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Geotechnical Instrumentation News
John Dunnicliff
IntroductionThis is the fifty-fifth episode of GIN.Two subjects
this time, both based onpapers that were presented at the Inter-nat
ional Symposium on FieldMeasurements in Geomechanics(FMGM) in
Boston in September lastyear. I selected these for re-publicationin
GIN because I want to maximize theaudience for two very important
contri-butions. There is also a review of a bookabout fibre optic
sensing.
Those of you who have already readthe FMGM paper on
fully-groutedpiezometers by Contreras et alpleasereplace it with
this version, in whichseveral issues have been corrected.
The Use of the Fully-groutedMethod for PiezometerInstallationThe
first article, in two parts, is by IvnContreras, Aaron Grosser, and
RichardVer Strate of Barr Engineering Com-pany in Minneapolis. Ive
been waitingfor this for 39 years, ever since I readPeter Vaughans
1969 technical note inGotechnique, A Note on SealingPiezometers in
Boreholes. That maysound flippant, but its true!
In 1949 the Journal of the Boston So-ciety of Civil Engineers
published Ar-thur Casagrandes paper, SoilMechanics in the Design
and Construc-tion of Logan Airport. He describedthe installation of
open standpipepiezometers (Casagrande
piezometers) in boreholes bysurrounding them with a sand pack
andplacing bentonite pellets over the sand.The drill casing was
left in place and thebentonite seal was within the casing, sono
grout was placed over the bentonite.A few years later it became
normalpractice to withdraw the drill casing andplace grout over
bentonite pellets orchips, and were still doing this forpiezometers
installed in boreholes.
For open standpipe piezometers thesand pack is necessary because
a sizableintake volume is required for obtaininga pore water
pressure reading withoutsignificant time lag. So this
normalpractice is still appropriate today, ex-cept that in my view
the grout should beplaced directly over the sand, omittingthe
bentonite pellets or chipsI explainthis in my discussion of the
article byContreras et al.
Since the development of dia-phragm piezometers, usually
pneu-
matic or vibrating wire, most of us havefollowed this same
normal practice,with a sand pack, bentonite pellets orchips, and
overlying grout. Forget thesand and bentonite seal! This is
nolonger the way to go! Use theful ly-grouted method!
Thefully-grouted method entails installinga piezometer tip in a
borehole which isbackfilled entirely with cement-benton-ite grout.
Its taken several years of dis-cussion and argument for me to
arrive atthis conclusion because I feared that thegrout surrounding
the tip might preventthe piezometer from responding cor-rectly to
changes in pore water pressure.If you have the same fears, read the
arti-cle, the discussion and the authors re-ply to the discussion,
and become abeliever!
If any reader has other data, pro orcon, about the fully-grouted
method, Idvery much welcome hearing about it,and will consider it
for publication in alater episode of GIN.
Geotechnical Alarm SystemsThe second article is by KevinOConnor,
and focuses on alarm sys-tems based on the technology of timedomain
reflectometry (TDR).
My reason for regarding this as avery important contribution is
be-cause the message is an umbrella one,applicable to all alarm
systems, notonly those based on monitoring withTDR.
I have very clear memories of Kevin
Geotechnical News, June 2008 29
GEOTECHNICAL INSTRUMENTATION NEWS
Forget the sand andbentonite seal! This isno longer the way
to
go! Use thefully-grouted method!
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The Use of the Fully-grouted Method forPiezometer
InstallationPart 1
Ivn A. ContrerasAaron T. GrosserRichard H. Ver Strate
IntroductionThe fully-grouted method described inthis ar t ic le
entai ls instal l ing apiezometer tip in a borehole which
isbackfilled entirely with cement-benton-ite grout. Part 1 of this
article presents adetailed discussion of the fully-groutedmethod,
including the installation pro-cedure and theoretical background,
aswell as a seepage-model analysis usedto evaluate the impact of
the differencein permeabilities between surroundingground and
cement-bentonite grout.Part 2 describes laboratory test resultsfor
six cement-bentonite grout mixesand field examples of applications
ofthe fully-grouted method. Both parts ofthis article are based on
the paper, TheUse of the Fully-grouted Method forPiezometer
Installation, presented atFMGM 2007: Seventh International
Symposium on Field Measurements inGeomechanics, and are
published inGIN with permission from ASCE.
A crucial parameter for the successof the fully-grouted method
is the per-meability of the cement-bentonitegrout. Vaughan (1969)
postulated thatthe cement-bentonite grout should havea permeability
no greater than two or-ders of magnitude higher than the
sur-rounding soil in order to obtainrepresentative pore-water
pressurereadings. Unfortunately, there is limitedpublished data on
the permeability ofcement-bentonite grout mixes.
Figure 1a shows the typicalpiezometer installation commonlyknown
as a Casagrande or standpipepiezometer. With this installation,
thetip of the piezometer (e.g., slotted PVCpipe or porous stone
filter) is sur-
rounded with a high permeability mate-rial, commonly referred to
as sand pack.Above the sand pack is a bentonite sealtypically
consisting of bentonite chipsor pellets. The installation is
finishedwith cement-bentonite grout to theground surface. This
installation relieson a sizable intake volume and a
narrowriser-pipe diameter to obtain a pore-wa-ter pressure reading
in the riser pipewithout significant time lag (Hvorslev,1951).
With the development of diaphragmpiezometers (e.g., pneumatic
and vi-brating wire), the method developed forstandpipe piezometers
was used for dia-phragm piezometer installations(Dunnicliff, 1993).
This has been acommon practice for decades and theresulting
installation is shown on Fig-ure 1b. However, because of the
GEOTECHNICAL INSTRUMENTATION NEWS
30 Geotechnical News, June 2008
Instrument sent to me from Australia by Craig Johnson:I was in
an antique storeon the weekend, saw this artifact (circa late 20th
century) and thought of you.Enjoy! Thank you Craig.
laying down some ground rules both atthe 2007 instrumentation
course inFlorida and during his presentation ofthis subject at the
FMGM symposium inBoston. He talked about responding toalarms and
said, with enormous empha-
sis, If there is an alarm, you have torespond. Failure to
respond is not anoption.
ClosurePlease send contributions to this col-
umn, or an article for GIN, to me as ane-mail attachment in
MSWord, [email protected], or byfax or mail: Little
Leat, Whisselwell,Bovey Tracey, Devon TQ13 9LA, Eng-land. Tel. and
fax +44-1626-832919.
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low-volume operation of diaphragmpiezometers, the sand pack
around theinstrument tip is unnecessary, and thediaphragm
piezometer can be installedin the borehole surrounded by
ce-ment-bentonite grout. This procedure iscommonly known as the
fully-groutedmethod (Mikkelsen and Green, 2003)and is shown on
Figure 1c.
Fully-grouted MethodFigure 1c shows a piezometer installa-tion
using the fully-grouted method, inwhich a diaphragm piezometer tip
is setin a drilled borehole and entirely back-filled with
cement-bentonite grout. Thefollowing is a detailed description of
theinstallation procedure for a vibrat-ing-wire sensor t ip in
typicalgeotechnical boreholes (i.e., 140 mm),including preparation
of piezometer as-sembly and materials, grout mixing,piezometer
construction, and theoreti-cal background.
