Strength in the Lab
Learning objectives
- Read a triaxial stress-strain curve: elastic rise, peak strength, then post-peak weakening or flow
- Show that confining pressure raises peak strength and turns brittle failure ductile
- Anchor the numbers with a strength table: weak Gulf sand near 10 MPa, Berea sandstone 60 to 80, granite around 200
- Read peak axial stress off the Mohr-Coulomb line as confinement climbs
The Experiment That Defines Strength
Strength is not a property a rock simply has; it is the outcome of an experiment, and the experiment is the triaxial test. A cylindrical core is wrapped in a jacket, squeezed all around by a confining pressure , then loaded axially until it fails while the machine records stress against strain. The curve tells the story in three acts: a straight elastic rise whose slope is Young's modulus, a peak where the rock can carry no more, and a post-peak descent as a through-going fracture forms and the two halves grind past each other at a lower residual strength. Run it with no confinement, , and the peak is the unconfined compressive strength, the UCS, the single most-quoted rock-strength number.
Run the rig above. Raise the confining pressure and two things change together: the peak climbs, because squeezing the flanks makes it harder to slide a fracture open, and the post-peak softens its drop, brittle shattering giving way to ductile flow. That brittle-to-ductile transition is real geology: the same limestone that snaps like glass near the surface flows like putty at depth, which is why deep salt and chalk creep while shallow granite fractures. The rock selector pulls from the course's strength table: a weak Gulf sand peaks near 10 MPa unconfined, our Berea-like reservoir sand near 74, an Indiana limestone near 55, and a Westerly granite near 200, values that span a factor of twenty and set expectations for every wellbore-stability calculation to come.
From Curve to Criterion
Do the test at several confining pressures and the peaks trace a line: peak axial stress rising steadily with . That line, with the slope, is the Mohr-Coulomb criterion the next section derives properly, and its two numbers, the unconfined strength and the slope, are what a lab report actually delivers to a geomechanicist. A caution the course will honor: these are intact-rock strengths from careful plugs. The rock mass around a real wellbore carries bedding, natural fractures, and weak clay-rich layers that no core captures, so field strength is routinely a fraction of the laboratory value. The lab gives the ceiling; the earth delivers less. With that honesty in hand, the next section turns the peaks into the failure line that organizes all of rock strength.
References
- Jaeger, J. C., Cook, N. G. W., & Zimmerman, R. W. (2007). Fundamentals of Rock Mechanics (4th ed.). Blackwell.
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.
- Paterson, M. S., & Wong, T.-f. (2005). Experimental Rock Deformation, The Brittle Field (2nd ed.). Springer.