4.1 General description
The principle of the Ménard pressuremeter test is shown in Figure 1.
- 1 ground surface
- 2 ground
- 3 pocket
- 4 expanding pressuremeter probe p applied pressure
- A-A axial section
- B-B cross section
The pressuremeter as shown schematically in Figure 2 shall include:
- tri-cell probe;
- string of rods to handle the probe;
- control unit (CU);
- lines connecting the control unit to the probe.
The control unit (CU) 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;
- device which permits the direct reading and, in the case of procedure B, the automatic recording of the parameters to be measured: time, pressure and volume.
The pressure measuring devices for the liquid in the central measuring cell and for the gas in the guard cells shall be located either
- above the ground surface, or
- inside the probe, less than 1 m above the centre of the central measuring cell.
In the first case, the CU shall be provided with means to check the stabilized pressure value at the probe. Some means of measuring the depth of the test with appropriate accuracy shall be provided.
4.2 Pressuremeter probe
Two types of probe shall be used according to ground type and condition:
- probe with a flexible cover;
- probe with a flexible cover and either an additional more rigid protection or a slotted steel tube.
These probes are described in Figure 3 a) and Figure 3 b), respectively, and their geometrical features are given in Table A.1.
When the probe is driven or pushed into the ground (see C.3), it shall be fitted with the more rigid protection or a slotted tube together with a extension tube completed by either a point or a cutting shoe.
NOTE If no slotted tube is involved, the probe body must be designed to withstand driving or pushing.
The probe shall be capable of a volumetric expansion of at least 700 cm3 (550 cm3 for a probe with a short central measuring cell within a slotted tube).
|1 control unit (CU):||5 pressuremeter probe:|
|1a pressurization, differential pressurization and injection devices||5a upper guard cell|
|1b pressure and volume measuring devices||5b central measuring cell|
|1c acquisition, storage and printing out of the data (required for procedure B)||5c lower guard cell|
|2 connecting lines:||6 ground|
|2a line for liquid injection||7 pressuremeter test pocket|
|2b line for gas injection||8 probe body, hollow|
|3 depth measurement system||9 probe rod coupling|
4.2.1 Probe with flexible cover
The probe shall be made up of three cylindrical cells of circular cross-section along the same axis (see Figure A.1). During a test these cells shall expand simultaneously against the pocket wall. The probe includes:
- one central measuring cell, with an outside diameter dc and a length lc (lcl for a "long probe" or lcs for short probe — see Table A.1), which shall expand radially in a pocket and shall apply a uniform stress to the pocket wall. This cell shall be inflated by injecting a liquid which is assumed to be incompressible;
- two guard cells with an outside diameter dg and a length lg (lgl or lgs) located above and below the central measuring cell. These cells shall be designed to apply to the pocket wall a stress close to, but not greater than, the stress induced by the central measuring cell. These cells shall be inflated by gas pressure.
The probe shall consist of a hollow steel core with passages to inject the proper fluids to inflate the cells. The probe shall be fitted with a central measuring cell membrane and a flexible cover sleeve. The steel core, on its outside curved surface, shall usually bear a network of grooves which uniformly distribute the liquid in the central measuring cell under the membrane. To this core shall be fixed the membrane and the flexible cover. The top of the core shall be threaded and coupled to the string of rods handling the probe from ground level; the central measuring cell membrane shall isolate the fluid in the central measuring cell from the gas of the guard cells. The flexible cover which overlies the central measuring cell membrane shall give form to the guard cells. A flexible protection made of thin steel strips usually 17 mm wide either overlapping (up to half-way) or isolated, running between fixing rings (see Figure A.1) may be added over the cover. Fluid lines shall connect the probe cells to the pressure and volume control unit (CU). The drain tap of the measuring cell shall protrude from the bottom of the steel core.
NOTE The flexible protection may be added to reduce damage to the cover from sharp fragments protruding from the pocket wall.
4.2.2 Probe with slotted tube
This probe shall consist of two parts:
- an inner part which shall be an assembly of three cylindrical cells of circular cross-section along the same axis; and
- an outer part which shall be made of a slotted steel tube (see Figure A.1). When this slotted tube is pushed or driven into the soil it shall be fitted with an extension pipe ending with a point or a cutting shoe.