Piezometer AssemblyConstruction of the piezometer assem-bly
commonly begins with attachmentof the sensor tip to a sacrificial
groutpipe. The sacrificial grout pipe, whichcan be either
belled-end electrical con-duit or threaded PVC well casing,
isgenerally constructed or laid out on theground in manageable
lengths for han-dling. The piezometer location is se-lected by
reviewing the soil stratigra-phy. The sacrificial grout pipe
willgenerally extend to the bottom of theborehole for support;
therefore, it is
possible to determine the location of thepiezometer tip from the
top or bottom ofthe borehole since the pipe is left inplace.
After drilling a borehole, thepiezometer tip is attached to the
groutpipe at the appropriate location. Forboreholes with a diameter
of 140 mm, atypical grout pipe (such as 25.4-mm di-ameter PVC well
casing) is used.Large-diameter or stronger grout pipemay be
required for deeper installationswith higher pumping pressures.
The sensor tip, which has been satu-rated following the
manufacturers di-rections, is typically set with the sensorin the
upward position to minimize thepossibility of desaturation. The
cableconnected to the sensor tip is attached tothe pipe at
approximate intervals alongthe grout pipe, leaving some slack in
theline. The grout pipe, sensor tip, and ca-ble are then lowered
into the boreholewith the grout pipe placed on the bottomfor
support. The piezometer tip is nowlocated within the desired
monitoringzone. The cable is brought to the surfacewhere readings
are taken with a readoutdevice.
One advantage of the fully-groutedmethod is that it can be used
for installa-tion of nested piezometers. In a nestedpiezometer
configuration, more thanone piezometer tip is attached to
thesacrificial grout pipe. The authors havesuccessfully installed
up to fourpiezometer tips in a borehole. Duringinstallation the
drill casing should be re-
moved carefully to prevent damage tothe cables and the cables
should be sep-arated around the grout pipe to prevent adirect
seepage path along a bundle ofcables.
Another advantage of thefully-grouted method is the
feasibilityof using a single borehole to installmore than one type
of instrument. Forexample, the piezometer tips can be at-tached to
an inclinometer casing, and asingle borehole is used for
measuringboth deformation and pore-water pres-sures, resulting in
reduced drillingcosts. However, the inclinometer casingjoints must
be sealed. This techniquehas been used successfully by the au-thors
on several projects.MaterialsThe cement-bentonite mixes describedin
this article use Type I Portland ce-ment and sodium bentonite
powdersuch as Baroid Aquagel Gold Seal orQuickgel. The water used
in the mixesshould be potable water to prevent pos-sible
interaction of chemical constitu-ents in the water with the
cement-ben-tonite mixture.
Grout MixingThe mixing procedure described in thisarticle
assumes the availability of a ca-pable drill-rig pump and a
high-pres-sure, jet-type nozzle attachment on theend of a mixing
hose. In most cases, thedrill-rig pump provides enough pres-sure
for the jet-mixing required to ob-tain a desirable mixture. Other
methods
Geotechnical News, June 2008 31
GEOTECHNICAL INSTRUMENTATION NEWS
Figure 1(a). Traditional standpipe piezometer with sand
pack.Figure 1(b). Diaphragm piezometer with sand pack.Figure 1(c).
Fully-grouted piezometer. Figure 2. Schematic computer model to
simulate seepage
around a fully-grouted piezometer (borehole not to scale).
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may use actual grout-mixing plants.Generally, the
cement-bentonite mix isprepared in a barrel or mud tank usingthe
drill-rig pump to circulate the batchwith a suction hose and return
line.Occasionally, a hydraulically-operated,propeller-type mixer is
used. However,it has been the authors experience that,in some cases
(depending on the mixviscosity, pump operability on the drillrig,
or grout volume), the use of a groutmixer/pump may be required.
Typicalbatch sizes are 200 to more than 2,000liters.
The mixing process begins withcalculation of the amount of grout
re-quired to fill the borehole. A measuredquantity of potable water
is pumpedinto the mixing barrel first and circula-tion begins.
During circulation, the wa-ter and cement are mixed first so that
thewater:cement ratio remains fixed andthe properties of the grout
mix are morepredictable. The measured quantity ofcement is
gradually added to the wateruntil both components have been
thor-oughly mixed. This is the most impor-tant step in the mix
preparation and runscontrary to the common practice ofmixing
bentonite and water first. An ini-tial measured quantity of
powderedbentonite, based on a mix design, isadded into the barrel
near the jet streamto minimize the formation of clumpswithin the
mix. Typically, additionalbentonite is added as mixing continuesto
achieve a creamy consistency.Mikkelsen (2002) describes the
consis-tency as drops of grout should barelycome off a dipped
finger and shouldform craters in the fluid surface.
Piezometer ConstructionAt the completion of the
grout-mixingprocess, and after measuring the finaldensity of the
mix, the piezometer tipassembly is lowered into the borehole.In
shallow boreholes (e.g., typicallyless than 30 m deep), grout is
thenpumped into the borehole through thesacrificial grout pipe
until it reaches theground surface. In deeper boreholes,staged
grouting using multiple groutpipes or multiple port pipes may be
re-quired so the piezometers are notover-pressurized during
installation. Incased boreholes, the drill casing is
slowly retrieved so that no gap is left be-tween the top of the
grout and the bot-tom of the casing. Typically, the entireprocess
takes approximately one hourfor a 30-m borehole. The hole is
typi-cally completed with concrete and aprotective top.
The field engineer should take pres-sure readings during and
immediatelyafter installation. One benefit of vibrat-ing-wire
technology is that readings canbe taken quickly. The readings
obtainedduring grouting can be used to deter-mine if the device has
beenover-pressurized during grouting. Themeasured pressures should
approxi-mately correspond to the pressure ex-erted by the column of
grout above thetip, provided the sensor and grout are atnearly the
same temperature, astemperature equalization may take sev-eral
minutes. However, with time, thispressure decreases as the
cement-ben-tonite mix sets up and pore-water pres-sure readings are
taken at the tiplocations. Typically, grout set-up takesone to two
days.
Theoretical BackgroundMcKenna (1995) clearly describes thetwo
basic requirements for anypiezometer to perform its function.
Themeasured pore-water pressure must befairly representative of the
actualpore-water pressure at the measurementlocation (i.e., small
accuracy error), andthe hydrodynamic time lag must beshort. At
first glance, it does not appearthat the fully-grouted method will
sat-isfy these requirements. It would seemthat the cement-bentonite
grout sur-rounding the tip might prevent thepiezometer from
responding quickly tochanges in pore-water pressures in theground
due to its low permeability. Onthe other hand, if the
cement-bentonitegrout is too permeable to enhance shorthydrodynamic
time lags, there wouldbe significant vertical fluid flow withinthe
cement-bentonite grout column.
