ROCK STRENGTH TESTS
Rock strength tests measure the fundamental mechanical resistance of rock materials when subjected to compression, tension or shear. These tests provide the core parameters used in geotechnical design, enabling engineers to understand how rock will behave under the loads imposed by foundations, slopes, tunnels, underground excavations and other structures.
Compared with index or durability testing, strength tests require more precise specimen preparation and more specialised equipment. They are used to define the true load‑bearing capacity of intact rock and to develop strength envelopes that describe how failure occurs under different stress conditions. These parameters form the basis for deriving Mohr–Coulomb or Hoek–Brown design values, which are essential for assessing stability and deformation in rock engineering applications.
Strength tests should be carried out on high‑quality, representative specimens from each significant rock unit. This ensures that the derived parameters reflect natural geological variability, anisotropy, fabric and mineralogical composition. A well‑distributed suite of strength tests supports reliable ground modelling and underpins safe, efficient design across a wide range of geotechnical projects.
Uniaxial Compressive Strength
'Baby' description:
“Squashes a rock to see how hard it is to crush.“
Applicability:
All rocks.
Parameter Obtained:
Unconfined compressive strength [MPa]
Sample Form:
Intact core / lump
Sample Size:
Min core diameter: 55mm
Min core length: 2.5 x diameter of core
Scheduling Guidance:
1 – [Exp] Estimated length of core needed = 250mm.
Reference Standard:
ASTM D7012
ASTM 4555-01(2005)
Point Load Test
'Baby' description:
“Squeezes a rock between two sharp points to see how strong it is.“
Applicability:
All.
Parameter Obtained:
Uncorrected (Is) and corrected Is(50) point load strength index values;
Sample Form:
Intact core / lump
Sample Size:
Min core diameter: 30mm, with 50mm recommended.
Min core length: >1.0x diameter of core.
Scheduling Guidance:
Minimum 10 specimens recommended when testing cores, 20 when lumps.
[Exp] Estimated length pf core required = 150mm.
Reference Standard:
ASTM D5731-16
ISRM Suggested Method – Determination of Point Load Strength
Brazilian Tensile Strength (Indirect Tensile Test)
'Baby' description:
“Squeezes a rock disc until it snaps in the middle to see how easily it pulls apart.“
Applicability:
All.
Parameter Obtained:
Tensile strength.
Sample Form:
Intact core / lump
Sample Size:
Min diameter 10x times largest grain. 50 mm recommended.
Scheduling Guidance:
The specimen shall be selected from the core to represent a true average of the type of rock under consideration.
The field moisture condition shall be preserved until the time of test.
At least ten specimens shall be tested to obtain a meaningful average value.
Reference Standard:
ASTM D3967
Triaxial Compression Test
'Baby' description:
“Squashes a rock while squeezing it from the sides to see how strong it is deep underground.“
Applicability:
All.
Parameter Obtained:
Peak axial strength at each confining pressure.
From this the value of internal friction angle φ and ‘apparent’ cohesion C may be calculated.
Sample Form:
Intact core
Sample Size:
Min diameter of 54mm with height of 2x to 3x of the diameter. Diameter should be min 10x the largest grain.
Scheduling Guidance:
For a full envelope at least five specimens per rock sample are preferred in addition to uniaxial tests.
[Exp] If possible, specify a UCS test alongside.
Reference Standard:
ASTM D7012
ISRM – Suggested Method for Determining the Strength of Rock Materials in Triaxial Compression (1978)
Direct Shear Test on Rock Discontinuities
'Baby' description:
“Slides two rock pieces past each other to see how easily the rock slips along a crack.“
Applicability:
All.
Parameter Obtained:
Peak shear strength.
Residual shear strength.
Shear stiffness.
Sample Form:
Intact lumps.
Sample Size:
Cross-sectional dimension min 10x the largest grain size. The test plane should have a minimum area of 1900 mm2 (3 in.2).
Scheduling Guidance:
Moisture condition of the sample must be preserved.
At least three samples are recommended.
Reference Standard:
ASTM D5607