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Grouted-ininstallation of
piezometers in boreholes:
Lessons from thefirst 20 years
Gord McKenna PhD, PEng, PGeolSenior Geotechnical Engineer
BGC Engineering Inc, Vancouver
Adjun ct Prof essor , U of Saskat chew anMarch 22, 2010
(With some pictures and data from the Internet)
Outline
Background / history
How it works
How it fails
Enhancements andopportunities
Being a geotechnical
instrumentation purist Efficacy and advocacy
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Background / history
A paper by Vaughan1969, largely overlooked
Ian Brown at Syncrudestarted experimenting in1988
McKenna experimentsand papers 1988-1995
Cautious adoption, workby Sinco, a few papers,better l ab testing
Gaining wideracceptance,but still novel
Vaughan 1969
A Note on Sealing Piezometersin Boreholes, P. R. Vaughan,Gotechnique, Volume 19,Issue 3, pages 405 413, 1969.
a piezometer is installed in aborehole which i s sealed with agrout of greater permeabilitythan the soil surrounding thepiezometer. It is shown that thegrout must have a permeabilityconsiderably higher than thatof the ground for thepiezometer reading to have asignificant error.
It is shown that in certaincircums tances the filtermaterial commonly used tosurround the piezometer tipmay be omitted
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How it works
The key to the success of the grouted-ininstallation method is that modern diaphragm-typepiezometer tips require only a very small fluidvolume change for pressure equalization, and thegrout can transmit this volume over the shortdistance from the formation to the tip quickly.
Some numbers help illustrate this. Suppose thereis a one psi i ncrease in water pressure in theformation. With a 3/4-inch standpipe, it would takean inflow o f 240 ml of w ater before this changecould be measured. However, with a VW piezometer,the same change could be measured with an inflowof only 0.00002 ml, approximately 12 million timesless.
Good performance
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Poor performance(dont forget the bentonite!)
More good performance
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But will it work in the lab?
New lab experiments
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But will i t work in theory?Time lag calculation
Hydrodynamic t ime lag
Piezometer type
Installationmethod
Typicalintakelength
(cm)
Typicalintake
diameter
(cm)
Approximateequalization
volume per 100 cm
water head (cm 3)
Approximate99% pressureequalization
time
Sinco pneumatic piezometer
Conventional 30 17 10 -4
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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 ofbentonite to produce a grout with the consistency of heavy cream. If the grout is toothin, the solids and the water will separate. If the grout is too thick, it will be difficultto 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 areaccustomed tomixing w ater andbentonite first, butthis will not allowyou to control thewater-cementratio. Mix waterand cement first.Then addbentonite.
The quantity o fthe bentonite to
be added mus t beadjusted to obtaina suitableconsistency.
Do not leave toowatery.
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Limitations(McKenna 1995)
The permeability of t he grout must be at equal toor lower than the formation permeability foralmost all situations.
The grouted-in installation may not be practicalfor a very high permeability soils whereexcessive grout may be lost in th e voids.
Stress transfer along stiff grout column in soft,consolidating soils may cause a piezometricerror.
Grout may crack due to groun d movements,which could lead to higher macro-permeability.
For installati ons in remot e areas, it may beeasier to transport sand and bentonit e sealingchips instead of a grout reservoir and pump.
Groundwater sampling using this method isimpractical because of the use of diaphragm-type piezometer tips and the low permeability ofthe grout.
Chemical contaminants in the soils (or extremelysaline pore water) may interfere with the long-term mechanical properties of grouts
Need to d evelop more docu mented experienceworldwide.
Why bother?(Change is hard)
It is a lot easier and morereliable than trying toestablish bentonite seals100ft down a borehole at -20C
Less training, lessexperience
Allows mult iple piezos ina sing le borehole (>3?)
Allows installationalongside a slopeinclinometer
$$
BUT YOU HAVE TO DO ITRIGHT!
