Checking the Lift-Off performance of the arms

From time to time it is as well to check that the arms on the instrument do not show an excessive amount of movement due to the application and removal of external pressure.

This is important because exactly this takes place every time the instrument is bored into the ground and a test carried out.

During the boring the insitu lateral stress is coming to bear on the instrument, and forces the arms tight against their stop. During the test the application of a gas pressure to the internals of the pressuremeter means that the load represented by the ground is gradually being removed. If there is any defect in the seating of the arms it can show as a small positive or negative strain, of sufficient magnitude that the 'lift-off' point is disguised.

In addition, because any movement is accompanied by a corresponding change in stress, it is possible that these small movements in material of high stiffness can result in stresses being generated that are sufficiently large to cause a significant change to the material being tested.

To carry out a check proceed as follows:-

  1. Set up the equipment exactly as described in Carrying out a membrane correction test - SBP

  2. Set the computer to make a test using the WINLog software, as described in Carrying out a Strain Controlled test.

  3. Initially, close valve E on the Pressure Control Panel. Raise the pressure using the hand regulator until 1000kPa is indicated on the gauge.

  4. Close regulating valve B, and vent the panel through regulating valve F and valve G.

  5. When vented, close regulating valve F and open valve E. Now crack open regulating valve B and allow the pressure in the external calibration cylinder to climb slowly to 1000kPa.

  6. Now shut regulating valve B and crack open regulating valve F, letting the pressure fall very gently.

There are no special facilities in the logging software for examining the data other than as a plot of the arm output against time. However the calibrated data file can be read by a spreadsheet using the pore pressure cells to give a direct reading of the pressure changes.

Pressuremeter references: