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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 3: Special cones: other cones

Lubricating cone

The friction between the soil and the push-in rods limits the maximum depth of a cone penetration test. Depending on the soil type, friction can increase rapidly at greater penetration depths. A new type of cone, the lubricating cone, has therefore been developed to enable investigations to be taken to greater depths without using more expensive and time-consuming drilling techniques. The same technique can be used to grout the probe hole during retraction of rods.
Bentonite wash
During the cone penetration test, a bentonite wash is pumped through the push-in rods to the outlet opening (Figure 34). The bentonite wash forms a lubricating layer along the entire length of the penetration rods so that friction along the rods is considerably reduced. The outlet opening is located behind the first friction reducer, approximately 0.5 m above the cone tip. This construction prevents the soil penetration results being influenced by the introduction of the wash. The pump pressure used is the same as the hydrostatic pressure acting at a given penetration depth.
An advantage of this method is that soil penetration testing can be
carried out at much greater depths or in very stiff clay. Depths of more than 100 m have been reached using the lubricating cone. Relatively great depths may also be reached at locations where only small and light equipment can be used. The system can be linked to any type of CPT equipment.
Retraction of rods
During retraction of the push rods, a hardening bentonite mixture (that is harmless to the environment) is injected at high pressure into the soil. This mixture stiffens and so seals the hole created by the CPT. A well sealed soil penetration hole is desirable in the following cases.

  • During environmental engineering investigations where the soil penetration test can cause contamination leakage
  • During soil investigations for bored tunnels, where ‘blow-out’ can lead to costly delays
  • During soil investigations on sites where there is a likely existence of well shafts or on the inner foot of a dike. 
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