mckenna grouted piezo 20 year update mar 22 2010f

8/12/2019 McKenna Grouted Piezo 20 Year Update Mar 22 2010f

1/17

rd McKenna, BGC Engineering

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


2/17


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


3/17


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


4/17


Poor performance(dont forget the bentonite!)

More good performance


5/17


But will it work in the lab?

New lab experiments


6/17


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


7/17


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.


8/17


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!


9/17


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


10/17


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?


11/17


12/17


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?


13/17


Adjust grout to mimic ground st iffness


14/17


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


15/17


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


16/17


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


17/17

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.

mckenna grouted piezo 20 year update mar 22 2010f

Documents