However, the fully-grouted methoddoes satisfy both of
McKennasrequirements . A diaphragmpiezometer, such as a vibrating
wirepiezometer, generally requires only avery small volume
equalization to re-spond to water pressure changes (10-2
to 10-3 cm3), and the cement-bentonitegrout is able to transmit
this small vol-ume over the short distance that sepa-rates the
piezometer tip and the groundin a typical borehole. A practical way
toreduce this distance is to set up the tipclose to the wall of the
borehole by re-ducing the thickness of grout betweenthe tip and
ground using pre-manufac-tured, expandable piezometer
assem-blies.
Grout PermeabilityRequirementsVaughan (1969) introduced
thefully-grouted method and developedclosed-form solutions which
showedthat the error in the measured pressure issignificant only
when the permeabilityof the borehole backfill is two orders
ofmagnitude greater than the permeabil-ity of the surrounding
ground. If thepermeability of the cement-bentonitegrout is lower
than the permeability ofthe surrounding ground, measuredpressures
will be without error. As a re-sult, for the fully-grouted method
towork, the grout mix used to backfill theborehole must meet
certain permeabil-ity requirements. A seepage model wasdeveloped by
the authors to better un-derstand those requirements.
Computer ModelingA finite-element computer modelsimulating
seepage conditions around afully-grouted piezometer installationwas
used to evaluate the impact of groutpermeability on the accuracy of
thepiezometer reading. The seepage modelwas conducted using SEEP/W,
a com-puter-modeling program developed byGEO-Slope
International.
Figure 2 shows the conceptualmodel developed to simulate the
seep-age around a piezometer installed usingthe ful ly-grouted
method. Theaxisymmetric flow model includes a7-cm radius,
cement-bentonite-groutcolumn surrounded by soil of
constantpermeability. The simulated ce-ment-bentonite grout column
extends27.5 m and the soil layer extends 33 mbelow the ground
surface with a radiusof 50 m. Underlying the soil, a sandlayer was
incorporated to simulate thelower boundary conditions.
GEOTECHNICAL INSTRUMENTATION NEWS
32 Geotechnical News, June 2008
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The seepage analyses were per-formed simulating upward and
down-ward flow using two sets of imposedtotal head conditions
(i.e., 10 and 20 m)that induce flow under steady-state con-ditions.
This set of boundary conditionscorresponds to the
one-dimensionalflow condition in the vertical direction.In all
cases, fully saturated conditionswere used for all the materials in
themodel. The error, , defined as the dif-ference in computed
pore-water pres-sure between the soil and the grout, wasdetermined
during each model run at
points in the soil and grout 20 m belowthe ground surface, as
shown on Fig-ure 2.
Results of Computer ModelingSeveral model runs were made in
whichthe permeability ratio, kgrout/ksoil, wasvaried from 1 to 107.
Figure 3 shows theresults of the seepage simulations interms of the
normalized error, i.e., di-vided by the pore-water pressure in
soil,usoil, against the permeability ratio. Fig-ure 3 also shows
that the normalized er-ror is zero for all practical purposes
up
to permeability ratios of 1,000 fordownward and upward flow and
the twosets of imposed total heads. As the per-meability ratio
increases beyond 1,000,the normalized error increases up toabout 10
percent at permeability ratiosof 10,000. As the permeability
ratiocontinues to increase to 107, the nor-malized error also
increases up to about23 and 40 percent, respectively, for the10-m
and 20-m imposed total heads.
In summary, the finite-element com-puter model revealed that the
perme-ability of the grout mix can be up tothree orders of
magnitude greater thanthe permeability of the surroundingground
without introducing significanterror. This finding differs from
previousassessments, which indicated that thepermeability of the
grout mix shouldonly be one or two orders of magnitudegreater than
the permeability of the sur-rounding ground. The minimum per-meabil
i ty that is l ikely to beencountered in natural soils is on the
or-der of 10-9 cm/s. As a result, the ce-ment-bentonite grout mix
used in thefully-grouted method needs to have apermeability of, at
most, 10-6 cm/s.
Part 2 of this article will discuss lab-oratory test results of
six cement-ben-tonite grout mixes and field examples ofapplications
of the fully-groutedmethod.
Geotechnical News, June 2008 33
GEOTECHNICAL INSTRUMENTATION NEWS
Figure 3. Normalized error versus permeability ratio.
The Use of the Fully-grouted Method forPiezometer
InstallationPart 2
Laboratory Testing ProgramA laboratory testing program was
de-veloped to evaluate the range in perme-ability and strength of
cement-benton-i te grout for piezometerinstallationsusing the
fully-groutedmethod. The test program was designed
so that small batches of grout could bemixed in a controlled
environmentwithout large grout-batch mixingequipment. Six mix
designs were cho-sen to represent a wide range of valuesthat would
reasonably be used on pro-jects.
Sample PreparationMixing the grout used for laboratorytesting
began with calculating the de-sired quantities of material
andthenweighing individual portions of ce-ment, water, and
bentonite. Additionalbentonite was prepared in anticipation
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of adjusting the mix viscosity. Theproperties of the individual
mix compo-nents used in the laboratory testing arelisted in Table
1.
To begin, the cement was added to
the water slowly while mixing. Thebenefit of adding the cement
first in themixing process is that it ensures the cor-rect
water:cement ratio before addingthe bentonite.
After the cement and water weremixed and the water-cement paste
ap-peared uniform, which generally tookfive minutes, bentonite was
slowlyadded to the bucket. The cement-ben-tonite grout was then
mixed for approx-imately five additional minutes until itappeared
uniform and did not containlumps. Viscosity was measured at
vari-ous times during mixing to evaluate thecondition of the mix.
Samples of the fi-nal mix were taken using plastic moldsand the
density was measured.
After a short cure period, the sam-ples were carefully extruded
out of theplastic molds and stored until the testdate. For the
Unconfined CompressiveStrength testing (UCS), a set of twospecimens
were tested at 7, 14, and 28days. Permeability testing was
com-pleted on specimens from each mix at 7and 28 days under three
different con-fining stresses. In addition to strengthtests, basic
index properties, such asmoisture content and dry density of
thesamples, were also measured.
Laboratory Test ResultsTable 2 summarizes the final
ce-ment-bentonite grout proportions usedin this study. The results
of the labora-tory testing are presented in a series offigures.
Figure 4 summarizes test results asthe average UCS at 28 days
versus thewater:cement ratio by weight. It showsthat the UCS
decreases with increasingwater:cement ratios. In fact, the UCS at28
days is approximately 1700 kPa at awater:cement ratio of 2:1; it
then de-creases to approximately 90 kPa withincreasing water:cement
ratio. Also in-cluded on Figure 4 are data presented byMikkelsen
(2002), which show a rela-tively strong correlation with the data
ofthis study.
