Soil Testing Equipment

Cone penetration tests (CPT)

An Electric Cone Penetration Test (CPT) is a  geomechanical probing technique for shallow  subsurface exploration. Probing through weak  ground to locate firmer strata at depth has been  practised since 1917, but CPT developed into its  final form in the Netherlands during the 1930’s  (Lunne et al. 1997). 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  sleeve. 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 in  MPa); indicative for the adhesive strength of the  material.

(from Lunne et al. 1997).

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

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.
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.

Ideally, site-specific CPT „facies“ have to be verified with borehole data or observations from exposures to ascertain relationships with actual lithofacies units.


Application of CPT in the study of buried valleys

CPT is a useful, fast and cost-effective technique that can be used for the following applications related to the characterisation of buried valleys;
1. the establishment of the occurrence, extenand upper boundary of “pot clay“ bodies
2. the establishment of protecting impermeable beds above buried valley aquifer systems (such as “pot clay“, boulder clay, etc.)
3. characterisation of the upper sedimentary records outside buried valleys.

Figure 5.6.8 presents a CPT transect, combined with lithological columns of boreholes, of the
Groningen Burval project area. Particularly between 2 and 4 km along the profile, a large clayey body is identified. The presence and rough outline of this unit was also demonstrated by Helicopter Electromagnetics . Below this unit other sediments associated with the Peelo Formation occur, while near the surface Weichselian deposits are present (Boxtel Formation). Figure 4.9.3 shows an enlarged part of Figure 5.6.8 to demonstrate the correlation more clearly.

The CPT characteristics of the “pot clay“ (PENI) are clearly defined. The deeper undifferentiated  deposits of the Peelo Formations are characterised by a strong lateral and vertical heterogeneity.

1. The empirical relationship presented in Table 4.9.1 is based on observations below the ground water table. Above the ground water table a clayey bed (for example) can be partly dried out, leading to higher cone resistance and lower sleeve friction, hence lowering the friction ratio number. However in a climate with excess rainfall like that in northwestern Europe, hanging water is likely in the  unsaturated zone. Hence it should be  possible to discriminate clayey beds from  their sand or gravely counterparts in the  unsaturated zone.
   
2. In buried valley environments glaciotectonized sediments can occur. The same is true for overconsolidated sediments due to glacial loading. In both situations geomechanical properties of the sedimentary record is potentially modified: sediments could respond different to CPT than expected in in situ sedimentary sequences following the stratigraphical rule (Bakker 2004). In situations with overconsolidated clay adhesive strength is relatively high, but possibly reduced due to mechanical expel of pore water. Cone resistance is enhanced (higher compaction), leading to a reduced Rf. Hence, Rf values can differ from normal sedimentaryconditions, with normal setting and compaction due to overburden.

The examples demonstrated above, attest that CPT is a powerful technique for the identification and mapping of large sedimentary units to a maximum depth of about 60 m below the surface. Combined with the low-costs CPT is a technique that is highly recommendable in environments with unconsolidated sediments such as buried valley systems.

References :

1. Bakker MAJ (2004): The internal structure of  Pleistocene push moraines. A multidisciplinary approach with emphasis on ground- penetrating radar. – PhD thesis, Queen Mary,  University of London, 177 pp.
2. Coerts A (1996): Analysis of static cone penetration test data for subsurface modelling. A methodology. – PhD thesis, Netherlands Geographical Studies 210, 263 pp.
3. Lunne T, Robertson PK, Powell JJM (1997): Cone Penetration Testing in Geotechnical Practice. – Blackie Academic & Professional, London, 312 pp.