of a metal electrical cone with a surface of 10 cm2 into the subsurface (Fig. 4.9.1). From beneath a heavy truck, the cone is penetrated at a constant rate of 1 cm/s. During penetration, a number of variables are recorded at the cone head or along the. At the cone head the cone resistance (qc) is recorded (in MPa), which expresses the resistance of the sediments to penetration. Along the cone the sleeve friction (fs) is recorded (also inMPa); indicative for the adhesive strength of the material.

Fig. 4.9.1: Terminology for cone penetrometers
(from Lunne et al. 1997).


Pore water pressure

Another useful parameter that can be recorded in CPTU surveying (the so-called piezocone test) is the pore pressure u (Fig. 4.9.1). In the saturated or vadose zone increasing values occur with increasing depth,expressed in MPa. Also perched ground water tables can be detected using this
technique.

Data interpretation

CPT interpretation mainly involves pattern analysis of the cone resistance and friction ratio curves. In common practice it is possible to define CPT „facies“ for certain sedimentary deposits. In buried valley environments for example, the friction ratio curve characteristics of “pot clay“ (or Lauenburger Ton) are well known. Similar typical CPT facies units can be defined for cover sands, boulder clay (till), several fluvial deposits and so forth. Figure 4.9.2 demonstrates an example of a CPT plot in which typical “pot clay“ patterns can be recognized between 22 and 29 m depth. Less distinct are the clayey deposits between 0 and 22 m depth.

From the cone resistance and the sleeve friction
the friction ratio (Rf) can be calculated according:

Rf = [(fs/qc)*100]

Numerous analyses of data have lead to an empirical relationship between Rf and inferred lithology (Table4.9.1). The friction ratio is, in combination with cone resistance, broadly used in geomechanical applications.

Table 4.9.1: Empirical relation between the dimensionless
friction ratio andinferred lithology in CPT.

Friction ratio (Rf) Inferred lithology
0.2 – 0.6 Gravel, coarse sand
0.6 – 1.2 Sand
1.2 – 4.0 Silt/loam
3.0 – 5.0 Clay
5.0 – 7.0 Heavy clay (incl. “pot clay“)
5.0 – 10.0 Peat

Pore water pressure

Another useful parameter that can be recorded in CPTU surveying (the so-called piezocone test) is the pore pressure u (Fig. 4.9.1). In the saturated or vadose zone increasing values occur with increasing depth, expressed in MPa. Also perched ground water tables can be detected using this technique.

Data interpretation

CPT interpretation mainly involves pattern analysis of the cone resistance and friction ratio curves. In common practice it is possible to define CPT „facies“ for certain sedimentary deposits. In buried valley environments for example, the friction ratio curve characteristics of “pot clay“ (or Lauenburger Ton) are well known. Similar typical CPT facies units can be defined for cover sands, boulder clay (till), several fluvial deposits and so forth. Figure 4.9.2 demonstrates an example of a CPT plot in which typical “pot clay“ patterns can be recognized between 22 and 29m depth. Less distinct are the clayey deposits between 0 and 22 m depth.