Approach for Pressuremeter
Principle
The principle of the test is to carry out an in-situ loading test, using an expandable cylindrical probe which is placed within the ground in a borehole. This probe, made up of three cells, is inflated with water and compressed air, thus exerting strictly uniform pressures on the wall of the borehole.
The strain of this wall are therefore accompanied by an increase in the volume of the probe which is then read, for each of the pressures, as a function of time. Thus, the pressureometer allows to obtain a relationship between stress (applied pressure) and deformation (variation in the volume of water in the probe, directly linked to the strain of the soil).
Please note that unlike others in-situ test such as the CPT test, the pressuremeter test give a « ponctual information », determinated by the measurement step chosen by the experimentator. This measurement step is itself limited by the size of the probe, and it can be, generally, at least one meter or more.
The purpose and main interest of the pressurmeter test is to mesure, trough the same test quantitative parameters of strength and deformation of the ground tested.
These parameters, calculated after the results of the test, are the following :
- _p_LM : Ménard pressuremeter limit pressure ;
- _p_f : p pressuremeter creep pressure ;
- _E_M : Ménard pressuremeter modulus.
These parameters could be used to do geotechnical calculations :
- Directly, through semi-empirical methods, example : fondations claculations
- Indirectly, trough the determination of others geotechnical parameters, for example, the Young Modulus of the soil used in numerical approach, which can be obtained from the Menard pressuremeter modulus
Indirectly, it is possible to link these parameters to a lithology, in particular the couple _p_LM _/ E_M, which could be used in the Pressiorama® (Baud, 2005), NF-P 94-262 annex B.
Figure: _Pressiorama® diagram (Baud, 2005)Extracted from NF-P 94-262 annex B
Please note that the diagram is defined for a range of _p_LM up to 10 MPa, wich corresponds to the actual application domain of the pressuremeter :
- _p_LM < 5 MPa : Range of values mesurable by a « classic probe », corresponding to the values encountered in most soil type, to soft and weathered rocks.
- 5 MPa <_p_LM < 8 MPa : Range of values mesurable by a « specific probe », corresponding to the values encountered in most soft and weathered rocks.
- _p_LM > 8 MPa : This range of values couldn't be mesaured with the actual equipment, and correspond to a roccky domain
In conclusion, please keep in mind that the pressuremeter has a relatively wide range of application, from all types of soils to soft and weathered rocks. However, it could not be properly use for the characterisation of solid rocks.
Equipement
The following is mostly extracted from the standard NF EN ISO 22476-4, which is about the Ménard procedure of the pressurmeter test (the « classical » procedure). Please notice the existence of some variants of this procedure, depending principally of the way to instal the probe whithin the ground. These others spécific procedures are exposed on the norms listed at the end of this page.
- Général description
The pressuremeter shown schematically in §2.2 shall include:
- the pressuremeter probe;
- the string of rods to handle the probe;
- the control unit (CU);
- The connecting lines between the control unit and the probe.
Some means of measuring the depth of the test with appropriate measurement error shall be provided.
Figure : Photography of a pressuremeter set (Extracted from https://www.apageo.com.
- Probe description
If expansion is followed by the volume of the measuring cell, the measuring cell shall be inflated by injecting a liquid of low compressibility. Alternatively air can be used to inflate the measuring cell and the expansion followed by displacement transducers.
Figures : Diagram of a Ménard pressuremeter (Extracted from NF EN ISO 22476-4).
| Key : | | | --- | --- | | 1a : pressurization, differential pressurization and injection devices :
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1b pressure and volume measuring devices
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1c acquisition, storage and printing out of the data (required for CU type B and C) 2 : connecting lines:
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2a : line for liquid injection
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2b : line for gas injection 3 : depth measurement system 2b line for gas injection 4 : rods | 5 : pressuremeter probe
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5a : upper guard cell
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5b : central measuring cell
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5c : lower guard cell 6 : ground 7 : pressuremeter test pocket 8 : hollow probe body 9 : probe rod coupling 10 : transducers |
- Pressure and volume control unit
The control unit shall include:
- equipment to pressurize, and so to inflate the probe, and to maintain constant pressures as required during the test;
- equipment to maintain an appropriate pressure difference between the central measuring cell and the guard cells, if relevant;
- device which permits, according to the type defined in Table 1, the reading and recording of the parameters to be measured: time, pressure and volume.
Table : Types of pressuremeter control units (Extracted from NF EN ISO 22476-4).
The control unit shall control the probe cell expansion and permit the simultaneous reading of liquid and/or gas pressures and injected liquid volume or radius of the measuring cell as a function of time.
The pressurizing device shall allow:
- reaching the pressuremeter limit pressure or a pressure p r at least equal to the maximum pressure defined for the test;
- holding constant each loading pressure level in the measuring cell and in the guard cells during the set time;
- implementing a pressure increment of 0,5 MPa in less than 20 s as measured on the control unit;
- controlling the pressure difference between the measuring cell and the guard cells;
- injecting a volume of liquid in the measuring cell larger than at least its volume at rest V c, i.e. 700 cm 3 for a 60 mm pressuremeter probe.
If volumetric measurement is used, a valve between the volumeter and the pressure measuring device shall allow stopping the injection.
- String of rod/connecting lines
Not described here, to have information regarding this subject, please consult the norm.
