John Dunnicliff is of course correct in drawing attention to the importance of good pre-installation procedures for stand vibrating wire piezometers (Ground Engineering, April 2000).
He describes a fairly complicated and timeconsuming process that can be eliminated for piezometers using the DiBiagio modification.
Dr Dibagio first described this during the BGS symposium Field instrumentation in geotechnical engineering which is reported in the proceedings on pages 565/6.
An air tube in the connecting cable keeps the back of the diaphragm at atmospheric pressure. By increasing the air pressure in controlled steps, the calibration of the piezometer can be checked while it is in insitu, at any time after installation. The metal diaphragm contains small holes and is covered on the water side by a neoprene membrane.
When the air pressure reaches that of the insitu pore pressure, the neoprene membrane is pushed off the metal diaphragm and this is revealed by a cessation of change of vibrating wire reading as air pressure increases, showing that the metal diaphragm is free and is showing its zero reading. A perforated back plate prevents the neoprene membrane from moving too far, limiting the volume of pore water displaced and preventing damage to the membrane.
I strongly recommend this type of piezometer, particularly for long-term use, as with earth dams, where reliable measured values are needed many years after initial installation. This type of vibrating wire piezometer is manufactured by Geonor in Oslo, type M603.
In fact, John Dunnicliff knows about the DiBagio modification because he describes it in his well known 'red book' on instrumentation. Why did he not mention it in his recommendations on pre-installation procedures?
Dr Arthur Penman, Geotechnical engineering consultant, Hertfordshire
The author replies:
You refer to the advantages of the Geonor M603 piezometer with an in-place check feature and, as you point out, I described this in my instrumentation book.
In Section 9. 8. 5 I state: 'Although an ingenious arrangement, it is incompatible with a hermetically sealed cavity within the piezometer tip and increases the potential for corrosion because any damage to the ventilating tube may provide a direct path for moisture to enter the cavity. Deterioration of the neoprene diaphragm may also result in malfunction. The instrument is therefore best suited to obtaining verified data of high accuracy over a short term, rather than as an in-place check over a long term. '
I recently discussed this with the designers of the Geonor M-603 piezometer at the opening of the Ralph B Peck Library at Norwegian Geotechnical Institute (News this month). They confirmed the modification was devised solely for such short-term applications. It was suggested that design could perhaps be improved to overcome the corrosion problem but no improvements have been made since the original 'first attempt' in the 1970s.
Field experience during recent years has shown the failure rate for the modified piezometer is significantly greater than the unmodified version and I was advised not to select it for applications that require data for periods longer than about one year.
I hope that we can now arrive at a mutual view that we cannot 'afford to use this type of piezometer'(in its current version) for normal applications and, if this means that we have to suffer 'a fairly complicated and time consuming process', so be it. After discussions with Arthur Penman we are in agreement.
John Dunnicliff, geotechnical instrumentation consultant, Devon