Drying out a flooded pressuremeter

Note: the procedures described refer to operation with the Strain Control Unit. The symptoms and remedies still apply when an operator is controlling the system manually using the Pressure Control Panel.

If the membrane becomes punctured during the course of a test the first indication is an increase in activity from the Strain Control Unit as it attempts to put more gas into the instrument. If the leak is severe, the UP valve light will be on continuously, and the pressure:displacement plot on the computer screen will show a sudden fall in pressure.

If the leak is small, it is possible that the only indication will be that the SCU is having to work unusually hard to keep up with the expansion โ€“ one expects, towards the end of a test, for activity from the SCU to fall off. If a small leak is suspected, then have a look down the borehole. If the hole is full of water a stream of gas bubbles often appears.

  1. If the leak is small, try to complete the test if possible, using higher settings on the PRESSURE RATE switch to increase the pressure available to the SCU.

  2. A small leak means that the membrane is punctured, but largely intact. In these circumstances it will collapse back on to the body of the instrument and will make a reasonable seal that will keep out dirt and water.

  3. A large leak can demand some thought. It is important to keep water out of the instrument, so connect the probe to the pressure control panel and let a supply of gas pass through the instrument. This should be a pressure greater than the hydrostatic head.

  4. Note that as the instrument is withdrawn from the ground, the head of water will be falling and the gas pressure applied should be reduced accordingly. It helps to watch the reading on the EIU of the strain signal, which, if all is working properly, will show if the membrane has expanded as a consequence of this gas pressure. The instrument must not be allowed to expand.

  5. When the instrument is recovered, strip off the membrane and sleeves.

  6. If there is evidence of water, then form some assessment of how serious this is. Salt water drowning is usually very serious, so that all parts that have come into contact with the salt need washing thoroughly in clean fresh water.

  7. However the place that is often overlooked is the hose. A large enough head of water will partly fill the hose โ€“ if nothing else, this is why gas must be passed through the instrument to stop the hose filling with water.

  8. Pass gas through the hose, and see if any moisture comes out. If it does, then there is no alternative but to keep passing gas until all moisture is gone.

  9. If the hose is OK, wash and wash again all parts of the instrument. Salt will eventually cause a build up on the arms that will lead to them becoming jammed in their slots. Although the transducers have some protection on them the salt will attack the wiring, and will eventually work its way onto the surface of the strain gauges.

  10. Distilled water does not seem to offer any advantage, ordinary tap water is sufficient. Pay particular attention to the exposed part of the ring of seals where the electronics compartment starts.

  11. In general, success is the instrument zero readings being the same after cleaning as they were before the membrane became punctured. Serious departure from these readings needs some judgment. If the transducer reading is stable over a long period of time then it is probably OK โ€“ a long time means about 1 hour.

  12. If the readings from a transducer wander then this may indicate that the transducer is breaking down, and needs changing. However it may also indicate some activity on the ring of seals. Measure the resistance to the body of the instrument from the + and - terminals. It ought to be a resistance greater than 1 MegOhm, so any departure from this suggests more cleaning is required.

Pressuremeter references: 
SBP