Report S5
Suggested method for pressure
measurements using GLÖTZL-Type
hydraulic cells
SUGGESTED
-.
METROD
RYDRAULIC
CELLS
FOR
PRESSURE
MEASUREMENTS
USING
GLOETZL-TYPE
General
There are several
methods
to measure pressures
generated
in soils,
rock and concrete.
(a} direct
method for measuring
pressures
by means of infinetely
thin
disces.
(two-dimensional}
(b} load measurement.
load measurement
by means of a three-dimensional
hard and soft
inclusion
cell.
(c) indirect
methods.
one must always
have a good correlation
between the measured
values
and the searched
pressures.
deformation
measurements
optical
stress
geophysical
measurements
aso.
Scope
1
{a)The
hydraulic
pressure
-flatjack
-Wöhlbier-Ambatiellc
-verin
plat
Freyssinet
cell
and
cell
{Glcetzl-Type)
is
a develcpment
cf:
cthers
This cell
consists
essentially
of a fluid
filled
pressure
pad of very
small
thickness
proportionally
to the total
area connected
to a diaphram transducer,
based on the compensation
method.
The diaphragm
transducer
which
in turn
is connected
by tvlin
flexible
tubing
to a
readout
unit.
Pressure
applied
from the surrounding
material
to the
pressure
pad is measured
by balancing
the fluid
pressure
in the cell
by apressure
applied
to the reverse
side of the transducer
diaphram.
(b)
The cell
is intended
as a means of measuring
total
pressure
changes
in soil,
fill,
concrete,
or at the interface
between
rock and any of these materials.
It may be also be used to measure
pressure
changes
in rock when installed
in a machined
slot.
Typical
applications
are in earth
or fill
embankments,
dams and dam foundations,
retaining
walls,
and behind
within
tunnel
linings.
(c)
Total
pressure
is defined
as the sum of effective
soil
or rock
pressure
and groundwater
pressure
at a given
location.
If effective
earth
or rock pressure
is to be distinguished
from water pressure
a pore water
pressure
cell
should
be installed
alongside
the total
pressure
cell.
Pressure
change is defined
as the difference
between
total
pressure
at any given
time and the total
pressure
value
at the
time of the cell
installation.
Measurements
of absolute
pressure,
the
ground pressure
before
cell
installation,
may be made using one of the
I.S.R.!\1.
Procedures
for stress
determination.
{d)
The Gloetzl
type of hydraulic
transducer
that
forms a component
of the total
pressure
cell
is used in other
applications,
for
example
to measure pore water
pressure
or settlements.
These alternative
applications
are not described
in the present
Suggested
Method.
Apparatus
2.
The hydraulic
(Gloetzl-type)
cell
(for
example
Fig.1)
, consisting
of:
(a) Apressure
pad, which is a flexible
fluid
filled
sandwich
connected by a steel
tube to the pressure
measuring
transducer
and compensating
tube when fitted.
The pressure
pad is typically
manufactured
from
two flat
sheets
of steel,
welded
around
the periphery
to form a leak
free
container
for the cell
fluid.
To reduce
it from edge effects
it is
necessary
to have a groove
round the welding.
A groove
of a depth of
about
1/3 of the plate
thickness
is made along
the edges of the pressure
pad to minimise
the edge effects
during
and after
the welding
operation.
(b) Cells
are typically
either
circular
or rectangular
in plan,
with
dimensions
from 6 to 40 cm. The largest
size
compatible
with
the size
and configuration
of the location
should
be selected
to obtain
abetter
representative
result.
(C} The stiffness
of the pressure
pad should
be of course
similar
to
that
of the material
in which
it
is embedded,
to ensure
that
stress
is
applied
to the pad without
arching
effects,
so that
pressure
measured is
similar
to that
in the surrounding
material.
Thus, cells
for installation
in soil
should
be more flexible
than those
for use in rock or concrete.
The influence
of the stiffness
in the Gloetzl-type
cell
is not so big
as the error
is reduced
by the small
thickness
of.construction.