The inner part includes:
- one central measuring cell, with an outside diameter dc and a length lc (lcl for a "long probe" or lcs for short probe — see Table A.1), which shall expand radially in the slotted tube and shall apply a uniform stress to the tube wall. This cell shall be inflated by injecting a liquid which is assumed to be incompressible;
- two guard cells with an outside diameter dg and a length lg (lgl or lgs), located above and below the central measuring cell. These cells shall be designed to apply to the slotted tube wall a stress close to, but not greater than, the stress induced by the central measuring cell. These cells shall be inflated by gas pressure.
During a test these cells shall act simultaneously on the inside wall of the slotted tube, which shall transfer the stresses to the pocket wall.
The outside steel tube shall carry at least six axial or helical slots evenly distributed round the circumference (Figure 3 b).The tube slotted length lm is measured along the tube axis. This length shall be the greater of:
1,3 (lc + 2lg) or 800 mm
Before and after expansion, the opening of each slot of the tube shall be less than or equal to 0,4 mm. After expansion the slotted tube and the slots shall be able to recover their original shape and size.
The assembly within the slotted tube shall be located by flexible spacers so as to allow the probe to expand radially with a minimum of resistance.
|a) pressuremeter probe with flexible cover||b) pressuremeter probe with slotted tube|
|1 hollow probe body||6 measuring cell drain outlet|
|2 measuring cell membrane||7 slotted tube|
|3 external sleeve or flexible cover||8 rods|
|4 liquid inlet to the measuring cell||9 probe-rod coupling|
|5 gas inlet to the guard Dimensions are given in Annex A|
4.3 Pressure and volume control unit (CU)
The control unit (CU) shall be built around a cylindrical volumeter fitted with a pressurizing device and a set of measuring devices. The CU shall control the probe cell expansion and permit the simultaneous reading of liquid and gas pressures and injected liquid volume as a function of time.
The pressurizing device shall allow:
- reaching the pressuremeter limit pressure or a pressure pr at least equal to 5 MPa;
- 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 CU;
- controlling the pressure difference between the measuring cell and the guard cells;
- injecting a volume of liquid in the measuring cell larger than 700 cm3.
Further, in the control unit a valve between the volumeter and the pressure measuring device shall allow stopping the injection.
4.4 Connecting lines
The flexible lines shall connect the pressure and volume control unit (CU) to the probe. They shall convey the liquid to the measuring cell and the gas to the guard cells. They may be parallel or coaxial. When the lines are coaxial the central line shall convey the liquid and the outer line the gas.
4.5 Injected liquid
The liquid injected into the measuring cell is either water or a liquid of similar viscosity and shall not freeze under the conditions of use.
4.6 Measurement and control
The accuracy of the device used to measure time shall be in accordance with Annex E.
4.6.2 Pressure and volume
The resolution of measurement of the devices measuring pressure and volume shall be in accordance with Annex E.
4.6.3 Display of readings
At the site the pressure and volume control unit (CU) shall give a simultaneous and instantaneous display of the following readings: time, pressure of the liquid injected into the measuring cell, volume of the liquid injected and pressure of the gas in the guard cell circuit.
4.6.4 Volume loss calibration cylinder
The main features of this steel cylinder (Figure B.1) shall be as follows:
- measured inside diameter di not more than 66 mm;
- wall thickness e not less than 8 mm;
- length lp more than 1 m or the slot length lm, whichever is greater.
4.7 Data logger
The data logger, the device to acquire and record the data under procedure B, shall be fitted with
- an internal clock,
- a printer, and
- a memory device readable by a computer.
The data logger shall be designed to record the raw data from the transducers, the zeros, calibration coefficients and identification of each and the resulting calibrated data of pressure and volume.
The data logger shall not interfere with the conduct of a test as specified in 5.7 and it shall not obscure any other measuring devices. It shall be designed so as to automatically:
- record its own identification parameters: date, hour, minute, second, CU number, data logger number, memory device number;
- require the input of the information necessary to identify the test, as described in 5.4;
- prevent the input of pressure and volume data or other information not obtained during the testing process. The data logger shall include an alarm device or a special display for the following events:
- no memory device in place;
- no test identification parameters recorded according to 5.4;
- no electric power.