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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 2: Special cones: geo-environmental cones

Electric conductivity cone

Subsoil conductivity
In addition to the measured cone tip and sleeve resistances, the electric conductivity cone (Figure 28) also measures the soil
conductivity with depth. As with the conventional CPT, the basic
penetration parameters, qc and fs, enable the classification of soil.
The measurement of conductivity in the subsoil facilitates separation of zones with differentiated water content including determining the water table depth and the thickness of the zone of capillary ascent. The most important application of the conductivity cone, however, is related to evaluating the degree of contamination of a soil medium containing electrolytes (the compounds dissociating into ions in water).
Inorganic acids
Since hydrogen and hydroxide ions have the highest conductivity, the easiest way to detect strong inorganic acids, hydroxides and their salts in soil is to use the electric conductivity cone. This method may be applied to identify zones with increased concentrations of nitrites and nitrates, sulphates, calcium and magnesium compounds and concentrations of heavy metals. If aromatic amines are found in the subsoil, compounds dissociating in the water solution which lead to an increase in conductivity may indicate an excess of N-nitrogen fertilizers.

Oil derivatives do not dissociate into ions, therefore rendering direct
detection of them with the conductivity method impossible. In certain
cases, however, it is possible to estimate indirectly the presence of
hydrocarbons when there are phenols in the subsoil – phenols are
products of the reaction of aromatic compounds which dissociate in
Levels of pH
The conductivity method also yields information on the aggressiveness
of groundwater towards concrete. If the pH value of the soil changes
from neutral, the conductivity increases. The presence of carbon
dioxide in the water results in formation of carbonic acid – the factor
that increases conductivity.
Inorganic salts
Water hardness related to the presence of magnesium, calcium and
iron ions and the presence of sulphates (inorganic salts) is another
factor which increases subsoil conductivity. Another application of the
method can be the determination of the range of fresh and salt water in
subsoil; the presence of sodium and chlorine ions in salt water
increase conductivity, facilitating discrimination of zones where salt
water occurs.
From the perspective of analysis of pollutants, the universality of the
application of this method is a significant drawback. The method
measures the sum of all influences against the background of the soil’s
natural conductivity based upon its structure, water content,
mineralogical composition, and the physical, chemical and biological
processes taking place in the subsoil. It is therefore best to use this
method to monitor zones, range and intensity of contamination of an
area under study.
Depending on the way in which the problem is formulated, execution of
the studies can take two directions. Firstly, in cases where the
contaminating agent is known but the range and intensity of
contamination is unclear, penetration is carried out in a systematic grid
to facilitate determining isolines (regions with similar concentrations of
the agent). Secondly, where there are several pollutants or the pollutant
is unknown, monitoring is carried out to select investigation points with
the highest and the lowest levels of contamination from which water and
soil samples are taken for special chemical investigation. 

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