Th~re
must be a factor
of 1/20 or better
between
the thlckness
and the dlameter or sidelength.
(d) The material
af the pressure
pad, transducer,
and all
ancilliary
campanents
shauld
be selected
ta resist
carrasian
fram the surraunding
materials
and graundwater.
{e) The fluid
filling
the cell
should,
to ensure
a suitable
stiffness,
be mercury
if
the cell
is to be installed
in rock or concrete,
but has
to be less
dense fluid
such as oil
if the cell
is to be installed
in
soil.
The cell,
connecting
and compensating
tubes and transducer
has to
be filled
completely
filled
with
fluid,
free
from air bubbles.
(f) The connecting
tube from the pressure
pad to the transducer
may
be either
welded
to the face of the cell
or welded in its
flange.
Connections
to the cell
and transducer
must be strong
enough to avoid
darnage. The connection
tube should
be bent in the factory
to ensure no
darnage or kinking.
(g) The hydraulic
or pneumatic
transducer
(for example Fig.2)
is a
valve
consisting
of a flexible
steel,
plastic
or rubberdiaphragmincorporated
in a metal
housing.
One side of the diaphram
is connected
to
the cell
fluid
and the other
to the measuring
fluid
delivery
and return
tubes.
The transducer
design
should
be such that
when pressure
in the cell
is slightly
greater
than in the measuring
fluid,
the diaphragm
prevents
return
flow
of the measuring
fluid.
When the applied
measuring
pressure
exceeds
the cell
fluid
pressure
by more than an amount corresp.onding
to
diaphragm
inertia
the diaphragm
will
deflate
allowing
flovl along the
measuring
fluid
return
line.
(h) The design
and material
of the transducer
should
be such as to
ensure
minimum diaphragm
inertia
irrespective
of operating
conditions
and length
of service.
Corrosion
of components,
or ch;anges in shape or
flexibility
of the diaphragm
during
use must be avoided,
since
they
will
adversely
affect
the operating
a characteristics
of the transducer.
2
Figure
1:
The Gloetzl-type
pressure
cell
A)
Gloetzl
type
pore
water
B)
Gloetzl
type
soil
pressure
al)
Cerarnic
filter
a2)
Pressure
in
Figur'e
2
:
200
rnrn in
cell
cell
width
and
300
rnrn
length
b)
Valve
c)
Pressure
external
d)
Return
and
pad
pressure
diaphragrn
line,
3 rnrn inside
diameter
of
6 rnrn
line
externa1.
with
internal
diameter
Hydraulic/Pneumatic
diameter
diameter
of
and
of
3 rnrn
6 rnrn
Diaphragm
Transducer
{i) The transducer
is generally
ewhedded along the pressure
pad
in the surrounding
material,
and should
be protected
from malfunction
due to pressures
transmitted
from these
materials,
either
by robust
design
or by coating
of rubber
or similar
material.
The transducer
should
be positioned
with
respect
to the pressure
pad at a location
that
will
not result
in an erroneous
or nonuniform
transfer
of pressure
to the
pressure
pad from the surrounding
material.
(j) When the cell
is to be installed
in or adjacent
to concrete
where shrinkage
may create
a gap between
the pressure
pad and its
surroundings,
the cell
must be fitted
with
a compensating
tube or similar
means os augmenting.
the cell
fluid
volume.
The compensating
tube may
consist
of a metal
tube filled
with
cell
fluid
and connected
to either
the pressure
pad or transducer.
The tube may be crimped
to re-inject
further
fluid
into
the cell.
3.
Hydraulic
connecting
tubes,
the readout
unit
to one ore more
structions:
manifolds,
couplings
etc.
to connect
cells
should
meet the following
in-
(a) One tube is required
per
fluid
directly
from the pump. The
coupled
and connected
by at least
(ringline)
It is of course
safer
cell
for delivery
of the measuring
return
lines
from several
cells
may be
two tubes
to the readout
location.
to have one return
line
per cell.
