[Click on image to obtain a large
view]
A DESCRIPTION OF THE CONE PRESSUREMETER AND ASSOCIATED PARTS
[Click on image to obtain a large
view]
The Cone Pressuremeter
The Cambridge Cone Pressuremeter is a rigid cylindrical probe 2 metres long
approximately and 43.7mm in diameter. Part of the probe, the central 0.5
metres, is covered with an elastic membrane. The foot of the probe is fitted
with a 15cm² friction cone, the top of the probe is connected by
conventional CPT rods to the surface.
Inside the cone
rods a special pressure hose and cable assembly conveys the combined
pressuremeter and friction cone electrical signals to data logging equipment.
When a pressuremeter test is carried out, dry nitrogen gas passes down the
hose to the pressuremeter. When the pressure behind the membrane exceeds
the pressure due to the soil bearing against the instrument the membrane
inflates in a controlled manner.
The pressure applied and the displacement of the membrane which results are monitored by transducers in the instrument itself. The pressure transducer consists of a strain gauged diaphragm which has a vacuum on one side and the pressure in the probe on the other. A displacement transducer (normally referred to as an 'arm') consist of a pivoted lever which bears on the inside wall of the membrane at one end and has a leaf spring forcing it to follow the movements of the membrane on the other side of the pivot. The leaf spring is strain gauged with a full bridge circuit so that movement is converted to an electrical signal. The leaf spring is de- coupled from the pivoted lever by a small ball race to minimise frictional effects.
There are three displacement followers in the CPM, all in the same plane and spaced 120° apart. They act at the centre of the expanding membrane and the electrical signal represents radial displacement.
There are some simple electronics in the probe that regulate the voltage supplied to the strain gauges and which amplify the small signal output from the transducers. The electronics are contained in a plug-in module and are sufficient to ensure that changes to the length of the cable connecting the pressuremeter to the surface do not affect the output of the pressuremeter.
The cable consists
of 18 conductors, 10 of which are dedicated to the cone and 8 to the
pressuremeter. The cone signals are completely independent of the pressuremeter
as this is the easiest way to ensure compatability with a range of possible
cones. The data output by the cone and by the pressuremeter are logged on
separate systems.
The CPM and all associated electronics operate from a single 12 volt car battery.
The arms of the pressuremeter are positioned one metre behind the tip of the cone. Deciding where to carry out a pressuremeter test is normally made from the cone trace, the one metre interval giving sufficient time to identify significant bands of material. For these tests the cone was not live, and an arbitrary decision was taken of where to place the tests. The interval between tests varied from 1.5 to 2 metres.
The hose/cable assembly must be threaded through the CPT rods before the instrument is placed in the borehole. It is possible for additional rods to be added after the borehole has started, but it is not possible to extend the length of the combined hose and cable.
The Electronics
The CPM data logging and testing system has three components, an interface
box, a portable notebook computer and a Strain Control Unit for inflating
and deflating the membrane in a controlled manner.
The Interface Unit
The interface box does the following:
In addition the box separates the pressuremeter lines from the cone lines and routes the cone signals to an isolated socket.
Continuous monitoring of the analogue pressuremeter signals is available via a rotary switch and a pair of terminals to which a multimeter could be connected. The same switched output can be observed on a built-in liquid crystal display.
The Computer
The RS232 signals output from the interface unit can be accepted by a variety
of computers. We use a notebook computer running purpose written data acquisition
software normally referred to as ‘the logging program’. The software
allows a pressure versus displacement plot to be seen in real time, and stores
the readings on floppy disk.
The Strain Control Unit
The inflation and deflation of the membrane during a pressuremeter test is
done automatically by the Strain Control Unit (SCU). This consists of a pair
of solenoid valves through which the gas supply to the pressuremeter passes.
These solenoid valves are controlled by an electronic feedback loop. The
SCU is so named because the rate at which the test cavity expands is used
to control the application of pressure to the instrument.
A signal representing the displacement of the membrane is passed to the SCU from the interface unit. This external signal is compared with an internally generated strain signal, and the unit acts to keep the two signals in step.
The system is complex, with a control over the rate of pressure change as well as strain. The effect of this is to ensure that when the soil response is elastic, the rate at which gas is passed to the probe is pressure controlled. When the soil deforms plastically, the strain rate is the dominant control. A strain controlled expansion is important for the cone pressuremeter test because once the material is close to limit pressure, small changes of pressure result in large strain changes. Pressure increments would be inappropriate in this context.
The direction of strain can be reversed by the operator, so enabling unload/reload loops to be taken. There is in addition a hold facility so that the expansion can be halted - this is necessary before a rebound loop is taken to allow rate effects to diminish.
Installing the Pressuremeter.
Conventionally the pressuremeter is pushed using CPT (Cone Penetration Testing)
equipment, usually a 20 tonne cone truck. The maximum force applied ought
not to exceed 15 tonnes. Where a CPT truck was not a viable option and the
ground is soft a cheaper solutions are possible. An example is given below,
for a series of tests in old waste.
The CPM was pushed with a set of free standing hydraulic rams whose base was a simple steel ‘H’ frame, the length of the frame being about 3 metres. A portable power pack provided the power for the rams. Reaction for the system was provided by leaning on each end of the frame with the bucket of a JCB.
At both positions a pit was scooped out of the ground, one JCB bucket wide and about 1.5 metres deep. This was sufficient to remove a surface layer of recently deposited rubble and reach the much older and decomposed waste. The frame was then laid across the pit and a pair of JCBs then positioned their buckets. The available kentledge was probably in excess of 4 tonnes. Once the CPM was in the older waste observation suggests less than 20% of this potential was utilised.
The picture shows the rams pinned to the frame in position on the first location. The instrument itself is already in the ground, and the CPT rods driving the probe run through the centre of the rams. The umbilical linking the probe to the surface electronics can be seen coming out of the CPT rods.
Written by Robert Whittle Oct 1999
Back to specifications index...