Grouted-in installation of piezometers in...

Gord McKenna, BGC Engineering 1

Grouted-in installation of

piezometers in boreholes:

Lessons from the first 20 years

Gord McKenna PhD, PEng, PGeol

Senior Geotechnical Engineer

BGC Engineering Inc, Vancouver

Adjunct Professor, U of Saskatchewan

March 22, 2010

(With some pictures and data from the Internet)

Outline

• Background / history

• How it works

• How it fails

• Enhancements and opportunities

• Being a geotechnical instrumentation purist

• Efficacy and advocacy


Background / history

• A paper by Vaughan 1969, largely overlooked

• Ian Brown at Syncrude started experimenting in 1988

• McKenna experiments and papers 1988-1995

• Cautious adoption, work by Sinco, a few papers, better lab testing

• Gaining wider acceptance,but still novel

Vaughan 1969

• A Note on Sealing Piezometers in Boreholes, P. R. Vaughan, Géotechnique, Volume 19, Issue 3, pages 405 –413, 1969.

• …a piezometer is installed in a borehole which is sealed with a grout of greater permeability than the soil surrounding the piezometer. It is shown that the grout must have a permeability considerably higher than that of the ground for the piezometer reading to have a significant error.

• It is shown that in certain circumstances the filter material commonly used to surround the piezometer tip may be omitted


How it works

• “The key to the success of the grouted-in installation method is that modern diaphragm-type piezometer tips require only a very small fluid volume change for pressure equalization, and the grout can transmit this volume over the short distance from the formation to the tip quickly.”

• “Some numbers help illustrate this. Suppose there is a one psi increase in water pressure in the formation. With a 3/4-inch standpipe, it would take an inflow of 240 ml of water before this change could be measured. However, with a VW piezometer, the same change could be measured with an inflow of only 0.00002 ml, approximately 12 million times less.”

Good performance


Poor performance (don’t forget the bentonite!)

More good performance


But will it work in the lab?

New lab experiments


But will it work in theory?Time lag calculation

Hydrodynamic time lag

Piezometer type

Installation

method

Typical

intake

length

(cm)

Typical

intake

diameter

(cm)

Approximate

equalization

volume per 100 cm

water head (cm3)

Approximate

99% pressure

equalization

time

Sinco pneumatic

piezometer

Conventional 30 17 10-4 <1s

Grouted 20 5 10-4 10s

Geokon vibrating-

wire piezometer

Conventional 30 17 10-5 <<1s

Grouted 20 5 10-5 1s

Sinco pneumatic

piezometer

Conventional 10 7.5 10-4 1s

Grouted 10 5 10-4 10s

2.5cm-diameter

standpipe

Conventional 60 17 500 10 days

Grouted 60 2.5 500 10 months

* based on formation permeability of 10-7 cm/s and grout permeability of 10-8 cm/s.


Grout Mixes for Piezometers

Grout Mix for Hard and Medium Soils

Materials Weight Ratio by Weight

Portland cement 94 lb (1 bag) 1

Water 30 gallons 2.5

Bentonite 25 lb (as required) 0.3

Mix cement with water first. Then mix in the bentonite. Adjust the amount of bentonite to produce a grout with the consistency of heavy cream. If the grout is too thin, the solids and the water will separate. If the grout is too thick, it will be difficult to pump.

Grout Mix for Soft Soils

Materials Weight Ratio by Weight

Portland cement 94 lb (1 bag) 1

Water 75 gallons 6.6

Bentonite 39 lb (as required) 0.4

Drillers and grout

• Drillers are accustomed to mixing water and bentonite first, but this will not allow you to control the water-cement ratio. Mix water and cement first. Then add bentonite.

• The quantity of the bentonite to be added must be adjusted to obtain a suitable consistency.

• Do not leave too watery….


Limitations(McKenna 1995)

• The permeability of the grout must be at equal to or lower than the formation permeability for almost all situations.

• The grouted-in installation may not be practical for a very high permeability soils where excessive grout may be lost in the voids.

• Stress transfer along stiff grout column in soft, consolidating soils may cause a piezometric error.

• Grout may crack due to ground movements, which could lead to higher macro-permeability.

• For installations in remote areas, it may be easier to transport sand and bentonite sealing chips instead of a grout reservoir and pump.

• Groundwater sampling using this method is impractical because of the use of diaphragm-type piezometer tips and the low permeability of the grout.

• Chemical contaminants in the soils (or extremely saline pore water) may interfere with the long-term mechanical properties of grouts

• Need to develop more documented experience worldwide.

Why bother?(Change is hard)

• It is a lot easier and more reliable than trying to establish bentonite seals 100ft down a borehole at -20C

• Less training, less experience

• Allows multiple piezos in a single borehole (>3?)

• Allows installation alongside a slope inclinometer

• $$

• BUT YOU HAVE TO DO IT RIGHT!


Greater adoption

• Vaughan, P. R , "A Note on Sealing Piezometers in Boreholes", Geotechnique 19, No 3, p405-413, 1969.

