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IS 12955:2000 (Part 1) is the Indian Standard (BIS) for in-situ determination of rock mass deformability using a flexible dilatometer, part 1: volume change. 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
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
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.
Standard Test Method for Determination of the In-Situ Modulus of Deformation of Rock Mass Using the Flexible Dilatometer Test
Defines the procedure for in-situ measurement of rock mass deformability using a flexible dilatometer, matching the core scope of IS 12955.
ISRM Suggested Method 1996International Society for Rock Mechanics and Rock Engineering (ISRM), International
HighCurrent
Suggested Method for Deformability Determination Using a Flexible Dilatometer
Provides a foundational, globally recognized methodology for the flexible dilatometer test, upon which both IS and ASTM standards are based.
NF P94-421AFNOR, France
MediumCurrent
Rock mechanics - Tests in place - Part 21 : Deformability test of a rock mass using a dilatometer
Covers in-situ rock mass deformability testing with a dilatometer, though may have procedural variations specific to French practice.
Key Differences
≠The creep stabilization criteria are different. IS 12955 requires the rate of volume change to be less than 5% of the total volume change in the last 10 minutes, whereas ASTM D4971 requires the volume change to be less than 1% of the total injected volume over a 1-minute interval, a more stringent and shorter-duration criterion.
≠The explicit formula for calculating the modulus of deformation differs. IS 12955 presents the formula as E = (1+ν) * Vₘ * (ΔP/ΔV), while ASTM D4971 uses E = 2(1+ν) * Vₐ * (ΔP/ΔV). The factor of 2 in the ASTM formula is theoretically significant for a plane strain condition.
≠IS 12955 requires at least one complete loading-unloading cycle for each test, while ASTM D4971 recommends performing at least two unloading-reloading cycles to better assess the elastic and plastic behavior of the rock mass.
≠The Indian Standard is split into two parts (Part 1 for volume change, Part 2 for diametral change), whereas ASTM D4971 is a single, comprehensive standard covering the entire flexible dilatometer test procedure and calculations based on volume change.
Key Similarities
≈All standards are based on the same fundamental principle: inserting a flexible cylindrical probe into a borehole, inflating it with fluid, and measuring the pressure-volume relationship to determine rock mass deformability.
≈Both IS 12955 and the international equivalents recommend a similar test section length to diameter ratio (L/D) of at least 6:1 to minimize end effects and approximate a plane strain condition.
≈The general test procedure involving incremental pressure application, holding pressure at each step, and performing loading/unloading cycles is a common feature across all standards.
≈All standards mandate a system calibration procedure, typically involving the inflation of the probe inside a thick-walled steel cylinder of known dimensions and properties, to correct for membrane stiffness and system compressibility.
Parameter Comparison
Parameter
IS Value
International
Source
Creep Stabilization Criteria
Volume change rate < 5% of total change in last 10 min
Volume change < 1% of total injected volume in 1 min
ASTM D4971-17
Minimum Unloading Cycles
At least one
At least two are recommended
ASTM D4971-17
Deformation Modulus Formula (E)
(1+ν) * Vₘ * (ΔP/ΔV)
2(1+ν) * Vₐ * (ΔP/ΔV)
ASTM D4971-17
Measuring Cell L/D Ratio
≥ 6
≥ 6
ASTM D4971-17
Typical Test Borehole Diameter
NX size (76 mm)
NX size (76 mm) or NQ size (75.7 mm)
ASTM D4971-17
Number of Pressure Increments
5 to 8
At least 6
ASTM D4971-17
Basis of Measurement
Volume change (Part 1)
Volume change
ASTM D4971-17
⚠ Verify details from original standards before use
Key Values5
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.
What is the primary purpose of the IS 12955 test?+
To determine the in-situ modulus of deformation (Ed) of a rock mass, a critical parameter for designing structures like dams, tunnels, and large foundations on or in rock.
What size borehole is typically used for this test?+
The standard specifies an NX size borehole, which has a diameter of approximately 76 mm (Clause 5.1).
How is the loading performed during the test?+
Pressure is applied in 5 to 10 equal increments, with each pressure step held constant for 5 to 10 minutes to allow for stabilization of deformation (Clause 5.4.3).
What is a 'flexible' dilatometer?+
It's a device, like a Goodman Jack, with curved steel plates that are pressed against the borehole wall by a hydraulic jack, allowing for the measurement of rock mass deformation under a known pressure.