(b) The tubes
should
be flexible
to allow
them to travel
along a
convenient
route
to the readout.
They may, for example,
be manufactured
from plastic
material,
provided
that
their
absorption
of water be equal
or less than 1.6% to make sure that
their
chemical
consisting
does not
practically
change over the years.
(c) The .length,
inside
and outside
diameter
and material
of the
tubing
should
be selected
to
ensure
that
no blockages
or leaks
develop
during
installation
or use, and that
the rate
of flow and pressure
drop
along
the tubing
are such that
rneasurernents
can be made within
the required
accuracy
limits.
Lengths
of tubing
rnay typically
be frorn 10-500 rn.
The inside
diameter
of the tubing
is 2-3 rnrn depending
on the value
of
the expected
pressures.These
.srnall
diameters
give better
possibilities
to get rid oI the air bubbles
eventually
trapped
in the circuit
as to
keep the oil
colurnn constant.
Manufacturer's
advice
should
be sought
in
selecting
appropriate
tubing,
and if
in doubt,
trials
should
be carried
out.
The burst
pressure
of the tubing
and fittings
should be at least
100% above the maximum to be applied
in use.
{d) When several
cells
are to be install.ed
these may be conveniently
be connected
to a valve-manifold
terminal
unit
fixed
to the structure
or
in an instrument
house.
~he terminals
should
be designed
to ensure
positive
leak-free
connection
9f the readout
pump to each cell
in turn,
and
to prevent
entry
of air
into
the measuring
lines.
Quick-connect
couplings
can be used.
The ..best solution
is a permanently
installed
and connected
readout
unit
to prevent
air
bubbles
or durt
to be introduced
into
the
circuit
and have.the
cells
ready
for measurements
anytime.
4.
The readout
equipment
usually
is
either
air,
nitrogen
or oil
pressure.
following
requirements:
3
a portable
The readout
unit'and
may deliver
unit
should
meet the
(a) The choice
between
cell
fluids
should
be made to suit
project
requirements.
Gas pressure
is convenient
and clean but is generally
only used at pressures
below 2 MPa. Liquid
is generally
used for higher
pressures
and also
for long delivery
lines
in order
to avoid
time laqs
due to compressibility.
(Figure
3: Filling
times
and looses
of pressure)
(b) The pressure
gauge
used
should
have a range
of about
120% of
the maximum
pressure
so as to have
abetter
sensitivity
and provided
a
pressure
gauge
of greater
range
is available
if
the" above
maximum is
exceeded.
5.
Calibration
requirements
are
equipment
obtained
should
be available
to
throughout
the project,
(a) The pressure
pad design
compression
testing
machine.
ture
(b) Calibration
effects
at the
may be required
cell
location.
(c} Before
installation,
to determine
diaphragm
inertia
and fluid
return.
and
(d)
after
The readout
the project.
should
pressure
to
ensure
that
for example:
be checked,
allow
for
accuracy
example
compensation
for
in
a
tempera-
the complete
assernbly
should
be checked
and the effects
of delay
between pumping
gauge
should
be
calibrated
at
least
before
Procedure
8.
Selecticn
cf
lccaticns.
(a) Cells
are generally
installed
in pairs
or clusters
to measure
pressure
in different
directions
at the same location.
Adjacent
cells
should
be seperated
by a distance
of at least
half
a cell
diameter,
in
such a way as to prevent
the presence
of one cell
effecting
readings
on
adjacent
cells.
(b) The location
for
on the specific
objectives
be located
in ground
that
typical
of the surrounding
the cell
or cell
cluster
is determined
of the measurements,
however each cell
is undisturbed
(for example by blasting)
materials.
depending
should
and
(c) The cell
must be in uniform
and complete
contact
with
the surrounding
material.
Soil
or rock adjacent
to the cell
should
be from
protrusions
or unrepresentative
material
that
would result
in stress
irregularities
on the pressure
pad.
(d) Cells
should
preferably
not be located
where they will
be exposed to appreciable
temperature
changes.
The influence
of the temperature
is of small
importance
inasmuch
as
the cell
is in its
design
virtually
insensitive
to temperature
changes.
The error
is generallyequal
to 1 millibar
per degree celcius.
9.
Installation
in
soils.
(a) \ihen installing
cells
in a natural
soil
or fill
embankment,
an
lenticular
excavation
is made to accomodate
the cell
cluster,
then individual pockets
for each cell
are hand-dug
at the correct
locations
and \vith
flat
faces
at the required
inclinations.
The lenticular
excavation
should
be stable
with
side
slopes
as necessary
and a depth of at least
50 cm.
-4
-
Filling-times
and
pressure-losses
Polyamid-ll-lines
613
for
rom depending
on
air-operated
given
pressure-cells
pressures
when
using
differential.
Min
1A
~
1n
11.
~
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-:7"'1
Q61
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.--1
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h
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lass
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pressure
pressure~iff~T~n~i~l
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~oo rn
rom
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~r
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11.
17
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h
rn
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Air
cansumPtian
7
accarding
~; n11rp
s
L
1
ta
1!
h
pressure
Fi , , i na
7
RAR
height
and pressure'
t- i mp~ ~na
l.()()~P-S ()f
differential
Dressure
1-3
l/min.
The cell
bance ta
packets
shauld
the sail,
each
be excavated
with
extreme
care ta avaid
af size
appraximately
twice
that
af the
disturcell.
(b) Rock fragments
of size greater
than 1/10th
of the cell
diameter
except
where the embankment
is of rock fill,
should
be removed and replaced
by fine
grained
material,
generally
hand compacted
into
the voids.
A bed of fine
grained
material
is placed
and compacted
to separate
the
cell
from any irregularities
in the pocket
walls.
The cell
is then fixed
in position
ensuring
that
it is fully
in contact
with
underlaying
materia]
and checked
to ensure
that
it is functioning
correctly.
(c)
campacted
Each packet
is then
ta a density
similar
backfilled
ta that
with
fine
grained
af the surraunding
material,
sail.
hand
(d)
pockets
coarse
to the
When the embankment
is of coarse
material
such as rock fill,
the
should
be larger
and the pocket
backfill
should
be graded with
material
adjacent
to the embankment and finer
material
adjacent
cell.
(e) The cluster
excavation
is then backfilled
and compacted
with
natural
embankment
material
having
rernoved rocks
larger
than the cell
dimensions.
No heavy vibratory
rollers
should
be used until
at least
1 rn
of fill
has been so placed.
Installation
10.
between
soil
and concrete
Generally
the cell
is attached
to the formwork
and placed
in the structure
or fastened
to the structure
prior
to backfilling,
or embedded in
the backfill
a short
distance
away from the structure.
Installation
at
the
interface
between
concrete
and
rock.
(a) The area of rock over which the cell
is to be placed
should
be prepared.
Loose material
should
be rernoved.
The area should
be precoated with
a 15 rnrn thick
layer
of cernent rnortar.
The cell
should
be
cleaned
of grease
and any aluminium
paint
and dipped
in cernent rnortar.
(b) A pad ofcement,mortar
is trowelled
into
the rock or concrete
surface
and the cell
is pressed
against
the pad squeezing
out the mortar
until
a layer
no more than 2-5 mm thick
remains
beneath
the pressure
pad.
The cell
must remain
in place
during
concreting
or shotcreting
operations,
Installation
in
concrete.
(a) The cell
is fixed
ta the reinfarcement
ar ta the structure.
Its pasitianing
shauld
be such as ta ensure
an all
raund af cancrete.
Entrapment
af air
must be avaided.
by
the
(b) Cell
client.
Alignment
should
be
Cells
must
be fixed
within
during
10 degrees
pouring
of
of that
specified
shotcreting.
A cell
installed
in concrete
has a dilatation
coefficient
different
from that
of the concrete
a gap will
appear and the arching
effect
will
result
in a smaller
value
being measured.
