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air the unit following steps 4.1.7 to 4.1.15 described above. Then proceed to a
saturation check to make sure that less than 15 cc are required for increasing
pressure up to 2500 kPa. Bring pressure and volume down to 0 and disconnect
saturation hoses.
2.
Initial readings.
Reconnect the Tecalan-probe assembly to the control box. Just
before lowering the probe in the borehole, write down the initial Pressure and Volume
readings. Make sure that the probe is at the same height then the gauge of the
pressuremeter. Normally these initial readings should be close to 0, however
sometimes due to temperature effects or other effects they are not. These initial
reading should be subtracted to the raw test data afterwards.
3. Once the probe is lowered in place, pressure reading may be different than zero. Do
not re-zero the pressure. Start the test.
7.3 STRAIN CONTROLLED TEST
The strain controlled test is the recommended method for general applications.
1. With the probe set at the test depth, the pressuremeter test can begin. The test is
carried out in 40 steps of equal volume increments. The volume increments are about
40 cc for the long 70 mm diameter probe and about 20 cc for the 44 mm diameter
probe. Rotate the crank handle clockwise at a uniform rate of about 1 rotation per 4
seconds to inject water, stopping to record the pressure 15 seconds after each step of
volume. The maximum volume injected is 1600 cc for long 70 mm probe and 800 cc
for 44 mm probe; this is equivalent to 40 increments.
2. Stop the test : a) when a sufficient number of data have been collected for the intended
purpose of the tests, typically for determining the limit pressure with a satisfactory
accuracy (typically between 1000 to 1200 cc for an NL probe) ; b) or when risk of
bursting the probe is high
– see chart at the end of this manual.
3. It is possible and suggested to carry on an unload-reload cycle in the pseudo-elastic
zone i.e. between Pf (creep pressure) and Po (in-situ horizontal earth pressure). This
cycle can be performed different ways. The ASTM-D4719 standard suggests running
this cycle in one step down to Pf / 2, and then back the same way up to Pf. Another
method consists in reducing pressure in three equal-pressure steps down to a point
slightly above Po, and then back the same way up to Pf. Roctest recommends using
the latter method.
During a test the end of the pseudo-elastic zone may be found by keeping track of
p,
which will start decreasing when the soil starts to yield. When the unload-reload cycle is
completed, the test is resumed in the plastic zone as a normal test.
4. When the test is over, slowly return the piston to its initial position.
Warning: Deflation of the probe must be done slowly
– see warning at section 5.1.
Make sure that the borehole is filled with fluid for helping deflating the probe.
Warning: If the pressure drops to zero or close to zero, this means that the sheath is
punctured. Do not bring volume back to 0 cc. Dirty water or mud would get into the
unit.
5. Withdraw the probe
. If this can’t be done because the probe is sticking to stiff plastic
clay or because of caving problem, follow this procedure: the probe is pulled up about 5
mm and pushed back by two persons with pipe wrenches on the rods. This is repeated
until this up and down cycling process weakens the soil and frees the probe. It is also
possible to apply some vegetable oil on the probe’s membrane to prevent it from
sticking to clayey soils.