The void ratios of the samples werecomputed based on the
measured watercontent of the specimens and the spe-cific gravity of
the grout-mix constitu-ents. The computed void ratios of themixes
are relatively high, in fact, theseare higher than soils with
similarstrength and permeability. However, thedata show that the
amount of cementcontrols the strength characteristics ofthe grout
mix. Bentonite appears to in-
GEOTECHNICAL INSTRUMENTATION NEWS
34 Geotechnical News, June 2008
Table 1. Properties of grout constituents
Mix Component Brand Specific Gravity MoistureContent (%)
Portland Cement Type I LaFarge 3.15 Bentonite Quickgel
(Mixes 1-4)Baroid 2.41 to 2.45 11
Aquagel Gold Seal Ben-tonite (Mixes 5 and 6)
Baroid 2.4 10
Table 2. Summary of cement-bentonite grout mixes used in the
study
Mix Water : Cement :Bentoniteby Weight
Marsh FunnelViscosity (sec)
Bentonite Type
1 2.5 : 1: 0.35 50 Quickgel2 6.55 : 1: 0.40 54 Quickgel3 3.99 :
1: 0.67 60 Quickgel4 2.0 : 1: 0.36 360 Quickgel5 2.49 : 1: 0.41 56
Aquagel Gold Seal6 6.64 : 1: 1.19 60 Aquagel Gold Seal
Figure 4. Variation of unconfined compressive strength versus
water:cementratio.
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fluence the amount of bleed water andvolume change of the
specimen duringcuring. Additional information on thestrength and
deformation properties ofcement-bentonite mixes can be found
inContreras, et al. (2007).
Figure 5 summarizes the test resultsin terms of the permeability
of the spec-imens at seven days for various confin-ing pressures.
The data show thatsamples with a higher water:cement ra-tio or void
ratio have higher permeabil-ity than those with lower
water:cementratios.
Figure 6 shows the permeability in
the same format for specimens at 28days. Data are very similar,
showingthat the permeability is relatively con-stant or decreases
slightly with confin-ing pressure. One important result isthat,
from seven to 28 days, the perme-ability continues to decrease. For
exam-ple, mixes with 2.49 water:cement ratioindicate a permeability
greater than1.0x10-6 cm/sec at 7 days and less than1.0x10-6 cm/sec
at 28 days. The data in-dicate that, as hydration of the
cementoccurs, the permeability of the mix de-creases. The high void
ratio and lowpermeability are two reasons the
fully-grouted method works; it allowstransmission of a low
volume of waterover a short distance yet maintains over-all low
permeability in the vertical di-rection.
Figure 7 shows the variation in per-meability data with respect
to void ratio.The data indicate that specimens withlower void
ratios typically exhibit lowerpermeability, while those with
highervoid ratios exhibit higher permeability.With grout mixes, the
cement has agreater influence on the void ratio thanthe bentonite
and is considered the con-trolling factor in the permeability of
thegrout. The difference between the sevenand 28-day permeability
is relativelysmall, as shown on Figure 7.
Field ExamplesThis section describes three field exam-ples in
which the fully-grouted methodwas successfully applied. The first
ex-ample compares pressure readings be-tween one installation using
thefully-grouted method in a nested con-figuration and the
traditional approachwith individual piezometer installationsin
separate boreholes. The second ex-ample descr ibes use of
thefully-grouted method with the installa-tion of nested
piezometers in an up-ward-flow condition. The third exam-ple is for
a nested, fully-grouted methodinstallation in a downward-flow
condi-tion.
Example 1. Comparison BetweenNested and Individual
InstallationsThis field example compares two meth-ods of
installation: Three vibrating-wire piezometers in
a single borehole using thefully-grouted method.
Four individual pneumaticpiezometers in separate boreholesusing
the traditional sand packaround the piezometer tips.The two
installations were within 7.5
m of each other. As a result, some differ-ences in the pressure
readings were ex-pected. Figure 8 shows a comparison ofthe
pore-water pressure profile with ele-vation for both installations.
The figureillustrates a fairly similar response con-sidering the
distance between the twosets. Similar data have been presented
Geotechnical News, June 2008 35
GEOTECHNICAL INSTRUMENTATION NEWS
Figure 5. Variation of permeability versus confining pressure at
7 days.
Figure 6. Variation of permeability versus confining pressure at
28 days.
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by McKenna (1995) , fur therconfirming the val idi ty of
thefully-grouted method.
Example 2. Upward-FlowConditionsThis field example illustrates
the use ofnested piezometers using thefully-grouted method in
upward-flowconditions. The site is in an area wherethree distinct
stratigraphy units arefound (alluvial deposits, Huot Clay
For-mation, and Red Lake Falls Formation).The upward-flow
conditions play a ma-jor role in the slope instability of thearea
(Contreras and Solseng, 2006).
Figure 9 shows the pore-water pres-sure and total-head profiles
at the site,illustrating the upward-flow conditions.Two
vibrating-wire piezometer tipswere installed in the Huot
Formationand one in the Red Lake Falls Forma-tion. The Huot
Formation is fairly uni-form and has a permeability in the rangeof
1.2x10-8 to 1.9x10-8 cm/s. The ce-ment-bentonite grout mix used in
thenested installation had a 2.66:1:0.27water:cement:bentonite
ratio with apermeability of approximately 2.0x10-6cm/s. This
example presents the resultsof the fully-grouted method in
alow-permeability unit.
Example 3. Downward-FlowConditionsFinally, this field example
demonstratesthe use of nested piezometers with theful ly-grouted
method in down-ward-flow conditions. A total of fourpiezometer tips
were installed in threeunits, with permeability ranging
from1.0x10-3 cm/s to 9.49x10-7 cm/s. Wherethere is a wide range of
permeability,the least permeable unit controls the
ce-ment-bentonite grout permeability. Asa general rule, the less
permeable the ce-ment-bentonite grout, the better, and asshown by
the computer model, for mostsoil, a cement-bentonite grout with
apermeability of 1.0x10-6 cm/s will beadequate. Figure 10 shows the
pore-wa-ter pressure and total-head profiles atthe site,
illustrating the downward-flowconditions. This example presents
theresults of an installation of nestedpiezometers with up to four
piezometertips in a single borehole.
Summary and ConclusionsThis two-part article presents a
detaileddiscussion of the fully-grouted methodfor piezometer
installation, including theprocedure and theoretical background.
Italso discusses the results of a laboratorytesting program on six
cement-bentonitegrout mixes, along with an evaluation ofa computer
model to determine the im-pact of the difference in
permeabilities
between the cement-bentonite groutbackfill and the surrounding
ground.The following summarizes the articlesmain issues and
findings: The practice of installing diaphragm
piezometers in a sand pack with anoverlying seal of bentonite
chips orpellets could be discontinued.
The fully-grouted method is a fairlysimple, economical, and
accurateprocedure that can be used to mea-sure pore-water pressures
in soilsand fractured rock. It allows easy in-stallation of a
nested piezometerconfiguration, resulting in drillingcost savings.
It can also be used incombination with other instrumenta-tion
(e.g., inclinometers) to measuredeformation and pore-water
pres-sures, provided the inclinometerjoints remain sealed.
The permeability of the cement-ben-tonite grout mix can be up to
threeorders of magnitude greater than thepermeability of the
surroundingground without a significant error inthe pore-water
pressure measure-ment. This finding differs from pre-vious
assessments.
Laboratory test results show that thepermeability of the
cement-benton-ite grout mixes is a function of thewater:cement
ratio. As the water:ce-ment ratio (void ratio) decreases,
thepermeability decreases.
Bentonite has little influence on thepermeability of the mix,
but ratherappears to stabilize the mix, keepingthe cement in
suspension and reduc-ing the amount of bleed water.
