10 ROCK DILATOMETER TEST (RDT)
(1)P This test covers the determination of the in situ deformability of rocks from measurements of the radial expansion of a borehole section under a known uniform radial pressure applied by means of a cylindrical dilatometer probe.
(2)P The basis of the test consists of inserting a cylindrical probe, having an outer expandable flexible membrane, into a borehole, and measuring, at selected intervals or in a semi-continuous manner at closely spaced test locations, the radial displacement of the borehole while inflating the probe under a known radial pressure.
(3) This test is used mainly in hard rock formations to obtain profiles of deformability variations with depth.
(4)P The tests shall be carried out in accordance with a method that complies with the requirements given in this section.
(5)P The test method used shall be reported in detail with the test results.
(6)The test method may be reported by reference to a published standard.
10.2 Definitions and symbols
10.2.1 Dilatometer probe and testing apparatus
(1)The main components of a dilatometer apparatus for testing in rocks are defined in fig. 10.1.
(2)P probe: cylindrical device consisting of an outer flexible membrane mounted on an inner steel core that can be filled with a pressurized fluid so that the membrane can be inflated from the inside and expanded against the wall of the borehole to apply a uniform radial pressure.
(3)P membrane stiffness: pressure that shall be applied inside the probe to inflate and expand the membrane in air.
(4)P membrane compression: change in thickness of the membrane as the pressure inside the probe is increased.
(5)P calibration cylinder: cylinder made of a material of known elastic properties and having an inner diameter slightly larger than the deflated diameter of the probe and length equal to that of the probe.
(6)P hydraulic system stiffness: increase of pressure applied at ground surface to the dilatometer apparatus that corresponds to a given volume increase of the hydraulic pressurizing system measured at ground surface.
(7)P borehole dilation ΔD: average increase of borehole diameter D along a test section under the applied pressure;
(8)P seating pressure ps: minimum pressure that shall be applied before expanding the probe to ensure permanent contact of the flexible membrane with the borehole wall with no sliding;
(9)P pressure-dilation graph: plot of applied pressure and corresponding dilation such as pi versus ΔD, pi versus probe volume.
10.2.2 Symbols of dilatometer parameters
(1)P The following parameters are defined:
- p1: the pressure inside the probe during testing;
- p1c: the pressure applied to the borehole wall during testing;
- νz: the Poisson's ratio of the tested rock;
- Ed: the dilatometric modulus of deformation of a borehole test section;
- Ed1: the secant dilatometric modulus of a borehole test section corresponding to a finite increase of pressure Δp1c.
10.3.1 Drilling equipment
(1)P The drilling equipment shall be capable of providing a clean and smooth walled borehole having a diameter slightly larger than the deflated diameter of the probe and smaller than the diameter of the probe when expanded in air under the expected range of pressures that will be applied during testing.
(2) When necessary the borehole wall should be supported by casing, except in the test section, or by cementation and subsequent redrilling. During cementation the head of fluid grout should not exceed 3 m to avoid pressure grouting. After redrilling and prior to testing the cement lining should be thinner than 1 mm.
10.3.2 Pressurizing system
(1)P The hydraulic system to pressurize the dilatometer probe shall be capable of storing and displacing the necessary volume of fluid to fill, inflate and deflate the probe, applying all specified ranges of fluid pressure and also performing cyclic loading and stress relaxation testing when required.
(2) For measurements in hard rock formations the application of pressures as high as 20 MPa should be expected.
(3)P The stiffness of the hydraulic system shall be such that the dilation of the system is minimal if the radial displacements of the borehole are backcalculated from measurements of volume changes taken at ground level.
(4) Glycerin, ethylene glycol, water and hydraulic oil should generally be used as pressurizing fluids.
10.3.3 Measuring system
(1)P The measuring system shall be capable of determining the applied fluid pressure with a sensitivity better than 2% and the volume of fluid displaced to pressurize the probe with an accuracy of 1% of the probe volume over the full range of pressures to be employed during the testing program.
(2)P When borehole displacements are obtained from direct measurements against the borehole wall, as with displacement transducers, the measuring device shall be capable of determining the borehole diameter with an accuracy of at least 0,02 mm.
10.3.4 Calibration equipment
(1)P At least one calibration cylinder of known elastic properties with internal diameter equal to that of the borehole and length equal to that of the probe shall be available on site to determine the stiffness of the dilatometer testing system.
(2) The calibration cylinder should have a stiffness similar to that of the rock mass to be tested and should allow calibration over the full range of pressures specified for the testing program.
(3) Two or more calibration cylinders of different stiffness should be used to improve accuracy and precision of the calibration procedure.
10.3.5 Dilatometer probe
(1)P The probe shall be fitted with the necessary components to ensure pressure and leakage control, system bleeding and connection with rods, high pressure tubing and cable to pressurize and position the probe inside the drillhole.
(2) The flexible membrane should be strong enough not to be damaged when inserted into and withdrawn from the borehole but flexible enough to transmit not less than 90 % of the pressure acting inside the probe.
(3)P For testing in a hard rock formation a probe equipped with the necessary measuring devices that allow to measure borehole dilations directly against the borehole wall shall be used. The measuring devices shall be mounted at mid point of the membrane and equally spaced.
(4) Probes not equipped with the necessary devices to measure borehole dilations directly against the borehole wall should be used only when testing in weak rocks or when large deformations compared to the probe volume are expected.