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

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.


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.


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


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.


NOTE: The support system must be robust enough to hold it’s own weight plus the cable

and sensor.


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


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


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


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.

SUMMARY

Documents

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