AcknowledgmentsThe support provided by the InnovationCommittee
of Barr Engineering Com-pany is gratefully acknowledged. Thecareful
performance of the laboratorytesting by Soil Engineering Testing
ofBloomington, Minnesota, is greatly ap-preciated. The continual
assistancefrom Erik Mikkelsen and his thoughtfulinsight and
contributions from the be-ginning of the research program havebeen
extremely helpful in pursuing theresearch and use of the
fully-groutedmethod. John Dunnicliffs thorough re-view and comments
on this manuscriptare also greatly appreciated.
GEOTECHNICAL INSTRUMENTATION NEWS
36 Geotechnical News, June 2008
Figure 7. Void ratio versus permeability.
-
ReferencesContreras, I.A., Grosser, A.T., and
VerStrate, R.H. 2007. BasicStrength and Deformation Proper-ties
of Cement-Bentonite GroutMixes for Instrumentation Installa-tion.
Proceedings of the 55th An-nual Geotechnical EngineeringConference
University of Minne-sota. pp. 121-126.
Contreras, I.A., Grosser, A.T., andVerStrate, R.H. 2007. The Use
ofthe Fully-grouted Method forPiezometer Installation. Proceed-ings
of the Seventh InternationalSymposium on Field Measurementsin
Geomechanics. FMGM, 2007.Boston, MA. ASCE GeotechnicalSpecial
Publication 175.
Contreras, I.A. and Solseng, P.B. 2006.Slope Instabilities in
Lake AgassizClays. Proceedings of the 54th An-nual Geotechnical
Engineering Con-ference. University of Minnesota.pp. 79-93.
Dunnicliff, J. 1993. Geotechnical In-strumentation for Measuring
Field
Performance.J. Wiley, NewYork, 577 pp.
Hvorslev, M.J.1951. TimeLag and SoilPermeabilityin Groundwa-ter
Observa-tions. BulletinNo. 36,U.S. Water-ways Experi-ment Station,
Vicksburg, MI.
McKenna, Gordon T. 1995.Grouted-in Instal la t ion ofPiezometers
in Boreholes. Cana-dian Geotechnical, Journal 32, pp.355-363.
Mikkelsen, P.E. and Green, E.G. 2003.Piezometers in Fully
Grouted Bore-holes. International Symposium onGeomechanics. Oslo,
Norway. Sep-tember 2003.
Mikkelsen, P. Erik. 2002. Ce-ment-Bentonite Grout Backfill
for
Borehole Instruments .Geotechnical News. December2002.
Vaughan, P.R. 1969. A Note on SealingPiezometers in Boreholes
.Geotechnique, Vol. 19, No. 3,pp. 405-413.
Ivn A. Contreras, Aaron T. Grosser,Richard H. Ver Strate, Barr
Engineer-ing Co., 4700 W. 77th Street, Minneapo-lis , MN 55435,
952-832-2600,[email protected], [email protected],
[email protected]
Geotechnical News, June 2008 37
GEOTECHNICAL INSTRUMENTATION NEWS
Figure 8. Comparison between a nested fully-groutedinstallation
and individual separate installations.
Figure 9. Field example of fully-grouted method in upwardflow
condition.
Figure 10. Field example of fully-grouted method indownward flow
condition.
-
Discussion of The Use of theFully-grouted Method for
PiezometerInstallation
Ivn A. ContrerasAaron T. GrosserRichard H. Ver Strate
Geotechnical News, Vol. 26 No. 2June 2008
John Dunnicliff
Thank you to all three authors for theirexcellent practical
contribution. Ivebeen waiting for this for 39 yearsseethe last
reference citation in Part 2 ofyour article, Vaughan (1969)!
Other Experiences with theFully-Grouted MethodIn my view, the
rationale for acceptingthe fully-grouted method is very
con-vincing. Despite that view, owners andtheir consultants may
tend to be wary ofwhat they consider to be a new and rad-ical
method. As youll see below,theres plenty of positive experienceout
there, and if were to convinceowners and their consultants, we
needas much supportive information as pos-sible. Among other
experiences are: The engineers at Applied
GeoKinetics, located in Irvine, Cali-fornia
(www.appliedgeokinetics.com) have used the fully-groutedmethod
successfully on approxi-mately 400 installations since 1990.Several
of these installations havebeen to depths of approximately 500feet,
with up to ten piezometers tipsinstalled within a single
borehole.For more information, please con-tact Glenn Tofani at
[email protected].
Colleagues in Austral ia ,Geotechnical Systems Australia PtyLtd.
, (www.geotechsystems.com.au) have used the fully-groutedmethod
with very good results onabout 15 sites since 2001. Installa-tions
have been up to 500m deep. For
more information, please contactMatt Crawford, matt@geotech
sys-tems.com.au.
Geometron of Seattle (Bellevue) andKleinfelder of Denver
installedabout 40 fully-grouted vibratingwire piezometers on a
multiple-damproject in Southern Oregon over thelast three years,
with real-time-mon-itoring. It includes several in unsatu-rated
embankments, some reactingto rainfall recharge. For more
infor-mation please contact ErikMikkelsen,
[email protected].
Many firms in Washington State reg-ular ly specify and use
themethod, including Camp DresserMcKee (CDM), CH2M Hill, andJacobs
Associates. It is in current useon major transportation and
tunnelprojects.
The US Army Corps of Engineers inOmaha started using
fully-groutedpiezometers on Oahe Dam on theMissouri River, SD in
2000, particu-larly for piezometers installed in Pi-erre Shale. A
pilot relief wellprogram showed that the vibratingwire piezometers
responded betterthan conventional open standpipes.
Syncrude Canada Ltd., in FortMcMurray, Alberta have
usedfully-grouted piezometers success-fully in 83 vibrating wire
piezometertip installations since 2003 in stiff insitu soils using
Syncrude installationprocedures . Syncrude hasalso grouted in about
65 vibrating
wire tips when installing in com-pressible fills, but in those
cases useda bentonite seal within 3m above thetip to protect
against the potential ofthe grout cracking due to the
settle-ment.
Strata Control Technology, a miningconsulting firm in Australia
that alsospecializes in geotechnical instru-mentation, have used
the method andconclude, Fully-grouted vibratingwire piezometers are
proving an ex-cellent tool for investigating the im-pact of coal
mining on groundwatersystems. For more information,please contact
Ken Mills ,[email protected].
DeJong et al (2004) describe com-parative tests between a single
vi-brating wire piezometer installed ina soft varved clay deposit
with a sandpack, and a ful ly-groutedpiezometer. They conclude,
Theperformance of the fully-groutedpiezometer was shown to be
nearlyidentical to that of the sand packedinstrument. This paper
also de-scribes the use of pre-manufactured,expandable piezometer
assemblies,to which the authors refer in Part 1 oftheir article, in
the context of reduc-ing the distance between thepiezometer filter
and the wall of theborehole.If any reader has other data, pro
or
con, about the fully-grouted method, Idvery much welcome hearing
about it,and will consider it for publication in alater episode of
GIN.
