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Published By Lankelma

Lankelma is the foremost contractor for onshore in-situ soil testing in the UK. An acknowledged specialist in CPT, Lankelma also offers a worldwide consultancy and training service.

A.P. van den Berg develops, designs and manufactures geotechnical and environmental soil investigation equipment for onshore and offshore applications. Specialists in CPT systems and equipment.


Gardline Geosciences offers worldwide marine geotechnics, in-house consutancy and services with marine investigations ranging from nearshore to full ocean depth (down to 3000m).

About the Author

Hans Brouwer studied civil engineering at Delft University in The Netherlands. He has worked as a part-time lecturer at Amsterdam Polytechnic and was senior partner in a structural engineering consultancy. He has written a standard textbook in Dutch about the design of building foundations. He now lives in England where he writes technical textbooks in English, hopefully to reach a bigger readership.

Chapter 4

Part 1: Special cones: geotechnical cones

Nuclear density probe


For many engineering purposes it is essential to obtain accurate
information on the in-situ density and porosity of the soil. This
requirement has resulted in the development of a variety probes that
contain a radioactive source and a detector that can be used in
conjunction with the CPT.

Types of probes
Essentially two types of probe are available:

  • A probe that has the radioactive source mounted near the tip of the
    probe and which is inserted into the ground by conventional
    penetration techniques
  • A probe that is inserted into the ground through a pre-formed hole.

In determining the soil density, the gamma radiation emitted by a
suitable source (normally caesium 137) is attenuated by either
absorption or back-scattering.
Compton scatter phenomenon
Most available probes have been designed on the Compton scatter
phenomenon. Compton scattering is proportional to the charge-tomass
ratio of the atoms of the substance, this being approximately
0.50 for carbon, oxygen and silicon, and approximately 1.0 for
The amount of back-scattered radiation is related to density with the
rate of radioactive back-scatter increasing linearly with a decrease in
soil density. Readings of radioactive back-scatter are readily converted
to soil density using appropriate calibration curves.
Calibration of the instrument can be carried out by lowering the probe
into fluids with known densities of between 1.0 and 2.2 Mg/m3. Typical
calibration fluids used would include water, bentonite and aluminium
Alternatively, suitable materials could be compacted to a known density
in layers into rigid calibration chambers. The calibration chambers are
accurately weighed and the materials’ water content determined. This
exercise is repeated at least three times and the probe inserted into the
centre of the calibration chamber and readings taken to enable a
calibration curve to be determined. A calibration chamber diameter of
1.0 m is recommended since this size should ensure that the sides of
the rigid chamber are beyond the normal range of the probe.
With the ever increasing environmental and legislative concerns relating
to nuclear devices, the potential for the penetration probe to suffer
damage – together with a loss of integrity of the source encapsulation
with associated ground contamination – expensive remediation is high.
Consequently these probes have lost favour as a prime method of

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