Geotechnical News December 2011
29
GEOTECHNICAL INSTRUMENTATION NEWS
length of time it took to get accurate
readings when pressure was immedi-
ately changed from 0 to 100 kPa was
only 120 seconds for the VWP installed
tip down and 60 seconds for the VWP
installed tip up.
Installing VWPs in bentonite chips
is not recommended by the manufac-
turers, but I tested the results of such
an installation out of scientific curios-
ity. As discussed by Dunnicliff (1988,
1993, page 161), using bentonite chips
as backfill can adversely affect pres-
sures recorded by VWPs, both by ab-
sorbing water from the formation and
therefore recording a falsely low pore
water pressure, and alternatively by
expanding and pressing on the sur-
rounding ground and therefore record-
ing a falsely high pore water pressure.
Figure 4 is a graph of the response
times of a VWP installed directly in
saturated bentonite chips compared
with the pressure applied by the tri-
axial compression test frame. I see an
even greater response time between
incremental pressure changes, and an
almost asymptotic approach to the true
value applied by the frame. Again, this
may due to incomplete saturation of the
bentonite chips. As they approach the
100 or 200 kPa level, they VWPs ap-
proach the pressure levels applied by
the frame, but do not reach them (reach-
ing 97 and 195 kPa, respectively). It’s
possible that, given enough time, the
VWP would reach the pressure reading
applied by the frame, but I didn’t have
enough time to test this process.
Field Considerations
I attempted to re-create as many of
the field conditions as I was able to
in a laboratory setting. To do this, I
mimicked the installation methods for
the surrounding material by dropping
chips and sand around the instrument,
and pouring grout around it. I attempted
to re-create accurate distances between
the instrument and applied pressure.
However, my investigation varied from
field techniques in several ways, which
are important to note. First, the grout I
poured was not cured under pressure,
as grout in the field would be. Grout in
the field would feel the effects of the
column of grout above it. Second, my
VWPs were allowed unlimited water.
I didn’t and couldn’t re-create the
effects of installing a VWP into a low-
permeability unit, which might restrict
the amount of water the VWP receives.
Inverting the tip is recommended by
some manufacturers to retain water in
the tip. This may allow the VWP to
function better in a low-permeability
soil situation. I was not able to test low-
permeability settings in the laboratory.
Recommendations
My results suggest that a VWP will
function well in a variety of installation
methods, including: diaphragm up,
diaphragm down, in water, sand,
and in grout - with a canvas bag full
of sand or without. In fact, I had
difficulty getting the VWPs to fail. In
laboratory tests, I found that the canvas
bags of sand, inverting the tip, or pre-
saturating the filter stone or VWP were
not necessary procedures for the VWP
to function properly. Based on my test
results, the absence of these procedures
made no difference to the accuracy
of the VWP or the response times. It
could be argued that the canvas bags
of sand assist with the protection of
the VWP during installations, but I
have no reason to believe that this is
the case. Manufacturers recommend
saturating the filter stone, and some
recommend inverting the VWP tip. As
I was unable to mimic an installation
in low-permeability soil in the lab,
my results don’t contradict these
recommendations. In some cases, the
use of the sand-filled bags can make
the installation process more difficult
and time consuming, but inverting
the tip and saturating the filter stone
are easy steps to take. This research
supports the capability of a VWP to
function properly when installed by the
fully-grouted method,
Acknowledgments
I would like to thank Shannon and
Wilson, Inc., for the grant that made
this research possible. I’d like to thank
the staff of the Shannon and Wilson
soils laboratory, primarily Joe Laprade
and Aaron Van Derslice, for help with
the triaxial compression tests, and
use of their equipment. I’d also like
to thank Rob Clark, whose technical
experience and assistance was crucial
to the success of this research.
References
(The four Geotechnical News articles
can be accessed from
technicalnews.com/instrumenta-
tion_news.php)
Contreras, I.A., Grosser, A.T., and Ver-
Strate, R.H. (2008). “The Use of
the Fully-grouted Method for Pi-
ezometer Installation”, Geotechni-
cal News, Vol 26, No 2, June, pp.
30-37.
Contreras, I.A., Grosser, A.T., and Ver-
Strate, R.H. (2011). “Practical As-
pects of the Fully-Grouted Method
for Piezometer Installation”, Proc.
Int. Symp. on Field Measurements
in GeoMechanics, September, Ber-
lin, Germany. J. Gattermann and B.
Bruns (Eds.). TU Braunschweig,
ISBN: 3-927610-87-9. For further
information:
Dunnicliff, J (2008). Discussion of
“The Use of the Fully-grouted
Method for Piezometer Installa-
tion” by Contreras et al (2008),
Geotechnical News, Vol 26, No 2,
June, pp. 38-40.
Dunnicliff, J (1988, 1993), “Geotech-
nical Instrumentation for Monitor-
ing Field Performance”, J Wiley,
New York, 577 pp.
Mikkelsen, P.E. (2002). “Cement-Ben-
tonite Grout Backfill for Borehole
Instruments.” Geotechnical News,
Vol 20, No 4, December, pp. 38-42.
Simeoni, L., De Polo, F., Caloni, G.,
Pezzetti, G., (2011). “Field per-
formance of fully grouted pi-
ezometers”. Proc. Int. Symp. on
Field Measurements in GeoMe-
chanics, September, Berlin, Ger-
many. J. Gattermann and B. Bruns
(Eds.). TU Braunschweig, ISBN:
3-927610-87-9. For further infor-
mation:
.
Webber D.S. (2009). “In Support of the
Fully-grouted Method for Piezom-
eter Installation”, Geotechnical
News, Vol 27, No 2, June, pp 33,34.
Garrett Bayrd, Geologist, Shannon
and Wilson, Inc. 400 North 34
th
Street,
Seattle, WA 98103,
email:
