KNOWLEDGE ARTICLE LR 18/08/2016 V1.0 This article outlines the challenges that may be faced during the installation of deep piezometers which are defined as having a depth greater than 50m. It is only a practical guide based on experience and is not intended to replace any Engineers’ design. Deep piezometer installations create a range of challenges, with the deeper the installation the larger the impact. These challenges include the positioning of the response zone, placing of the piezometer, protection of the sensor and cable and backfilling of the borehole all of which will have significant impacts on a successful installation. No one piezometer installation is the same and careful planning of each project should be carried to ensure a successful installation. Deep Piezometer Installation - considerations There are major challenges to be considered when installing deep piezometers and the deeper the installation the more difficult these challenges become and therefore, careful consideration on how to overcome these challenges should be undertaken. The main considerations are: • Borehole construction • The piezometer response zone • The piezometer seal • The sensor NOTE: There are major challenges to be considered when installing deep piezometers.
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Deep Piezometer Installation considerations · the sand not going to the depth required. Swelling of bentonite pellets before reaching required depth may take place. In addition the
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KNOWLEDGE ARTICLE LR 18/08/2016 V1.0
This article outlines the challenges that may be faced during the installation of deep piezometers
which are defined as having a depth greater than 50m. It is only a practical guide based on experience and is not intended to replace any Engineers’ design.
Deep piezometer installations create a range of challenges, with the deeper the installation
the larger the impact. These challenges include the positioning of the response zone, placing of the piezometer, protection of the sensor and cable and backfilling of the
borehole all of which will have significant impacts on a successful installation.
No one piezometer installation is the same and careful planning of each project should be carried to ensure a successful installation.
Deep Piezometer Installation - considerations
There are major challenges to be considered when installing deep piezometers and the deeper the installation the
more difficult these challenges become and therefore, careful consideration on how to overcome these challenges
should be undertaken.
The main considerations are:
• Borehole construction
• The piezometer response zone
• The piezometer seal
• The sensor
NOTE: There are major challenges to be considered when installing deep piezometers.
KNOWLEDGE ARTICLE LR 18/08/2016 V1.0
Item Considerations Possible solutions
Cased or un-cased borehole
The use of casing will be
dependent on the type of
formation and/or the drilling
contractors preferred method of
drilling and therefore, should be
carefully considered during the
planning stage.
Drilling a fully cased borehole will be
more difficult to ensure:
• Correct location of the response zone
& seal during casing extraction;
• The support and integrity of the
piezometer and cable during casing
extraction.
Use of techniques such as drilling
fluids to remove the need for casing.
Borehole diameter
It is likely that a decision to drill the
smallest possible borehole will be
made based on financial reasons.
Having a small annulus within the
borehole will create problems with the
placing and location of the response
zone and seals, especially if several
sensors are required within the same
borehole. This becomes more acute the
deeper the borehole.
See picture on page 6.
Ensure the borehole is the correct
size for the planned installation.
Use of the fully grouted method
especially for multi-level installations.
Limit the number of piezometers in
each borehole.
Practical information from installers
has suggested the following:-
Size No of sensors
NQ (75mm) 3-4
HQ (95mm) 4-5
Borehole formation
Previous knowledge of the kind of
ground that is likely to be
encountered is recommended to
have all the drilling tools and
accessories onsite.
Rough or jagged formations could cause
damage to the cable during installation,
placing of the filter and seal and
grouting.
Ensure good protection of the cable.
NOTE - protection of standard cable
within a protective pipe requires the
sensor to be supported from the
bottom by a wire rope (see section
4).
Use of heavy duty cable.
Drilling fluids
Experienced drillers will know
which type of drilling fluid is to be
used.
Drilling fluids will increase the density of
the fluid into which the sensors will be
lowered. No drilling fluids should
remain in the borehole after piezometer
installation.
Add a weight to the sensor to
overcome buoyancy effects.
Use additional support pipe to push
down sensor.
Always flush out the drilling fluid.
BOREHOLE CONSTRUCTION
NOTE: Great care must be taken when retrieving temporary borehole casing with the
sensors in place, so to avoid damage / movement of the sensors and the cables.
KNOWLEDGE ARTICLE LR 18/08/2016 V1.0
Traditional Method
Sand pocket to create response
zone around piezometer tip, with a
conventional bentonite seal to
separate sand and grout
NOTE: Ensure correct design of grout mixture; when preparing the mix on site, mix water
and cement first before adding bentonite.
Fully Grouted Method
No sand pocket required as
grout is pumped in from the
bottom of the borehole
upwards
Item Considerations Possible solutions
Filter pack
Washed sand (1-2mm) is
typically used to create the
defined response zone.
Seal
The most commonly used
method to create a seal is by
using Bentonite pellets for the
seal/plug and cement/Bentonite
grout for the remainder of the
borehole.