GEOTECHNICAL INSTRUMENTATION NEWS
38 Geotechnical News, June 2008
-
Use of Bentonite Pellets andChips over Sand Packs forDiaphragm
PiezometersThe authors conclude, The practice ofinstalling
diaphragm piezometers in asand pack with an overlying seal of
ben-tonite chips or pellets can be discontin-ued. As I wrote at the
beginning of thisdiscussion, owners and their consul-tants may tend
to be wary of thefully-grouted method. In these cases Isuggest that
we ask them how confidentthey are that bentonite pellets or
chipsarrive at the depths shown so neatly inyour Figure 1(b). Ive
had numerous ex-periences of these infuriating thingsarching part
way down the borehole,and have little confidence that Figure1(b)
represents reality. As an aside here,Ive tried to retard the onset
of swellingso that they dont become sticky for afew minutes,
including freezing, coat-ing with hydraulic oil, and sprayingwith
hair sprayforget it!
If owners and their consultants fullyappreciate these
uncertainties, perhapsthey may be more willing to accept
thefully-grouted method.
Use of Bentonite Pellets andChips over Sand Packs forOpen
Standpipe PiezometersFigure 1(a) shows a bentonite sealabove the
sand pack for an openstandpipe piezometer. For the reasonsgiven
above, I believe that a more reli-able installation method is to
omit thebentonite seal and to place the grout di-rectly over the
sand pack. To preventcontaminating the sand pack with grout,a
bottom plug should be used on thegrout pipe, side-discharge holes
drillednear the bottom of the pipe, and thegrout pumped very
slowly.
Use of the Method in SoftGround below FutureEmbankmentsWhen
there is a need to monitor porewater pressure in
highly-compressibleground as an overlying embankment isconstructed,
it is usual to do so withpiezometers at various elevations. Thishas
been done where vertical compres-sion in the soft ground has been
up to35% (e.g. Handfelt et al, 1987). Be-cause it allows several
piezometers tobe installed in a single borehole, the
fully-grouted method is very attractivein this application, but
I dont know ifthis has been done. Some of the installa-tion
procedure that is described by theauthors would have to be changed
forthis application.
First, a sacrificial grout pipe couldntbe used, because it would
impede con-formance as vertical compression pro-gresses, and I
believe that a telescopinggrout pipe would introduce too
manyproblems. Perhaps the piezometerscould be attached to aircraft
cable(stranded flexible and thin stainlesssteel cable), which would
readily copewith the vertical compression. Aflush-coupled (inside
and outside) PVCgrout pipe would be used, and with-drawn after
grouting. If drill casing hasbeen used, care would need to be
takento maintain piezometer depths whenpulling the casing.
Second, arrangements would have tobe made to ensure that the
piezometertubing or cable doesnt fail in compres-sion. I know that
this can happen. Forpneumatic piezometers the tubing canbe
pre-spiraled as shown in Figure 9.31in the red bookthis was for a
projectin Hong Kong, described by Handfelt etal (1987), where the
pneumatic tubeswere wrapped around a 3 steel pipe,placed in very
hot water for a few min-utes, removed and allowed to cool. Butfor a
future project Id prefer to use vi-brating wire piezometers, and
wouldntwant to trust that the cable would movearound in the grout
and survive. Variousunhappy experiences have taught me agolden rule
about installation of instru-ments: if you can think of
somethingthat might go wrong, deal with it bychanging the planned
procedure. Ap-parently it isnt possible to pre-spiralthe types of
electrical cable that are usedfor field instrumentation. One
possibleway could be to coil the piezometer ca-bles loosely around
the grout pipe as allcomponents are lowered together, butwould that
run the risk of cable damageand possible lifting of piezometerswhen
removing the grout pipe? Anotherpossible way could be for the
manufac-turer to insert the cable in a plastic tubeand coil the
tube as was done for theHong Kong project, but would that runthe
risk of tubing damage and creation
of a bleed path for pore water pressure?Third, the
compressibility of the
grout must not be less than that of thesurrounding groundthis
would needto be taken care of during design of thegrout mix.
For this application it is necessary tokeep track of the
elevation of thepiezometers as they settle, because themeasurements
of pressure need to beconverted to piezometric elevations.This is
done by monitoring settlementnearby, usually with probe
extensom-eters, and ensuring that the probe exten-someters can also
cope with the verticalcompression.
Id very much welcome the authorscomments on these
suggestions.
Borehole DiameterIn Part 1 the authors refer to
typicalgeotechnical boreholes (i.e., 140 mm).In my experience many
piezometers areinstalled in smaller diameter boreholes,often as
small as 76 mm. Do the authorshave any recommendations if we
dothis?
Use of a Single Borehole forFully-grouted Piezometers
andInclinometer CasingIn Part 1 of the article the authors saythat
a single borehole can be used to in-stall both piezometers and
inclinometercasing, resulting in reduced drillingcosts. They add
the caveat, However,the inclinometer casing joints must besealed. I
want to emphasize thatsealed must be taken very seri-ouslyany lack
of sealing will create apath for dissipation of pore water
pres-sure, therefore false readings.
ReferencesDeJong, J.T., Fritzges, M.B., Sellers,
J.B. and J.B. McRae (2004), PorePressure Character izat ion
ofGeotechnical Experimentation SiteUsing Multilevel Vibrating
WirePiezometers, Proc. 57th CanadianGeotechnical Conference,
QuebecCity, Session 7B, pp 17-24.
Handfelt, L.D., Koutsoftas, D.C. and RFoott (1987),
Instrumentation forTest Fill in Hong Kong, J. Geotech.Eng. Div.
ASCE, Vol. 113, No. 2,Feb.., pp. 127-146.
Geotechnical News, June 2008 39
GEOTECHNICAL INSTRUMENTATION NEWS
-
Authors Reply
The authors appreciate the opportunityof discussing the details
and concerns ofthe fully-grouted method with othercolleagues.
Other Experience with theFully-Grouted MethodThe authors have
successfully installedover 100 piezometers using thefully-grouted
method to depths greaterthan 100 feet since 2003. Some
installa-tions have had up to four piezometertips in a single
borehole.
Use of Bentonite Chips andSand PacksThe best way to obtain
confidence in theuse of the fully-grouted method is toconstruct a
trial application using boththe traditional sand pack method
andfully-grouted method in adjacent bore-holes. This comparison
test will quicklyreveal the benefits of the fully-groutedmethod
regarding the ease of installa-tion, time, and cost reduction. This
isthe way the authors became convincedthe method works. Arching of
sandpack and bentonite chips can be a veryfrustrating and costly
problem duringconstruction and the fully-groutedmethod eliminates
this problem.
Use of the Method in SoftGround below FutureEmbankmentsThe
authors have successfully used thefully-grouted method in soft
groundwith sacrificial grout pipes; however, intheir experience,
the magnitude of set-tlement has generally been much lessthan 35
percent. It is recognized that inground with high compressibility,
thegrout pipe would impede conformanceas vertical compression
progresses.
The application of the aircraft ca-ble installation sounds
reasonable andwill be considered for future installa-tions in the
authors practice. It appearsto be the easiest and most sound
solu-tion to the problem.
The installation using traditionalmethods and fully-grouted
method ap-pear to have the same concerns regard-ing the
compressibility of the grout andthe formation.
