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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

Flow penetrometers

Undrained shear strength
Undrained shear strength profiling is an important part of many ground
investigations, especially in soft clay. This is normally achieved by insitu
vane testing or laboratory strength measurements on undisturbed
soil samples. This approach suffers the disadvantage that such tests
can only be taken at discrete, well spaced depths in the profile and the
presence of stiffer materials can affect the results obtained.
In contrast, the CPT provides a continuous measurement of undrained
shear strength, but, regrettably, the CPT is not very accurate in soft
clay deposits due to the low tip resistances being measured. Also the
deformation mechanism around the cone during penetration is
asymmetric in the vertical plane; consequently a correction for
overburden pressure and porewater pressure is also required. The
empirical and theoretical solutions relating undrained shear strength to
cone tip resistance are difficult to apply objectively and resulting
estimates can therefore be erroneous.
Many of the limitations outlined above can be overcome by varying the
shape of the penetrometer such that it causes symmetrical flow during
penetration. Two such devices, the T-Bar and the spherical ball are
shown in Figure 27.
The T-bar consists of a short cylindrical bar measuring 250 mm in
length and 40 mm in diameter, while the ball cone is 163 mm in
diameter. Both devices are attached at right angles to penetrometer
rods, just below a calibrated load cell. Also included in the shaft is an
inclinometer to indicate any deviation from the vertical during insertion.
The device can also incorporate porewater pressure transducers. 
Flow penetrometers have two major advantages over the conventional
electric cone.
Firstly, for both flow penetrometers (T-bar and spherical ball), the soil
deformation mechanism is symmetrical in the plane perpendicular to
the axis of the penetrometer, and the load cell measures what is
essentially a differential force (or net pressure) on the bar with no
adjustment being made for the overburden stress and ambient
Secondly, the correlation between net pressure on the flow
penetrometer and the shear strength of the soil is made via an exact
plasticity solution; this operates within a potential range of flow
penetrometer factor of less than ± 10% (due to different roughness of
the bar surface) compared with cone factors which may vary from as
low as 7 in sensitive clays to over 15 – that is a range of ± 35%.
Remoulded strength
An additional use of the flow penetrometer is to assess the remoulded
strength of the soil by monitoring the bearing resistance during
extraction along the same path as insertion. 
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