By meansof
the compensation
.
tube it will
be possible
to regain
contact
between
the cell
and the concrete.
The compensation
tube will
be crimped
with
a special
tool
in
order
to in-ject
the fluid
into
the pressure
pad and a measurement
is
made simultaneously.
5
A linear
corelation
will
at first
be obtained
between the crimped
length
and the measured
values.
The pressure
increase
is generated
by the mechanical
affects
of the pressure
pad resting
in
a void.
The regain
of contact
can be monitored
when the corelation
factor
increases.
The full
contact
is achieved
when this
factor
reaches
a constant
value
which indicates
a linear
corelation
between
the crimped
lengtr
and the measured
pressure.
It is recommended
to apply
an overload
between 0,2 MPa to 2 MPa depending
on the quality
and thickness
of the
surrounding
concrete
in order
to have a possibility
to measure tension.
It is very important
to record
all
the data and plot
them to observe
the characteristics
of the curve
obtained.
For a shotcrete
in the tension
thickness
bigger
than
and compression
areas
20 cm measures
must
(outer
and inner)
.
be taken
both
The experience
has shown that
the greatest
annular
(tangential}
tensions
develop
in the region
of the base of the vault.
This must imperatively
be taken
into
consideration
when determining
the location
of tunnels
without invert
it
is imperative
to strongly
recornmend to measure the stress
within
the piers.
Connecting,
filling
and checking
the
cells
(a) The terminal
equipment
is fixed
securely
in place,
either
near
the cells
for example
as a wall
fixture,
or remote
from them in an instrument
house.
Terminal
panels
should
be pre-checked
to ensure
correct
functionning
of all
valves,and
freedom
from leakage,
and should
clearly
and permanently
labelled.
(b) The labelled
tubing
is connected
to the appropriate
terminals
and is secured
in place.
Atest
should
be made on each cell
while
still
accessible
for repair
and replacement,
to ensure
that
the completed
hydraulic
circuit
is functioning
correctly.
(c)
Acheck
Procedure
{a)
flow
for
should
be
taking
readings.
The readout
(b) The supply
is recorded.
unit
pressure
made
is
along
connected
is
increased
the
with
complete
the
tube
cells
gradually
length,
and checked.
until
areturn
(c) The pressure
is released
and again
increased
at a very slow
and constant
rate
(3-4cm3/min.)
until
flow
is observed.
This pressure
is noted,
being
the minimum pressure
at which,
under conditions
of minimum flow,
a steady
return
flow
is achieved.
A false
reading
will
also
be obtained
if the flow rate
is too great.
(d) The frequency
the requirements
for the
from time to time.
of readings
should
be specified
project,
and should
preferably
6
depending
be adjusted
on
Calculations
15.
(a )
The
cell
into
account
taking
p =
pressure
corrections
(Pr-Pi-Ph-Pf)
Pis
obtained
as follows:
from
the
reading
Pr
after
xE
Pr is the pressure
reading
Pi is the initial
cell
pressure
applied
d~ring
manufacture
and (in concrete)
subsequently
adjustedby
.compensation
for
shrinkage.
Ph is the static
head correction
for the pressure
due to difference
in elevation
between
the cell
and readout
(liquid
onlyi
for gas, Ph=O) .
pf is a correction
for friction
losses
in the fluid
delivery
line
Eis
amultiplication
factor
(less
than 1.0) to compensate
for cell
edge effects.
where
(b)
In most applications
only
changes
in pressure
P are of interest.
In these
cases an initial
reading
Pi is taken
after
completion
of
installation
and
includes
the
effects
Ph
and
pf
which
remain
constant
throughout
the project.
In these
cases:
.
P =
(Pr-Pi
xE
It shouldbe
noted
recorded
using
this
are to be measured
(c
however
that
the minimum pressure
change that
cell
is
(-Pi+Ph+Pf) , so that
if small
pres.sure
agas
is preferred
as the measuring
fluid.