Borehole DiameterTypical hollow-stem auger drill casingused in
the Midwest region of theUnited States has inner diameters of 82mm
to 108 mm, which allows the use ofa 25 mm grout pipe and
multiplepiezometer tips and cable bundles. Cas-
ing diameters less than 108 mm maycause casing removal problems
as thepiezometers and cables may catch onthe casing and damage the
installations.For multiple-device installations, thelarger casing
is preferred. Some manu-facturers also offer protective casing
forthe piezometers that helps reduce dam-age to the devices. For a
single-tip in-stallation, the small-diameter casingshould be
adequate.
Use of a Single Borehole forFully-grouted Piezometers
andInclinometer CasingThe authorsexperience of installing
in-clinometer casing and piezometers hasbeen successful. Care has
been taken toseparate the tips from the joints. Addi-tionally, care
has been taken to ensurethe joints are as watertight as possible.A
verification test can be performed byadding and removing water
inside thecasing and measuring readings at thepiezometers to
identify any impact onthe readings from leaks. A stable read-ing
may indicate a successful installa-tion. However, the readings
should beevaluated during monitoring in theevent casing movement
has caused ajoint to leak.
GEOTECHNICAL INSTRUMENTATION NEWS
40 Geotechnical News, June 2008
Geotechnical Alarm Systems Based onTDR Technology
Kevin M. OConnor
AbstractGeotechnical applications of time do-main reflectometry
(TDR) are continu-ing to evolve and usage is
increasing,particularly for monitoring deformationover large
lateral distances when a pri-ori knowledge of movement locations
islimited. Applications include monitor-ing subsidence along major
roadwaysover active and abandoned mines andmonitoring movement
along the toe ofdams. It has evolved into real time mon-itoring
with alarms and a variety of noti-
fication schemes. Fully automated sys-tems have been installed
which detectwhen measured deformation on the or-der of millimeters
has exceeded speci-fied magnitudes and/or rates, and initi-ate
phone calls to responsible parties.The diversity of
project-specific details(e.g., cables installed in trenches,
inhorizontal directionally drilled holes, inangled holes,
notification via telephoneor radio, etc.) is a reflection of the
rangeof site conditions and owner require-ments. This article is
based on the paper,
Geotechnical Alarm Systems Basedon TDR Technology, which was
pre-sented at FMGM 2007 and is publishedin GIN with permission from
ASCE.
Representative ProjectsThe following projects illustrate
alarmsystems utilizing TDR technology.Common features of the
hardware andsoftware include: 22 mm diameter solid aluminum co-
axial cable robust datalogger and TDR unit
-
cable lengths up to 610 m maximized number of interrogation
points per cable baseline reading, or specified refer-
ence values, stored on datalogger difference between baseline
reading,
or specified reference value, and cur-rent reading computed by
datalogger
datalogger initiates alarm sequencewhen difference exceeds
specifiedthreshold value(s), and
external data storage moduleWhile there are common features,
project-specific installation details,data requirements, and
interrogationdetails reflect the flexibility of TDR
technology in these applications.There are also project-specific
ratio-
nale and objectives that reflect the flexi-bility of TDR
technology.
I-77 Summit County, OhioThis project was motivated by
previousexperience of the Ohio Department ofTransportation during
stabilization of asection of interstate highway impactedby
abandoned mine subsidence inGuernsey County. At that location,
thehighway was closed as the mine wasbackfilled with grout. As
grout was in-jected, water within the mine was dis-placed and
subsidence sinkholes devel-oped under the roadway.
For the project in Summit County, itwas specified that the
highway remainopen for traffic during grout backfill in-jection and
it was necessary to providean early warning system to detect if
sub-sidence was occurring beneath the high-way as water was
displaced. Holes werehorizontal directionally drilled (HDD)beneath
the centerline of each lane ofthe highway. Coaxial cables
werepulled back through the holes and thenconnected to a remote
data acquisitionsystem. Cables were also installed in atrench along
the road. When the systemdetected an alarm condition along
anycable, it communicated via phonemodem with on-duty
GeoTDRpersonnel.
State Route 91 Plasterco, VirginiaWhen the United States Gypsum
Com-
pany was decommissioning its facilityin Plasterco, it was known
that subsi-dence would occur when pumps wereturned off and water
levels rose withinthe mine.
One component of the decommis-sioning plan was construction of a
newalignment for SR91 outside the pro-jected influence of
subsidence. Con-struction of the new alignment involvedblasting for
rock excavation and com-pany personnel were concerned
thatblast-induced vibrations would acceler-ate subsidence of the
soft overburdenbeneath the existing highway. Coaxialcables were
installed in angled holesdrilled under the road and also in atrench
along the road.
In addition, mine personnel wereconcerned that subsidence would
occurin the former plant area where exca-
Geotechnical News, June 2008 41
GEOTECHNICAL INSTRUMENTATION NEWS
Table 1. Installation Details
Site Orientation CablesI-77Ohio
HDD holesand trench
8 cables317 to 374m (1040 to1227 ft)
SR 91Virginia
Angledholes andtrench
18 cables36 to 447m (118 to1466 ft)
TuttleCreekDamKansas
Trench 4 cables610 m(2000 ft)
McMickenDamArizona
Trench 6 cables416 to 497m (1366 to1630 ft)
Table 2. Interrogation Details
Site Data Points IntervalI-77Ohio
1 point/ m 3 hrs
SR 91Virginia
3 points/m 3 hrs
TuttleCreekDamKansas
2 points/m 10 minutes
McMickenDamArizona
20 points/m 24 hour
Figure 1. TDR data acquisition system. Four coaxial cables
areconnected to the multiplexer inside the smaller cabinet.
Thedatalogger, TDR unit, external storage module, phone modem,and
auto-dialer are also in the smaller cabinet.
Figure 2. Holes being drilled for grout injection intoabandoned
mine along I-77 in Summit County, Ohio.
-
vated rock was being stockpiled. Cableswere also installed in
trenches in thisarea. As each of the four remote sys-tems detected
an alarm condition alonga cable, a datalogger would activate
anauto-dialer to call on-duty U.S.Gypsum personnel.
Tuttle Creek Dam, Manhattan,KansasUnder contract with the U.S.
ArmyCorps of Engineers, URS Corporationinstalled a warning system
at TuttleCreek Dam. The concern was motivatedby a possible seismic
event within theNew Madrid Fault Zone that could initi-ate movement
of the downstream slopeof the dam. URS installed a multi-fac-eted
instrumentation system to monitorsurface and subsurface movement
inreal time. When multiple sensors detectchanges that exceed
specified threshold
values, a sequence is initiated that canmobilize evacuation of
downstreamresidents.
If slope movement should occur,modeling by URS has indicated
bulgingof the toe would intersect the down-stream trench in which
cables are in-stalled. Two adjacent cables extendwest from the data
acquisition system(DAS) and two adjacent cables extendeast from the
DAS. When deformationexceeds a threshold value on adjacentcables
simultaneously, the dataloggeractivates one channel of a control
mea-surement unit. The CMU polls severaldifferent sensors and
communicateswith the base station via radio.
