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Simplify Piezometer Installations, Lower Costs Simplify Piezometer Installations, Lower Costs and Get Better results withand Get Better results with
Fully Grouted PiezometersFully Grouted Piezometers
P. Erik Mikkelsen, P.E.Consulting EngineerGeoMetron Inc PSBellevue, WA, USA
Univ. of Florida Short Course, March 19, 2007
Open Standpipe PiezometerOpen Standpipe PiezometerObviously low material costs, but …...– Time consuming recording– Time-lag potential– Minimal and un-timely observations
Installation sensitivity/problems– time consuming backfill procedures– complicated backfill– risk of incomplete backfill
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VW Piezometer SensorsVW Piezometer SensorsAdvantages– instant reading, not influenced by observer– can be fully grouted– best suited for automated readings– calibration not influenced by lead length– high zero stability (minimal drift for decades)
Disadvantages– vulnerable to over-voltage, need protection– calibration factors, barometric influence– can be damaged by over-pressure
(After Slope Indicator)
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Fully grouted piezometerFully grouted piezometer
Why grout? …Eliminates complexityWhy does it work? …It is like saturated clay!Works like piezo sensor pushed into clayMix design? …Design based on cement content.Grout properties (new research results)
Mandrel pushedMandrel pushed--in VWP in soft clayin VWP in soft clay
4
Casagrande missed the point that pressure sensors are different!
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Darcy’s Law for FlowDarcy’s Law for Flow
Grout Type Characteristics k (cm/sec) SourceNeat cement w/c = 0.53, γ = 112 pcf 10-7 Baroid
Bentonite chips hydrated 10-8 BaroidBentonite slurry 6 % solids 10-5 BaroidBentonite slurry 20 % solids 10-8 Baroid
Cement-bentonite water/solids = 4 to 1 10-6 Vaughan, 1969Cement-bentonite w : c : b = 4 : 1 : 1 5 x 10-8 Vaughan, 1973
Table 2. Permeability of some grouts
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CementCement--Bentonite GroutBentonite GroutMix ratio by weight - 2.5 water : 1 cem : 0.37 bent + or – according to consistencyMix water and cement first Add bentonite slowly, high agitation, make grout creamy, yet pumpable (Marsh # 50-60 sec)Density - 80 pcf Water Content - >100 % Modulus - about 10,000 psi30- day QU strength - 50 to 100 psiPermeability – k = 1.0E-06 cm/sec.
0
20
40
60
80
100
120
0 2 4 6 8 10 12 14
Water-Cement Ratio by Weight
Unc
onfin
ed C
ompr
essi
ve S
treng
th in
psi Marsland, 1973
Slope Indicator, 1982
Dunnicliff, 1980
Anderson, 1984 Minnitti, 1980
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VWP in Grout, 200mm thickness, 288 days curing
0
5
10
15
20
25
30
35
40
0.00 5.00 10.00 15.00 20.00
Lapsed Time in Minutes
Pres
sure
Hea
d in
Fee
t
PiezometerChamber
8
VWP in Grout, Response Time vs. Curing in Days200mm grout thickness
01234
0 50 100 150 200
Curing Time in Days
Tim
e in
Min
utes
50%90%95%100%
9
10
Normalized Cement-Bentonite Grout Properties(Marsh Number = 50 to 60 sec.)After Barr Engineering 3/9/05
0
0.5
1
1.5
2
2.5
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Water-Cement Ratio - w/c
Nor
mal
ized
Val
ue o
f Den
sity
, Voi
d R
atio
&
Stre
ngth
0
20
40
60
80
100
Nor
mal
ized
Val
ue o
f Per
mea
bilit
y - k
Density(pcf) = 83.8 xVoid Ratio = 4 xUCC strength = 240 xk (cm/sec) = 1.0E-7 x
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Fully grouted multiFully grouted multi--level VWPslevel VWPs
Barometric Pressure and Rainfall, Oregon Coast
950
960
970
980
990
1000
1010
1020
1030
10/1/990:00
10/31/9912:00
12/1/990:00
12/31/9912:00
1/31/000:00
3/1/0012:00
4/1/00 0:00
1999 / 2000
mill
ibar
s or
cm
of H
2O
0
0.6
1.2
1.8
2.4
3
3.6
4.2
4.8R
ainf
all p
er 1
2 ho
urs,
In
ches
12
VWP - Altitude Correction
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0 2000 4000 6000 8000 10000 12000
Altitude - Feet
Cor
rect
ion
- Fee
t
0.00
1.00
2.00
3.00
4.00
5.00
6.00C
orre
ctio
n - P
SI
feet of H2OPSI
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Fully Grouted Piezometer ProjectFully Grouted Piezometer Project
Raw data for fully grouted VWPRaw data for fully grouted VWP
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PROFILE 2: Pore Pressure Elevations Grnd. Elev. Assumed = 20 m
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
1-May-01 1-Jul-01 31-Aug-01 31-Oct-01 31-Dec-01 2-Mar-02
Pie
zom
etric
Ele
vatio
n, m
eter
s
#9, elev. 18.1 m#8, elev. 18 m#10, elev. 12 m#12, elev. 10.5 m#11, elev. 9 m#13, elev. 8 mEmb. Elevation
#9 #8
#10#12
#11#13
PROFILE 1: Pore Pressure Elevations,Grnd. Elev. Assumed = 20 m
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
1-May-01 1-Jul-01 31-Aug-01 31-Oct-01 31-Dec-01 2-Mar-02
Pie
zom
etri
c El
evat
ion,
met
ers
#1, elev. 18.3 m#3, elev. 11.5 m#2, elev. 11 m#5, elev. 11 m#7, elev. 10 m#6, elev. 6.5#4, elev. 5 mEmb. Elevation#4
#6
#7
#5 #2#3
#1
15
CC--B grout: 55 gal. paddle mixerB grout: 55 gal. paddle mixer
400400--m deep Mm deep M--L VWP installationL VWP installation
16
ConclusionsConclusionsCement-bentonite grout works!Simpler and faster installationLess chance of failed installationWorks as a high air-entry filterIt always starts out fully saturatedEliminates risk of bentonite “activity”Applicable to both boreholes and embankments