• Laing et al Geotechnical Instrumentation of the AOSTRA Mine-assisted Underground Steaming Trial, Fourth Unitar/UNDP Conference on Heavy Crude and Tar Sands, #112-3, p17-32. 1988

• McKenna, G.T, "Grouted-in Installation of Piezometers in Boreholes," Geotechnical Journal 32, pp 355-363, 1995.

• Mikkelsen, P. Erik. 2002, Cement-Bentonite Grout Backfill for Borehole Instruments, Geotechnical News, December 2002.

• Mikkelsen, PE and Green, E.G. 2003. "Piezometers in Fully-Grouted Boreholes." International Symposium on Geomechanics, Oslo, Norway. September 2003.

• Contreras, Grosser, Ver Strate, "The Use of the Fully-grouted Method for Piezometer Installation, " Geotechnical News, pp 30 - 37, Vol 26, June 2008.

Promoted by supplierswww.slopeindicator.com


New technology often takes 25 years19

80s 19

90s

2000

s

???

• Time lag too long

• Grouter higher than formation

• Very low k ground

• Forgot bentonite

• Cross-aquifer contamination

• Cracking due to moving ground

• Grout in the sinter stone

How does it fail?


Time lag too high

Grout permeability higher than formation

“If you know the ground conditions, you can get away with high k grout”


Beware – some of the analysis may have not considered the most severe cases

Frank Patton would ask…

Just how well do you know your hydrostratigraphy anyway?


Adjust grout to mimic ground stiffness


Grout in the sinter stone

• Sinter stones often 50 micron

• Cement is submicron

• Seems to lock in the formation pressure at time of installation

• May not appear as problem in lab (may occur under higher grout column pressures)

• Does not always happen

• Easily solved with a sand sock

Enhancements?

• Preplug of bentonite cement

• Unsaturated readings –intimate contact

• Mandatory permeability or strength test


Gord’s recommendations

• If you think the ground might be lower permeability than your grout

• When in doubt, chicken out

• Go traditional with bentonite seals and sand packs

• Use cement bentonite grout elsewhere in hole

• Use a sand sock because it works. Why not?

• Take extra care to make sure your grout mix is right. Trust no one.

• Let’s look at seeing how we might use the method for measuring in situ suctions

• Be a geotechnical instrumentation maven, maybe even a purist. Publish in Geotechnical News.

Good practice in completing your installation


Philosophy / Advocacy• Philosophy

• Don’t put something in the ground you don’t understand

• Don’t put something in the ground that may give you the wrong reading (unless you can tell)

• For a new technology to replace the old one, the benefits have to outweigh the risks (efficacy = the achievement of desired results)

• Advocacy• active support of an idea or cause etc.; especially the act of

pleading or arguing for something • Promoting / Over promoting?

• Responsibility?

• Community

EXTRAat no extra charge


Some references

• Chapuis, R.P. and Sabourin, L., 1989. Effects of installation of piezometers and wells on groundwater characteristics and measurements. Canadian Geotechnical Journal, 26, pp 604-613.

• Cioffi, R., Marroccoli, M, and Mascolo, 1989. On permeation effects of aqueous solutions through non-mature pastes of Portland-pozzolana cement, Cement and Concrete Research, 19, pp 189-193.

• Dunnicliff, J., 1988. Geotechnical Instrumentation for Monitoring Field Performance, John Wiley and Sons, New York, 577p.

• Filho, P.R., 1976. Laboratory tests on a new borehole seal for piezometers, Ground Engineering, 9, pp 16-18.

• Honda, A., Mashima, M., and Kunito, T., 1988. Influencing factors for impermeability of soil cement, Memoirs of the Faculty of Engineering, Osaka City University, 29, pp 205-212.

• Hvorslev, M.J., 1951. Time lag and soil permeability in ground-water observations, Bulletin Number 36, Waterways Experiment Station, Corps of Engineers, U.S. Army, Vicksburg, Mississippi, USA.

• Kurt, C.E., and Johnson, R.C. Jr., 1982. Permeability of Grout Seals Surrounding Thermoplastic Well Casing, Ground Water, 20, No. 4, p415-419.

• Laing, J.M., Scott, J.D., Stokes, A.W., Suggett, J.C., and Wood, D.F., 1988 Geotechnical Instrumentation of the AOSTRA Mine-assisted Underground Steaming Trial, Fourth Unitar/UNDP Conference on Heavy Crude and Tar Sands, Paper No. 112, 3, pp 17-32.

• Morgenstern, N.R., Fair, A.E. and McRoberts, E.C., 1988. Geotechnical engineering beyond soil mechanics - a case study, Canadian Geotechnical Journal, 25, pp 637-661.

• Mossop, G.D., 1980. Geology of the Athabasca Oil Sands, Science, 207, pp 145-152.

• Senger, J.A., and Perpich, W.M., 1983. An alternative well seal in highly mineralized ground water. 3rd National Symposium on Aquifer Restoration and Ground-water Monitoring, Columbus, Ohio.

• Vaughan, P.R., 1969. A note on sealing piezometers in boreholes, Geotechnique, 19, No. 3, pp 405-413.

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