We have the capabilities to undertake research into deformation of rocks. Our computer- controlled servoed triaxial system can be programmed to carry out various testing schemes (uniaxial stress or strain, hydrostatic compression, triaxial failure, multistage testing, etc.) and monitor load, lateral and axial strains as well as velocities and permeabilities. Tests can be run at controlled strain rates and saturations. An externally heated pressure vessel allows experiments to run as high as 200 ^oC. There is a setup charge and a run cost. Part of the setup charge involves disposal instrumentation such as strain gages and labor. Running experiments at elevated temperatures limits measurement capabilities. We request that the user provide stress protocols for each program.

Specimen requirements:

Test samples should have a minimum of 2:1 length to diameter ratio. We can handle both 1 inch and 1.5 inch diameter samples. Samples will normally be cleaned and saturated with the required fluids before testing.

Typical Tests:

Failure Envelope Definition:

1) Involves a series of specimens which are subjected to uniaxial and triaxial test conditions (varying confining pressures and pore pressures) at a fixed strain rate, typically 10^-6 sec^-1. Failure strength and stress conditions are used to define a Mohr’s failure envelope.

2) A multistage test is conducted on specimen and a Mohr’s failure envelope is constructed from the data recorded during each cycle.

Static Moduli and Failure Strength:

Specimens are loaded in uniaxial stress or strain, triaxial or hydrostatic stress conditions while monitoring the axial and lateral strains. Analysis of stress-strain data can provide values for Young’s modulus, E, bulk modulus, K, and Poisson’s ratio, n. Ultimate strength and failure angle are also recorded and reported. Stress-strain data is provided to customer along with the processed results. Crack porosity can be estimated from a plot of volumetric strain against hydrostatic pressure. For all but the weakest rocks, crushing strength is obtained from hydrostatic tests carried out in a separate 10 kilobar hydrostatic pressure vessel. 

Figure 1: Schematic of sample assembly inside the triaxial testing system. Sample is instrumented for axial velocity measurements.

Pictures of the actual apparatus and sample instrumented for displacement measurement suing LVDTs.

Figure 2: Specimen jacketed in polyolefin heat shrink tubing is outfitted with a roller chain extensiometer to measure circumferential displacement and an axial extensiometer. This sample is also mounted on platens which house the piezoelectric transducers shown schematically in Figure 1.

Figure 3: Instrumented specimen inserted in the load platen and pressure vessel.

Figure 4: A series of Mohr’s circles constructed from uniaxial and triaxial tests. All samples are Berea sandstone.

Send mail to csondergeld@ou.edu with questions or comments about this web site.
Last modified: 08/14/04