McMicken Dam, Maricopa County,ArizonaBased on the subsidence
history of theMcMicken Dam embankment crest and
other informa-tion, AMEC andthe MaricoupaCounty FloodControl
Districthave determinedthat groundstrains and fis-suring are
devel-oping due toconsolidation ofthe underlyingalluvial
aquifercaused byground waterw i t h d r a w a l .Based on
furtherstudies, it was
determined that there exists a high prob-ability of earth
fissures being present inthe soils underlying McMicken Dam.
As a component of the Fissure RiskZone Remediation Project, two
adja-cent coaxial cables were installed in atrench downstream of
the dam to detectdevelopment of earth fissures. Whendeformation
exceeds a threshold valueon adjacent cables simultaneously,
thedatalogger initiates a call via radio tothe ALERT-protocol
control center.
Alarm ActivityWhen calls are received from a remotedata
acquisition system, informationtransmitted includes the cable
identifi-cation number and location along thecable where the alarm
condition exists.Algorithms, which have been pro-grammed into the
dataloggers, do notdistinguish the cause of the alarm con-dition.
They only alert responsible per-sonnel to the fact that a condition
existsin which the difference between the cur-rent reflection
magnitude and baselinevalue is greater than the specified
alarmlevel threshold. These alarms can betriggered by causes other
than cable de-formation and the alarms must be fil-tered.
A context for the performance of thealarm systems is provided by
some his-torical data for police alarms and debrisflow alarms. The
false alarm rates forthree systems listed in Table 3 rangedfrom 70%
to 90%:
Monthly alarm activity for the TDRbased systems in Ohio and
Virginia issummarized in Table 4. These projects
GEOTECHNICAL INSTRUMENTATION NEWS
42 Geotechnical News, June 2008
Figure 3. Installing coaxial cable in the trench along SR91in
Plasterco, Virginia.
Figure 4. Installing coaxial cable in the downstreamtrench at
Tuttle Creek Dam in Manhattan, Kansas.
Figure 5. Data acquisition system at McMicken Dam inMaricopa
County, Arizona.
-
used single cables in each borehole ortrench without redundant
measure-ments, and false alarms ranged from 0to 100% with averages
of 52% and76%.
Average rates are not really mean-ingful since weekly and
monthly ratesprovide a more realistic assessment ofthe impact on
the response of personnel.
Alarm ResponseVarious techniques have been used tofilter alarm
calls. Personnel assigned re-sponsibility for responding to
alarmshave developed operational proceduresto filter calls as they
gained experience.Subsequent system designs have beenmodified to
incorporate redundancy toimprove the reliability of alarm
calls.
Time identifier - each cable is as-signed a time when it is
interrogated sothe cable is identified simply by the timeat which
an alarm call is received. Thedatalogger is programmed to stop
call-ing after a specified number of retries,and this information
has been utilized tofilter calls. If a call is not received fromthe
same cable at the next assigned timefor that cable, the alarm
condition istypically intermittent and not associatedwith ground
movement. This techniquethat has been used to respond to alarmcalls
when the cause of the alarm wasdetermined previously and the
alarmcondition is being addressed.
Adjustment of alarm levels this is arelatively straightforward
measure inwhich the specified threshold value isincreased either
temporarily or perma-nently,
Specific portions of cable are inter-rogated it is possible to
specify if theentire cable is interrogated or specific
segments of the cable are interrogated,Simultaneous deformation
of adja-
cent cables two cables can be placedin one trench and the alarm
condition isnot verified unless deformation has oc-curred on both
cables simultaneously.
Action PlanConsider the following action plan thatwas
implemented for the U.S. Gypsumproject in Plasterco.Action Level 1:
Receive call from remote datalogger Down load data, identify cause
of
alarm conditionAction Level 2: Visual inspection of alarm
location Increased frequency of data acquisi-
tion Confirmed movement (based on vi-
sual inspection and/or redundant
measurements) Notify managementAction Level 3: Accelerating
movement Confirmation with visual evidence
or redundant data Shut down highway
This type of action plan is based onreaction to an alarm call
from the re-mote monitoring system. It inherentlyinvolves: decision
making within acompressed time frame, and personnelon call
24/7.
During the USG project, the tasks ofmonitoring and response were
assignedto in-house personnel to control costsand to expedite the
decision makingprocess. For the other projects, callswere handled
by an in-house centralcontrol center or out-sourced person-nel.
Monitoring a phone 24/7 is reactiveand can lead to burnout when
alarmlevels are being exceeded frequently.This operational issue
has been ad-dressed by the call-filtering techniquesand system
design features mentionedabove. Equally significant are
periodsduring which there is no alarm activity(e.g., Oct-Nov 2002
in Table 4). Foreach project, dummy cables are in placethat are
used to create an artificial alarmcondition, verify system
operation, andverify personnel response duringperiods when there is
no alarm activity.
ClosureTDR technology is capable of monitor-ing movement over
large lateral extentsand to great depths with a high densityof
monitoring points. It is being used tomonitor deformation over
active andabandoned mines, deformation alongdams and slopes,
movement in land-slide areas, and sinkhole movement inkarst
areas.
TDR-based systems are similar toother geotechnical measurement
alarmsystems. The rationale for these mea-surements is
significantly differentfrom the rationale for performancemonitoring
where measurements aremade to compare actual and
anticipatedbehavior. Alarm calls are received thatmay not be
associated with actual defor-mation, but an action plan must be
inplace to respond to each call and deter-
Geotechnical News, June 2008 43
GEOTECHNICAL INSTRUMENTATION NEWS
Table 3. Public Safety AlarmActivity
Location Alarms ReferenceBellevue,Washing-ton
75% - 90%filtered out
AIREF,2002
Columbus,Ohio
90% falsealarms
Andes,2005
Taiwan de-bris flow
70% falsealarms
Wu, 2005
Table 4. TDR Alarm Activity
Period TotalCalls
GroundMoving
OtherCause
Summit County, Ohio8/01 10 4 6 60%9/01 82 40 42 51%Total 92 44
48 52%
Plasterco, Virginia6/02 5 0 5 100
%7/02 17 4 13 76%8/02 14 2 12 86%9/02 7 0 7 100
%10/02 2 2 0 011/02 0 -- -- --12/02 52 0 52 100
%1/03 6 0 6 100
%2/03 0 -- -- --3/03 108 49 59 55%4/03 42 3 39 93%Total 253 60
193 76%
-
GEOTECHNICAL INSTRUMENTATION NEWS
44 Geotechnical News, June 2008
mine the cause. Unless this is accom-plished, the alarm calls
will continue.
Proactive, scheduled data acquisi-tion and display has been the
most ef-fective monitoring plan to observemovement before alarm
levels were ex-ceeded.
ReferenceOConnor, K.M. (2007). Geotechnical
Alarm Systems Based on TDR Tech-nology, Proceedings of the 7th
In-ternational Symposium on FieldMeasurements in Geomechanics,ASCE
Geotechnical Special Publi-cation 175, Boston, Sept 24-27.
Kevin M. OConnor, Manager of theGeoTDR subsidiary of
GeotechnicalConsultants, Inc., 720 GreencrestDrive, Westerville,
Ohio 43081; Tel:(614) 895-1400; Fax: (614) 895-1171;email:
[email protected]