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Greater adoption
Vaughan, P. R , "A Note on Sealing Piezometers inBoreholes", Geotechnique 19, No 3, p405-413, 1969.
Laing et al Geotechnical Instrumentation of the AOSTRA Mine-ass ist ed Underg round Steaming Trial,Fourth Unitar/UNDP Conference on Heavy Crude andTar Sands, #112-3, p17-32. 1988
McKenna, G.T, "Grouted-in Installati on of Piezometersin Bo reholes," Geotechnical Journ al 32, pp 355-363,1995.
Mikkelsen, P. Erik. 2002, Cement-Bentoni te GroutBackfill for Borehole Instruments, Geotechnical News,December 2002.
Mikkelsen, PE and Green, E.G. 2003. "Piezometers inFully-Grouted Boreholes." International Symposium onGeomechanics, Oslo, Norway. September 2003.
Contreras, Grosser, Ver Strate, "The Use of t he Fully-grouted Method for Piezometer Installation, "Geotechnical News, pp 30 - 37, Vol 26, June 2008 .
Promoted by supplierswww.slopeindicator.com
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New technology often takes 25 years
1 9 8 0 s 1
9 9 0 s
2 0 0 0 s
? ? ?
Time lag too long
Grouter higher thanformation
Very low k ground
Forgot bentonite
Cross-aquifercontamination
Cracking due tomoving ground
Grout in the sinterstone
How does it fail?
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Beware some of the analysis may havenot considered the most severe cases
Frank Patton wo uld ask
Just how well do you know yourhydros tratigraphy anyway?
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Adjust grout to mimic ground st iffness
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Grout in the sinter stone
Sinter stones o ften 50micron Cement is submic ron
Seems to lock in theformation pressu re attime of installation
May not appear asproblem in lab (mayoccur under higher groutcolumn pressures)
Does not always happen
Easily so lved with a sandsock
Enhancements?
Preplug of bentonitecement
Unsaturated readings intimate contact
Mandatorypermeability orstrength test
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Gords recommendations
If you think the ground might belower permeability than yourgrout
When in doubt, chick en out Go traditional with bentonit e
seals and sand packs Use cement bentonit e grout
elsewhere in ho le
Use a sand sock because itworks. Why not?
Take extra care to make sure yourgrout mix is r ight. Trust no one.
Lets look at seeing how we mightuse the method for measuring insitu suctions
Be a geotechnical ins trumentationmaven, maybe even a purist.Publish in Geotechnical News.
Good practice in completingyour installation
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Philosophy / Advocacy
Philosophy Dont put something in the ground you do nt understand Dont put something in the ground that may give you the wrong
reading (unless yo u 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 som ething Promoting / Over promoting? Responsibility?
Community
EXTRAat no extra charge
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Some references
Chapuis, R.P. and Sabourin, L., 1989. Effects of installation of piezometers and wells on groundwater characteristics andmeasurements . Canadian Geotechnical Jour nal, 26, pp 604-613.
Cioffi, R., Marroc coli, M, and Mascol o, 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 o n a new borehole seal for piezometers , Ground Engin eering, 9, pp 16-18.
Honda, A., Mashima, M., and Kunito, T., 1988. Influencing factors for impermeability of soil cement , Memoirs of the Faculty ofEngineering, Osaka City University, 29, pp 205-212.
Hvorslev, M.J., 1951. Time lag and soil permeability in ground-water observations , Bulletin Number 36, Waterways ExperimentStation, Corps of Engin eers, U.S. Army, Vicksburg, Missi ssippi, USA.
Kurt, C.E., and Johnso n, 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 , CanadianGeotechnical Journ al, 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 alt ernati ve well seal in high ly mi nerali zed gro und w ater. 3rd Nation al Sympo sium on Aqui fer Rest orati on and Groun d-water Monit ori ng , Columbus, Ohio.
Vaughan, P.R., 1969. A not e on seali ng pi ezometers in b oreho les , Geotechnique, 19, No. 3, pp 405-413.
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