PROBEX Borehole Dilatometer

The PROBEX Borehole dilatometer (rock pressure) is a radially expandable borehole probe mainly used to determine in situ the modulus of deformation of soft and hard rocks.


The dilatometer test is a loading test run inside a borehole with a radially expandable cylindrical probe. It is used to determine in situ the deformability of rock and the creep properties of very soft rock.

The PROBEX determines the deformations by measuring the total volume change of the probe. This method is the same well-proven method used with the pressuremeter. It provides a mean modulus value of a large volume of rock, contrary to the use of callipered probes which can be affected by local heterogeneities.The volume changes of the probe are measured by monitoring the displacement of a piston. This configuration eliminates the parasitic expansion of the tubing and pumping system.

The PROBEX consists of:

  • An inflatable membrane mounted on a steel core
  • A hydraulic module comprising a dual piston and cylinder assembly, to inflate and deflate the membrane
  • A measuring module containing a linear transducer, which monitors the injected volume
  • The hydraulic and electrical lead lines
  • A hydraulic hand pump and pressure gauge
  • A digital readout
  • A pressure transducer

Test & interpretation:

The leads are threaded inside a B size casing or equivalent that is used to lower the probe to test depth. The tests are stress controlled. Increments of pressure are applied in stages using the hand pump. The compressibility of the probe is determined by calibration tests ran in a thick-wall cylinder. The method used to interpret the data is the same one used to reduce pressuremeter data. It is based on Lame’s equations and yields a mean modulus of deformation for the rock mass tested.

Maximum pressure 30 000 kPa
   Minimum (deflated) 73.7 mm
   Maximum (inflated) 85.5 mm
Effective length 460 mm
Probe upper threads BW Casing and NQ rod female
Function Volume and pressure indicator
User interface Keyboard and graphical LCD,

64 x 128 pixels with backlight

Power Supply Rechargeable battery pack


  Diametrical change 0.001 mm (0.05 cc)
  Pressure 0.25 % F.S.
Logging capacity Up to 20 000 readings
Communication Serial port (USB adapter
interface included)
Power supply interface Universal AC wall plug with US/Euro adapter + adapter cable for car (lighter) and for external battery
Operating temperature -5 to +60 deg. C
Autonomy > 8 hours
Battery 12 V 2.3 A, rechargeable