The deeper the installation the more
difficult it is to guarantee the location
and successful placing of the sand filter
pack and seal.
If placed too fast or poured in from the
top, bridging can easily occur resulting in
the sand not going to the depth required.
Swelling of bentonite pellets before
reaching required depth may take place.
In addition the sand filter pack and seals
can be pulled up when the temporary
casing is withdrawn.
Use of pre-assembled sand filter pack
and bentonite pellets within a
geotextile sock.
Wash in sand using a Tremie pipe
(sacrificial or removable). Stage
grouting may be needed to eliminate
uplift.
Use a packer to create a seal.
Use the fully grouted method where
the grout becomes the filter pack and
seal.
PIEZOMETER RESPONSE ZONE & SEAL
Typical grout mixing equipment
Placing bentonite pellets.
KNOWLEDGE ARTICLE LR 18/08/2016 V1.0
NOTE: The support system must be robust enough to hold it’s own weight plus the cable
and sensor.
Item Considerations Possible solutions
Pressure range
Sensors should be chosen based
on the range of pressure that the
sensor is expected to be subjected
to, with sufficient range to avoid
over pressure especially during
grouting.
The greater the sensor range the greater
the potential systematic erroring the
readings and therefore for the most
accurate readings sensors should be
chosen to have the lowest appropriate
range that can still withstand the
pressure during grouting.
When the pressure is known select
the range so that it doesn’t exceed
75% of the maximum pressure.
When the expected pressure is
unknown it can be approximated
based on the installation depth.
Monitor pressure during installation
and grouting.
Sensor construction
Special care has to be taken with
operations whilst lowering and
keeping the piezometers in place
during installation.
The deeper the installation the more
weight will be needed to overcome
buoyancy effects if there is fluid in the
borehole.
With increased depth and pressure the
piezometer can start to deform which
will cause erroneous readings.
Potential sensor damage increases with
depth.
Add weight to the piezometer or use
a support pipe, GRP rod to push it
down.
The use of a heavy duty piezometer
eliminates the possibility of
deformation and is more robust
against damage.
Cable & support
Lowering cable down a borehole is
always a potential area for damage.
Suspending the piezometer by its cable
will potentially cause damage due to
elongation and possible breaking of the
internal conductors. This will increase
directly with depth.
Use heavy duty cable which has a
high tensile strength (14N/mm2) and
an elongation value of 260%.
For standard cable use suitable
protection & support from the base
using a steel wire, GRP rod, drillers
wireline or sacrificial pipe which can
also be used as a tremie pipe and
ensures correct location of the
piezometer.
SENSOR
KNOWLEDGE ARTICLE LR 18/08/2016 V1.0
Multipoint piezometer being tied to the sensor
cable, and therefore fixed into the upwards
position when lowered into borehole. Fully grouted method, filter up.
NOTE: It is important to install piezometers facing up when installed in soil which may
become unsaturated, as well as marginal soils which will have fluctuating saturation levels.
Ready to install several piezometers at the same site.
Multi-core cable used for multipoint piezometer installations.
Standard cable
CABLE & ASSEMBLY
Heavy duty cable
KNOWLEDGE ARTICLE LR 18/08/2016 V1.0
NOTE: It is crucial to evaluate the borehole diameter before starting to drill a borehole to
install a piezometer.
Steel wire
armoured
50 mm
The total external diameter of the piezometer plus the cable will determine the minimum
borehole diameter. 30 mm should be added to the dimensions above.
Heavy duty
50 mm
Standard
40 mm
ASSEMBLY DIMENSIONS
KNOWLEDGE ARTICLE LR 18/08/2016 V1.0
BOREHOLE Diameter Ensure borehole size is adequate for chosen installation.
Restrict number of sensors in one borehole.
Use multipoint piezometer system to reduce excess cable.
Formation profile Adopt the Fully Grouted Method.
Ensure protection of cable during installation.
Casing Drilling Fluids can remove the need for casing.
Always flush out drilling fluid left in the borehole.
SEAL/GROUT Buoyancy Add ballast weight to piezometer if pre-grouted.
Response Zone Use pre assembled sand filter around piezometer using a geotex-
tile sock.
Use wash in sand filter using a tremie.
Seal Assemble bentonite pellets in a geotextile sock.
Use packers as a seal.
Method Adopt the Fully Grouted Method.
CABLE Type Use heavy duty cable.
Elongation Support cable with rope rod or wireline.
SENSOR Pressure Do not exceed 75% of maximum pressure when selecting range.
Attachment Attach piezometer to a sacrificial pipe - not advised for installations
within soft ground where settlement is likely.
Overpressure Monitor pressure during grouting and installation.
Orientation Install piezometer facing up when unsaturated soils or changing
vadose zones are expected.
NOTE: Positive installations need planning and good care onsite.