Annex D

(normative)

Obtaining pressuremeter parameters

D.1 Obtaining a corrected pressuremeter curve

D.1.1 General

Values of pressures and volumes read during the test shall be corrected for:

• hydraulic head p_h;
• probe pressure loss p_e;
• volume loss of the whole equipment during pressurization.

D.1.2 Probe hydraulic head correction

During a test at a given elevation z_s,the pressure in the central cell shall be equal to the pressure regulator pressure plus the hydrostatic head, p_h, between the elevation of the pressure measuring device and the centre of the pressuremeter probe (see Figure D.1).

p_h = γ_I(z_c – z_s)

D.1.3 Probe pressure loss correction

This pressure correction involves the pressuremeter probe pressure loss p_e as a function of V_r (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

p_e(V_r) = b(V_r)^m + c

where

• 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 = p_r(V_r) – p_e(V_r)

a) in a marine or river environment	b) in a land environment

Key

• 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

The volume loss correction involving the experimental pressuremeter probe volume loss curve obtained in B.4.2.2 shall be applied using the a factor obtained by linear regression (see B.4.2.1).

For a given value of pressure p_r, the volume V_r shall be corrected so as to take into account the volume losses of the probe, the lines and the measuring system:

V = V_r(p_r) – ap_r

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 = p_r + p_h – p_e(V_r)V = V_r – V_e(p)

The pressuremeter curve shall be plotted with pressures on the horizontal axis and volumes on the vertical axis (EN 1997-2:2007, 4.4.3(5), Table 4.1).

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 m_i of straight line segments between data points

— and ΔV_60/30 Ménard creep values (see Figures 5 and D.2).

The corrected pressuremeter curve shall be analysed together with the corrected creep curve, considering m_i slopes and ΔV_60/30 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 m_i 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 p_fM shall be found in the transition zone between the last two groups (see D.3). Ménard modulus E_M shall be obtained from the second group of readings (see D.5). Ménard limit pressure p_LM 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 p_fM, 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_fM 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_fM 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.

Key

• 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 E_M after final check for p_LM and p_fM.
• ^e The black point retained for p_LM (D.4.2).
• ^f The 2 grey points initially limiting the pseudo-elastic range (D.5.1).

"i" stands for "initial".