Pressure sounding

from Wikipedia, the free encyclopedia

The pressure probing (cone penetration test, CPT ) is a probing method for subsoil investigation in which a measuring head with a conical tip is pressed into the ground at constant speed via a rod. In general, the peak pressure and the skin friction are measured, sometimes also other parameters such as the pore water pressure . In this case the attempt is abbreviated as CPTU .

Pressure sounding protocol, Dutch, plotted on the left are peak pressure and skin friction

standardization

In Germany, the structure and implementation of pressure sounding is regulated in the DIN EN ISO 22476-1 standard (previously in the DIN 4094-1 standard).

Further standards are e.g. B. the US-American standards ASTM D 3441 and ASTM D 5778, the Dutch standard NEN 5140, the Swedish standard or the recommendations of the ISSMGE , which formed the basis for EN ISO-22476-1.

Test setup and implementation according to DIN 4094-1

Special USGS truck for pressure sounding

With pressure probing, the probe with a conical tip is pressed vertically into the ground at 2 ± 0.5 cm per second. Measured values ​​should be recorded every 2 cm.
With the CPT, these are the skin friction and the peak pressure . With the CPTU, the pore water pressure is also measured. The results are usually reported in MPa .

The soundings on land are usually carried out with specially equipped trucks that also apply the counterweight for the hydraulic injection. 100 kN (≈ 10 tons) load are required to measure up to a peak pressure of approx. 50 MPa. Vehicles that are twice as heavy are increasingly being used.

Due to the necessary equipment, pressure soundings are usually carried out by specialist geotechnical companies.

geometry

Probes with a conical base area of ​​10 cm² ( CPT 10 ) and 15 cm² ( CPT 15 ) are permitted according to DIN. The cone base diameter must accordingly be 35.7 ± 0.3 mm and 43.8 ± 0.3 mm. The opening angle of the cone is always 60 °. The force measured when the tip is pressed in, divided by the base area of ​​the tip, gives the tip pressure.

A friction sleeve with a surface area of ​​150 cm² (for the CPT 10 ) or 225 cm² (for the CPT 15 ) is arranged above the tip . Similar to the peak pressure, the skin friction is determined via the measured force and the surface of the sleeve.
Instead of a separate skin friction sleeve, the skin friction was previously also determined from the difference between total resistance and peak pressure resistance. However, this is not regulated in the current standard.

Measurements

The force measurement can be carried out electrically, mechanically or hydraulically. Nowadays, however, electrical measurement is common. The accuracy of the measurement is prescribed and is divided into several classes in some national standards.

There are also special measuring tips for a wide variety of applications such as seismic, radiometric, magnetometer and chemical investigations.

Pore ​​water pressures are recorded with a piezometer in specially designed measuring probes ( piezocone ), which are classified according to type depending on the position of the pore water inlet. Depending on the type, the inlet has an influence on the peak pressure measurement, so that it is necessary to correct the measured peak pressure value.

evaluation

The peak pressure allows direct conclusions to be drawn about the density of non-cohesive soils .

The friction ratio (English f riction r atio )

With:
Skin friction ( s leeve f riction )
Peak pressure ( c one resistence )

provides information on the type of soil (cohesive or non-cohesive soils ) and, in the case of cohesive soils, also on the consistency . The pore water pressure also provides information on the type of soil. Further soil parameters that can be derived from the pressure sounding are, for example, the angle of friction of non-cohesive soils, the stiffness modulus and the undrained shear strength of cohesive soils.

field of use

The pressure probes are used, for example, to check the compaction of soils or to assess the load-bearing capacity for pile foundations, but also generally to provide a quick overview of the structure of the building site. Clues for soil classification from CPT data were published by Peter Robertson, for example, who is also co-author of a monograph on the CPT.

The use (or the penetration depth) is limited in very firm soils and very dense soils (if only because of the maximum pressing force, which is determined by the counterweight of the device or the anchoring forces), layers of gravel and stone deposits in the soil that are not only can damage the tip, but can also lead to intolerable deflections of the rod.

Alternatives to pressure soundings are pile-driving soundings , which can also be carried out by smaller engineering offices and construction companies. In the USA, the standard penetration test is also widespread (in Germany, the variant of the borehole drilling is standardized).

history

The first pressure probes were developed in 1932 by the Dutch engineer P. Barentsen from the state water engineering authority Rijkswaterstaat and therefore also called the Dutch Cone Test . They already had a base area of ​​10 cm², but only measured the peak pressure and were purely mechanical. They were tested and used by TK Huizinga, director of the earthworks laboratory in Delft founded by Keverling Buisman , in the 1930s.

