1

In situ shear resistance of soils and particular materials measured by the

Phicometer
Moulay I. Zerhouni1 and Rabah Arab2
1

Expert Engineer, FONDASOL, Service Conception Gotechnique, Argenteuil, France.

2
Technical Manager, AFITEX, Champhol, France.

ABSTRACT: Since the development, in 1986, of the Phicometer in situ shear test (French standard XP
P94-120), more than a few thousands tests have been carried out. Various types of natural or artificial soils
and materials have been thus tested. After a brief description of the equipment and the testing procedure, the
paper presents results of in situ shearing tests carried out in particular grounds and materials, which are
difficult to sample and test in laboratory. Among these materials and grounds, one quotes : coarse grounds,
fine sands which can hardly be sampled, treated soils with lime and/or cement and also compacted municipal
solid waste (MSW), as well as clinker and ash resulting from MSW incineration. The carried out experiments
demonstrate the feasibility of a direct measurement of the in situ direct shear strength. The shear strength
characteristics (c and ) obtained from the correlation rules established for soils from previous research
appear, in first approach, satisfactory. However, for a better validation of this, they need to be compared to
those which correspond approximately to the observed behaviour of the concerned works and structures and
established for example by means of back-analysis calculation or simulation.
1

INTRODUCTION

The shear resistance of soils and materials,

characterised by the friction angle and the cohesion,
represents an important parameter for the
geotechnical engineer while studying the stability of
construction works and structures in relation with
soils and materials.
Usually, this resistance is measured in the
laboratory using triaxial tests or direct shear tests
carried out on field samples. Of course, this measure
makes sense only if sampling, conservation and
preparation make it possible to consider the samples
as non remolded and thus the test specimens as
sufficiently representative.
Many categories of grounds and materials do not
allow to meet these conditions. Among those, it is
possible to quote granular medium to coarse sands,
friable soils (tender chalks, sandstones), coarse and
heterogeneous grounds and other various materials
like fills, Municipal Solid Waste, slag heap, clinkers
of incineration, etc.
In the event of such soils and materials,
Phicometer in situ shear tests allow a very useful
measurement of the limit resistance to shearing and
the determination of the usual parameters c and .

2 PHICOMETER TEST PRINCIPLE

2.1 Description of the apparatus
The Phicometer device is entirely described in the
AFNOR standard XP P94-120. It includes three
principal bodies (figure 1).
- The Phicometer probe (A), which consists in a
hollow cylindrical slotted tube includes a radially
expandable cell. A series of annular teeth are
provided in the central part of the probe to form the
cylindrical surface of shearing. The initial diameter
at rest of the probe (deflated state) is 58 mm. The
surface of shearing during the test varies but
remains close to 500 cm2.
- Link and connection devices (B) of the
Phicometer probe to the control and measurement
equipment located at the ground surface, such as
rods and tubes for injecting cell fluid.
- The control and measurement equipment (C)
located at the ground surface which includes a
pressurevolume controller to measure the volume
of the inflatable cell and control the pressure applied
to the ground through the cylindrical surface of
shearing of the phicometer probe, two bearing plates
and a hollow jack for exerting the pull out shearing
effort and a force measuring device. The pull out

shearing speed is maintained constant to 2mm/min.

2.3 Data reduction Calculation of the limit shear

stress
After having determined, by means of a calibration,
the relation between the measured cell volume and
the external diameter of the shear surface of the
probe and the relation giving the probe pressure loss,
the data reduction of the measurements is carried
out as described hereafter, following the parameters
defined in figure 2.

1 Pressure-volume controller

4 Force measurement

2 Bearing plates

5 Blocking device

3 Hollow jack

6 shear speed control

Figure 1. Diagram of the Phicometer apparatus.

2.2 Phicometer test procedure

The Phicometer shear test consists in introducing at
a defined depth, in a previously made borehole, the
Phicometer probe, to inflate this probe to make
penetrate the annular teeth into the material at the
wall of the borehole, then to shear the material by
slowly pulling out the probe from the ground
surface. During a pull out shearing step the pressure
in the probe is maintained constant. Several shearing
steps are carried out under successive increasing
stages of pressure holds, thus giving the relation
between the applied normal pressure and the
corresponding shearing resistance of the tested
material.
The AFNOR standard XP P94-120, details the
test procedure and the loading steps program to
achieve regarding the strength and type of material.

Figure 2. Phicometer test principle and parameters.

