4 Equipment

4.1 General

The test with the flexible dilatometer is performed by the expanding of a flexible dilatometer membrane placed in the ground (see Figure 1). The pressure applied to, and the associated expansion of the probe are measured and recorded so as to obtain a stress-displacement relationship for the ground as tested.

Example of a flexible dilatometer test


  • 1 ground surface
  • 2 borehole wall
  • 3 pocket
  • 4 expanding dilatometer probe p applied pressure
  • A—A axial section
  • B—B cross section
Figure 1 — Example of a flexible dilatometer test

The equipment to carry out dilatometer tests shall consist of the components shown in Figure 2.

Schematic diagram of flexible dilatometer equipment


  • 1 setting rods (optional)
  • 2 displacement measuring unit (obligatory)
  • 3 pressure control unit (obligatory)
  • 4 pressure source (obligatory)
  • 5 signal cable (obligatory)
  • 6 pressure line (obligatory)
  • 7 sediment collection tube (optional)
  • 8 flexible dilatometer probe (obligatory)
  • 9 data logger (optional) z test depth
Figure 2 — Schematic diagram of flexible dilatometer equipment

NOTE Sometimes, setting rods are necessary to push the probe into a tight pocket. They also allow orientation of the instrument. They are also needed in case it becomes difficult to extract the probe at the end of the test and hammering out is required.

Borehole diameters should be 76 mm, 96 mm, and 101 mm, according to ISO 22475-1.

The external diameter dd of the flexible dilatometer when deflated shall be some 3 mm to 6 mm smaller than the nominal diameter of the borehole.

The pressure applied to the membrane shall be measured by one or more electric transducers in the instrument (see Figure 3).

4.2 Dilatometer probe

The expansion of the borehole shall be monitored by three or more electric transducers.

In variant A, the diametral displacement shall be measured with electric transducers, which shall penetrate the membrane and shall directly bear on the borehole wall (Figure 3, left.). This variant shall be primarily used in rocks (Rock dilatometer, RDT, see EN 1997-2:2007, 4.5).

Sketch of flexible dilatometer variant A Sketch of flexible dilatometer variant B
a) Variant A b) Variant B


  • 1 membrane
  • 2 pressure transducer
  • 3 fluid or gas
  • 4 displacement transducers
  • 5 setting rod
  • 6 pressure line
  • 7 signal cable
  • 8 metal insert at both ends of each displacement transducer (variant A)
  • 9 sediment collection tube
  • 10 membrane clamping ring
  • 11 compass (if applicable)
  • dd external diameter of the dilatometer
  • LFD length of the expanding part of the dilatometer
  • Lg axial distance between transducer and clamping ring
  • Ld length of the measuring segment of the dilatometer

NOTE 1 On this sketch there are three displacement transducers (No. 4) at 120° from each other.

NOTE 2 For variant A, the third No. 4 transducer is represented lengthwise with its metal inserts (No. 8) at both ends.

Figure 3 — Sketch of flexible dilatometer (not to scale)

The expanding length LFD of the probe shall exceed (5,5dd + Ld). The measuring segment Ld shall not exceed 1,5dd.

In variant B, the diametral displacement shall be measured by electrical transducers placed at the inner wall of the membrane (Figure 3 b). Because membrane compression influences the readings of pressure and displacement, proper corrections shall be determined by corresponding calibration (see A.3). Variant B shall be primarily used in soils (Soil dilatometer, SDT, see EN 1997-2:2007, 4.5).

4.3 Pressure control and displacement measuring units

The pressure control and displacement measuring units shall control the probe expansion and permit the reading of liquid or gas pressure and displacement as a function of time.

The pressurizing system (3 and 4 in Figure 2) shall allow:

  • reaching a pressure at least equal to 20 MPa;
  • implementing a pressure increment of 0,5 MPa as measured on the pressure control unit in less than 20 s;
  • stopping the injection when necessary.

4.4 Connecting lines

The pressure line and signal cable shall connect the pressure control and displacement measuring units to the probe. The pressure line shall convey the fluid to the probe either parallel or coaxial with the signal cable.

4.5 Measurement and control accuracy

4.5.1 Time

The accuracy of the device used to measure time must be one second.

4.5.2 Pressure and displacement

The maximum uncertainty of measurement of the devices measuring pressure and displacement shall be as specified in 5.4.

4.5.3 Display of readings

At the site the pressure control and displacement measuring units shall give a simultaneous and instantaneous display of the following readings: time, pressure of the fluid injected into the probe and diametral displacements.

4.5.4 Membrane compression calibration cylinder

The main dimensions of the steel cylinder serving the calibration for membrane compression shall be as follows:

  • a known inside diameter which closely fits the deflated instrument;
  • a wall thickness appropriate to the maximum pressure to be applied;
  • a length appropriately greater than the expanding length of the instrument.

4.6 Data logging

If the data are not recorded manually, a data logging system shall be available to record the readings from the transducers, calibration data and the resulting readings of pressure and displacement.

ISO 22476-5:2012 Field testing — Part 5: Flexible dilatometer test