B.4 Reading corrections
The stiffness of the membrane and cover assembly decreases during their first expansions and this decrease shall be minimized by some preliminary exercising as described in B.4.1.
The operations described in B.4.2 and B.4.3 shall then be carried out as follows:
- at each change of pressuremeter probe configuration;
- at each change of lines between the probe and the pressurizing and read-out unit;
- at intervals appropriate to the use the probe has received, e.g. weekly for daily operation.
These operations shall be performed when the probe is ready to be inserted in the pressuremeter pocket, that is when the correct tube lines are fitted and gas bubbles have been purged from the central measuring cell and the liquid circuit.
B.4.1 Probe pre-inflation and zeroing of the volume measuring device
Before use, any probe shall first be inflated at least three times in the open air by injecting 700 cm3 of liquid into the central measuring cell (550 cm3 in a short probe fitted with a slotted tube).
- the volume measuring device shall be zeroed by adjusting the volume of liquid while keeping the centre of the measuring cell at the level of the pressure measuring device;
- the acquisition and recording device, i.e. the data logger, shall be initialized (procedure B);
- the initial reading of each transducer shall be checked and, if appropriate, recorded (procedures A and B).
B.4.2 Equipment volume loss calibration test
The probe, either simply clad by its rubber cover or fitted with its slotted tube, shall be placed into the pressure loss calibration cylinder as described in 4.6.4 and Figure B.1. The probe shall be pressurized by increments Δp initially of 100 kPa until the probe cover or the slotted tube comes into contact with the calibration cylinder. After this point, equal increments shall be applied up to the maximum pressure rating of the probe. During the second part of this test each pressure level shall be applied within 20 s and held for 60 s.
The pressure in the guard cells shall fulfil the conditions given in B.4.4 below.
B.4.2.1 Obtaining the volume loss correction for the equipment
The injected volume at the end of each pressure hold shall be recorded and used to plot a graph of
Vr = f(pr),
resulting in the volume loss correction curve.
The volume loss factors, referred to in D.1.4, shall be the slope of the straight line which is the best fit for the part of this graph that appears after the probe comes into contact with the calibration cylinder (see Figure B.2):
Vr = Vp + apr
- Vp is the intercept on the volume axis of the straight line best fitting the data points.
The value of a shall be less than 6 cm3/MPa (when the pressuremeter is fitted with not more than 50 m long lines).
Higher a values suggest inadequate liquid filling, a leak in the liquid circuit or other problem. The whole equipment, including control unit, lines and probe, shall be checked again.
- 1 calibration cylinder
- 2 pressuremeter probe
B.4.2.2 Obtaining central measuring cell volume Vc
The initial external volume of the central measuring cell shall be obtained from the following equation: Vc = 0,25 π lcdi2 – Vp
- Vp is the intercept on the volume axis of the straight line best fitting obtained in B.4.2.1;
- lc is the length of the central measuring cell measured when the cell membrane is fixed on the probe steel core, as shown in Figure B.1 and in Table A.1;
- when the probe is fitted with a slotted tube, lc is equal to lcs for a short probe or to lcl for a long probe;
- di is the inside diameter of the calibration cylinder. This value shall be recorded on the pressuremeter test report.
- Vr Injected liquid volume at the end of each pressure hold
- pr Pressure in the measuring cell
- Vp Intercept of the straight line Vr = Vp + apr
B.4.3 Probe pressure loss calibration test
The probe shall be placed close to the pressure measuring device, as shown on Figure B.3, and in the open air. The probe shall be inflated as if it were in the ground, with pressure increments Δp equal to 1/5 of the expected pressure loss of the probe pel. Each pressure increment shall be held for 60 s. A volume of at least 700 cm3 shall be injected in the central measuring cell (550 cm3 for the short probe fitted with a slotted tube).
NOTE The pressure loss pel of the probe is a function of the type of membrane, cover and slotted tube, if any, which are used. It is essentially adapted to the type of ground to be tested. It can vary between 0,05 MPa and 0,2 MPa.
The resulting pressure versus volume curve, V60 = f(pe) is illustrated in Figure B.4. The value zc – zs must be minimized so as to neglect any correction on the pressure readings (see D.1.1 and Figure D.1)
The pressure values obtained from this curve for each pressure hold shall be used for the pressure loss correction (see D.1.3).
The ultimate probe pressure loss pel (Figure B.4) shall be the pressure reading for an injected volume of liquid equal to 700 cm3 (or 550 cm3 for the short probe in a slotted tube).
- 1 pressuremeter probe
- 2 pressure measuring device
- 3 pressure regulator
- 4 ground surface z elevation
For zc, zs and zN, refer to Figures B.5 and D.1.1.
B.4.4 Estimation of gas pressure in guard cells for a given test
The gas pressure in the guard cells shall not be higher than the pressure in the central measuring cell. The value of the guard cell pressure shall be determined before each test and fixed at the first pressure hold.
During the application of the pressure pc in the central measuring cell, the gas pressure pk. in the guard cells shall be regulated according to the following rules (see Table 1 in 3.2, Figures B.5 and D.1 for the meaning of symbols).
For the G type probe, where the guard cells are created by the overall cover, the gas pressure pk in the guard cells shall be lower than the pressure in the central measuring cell, but high enough to maintain the pressuremeter probe cover in a cylindrical shape:
pc – 3pm ≤ pk ≤ pc – 2pm
pr + (ph – 3pm) ≤ pk ≤ pr + (ph – 2pm)
and pk = 0 as long as pr + (ph – 2pm) = 0
- pc is the liquid pressure in the central measuring cell: pc = pr + ph;
- pm is the central measuring cell membrane pressure loss;
- pk is the gas pressure in the guard cells. Since the unit weight of the gas changes with the gas pressure:
pk = pg [1 + λg(zcg – zp)]
Since values of zc and zcg will normally be positive, value of zp shall be negative and (zcg – zp) is the sum of absolute values |zcg| plus |zp|.
- pr is the liquid pressure reading at the CU at the elevation zc;
- ph is the head in the liquid line between the liquid pressure transducer and the central measuring cell, ph = γI(zc – zs) as explained in D.1.1;
- pg is the pressure reading at the CU of the gas in the guard cells. The measuring device in the CU elevation is zcg and the probe elevation is zs;
- λg is the rate of change of pressure head of gas at pressure pk per metre depth. When no data on X is known, it is recommended to use: λg = 1,15 × 10-4 per metre, or λg = 1,15 × 10-4 m-1
NOTE 1 For practical purposes, the gas unit weight change can be ignored when the elevation difference between the CU and the probe is less than 30 m and the gas pressure less than 5 MPa. If this is the case then pk = pg.
NOTE 2 For most purposes, the assumption zcg = zc is valid.
- 1 ground surface
- 2 CU
- 3 pressuremeter probe
- 4 gas line
- 5 liquid line
zc and zcg will be positive and zp will be negative.