This standard specifies the in-situ method for determining the modulus of deformation of a rock mass using a flexible dilatometer, commonly known as a Goodman Jack. It details the apparatus, test procedure within a borehole, and the calculations required to derive the modulus from pressure and radial displacement measurements.
Code of practice for in-situ determination of rock mass deformability using a flexible dilatometer, Part 1: volume change
Quick Reference Values
Required borehole diameter76 mm (NX size)
Length to diameter (L/D) ratio of test section3 to 6
Number of pressure increments5 to 10
Pressure hold time per increment5 to 10 min
Maximum test pressureShould not exceed one-third of the unconfined compressive strength of the rock
Key Formulas
Ed = K * (ΔP / Δur) — Modulus of deformation, where K is dilatometer constant, ΔP is pressure increment, and Δur is corresponding radial displacement.
Practical Notes
The quality of the borehole (smoothness, cleanliness, and correct diameter) is critical for accurate results.
This test provides a localized measurement; results must be interpreted in the context of the overall geological conditions and rock mass quality (e.g., RQD, discontinuity data).
The test is particularly sensitive to the orientation of discontinuities intersecting the test section, which can significantly influence the measured deformability.