The first electrical measurements were made at Degebo in Berlin during World War II . Under managing director R. Hoffmann, the pressure probe ( Degebo probe ) was developed during the war, which was continued after the war, including a peak pressure measurement from the Maihak company, which worked through strain measurements with the help of a vibrating string.

From 1948 pressure sounding was further developed in the Netherlands by the urban engineer Bakker in Rotterdam ( Rotterdam Cone ). Here the development was particularly advanced in the Delft soil mechanics laboratory.

In 1965, an electric pressure probe tip was developed by the Fugro company in collaboration with Dutch government agencies. HKS Ph. Begemann introduced devices for measuring skin friction in 1953. Pore ​​water pressure measurements during CPT measurements were presented in 1974 by Nilmar Janbu from Norway and by John H. Schmertmann in the USA.

In 1975, electrical piezometers integrated into the CPT measuring tip were developed in Sweden and the USA (Wissa and others).

In the 1970s, pressure probing experienced a strong upswing in Europe and is now also advancing worldwide beyond Europe, in contrast to the standard penetration test, which is widely used in the USA .

literature

  • Klaus-Jürgen Melzer, Ulf Bergdahl: Building ground investigation in the field. In: Grundbau-Taschenbuch 2001, Ernst and Son, Volume 1, p. 80
  • DIN 4094-1: Subsoil, field investigations, part 1: Pressure soundings . June 2006
  • Tom Lunne, Peter K. Robertson, John JM Powell: Cone Penetration Testing in Geotechnical Practice , Blacky Academic and Professional and Spon Press 1997
  • AC Meigh : Cone penetration testing: Methods and interpretation , Butterworths 1987

Individual evidence

  1. ↑ ASTM D 3441: Standard Method of Deep Quasi-Static Cone and Friction-Cone Penetration Tests of Soil , ASTM International, West Conshohocken, Pennsylvania, 7 pp.
    ASTM D 5778: Standard Test Method for Performing Electronic Friction Cone and Piezocone Penetration Testing of Soils
  2. ↑ Recommended standards for cone penetration tests . SGF Report 1:93 E, from 1992. Printed in the appendix to Lunne, Robertson, Powell Cone Penetration Testing in Geotechnical Praxis , Spon Press 2002.
  3. ^ On the CPT: Report of the Technical Committee on Penetration Testing - TC 16, TC 16 with Reference Test Procedures CPT-SPT-DP-WST, Swedish Geotechnical Institute Information, Information 7, Linköping 1989. On the CPTU: Report of the Technical Committee for Ground Characterization from in situ testing - TC 16, International Reference Test Procedures for CPT and CPTU, Proc. 12. European CSMGE, Amsterdam 1999, Volume 1, pp. 2196-2222
  4. ↑ EN ISO-22476-1: Geotechnical investigation and testing - Field testing - Part 1: Electrical cone and piezocone penetration test.
  5. ↑ Grundbau-Taschenbuch, Volume 1, 2001, p. 82
  6. ↑ DIN 4094-1: 2002-06, Subsoil: Field investigations - Part 1: Pressure soundings
  7. ↑ see DIN 4094-1, appendix
  8. ^ Robertson Soil classification using the cone penetration test , Canadian Geotechnical Journal, Volume 27, 1990, pp. 151-158. Robertson, Campanella Interpretation of Cone Penetration Tests , Part 1 Sand, Part 2 Clay, Canadian Geotechnical Journal, Volume 20, 1983, pp. 718, 734
  9. ↑ A corresponding diagram can also be found in DIN 4094-1, Appendix C 1
  10. ↑ Tom Lunne, Peter K. Robertson, John JM Powell Cone Penetration Testing in Geotechnical Practice , Blacky Academic and Professional and Spon Press 1997
  11. ↑ Based on the monograph by Lunne, Robertson, Powell, loc. cit. They cite as sources G. Sanglerat The penetrometer and soil exploration , Elsevier 1972 and BB Broms , N. Flodin History of Soil Penetration testing , Proc. Boarding school Symposium on Penetration Testing, ISOPT-1, Orlando, Balkema, Rotterdam, Volume 1, 1988, pp. 157-220
  12. ↑ Schultze, Muhs Bodenuntersuchungen für Ingenieurbauten , Springer 1966
  13. ↑ Klaus Weiß 50 years Degebo , Mitt. Degebo No. 33, 1978, Heinz Muhs work of Degebo in the years 1938–1948 , Bautechnik-Archiv, No. 3, 1949, p. 20.
  14. ↑ An early report is J. De Reister Electric Penetrometer for Site Investigations , Journal of SMFE Division, ASCE, Volume 97, SM-2, 1971, pp 457-472
  15. ^ Begemann The Friction Jacket Cone as an Aid in Determining the Soil Profile , Proceedings, 6th ICSMFE, Montreal, Volume 1, 1965, pp. 17-20

Web links