Discussion of Grout-In Method

Erik Mikkelsen
I've been asked to look at data from some grouted-in piezometers that were showing negative readings. There was some question to the validity of the data. However, when site conditions were considered, it appears that the grout around the piezometers was acting as a fully saturated high air-entry filter and the readings were actually due to negative pore-water pressure. So long as the grout does not dry out, the ability to measure negative pressures should remain intact. Where the soil is not fully saturated, there will be long-term migration of moisture from the grout, and this will eventually degrade the ability of the piezometer to measure this negative pore-pressures.

Andre Mazur
I have frequently installed piezometers without filter sand in a tight hole and tremmied back with bentonite mud. Results have been superb. Benefits of an expanding seal cannot be denied, and data attained from a piezo installed under non-shrink grout may not be representative of prevailing ground conditions..why risk it?

Drum Cavers
We have been using this method for some time with good success. We have also doubled up installations be installing piezometers on the outside of slope indicator casings. This has worked well except for cases of large movements where the movement induced cracking of the grout along casing. The piezometers should not be installed near a joint in the casing. We saturate the tip and install it facing up (like a cup) with no filter cloth since the filter cloth (or other sand filters and so on) are likely to introduce air bubbles which would slow the response time.

Rick Kobernik
We wanted to reduce drilling costs by installing piezometers and inclinometer casing in the same borehole. We installed the transducers directly in the grout. We de-aired the tip by pulling off the filter underwater, inverting the sensor so that all of the air could escape, and then replacing the filter. We taped the piezometers to the inclinometer pipe with their filter ends pointing up (so water would not run out, and then made a loop in the cable and taped it to the inclinometer pipe every 2 to 3 meters. The transducers are giving us good readings.

Gorden McKenna
I always put the piezo in a very small sock with some frac sand -- but had suspected that it wasn't really necessary. I worried about the grout clogging up the filter stone on the piezo intake. Like you, I figured the bit of geotextile (and the sand) was a bit of insurance. I'm hoping that you'll publish your test so that others might catch on. Out latest (minor) innovation in piezometers is to place them first, then pour the ground around them.... we install tips on power poles in the bottom of our mined out pits, then fill the tailings around them. The tips are not saturated when the tailings makes it's way up to them, but we had Bill Liu of Golders do some work in the lab with a tip and some soft tailings and found that the tip read correctly pretty much no matter how we filled the tailings. When we get a few minutes, we'll put the results of this little bench study into Geotechnical News.

Gil Aubertin
We installed four VW piezos on Dec 4-6, 1999 in general accordance with the procedures outlined in your September Newsletter and, to date, the 4 transducers appear to be working fine.

Jon Bounds
We have been installing VW piezos taped along the side of inclinometer casing since 1996. We saturate the filters, invert the piezo, and tape it to the inclinometer tube. All of the installations are still operating well except where major slide movement has pulled the cable apart. Using dataloggers with grouted in piezos have really helped us in our work.

Rick Kelsic
I have installed (grouted in) a VW piezometer outside of an inclinometer casing to monitor water pressesurers adjacent to the inclinometer. In addition I have also installed a porous-tube piezometer at the same depth (strata)within 10 feet of the inclinometer installion. Water pressures are very comparable.

Return to References and Papers on the Grout-in Method

Some Questions and Answers:

Question: What grout mixes are recommended?

Answer: Go to this link: Grout Mixes for Piezometers

Question: Doesn't the piezometer sense the weight of the grout column above it - when the grout is still fluid? Doesn't this damage the piezometer?

Answer: During installation, the piezometer sees full hydrostatic pressure of the column of grout. The pressure of the grout column is typically about 1.5 times greater than the pressure of a water column. Can the piezometer withstand this pressure? Yes, assuming that an appropriate pressure range is selected. VW piezometers have a 1.5 to 2x over-range capacity (i.e. the diaphragm can be loaded 1.5 to 2x the pressure rating of the sensor).

If you installed a 100 psi piezometer in a 100 foot borehole and backfilled with grout, it would see an initial pressure that is equivalent to 150 to 200 feet of water. This is still within the calibrated range of the piezometer (which goes to 230 feet of water). However, if you installed the same piezometer in a 200 foot borehole, then the piezometer would see a pressure equivalent to 300 to 400 feet of water, which is well into its over-range zone.

As the grout cures, the hydrostatic pressure will decrease to that of the formation. Setting time is primarily a function of how much cement is used. In a lab test, Gordon McKenna reports a 4.5 hour setting time for a short column of grout. We normally think of cement reading its full strength after about 28 to 30 days. Thus, we would not expect that reponse time would increase after that. Perhaps that is something we can look at in our current testing. Like you, we have mixed cement and water first and then added bentonite to achieve the consistancy that is pumpable.

Question: In light of the above comment by Gordon McKenna, is it considered necessary to wrap the piezometers in geotextile or place them in a sand-filled PVC tube? Or is it acceptable to simply saturate the filters, invert the piezometers and tape them directly to the inclinometer casing?

Answer: We think wrapping the piezometers in geotextile or placing them in a tube of sand is unnecessary. However, it might be useful in protecting the piezometer from scraping on the sides of the borehole.