Test procedure
The test procedure will not be detailled her, to consult the exact procedure, please consult the associate norm.
However, the principle is to establish a loading programm in at least 8 to 10 steps, in wich the volume of the probe increases, leading to a raising of the radial stess imposed to the ground. In each step, a constat stess is applied to the ground, during a certain time increment.
A template of loading program is exposed in the following figure, extracted from the norm.
Results and interpretation
- General
The application of the loading programme results in a raw pressuremeter curve. Some correction should be applied to this raw curve in order to avoid experimental biaises. The corrections procedure are well detailled in the norm.
After this step, the corrected pressuremeter curve is used to calculate the pressurementer paramters described above. A typical pressurmeter curve is presented in the figure below.
At least three data points in the second group of readings and three data points in the third group shall be available to determine all three parameters p f, p LM and E M.
If in a test, one group of readings is incomplete or missing, the following effects on the determination of the three parameters shall be considered:
- when the pressuremeter curve includes only the second and third groups of readings and with fewer than two data points in the second group, values of E M and p f cannot be obtained;
- when the pressuremeter curve includes only the first and second groups of readings (i.e. only one or no points in the third group), p LM and p f cannot be obtained.
In the folowing, the methods of determination of the parameters will be synthetised to give a brief overview of the process, and may not be exhaustive. Please consult the norm to have the complete procedure of determination
- Pressuremeter creep pressure p** f**
If there are at least two sets of readings both in the second and in the third group, the creep pressure pf shall be estimated, using the following graphical analysis of the (p, V 60/30) diagram: 2 straight lines shall be drawn on the (p, V 60/30) graph, one involving the data points in the second group, the second one involving the data points in the third group, as illustrated on Figure D.4; the abscissa of the intersection of the 2 straight lines give p f.
Figure: Pressuremeter creep pressure determination (Extracted from NF EN ISO 22476-4)
- Ménard pressuremeter limit pressure p** LM**
The Ménard pressuremeter limit pressure is conventionally defined as the pressure leading to the doubling of the initial volume of the pocket.
It can be either obtained by direct measurement or determined using extrapolation methods.
Figure : Ménard pressuremeter limit pressure, with reciprocal fitting and extrapolation method (Extracted from NF EN ISO 22476-4)
- Ménard pressuremeter modulus E** M**
The Ménard pressuremeter modulus is defined as the modulus of the pseudo-elastic part of the curve. The determination of this parameter should be done with great precaution and following the recommandation of the norm, that couldnot be detailled here.
Figure Plot of a corrected pressuremeter curve, creep curve and slopes (Extracted from NF EN ISO 22476-4)
Please note that the The Ménard pressuremeter modulus is not a Young Modulus of the tested soil. This Young Modulus could be calcaulated using the rhéologic coefficent α, defined by Ménard, and depending on the soil type.
In addition, please note that the young modulus of soils depend on the strain level : it should be corrected to be properly use in geotechnical calculation.
Figure : Variation of the deformation modulus in function of the strain level (Reiffsteck 2002)
Figure : Degradation laws E/EM = f(ε) (ARSCOP, 2017)
Normalization / Variants
There is differents variants of this kind of geotechnical test, depending mostly on the probe type and on the method to insert this probe within the groude. These variats are described in the folowing norms :
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NF EN ISO 22476-4, septembre 2021, Index : P 94-521-4, P 94-521-4 : Geotechnical investigation and testing - Field testing - Part 4 : prebored pressuremeter test by Ménard procedure
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NF EN ISO 22476-5, French Standard approved and published by AFNOR, 2023-04-12 : Geotechnical investigation and testing - Field testing - Part 5: prebored pressuremeter test
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NF EN ISO 22476-6, octobre 2019, Index P 94-521-6 , ICS : 93.020 : Geotechnical investigation and testing - Field testing - Part 6 : self boring pressuremeter test**
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NF EN ISO 22476-8, janvier 2018, Index : P 94-521-8, ICS : 93.020 : Geotechnical investigation and testing - Field testing - Part 8 : full displacement pressuremeter test
The PressumeterTest instance has to be declared as a Sensor with a SensorType refering to its nature, eg. https://data.geoscience.fr/ncl/Proc/94
For each PressuremeterTest performed, one DataStream shall be declared per ObservedProperty.
BhCollarThing and BhTrajectory shall be used to respectively describe the Borehole and its trajectory
BhSampling enable to declare each depth at which a test has been performed. For example, if a test has been performed from the Depth 1m to the Depth 10m every 1m, then there shall be BhSamplings for the Depths = 1m, 2m, ..., 9m, 10m.
One BhFeatureOfInterest shall then be declared per Sampling.
As a PressuremeterTest is an in-situ test with ponctual measures (at depth), the BhFeatureTypes that shall be declared are : Hole & Point
In this case, there shall be one Observation and Result per combination of DataStream / FeatureOfInterest. For example, if there are 3 DataStreams (the different ObservedProperties) and 10 FeaturesOfInterests (the different depth), then there shall be 30 Observations declared.
The Pressuremeter test imply several measurements of the same parameter at the same depth. Each measurement being associated to a step. In this case, there shall be for each FeatureOfInterest (the different depth) as much Observations as steps. For example, if the test imply 10 steps at one depth, then there shall be 10 observations associated to that depth. The number of the step shall be declared as a Parameter of the Observation.