ROCK INDEX TESTS
Rock index tests provide rapid, cost‑effective measurements of the fundamental physical and mechanical characteristics of rock materials. These tests are used to classify rock, understand its inherent variability, and support the selection and interpretation of more advanced laboratory tests.
Compared with full rock strength or durability testing, rock index tests are relatively quick and inexpensive, making them suitable for use in larger numbers. This allows engineers to build a detailed picture of how rock properties vary both vertically and laterally across a site, particularly within complex geological sequences.
Rock index tests should be distributed throughout each rock stratum to support robust geological logging, engineering classification and the development of appropriate design parameters.
NOTE: An “[Exp]” prefix signifies the guidance, which is based on professional experience of practicing geotechnical engineers and GEOCompass creators.
Natural Water Content
'Baby' description:
“Tells how much water is inside the rock compared to how much dry rock is there.”
Applicability:
All rocks.
Parameter Obtained:
Water Content [%].
Sample Form:
Intact lumps
Sample Size:
At least 10 lumps each having either a mass of at least 50 g or a minimum dimension of 10x the maximum grain size, whichever is the greater.
Scheduling Guidance:
1 – [Exp] Estimated length of core needed = 30mm.
Scheduling Guidance:
The Complete ISRM Suggested Methods For Rock Characterization, Testing And Monitoring (“The Blue Book”).
Porosity / Density - by Caliper
'Baby' description:
“Measures how many tiny holes are inside the rock and how heavy it is, by soaking it and measuring its size.“
Applicability:
All. Non-friable, coherent rocks that can be machined.
Parameter Obtained:
Density [kg/m³]
Porosity [%]
Sample Form:
Intact lumps.
Sample Size:
Three samples, min 50g each or min dimension >10x the maximum grain size, whichever is the greater.
Scheduling Guidance:
None
Reference Standard:
The Complete ISRM Suggested Methods For Rock Characterization, Testing And Monitoring (“Blue Book”).
Porosity / Density - Saturation and Buoyancy
'Baby' description:
“Finds how many tiny spaces are in the rock by weighing it in air and water to see how much it ‘floats’.”
Applicability:
All.
Parameter Obtained:
Density [kg/m³]
Porosity [%]
Sample Form:
Intact lumps.
Sample Size:
Min 10 lumps, each min 50 g or a min dimension of 10x the maximum grain size, whichever is the greater.
Scheduling Guidance:
None
Reference Standard:
The Complete ISRM Suggested Methods For Rock Characterization, Testing And Monitoring (“Blue Book”).
Swelling Pressure Index
'Baby' description:
“Shows how hard a rock pushes back when it soaks up water and tries to swell but isn’t allowed to grow.”
Applicability:
Mostly siltstones and mudstones. Softer rock types, or harder rocks at advanced weathering state.
Parameter Obtained:
Swelling Pressure Index
Sample Form:
Intact lumps / Intact core
Sample Size:
Diameter >2.5x thickness. Thickness >15mm or 10x maximum grain size.
Scheduling Guidance:
None
Reference Standard:
The Complete ISRM Suggested Methods For Rock Characterization, Testing And Monitoring (“Blue Book”).
Swelling Strain Index (confined sample)
'Baby' description:
“Measures how much the rock wants to swell when it gets wet, even when squeezed in tight.”
Applicability:
Mostly siltstones and mudstones. Less durable rocks.
Parameter Obtained:
Confined swelling strain [%].
Sample Form:
Intact lumps.
Sample Size:
Diameter >4x thickness. Thickness >15mm or 10x maximum grain size.
Scheduling Guidance:
None.
Reference Standard:
The Complete ISRM Suggested Methods For Rock Characterization, Testing And Monitoring (“Blue Book”).
Swelling Strain Index (unconfined sample)
'Baby' description:
“Shows how much the rock actually puffs up when it gets wet and nothing holds it in.”
Applicability:
Mostly siltstones and mudstones. More durable rocks.
Parameter Obtained:
Unconfined swelling strain [%].
Sample Form:
Intact lumps.
Sample Size:
Right cylinder or rectangular prism. Min 15mm or 10x maximum grain size.
Scheduling Guidance:
None.
Reference Standard:
The Complete ISRM Suggested Methods For Rock Characterization, Testing And Monitoring (“Blue Book”).
Petrographic Testing
'Baby' description:
“Looks at the rock under a microscope to see what it’s made of, like checking the ingredients in a recipe.”
Applicability:
All.
Parameter Obtained:
Mineral content, grain size and texture.
Sample Form:
Intact lumps.
Sample Size:
Rock lump, minimum 30x30x30 mm.
Scheduling Guidance:
Wherever possible, oriented specimens should be collected and the original strike and dip of one face of the specimen should be recorded.
Reference Standard:
The Complete ISRM Suggested Methods For Rock Characterization, Testing And Monitoring (“Blue Book”).