How to decide what pressuremeter to use

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The decision about what pressuremeter to use for a particular project is not clear cut and there will be budgetary constraints in addition to technical considerations. This page focuses on the technical issues. It is divided up by instrument, as there is considerable overlap between the probes and the materials they can test.

Self boring pressuremeter

Insertion methods

Self boring

Initial Diameter

83-89mm, depending on the configuration

Length of material sacrificed

At least 1 metre of material must be self bored before testing

Displacement system

Direct strain sensing at 3 points equally spaced around the centre of the expanding region

Displacement resolution

Better than 1 micron

Pressure resolution

0.1kPa

Maximum expansion capability

15% greater than the at rest diameter

Maximum working pressure

10MPa

Suitable for:

Homogeneous clays (soft to very stiff), silts and sands, soft rocks such as flint-free chalk.

Strengths

The SBP gives the highest quality pressuremeter test with minimal insertion disturbance. It is the only device able to measure the external pore water pressure and so can provide effective stress parameters. As an addition to the expansion test it can incorporate a consolidation phase. With a slight modification it can also be used to obtain good quality measurements of the permeability of the formation [Reference 105].

Weakness

If the cutting shoe edge is damaged (by gravel or a hard layer) then the insertion disturbance is not minimal and the expansion capability may not be enough to erase the consequences.

There is no core recovery as such but all the cut material is returned to the surface as a completely disturbed sample.

Additional notes

In general self boring is a faster system than other methods for making a test pocket. It can also be less demanding on supporting equipment. In some circumstances it can operate as a portable stand alone system and It is often used in conjunction with a cable percussion rig.

There are versions of this instrument that have 6 displacement sensors and incorporate a three axis inclinometer.

73mm High Pressure Dilatometer (HPD73)

Insertion methods

Pre-bored hole or pocket

Initial Diameter

73mm

Allowable pocket diameter

75mm to 83mm

Length of material sacrificed

At least 2 metres of material must be cored to give a pocket long enough to test.

Displacement system

Direct strain sensing at 6 points equally spaced around the centre of the expanding region

Displacement resolution

Better than 1 micron

Pressure resolution

0.3kPa

Maximum expansion capability

33% greater than the nominal pocket diameter (76mm).

Maximum working pressure

20MPa in normal use, 30MPa with some modifications

Suitable for:

Stiff clays, sands and rock of all kinds

Strengths

Pre-boring a hole means that core can be recovered, giving the possibility of carrying out laboratory tests on the same material as the pressuremeter tests.

Weakness

It can be difficult to core at this diameter in highly fractured or friable materials. If the material is prone to collapse, and a pocket it lost, this can give rise to substantial gaps in the information obtained from a borehole.

Additional notes

If the pocket size is 83mm then the expansion capability falls to 22%. Because a large pocket size implies a high level of disturbance it is likely to be difficult to achieve a test that gives representative properties for the material.

The instrument also has a magnetic compass so that the orientation of the displacement axes can be known.

95mm High Pressure Dilatometer (HPD95)

Insertion methods

Pre-bored hole or pocket

Initial Diameter

95mm

Allowable pocket diameter

98mm to 101mm

Length of material sacrificed

At least 2 metres of material must be cored to give a pocket long enough to test.

Displacement system

Direct strain sensing at 6 points equally spaced around the centre of the expanding region

Displacement resolution

Better than 1 micron

Pressure resolution

0.3kPa

Maximum expansion capability

49% greater than the nominal pocket diameter (101mm).

Maximum working pressure

20MPa in normal use, 30MPa with some modifications

Suitable for:

Stiff clays, dense sands and rock of all kinds

Strengths

Pre-boring a hole means that core can be recovered, giving the possibility of carrying out laboratory tests on the same material as the pressuremeter tests. Provided the pocket stands open then a test is almost certain. Because it has a large expansion capability it is often used in transition materials where core recovery is likely to be poor.

Weakness

If the material is prone to collapse, and a pocket it lost, this can give rise to substantial gaps in the information obtained from a borehole.

Additional notes

This HPD has sometimes been fitted with a point and used as a push-in probe in very soft materials, typically alluvial clay.

The instrument also has a magnetic compass so that the orientation of the displacement axes can be known.

47mm Reaming Pressuremeter (RPM)

Insertion methods

Pre-bored hole and pushed

Initial Diameter

46mm

Allowable pocket diameter

46mm to 52mm

Length of material sacrificed

Only 0.6 metres of material is required to make a test.

Displacement system

Direct strain sensing at 3 points equally spaced around the centre of the expanding region

Displacement resolution

Better than 1 micron

Pressure resolution

0.1kPa

Maximum expansion capability

52% greater than the at rest diameter

Maximum working pressure

12MPa

Suitable for:

Medium to stiff clays, loose to dense sands and weathered or soft rock

Strengths

Extremely compact, portable and versatile.

Weakness

Due to the small diameter the displacement sensing system is slightly more affected by instrument compliance than the larger probes.

It can be difficult to make a hole for the probe at the required tolerance, as this is not a common size.

Although it can be pushed, in practice it will be difficult to do this in stiff material because of the high loads that will be required.

Additional notes

Because the probe is dimensionally similar to a Ménard pressuremeter it is often used to carry out this style of testing, with the advantage that the high resolution of displacement allows good quality unload/reload cycles to be incorporated.

The probe has also been used down a borehole formed by a 102cm cone penetrometer, with the cone profile used to identify suitable locations for the pressuremeter test.

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