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.
- 1 ground surface
- 2 borehole wall
- 3 pocket
- 4 expanding dilatometer probe p applied pressure
- A—A axial section
- B—B cross section
The equipment to carry out dilatometer tests shall consist of the components shown in Figure 2.
- 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
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).
|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.
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
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.