What is a Pressuremeter?
A pressuremeter is a device which is used to determine the stress strain relations of in-situ soil by pumping it into the certain depth of borehole. It is designed by Menard of France so, it is also called as Menard pressuremeter. From this relations at rest horizontal earth pressure of soil, elastic modulus of soil etc., can be determined.
Pressuremeter test on soil is very quick and easy way of finding stress strain relationship of in-situ soil. The results obtained by this test are useful for the design of foundations.
Parts of Pressuremeter
Pressuremeter contains three parts namely:
- Control unit
Probe consists of three cells which one above the other as shown in figure. All the three cells are inflatable. The middle cell is the measuring cell or main cell which is filled with water during test. The other two cells which are at the top and bottom of measuring cell are known as guard cells which protects the main cell from the end effects caused by finite length of cable.
The whole arrangement of probe is generally protected by the metal shield as shown in picture. To pump the water and gas into measuring cell and guard cells a rigid hollow tube is arranged through the three cells. Control unit is set up near the borehole and is connected to tubing by hollow cables to control the pressure in the cells by pumping water and gas as well as to read the results of the test.
Procedure of Pressuremeter Test on Soil
The test procedure consists of three steps as follows:
- Drilling borehole
- Positioning of probe in the bore bole
- Conducting test
The Menard’s pressuremeter is not used to drill the borehole. To drill a borehole separate drilling equipment is used and preferably which causes least disturbance to the soil while drilling. The diameter of the borehole should be in between 1.03 times to 1.20 times the diameter of the probe.
1.03Dp < Dh < 1.20Dp
Different designations are there for the diameter of borehole with respect to diameter of probe and length of measuring cell (l0) and probe (L) which are tabulated below.
|Designation of borehole diameter||Diameter of probe (mm)||Length of measuring cell (l0) (mm)||Length of probe (l) (mm)||Nominal diameter of borehole (mm)||Maximum diameter of borehole (mm)|
Positioning of Probe in Borehole
After drilling the hole, the probe is lowered down to the required elevation using cables. The probe should be lowered slowly without disturbing the surrounding soil and the apparatus itself. After reached desired elevation, the probe is fixed using clamping device.
Conducting Pressuremeter Test on Soil
After the positioning of probe, now it’s time to fill the cells of probe with water and gas. This action is done by using control unit of the pressuremeter. The valves of the control unit are opened which admits water into the measuring cell and gas into the guard cells respectively. Equal pressure is maintained in both the measuring and guard cells.
Now, using measuring cell pressure is applied on the soil wall of borehole. The application of pressure can be done by any of the two methods as follows:
- Equivalent pressure increment method
- Equivalent volume increment method
Equivalent pressure increment method, in which certain amount of time (generally one minute) and pressure increment value is fixed for that time. After the completion of time, the volume change is noted.
Similarly, same pressure increments for the next one minute of time is applied and volume change is noted. This process is repeated until limited pressure is reached. In general, ten equal pressure increments for 10 minutes of time are enough to reach the pressure limit.
Equivalent volume increment method, in this method the probe volume is increased by 5% for each increment. After each increment the probe is held constant for 30 seconds. After every 30 seconds the pressure readings are noted.
Finally the reading will helps to obtain the stress strain curve of soil. This test can be done for every 1meter interval between the boreholes.
Results of Pressuremeter Test
The volume of water used for each increment of the probe volume is taken on x- axis and the pressure value obtained for each increment is taken on y-axis. The curve obtained may contain some errors. To overcome this, the pressuremeter should be calibrated for pressure loss, volume loss and hydrostatic pressure head before it is used in the design.