Civil Engineering Reference
In-Depth Information
contact with the saturated soil and is pushed inwards within the housing as a result of the pore water
pressure acting on it. A flow of dry compressed air is passed from the sensor to the inner aspect of the
diaphragm until the point that the diaphragm is forced back outwards. At this point the gas pressure is
equal to the pore water pressure and this value is simply read from the sensor unit.
Electrical system
The electrical piezometer also incorporates a diaphragm within a protective housing operated within a
borehole. The diaphragm is connected to a transducer and the water pressure acting on the diaphragm
causes a measurable response. The signal from the transducer is transmitted to a readout device at the
surface and then converted to display the pore water pressure.
The advantages of the electrical system are that (i) pressure is measured at the tip so that piezometric
levels below the gauge house level can be recorded, (ii) the ancillary equipment is compact, and (iii) the
time response of these instruments to pore pressure changes is fairly rapid. Disadvantages include the
fact that the readings from an electric tip depend upon an initial calibration that cannot be checked once
the tip has been installed, and the risk of calibration drift (especially if the tip is to be in operation for
some time).
Instrumentation in geotechnical engineering is dealt with in detail by Dunnicliff (
1993)
and a review of
piezometers within boreholes has been provided by Mikkelsen and Green (
2003
).
6.6 Field tests in soil and rock
During a ground investigation, field tests can be conducted to provide additional ground stratification
information and to obtain geotechnical parameters for the design. The tests are arranged such that the
data they reveal complement the soil and rock sampling so that all the information retrieved from the
ground investigation is linked.
A range of tests exist, each of which is used to gain specific information, and the following tests are
recognised in EN1997-2:2007 and ISO 22476 (the
International Standard for Geotechnical Investigation
and Testing - Field testing
):
●
cone penetration test;
●
pressuremeter and dilatometer tests;
●
standard penetration test;
●
dynamic probing;
●
weight sounding test;
●
field vane test;
●
flat dilatometer test;
●
plate loading test.
Some of these tests are more commonly used than others. Some of the most common in the UK are
described below.
6.6.1 Cone penetration test (CPT)
This test, sometimes referred to as the Dutch cone penetrometer as it was originally developed in The
Netherlands, is described fully in EN ISO 22476-1:2012 and involves a cone penetrometer at the end of
a series of stiff cylindrical rods being pushed vertically into the ground at a constant rate of penetration.
A record of the resistance to the movement of the cone against depth is taken so that changes in soil
strata and other soil strength considerations can be identified. The movement of the cone is resisted by
both the ground ahead of the cone as it advances through the soil, plus the friction acting on the side of
the cone as it is pushed into the ground.
