Obtaining pressuremeter parameters
D.1 Obtaining a corrected pressuremeter curve
Values of pressures and volumes read during the test shall be corrected for:
- hydraulic head ph;
- probe pressure loss pe;
- volume loss of the whole equipment during pressurization.
D.1.2 Probe hydraulic head correction
During a test at a given elevation zs,the pressure in the central cell shall be equal to the pressure regulator pressure plus the hydrostatic head, ph, between the elevation of the pressure measuring device and the centre of the pressuremeter probe (see Figure D.1).
ph = γI(zc – zs)
D.1.3 Probe pressure loss correction
This pressure correction involves the pressuremeter probe pressure loss pe as a function of Vr (see B.4.3 and Figure B.4). This experimental curve shall be modelled by one of the following mathematical functions, depending on the purpose of the test analysis. The methods are listed from the less elaborated (rough analysis) to the more elaborated one (research work):
- First method: linear interpolation between experimental points.
- Second method: power law type interpolation
pe(Vr) = b(Vr)m + c
- m is chosen between 0 and 1,
- b and c are obtained by the mean square regression method.
- Third method: double hyperbolic adjustment (see D.4.3.3).
As this pressure loss is a function of the type of membrane and cover, of the slotted tube if any, and of the injected liquid volume, the corrected pressure shall be:
p = pr(Vr) – pe(Vr)
|a) in a marine or river environment||b) in a land environment|
- 1 pressuremeter probe
- 2 pressuremeter borehole
- 3 casing
- 4 water table
- 5 ground surface
- 6 liquid pressure measuring device
- 7 control unit (CU)
- z elevation
D.1.4 Volume loss correction
For a given value of pressure pr, the volume Vr shall be corrected so as to take into account the volume losses of the probe, the lines and the measuring system:
V = Vr(pr) – apr
NOTE 1 The volume loss correction is not necessary in soft to medium stiff soils.
It is possible to use more elaborated methods than linear regression, such as hyperbolic model, or direct linear links between experimental points.
D.1.5 Corrected pressuremeter curve
The reduced values of volume and pressure, read at each pressure level for an elapsed time of 60 s, are obtained from the following equations:
p = pr + ph – pe(Vr)V = Vr – Ve(p)
D.2 Assessing the quality of the pressuremeter test
D.2.1 Analysis of a pressuremeter test
The corrected pressuremeter curve shall be analysed together with the corrected creep curve, considering — slopes mi of straight line segments between data points
— and ΔV60/30 Ménard creep values (see Figures 5 and D.2).
The corrected pressuremeter curve shall be analysed together with the corrected creep curve, considering mi slopes and ΔV60/30 Ménard creep values at each pressure hold (see Figures 5 and D.2). In a completed test, the sequence of readings can be divided into three successive groups:
— The first group consists of the sets of readings obtained during probe expansion up to the contact between the surface of the probe and the pocket wall; they usually exhibit high Ménard creep values.
— The second group in the lower pressure range includes readings which exhibit low mi slopes and low Ménard creep values. This group identifies the pseudo-elastic section of the curve.
— The third group in the higher pressure range exhibits increasingly higher slopes and higher Ménard creep values. This group identifies the plastic phase.
Ménard creep pressure pfM shall be found in the transition zone between the last two groups (see D.3). Ménard modulus EM shall be obtained from the second group of readings (see D.5). Ménard limit pressure pLM shall be obtained from the third group of readings (see D.4).
On the pressuremeter curve the region between the first and the second group is used to define the contact of the probe against the pocket wall.
D.2.2 Quality of the pressuremeter test
The magnitude of scatter of the test points and the shape of the pressuremeter curve shall give an indication of the test pocket quality.
If the test pocket wall is almost perfect and the test performed in ideal conditions, the first group shall be limited to the readings of the first pressure hold, indicating a high quality test.
At least two data points in the second group of readings and two data points in the third group shall be available to determine all three parameters pfM, pLM and EM.
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 EM and pfM 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) pLM and pfM cannot be obtained.
NOTE A pressuremeter curve that includes only the last two groups of readings can result from a test performed in swelling ground or in too small a pocket. Too large a pocket can give a pressuremeter curve which includes only the first two groups of readings.
- 1 initial evaluation
- 2 final check
- 3 double hyperbolic fitted curve
- 4 inverse volume straight line fitting the last three values
- 5 example of creep data points fitting
- a Corrected pressuremeter test data points fitted with double hyperbolic curve.
- b Pressuremeter creep data points (volume scale enlarged 10 times).
- c Corrected pressuremeter test data points on 1/V scale (volume reciprocal scale on the vertical axis, right hand side).
- d Points retained to obtain EM after final check for pLM and pfM.
- e The black point retained for pLM (D.4.2).
- f The 2 grey points initially limiting the pseudo-elastic range (D.5.1).
"i" stands for "initial".