The elevation
correctiori
Ph may be calculated
can be
values
as follows:
Ph =
(h1-
where
h2)
= unit
weight
of
h1- h2 = difference
measuring
fluid,
g/cm3 (0 for gas)
in elevation
(cm) between
readout
(positive
when cell
is below
Ph is then obtained
in g/cm2
by multiplying
by 0.0981.
readout)
.
and should
be converted
and cell
to
KPa
(d)
The tube friction
correction
pf should
be measured
during
installation,
before
connecting
the cell,
being
the pressure
reguired
to maint~in
a steady
flow
through
the tubing
at a flow rate
similar
to
that
obtaining
during
measurement.Under
normal
conditions,
with
unobstructed
and correctly
selected
tubing,
this
correction
should
be small.
(e)
The edge effect
correction
E should
be established
by
nufacturer
on the basis
of control
tests
in a compression
machine.
this
purpose,
a representative
cell
should
be cast
in a concrete
of at least
3 times
the
cell
dimensions.
The correction
is usually
significant
for
cells
of small
size.
(f)
In Addition,
specialized
applications
The correction
Pt to
be
the
maFor
block
only
a ternperature
correction
rnay be required
in
such
as for
cells
installed
in rnass concrete
subtracted
frorn
the reading
rnay be expressed
Pt
= Kt(tr-ti
7
sorne
.
as:
where
(tr-ti)
is the temperature
increase
(oC) from the time of the
initial
reading
Pii
and Kt is a coefficient
expressing
the response
of
the system
(cell,fluid
and surrounding
material)
to temperature.
Experimentally,
Kt has been found to lie
in the range 20-50KPa/OC
for cells
installed
in concrete.
The actual
value
will
depend on the size of cell.
(g)
The magnitude
an.d importance
of the above corrections,
which
in many cases will
prove negligible,
should
be investigated
at the ~tart
of the project
and reported,
whether
or not they are found to be significant.
(h}
The corrected
and plotted
graphically
of time
after
installation
Reporting
of
values
to show
(see,
of cell
pressure
P should
fluctuations
of pressure
for
example,
Figure
7) .
be tabul~ted
as a function
Results
16.
Results
should,
unless
otherwise
specified,
be presented
in two
forms of report:
an Installation
Report
giving
basic
qata on.the
instrumentation
system at the time of installation;
followed
by Monitoring
Reports
presenting
periodically
the results
of routine
observations.
The Monitoring
Reports
will
generally
be required
at freque~t
intervals
to minimise
delay
b~tween
the detection
of adverse
behaviour
and the
implementation
of any remedial
measures
that
may be necessary.
The
Installation
(a)
A description
and diagrams
of the
detailed
performance
specifications
including
ture.
(b)
manitaring;
ting
anly
Report
shoüld
include
the
following:
monitoring
equipment
used
and manufacturers
litera-
Details
af methads
used ,:far installation~
calibratian
and
:r:eference
may be made ta this
I.S.R.M.
Suggested
Methad stathe departures
fram the recammended praced-ures.
.(c)
J\ location
plan showing
det;;ai:;Ls of the pressure
cell
locations
with
respect
.to the structural
configuration
and the surrounding
soil,
rock or concrete
conditions.
.~
(d)
For each cell,
areport.
giving
the initialinstallation
pressure
and, if applicable,
the pressure
after
compensation
for shrinkage.
Details
of calibratio~s
and determinations
6f correction
factors
should
be included,
along with
any. pertinent
cominents on the peculiarities
or problems
encountered
during
installati6n
of each c-~ll.
The
Monitoring
thase
(a)
An updated
shawn in Figs.
changes
(b}
Abrief
and to all
ding
Reports
field
6 and
should
data
7.
comrnentary
instrument
include
sheet
and
the
following:
results
drawing
attention
to
malfunctions
occurring
graphs
similar
significant
since
the
ta
pressure
prece-
report.
(c)
since
The results
the preceding
of any
report.
calibrations
8
or
instrument
checks
carried
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