Under a pressure hold , the area of the shearing

surface is given by :
S = .d / l
Where, d is the external diameter depending on
the cell volume and l is the length of the shear
surface. The corresponding limit shear resistance t is
given by :
=T/S
Where T is the maximum measured pull out
force.
Thus, couples of values (i , i) are determined
for
3

each load step. A linear adjustment is then made on

the significant couples of values to determine the
shearing parameters c and corresponding to the
cohesion and friction angle of the coulomb
envelope. These in situ shear parameters are called
cohesion ci and friction angle i. They are short
term type parameters from which it is possible to
derive the usual design shear parameters
(Philipponnat and Zerhouni, 1993).

In the same manner, another test, carried out at

14m depth (figure 6), has given quite different
values, showing another behaviour of the MSW in
the tested zone. The corresponding in situ cohesion
and friction angle are 26 kPa and 18.
In this case, the behaviour can be considered as
similar to a clayey soil.
ESSAI AU PHICOMETRE
Effectu selon la norme XP P 94-120

3 TEST RESULTS IN PARTICULAR

MATERIALS

400

3.1 Test Results in a Municipal Solid Waste

300

More and more, there is a need to characterise the

geomechanical behaviour of Municipal State Waste
in landfills. The knowledge of the shear resistance is
necessary to study stability in landfill design and
management. The MSW materials are very
heterogeneous. Their composition can vary from
one area to another and with time. Their
granulometry includes either bulky and large size
elements and small and very fine elements as well,
which can influence their shear strength.
Figure 3 shows the results obtained from a
Phicometer test in a MSW landfill in Belgium
carried out at 18m depth.
This waste has been compacted following the
landfill management rules.
ESSAI AU PHICOMETRE

SONDAGE Prof. en (m) : 18

S1

Effectu selon la norme XP P 94-120

600

(kPa)

500

400

300

SONDAGE Prof. en (m) : 14

S3

(kPa)

200

100

(kPa)
0
0

100

i = 18

200

300

400

500

c i = 26 kPa

Figure 4. Phicometer shear test results obtained at 14m depth

in a compacted municipal solid waste landfill.

These two tests show a different behaviour for a

same material and an important difference in their
shear characteristics and therefore, the need for
being careful in choosing the corresponding
characteristics and design values of shear strength,
especially in the case where a few number of tests
are available.
The description of such a dispersion is shown in
the diagram of Figure 5, where the superposition of
the measured couples of all the in situ shear tests
carried out in the same MSW landfill are
represented.

200

(kPa)

100

0
0

100

200

i = 33

300

400

500

600

700

800

900

c i = 14 kPa

Figure 3. Phicometer shear test results obtained at 18m depth

in a compacted municipal solid waste landfill.

Except the two first points in the diagram, the

other experimental measured points follow the same
linear adjustment giving thus an in situ cohesion of
14 kPa and an in situ friction angle of 33.
These values correspond to a high shear strength
material, mainly like a granular soil behaviour.

Figure 5. Superposition of all the measured shear resistance in

a same MSW landfill, at different depths

The interpretation of this diagram leads to

possible design values for this compacted Municipal
Solid Waste ranging from 17 to 34 for the friction
angle and from 30 to 50 kPa for cohesion.
3.2 Test Results in a MSW incineration clinker
The Incineration of Municipal Waste generates ash
and clinker. Depending on the corresponding
regulations, some categories of MSW incineration
clinker can be reused in some construction works,
like filling under certain conditions.
The knowledge of their geomechanical properties
becomes then interesting.
Figure 6 shows a typical result of a Phicometer
shear test carried out in a heap of incineration ash, at
1.4m depth.
This ash/clinker material was approximately one
year old, corresponding to a certain degree of
maturation.
ESSAI AU PHICOMETRE
Effectu selon la norme XP P 94-120

400

4 CONCLUSION
Through a selection of some Phicometer in situ
shear test results, it is possible to characterise the
shear strength of grounds and particular materials,
usually difficult to test in laboratory, like coarse
soils, heterogeneous municipal solid waste, ash and
clinker resulting from MSW incineration and many
other materials in which intact sampling is difficult
to achieve.
The Phicometer in situ measured characteristics are
short term type characteristics giving thus a direct
measurement of typically undrained properties.
Depending on the nature and the permeability of the
materials, the effective characteristics can be
estimated through correlations. These correlations
have been established for soils from comparisons
with test results of shearing in laboratory. It seems
appropriate however, before generalising these
correlations to particular non-soil materials, to
confirm these estimates by the observation and
analysis of the real behavior of the works concerned.

SONDAGE Prof. en (m) : 1.4

PH1

(kPa)

REFERENCES

300

200

100

(kPa)
0
0

100

= 42

200

300

400

500

c i = 0 kPa

Figure 6. Phicometer shear test results obtained at 1.4m depth

in a one year old MSW incineration clinker.

Most of the experimental points lie on a same

linear adjustment giving an in situ friction angle of
42 and no cohesion, showing a behaviour similar to
a coarse granular material one.

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