Historical Background
The emergence of the enhanced design Ergotech Instrumented Triaxial Cell in 1990 was made possible by the prior existence of the original Hoek-Franklin “Simple Triaxial Cell” which was the result of ground braking development in the mid 1960s by Dr John A. Franklin under the guidance of Professor Evert Hoek in the Rock Mechanics Department, Imperial College, Royal School of Mines, London.
Several rock mechanics testing equipment companies around the world have recognised the elegance and indeed simplicity of the cell’s design which was published with a considerable quantity of test results*. To date, they continue to make and market their own variant of the original non-instrumented cell for room condition testing. The published paper suggested the use of mounted strain gauges on the surface on the cylindrical core for the measurement of axial and circumferential (radial) strain during deformation test.
Key elements of the original Hoek-Franklin Triaxial Cell
· A robust alloy steel externally threaded cylindrical pressure vessel at each end with smooth parallel bore surface finish to 0.4Ra to accept a centrally positioned special shape flexible polyurethane rubber jacket with bore matching the test core diameter.
· The smooth internal bore of the cell is necessary to seal the novel shape rubber jacket with integrally moulded “U Lip Seals” at its each end. The “Double Lip Seals” of the jacket are designed for an interference fit with the cell bore, so when installed it forms an oil pressure tight enclosed annulus with it.
· The threaded ends of the cell body are fitted with matching internally threaded end caps with a centre hole to allow insertion of a cylindrical test core followed by hardened steel platens with minimal annular clearance for axial stress application to the mounted core in the jacket by a hydraulic ram loading straining frame. The cell end-caps are also designed to oppose the axial bursting load developed by the confining pressure in the enclosed annulus as applied to the axial platens and test sample.
· There are two cross bores in the cell wall, one for application of confining pressure oil and the other for connecting a pressure sensing device in the oil.
· The key innovation of the H-F Triaxial Cell is in its special shaped sample jacket with the ability to form a pressure tight reduced volume permanently enclosed oil filled annulus which allows rapid core and axial platen loading for testing and unloading of the same elements after testing. An additional benefit of the design is that the applied axial and confining stress during testing are at 90° to each other, thus they are acting totally independently on the core. A small positive differential of 2MPa/20bar/290psig in the axial stress ensures that the confining rubber does not intrude between the sample and axial platen boundary planes to cause premature jacket failure.
*Hoek E & Franklin J.A. (1968) A Simple Triaxial Cell for Field and Laboratory Testing of Rock. Trans.Inst.Min.Metall,
Key elements of the Ergotech Triaxial Cell
Whilst the original H-F Tx. Cell was the first major innovation since the advent of classical rock mechanics cells of very cumbersome design and large dead volume of confining oil, it too had limitations. The polyurethane rubber jacket was too hard and not capable serious elevated temperature use. The strain gauges were laborious to apply and expensive plus prone to damage even with dry rocks and not suitable with water or brine saturated cores. They were also not reliable for post failure deformation testing as the gauges or wires would invariably break prematurely.
The impetus for advancing the H-F Tx. Cell came in 1988 from Robert J. Marsden, lecturer in Rock Mechanics, Imperial College, RSM who outlined the desirability of a permanently mounted two channel orthogonally aligned (0°-180° & 90°-270°) strain gauged cantilever transducer suspended on four bonded metal stand offs in the sample jacket to measure the radial strain in two directions. Bonded metal stand offs go right through the rubber subtending directly on the tested core having the same diameter as the core .The cantilever transducer would be permanently left in the pressurised oil filled confining pressure annulus.
Axial deformation would be measured by two LVDTs (0°-180°) mounted on cross bars firmly attached to the axial platens so that they would move in unison.
The first generation of room temperature multi-core diameter instrumented cell for Ø 1”, Ø1.5” and Ø2” cores capable of 69MPa/10,000psi was made in 1990.
With existing in-house pressure vessel and rubber jacket design technology coupled with rubber compatibility advice from associates with fluids at elevated temperature, cells capable 200°C at 103MPa/15,000psi were developed soon.
At the encouragement of Professor Michael S. King,
Recently P, S1 & S2 lateral transducers (4 transducers at 0°-180° & 90°-270° around the circumference) and AE Pinducers were developed for mounting in axial platens and in special spatial array in the sample jacket ( 18 AE pinducers mounted) for the Ø2”/Ø50.8mm x 127mm long core .
Ergotech is endeavouring to make its Instrumented Cell really versatile and less strenuous for sample loading by mounting it in a cantilever design swing out mechanism on one column of a rock mechanics straining frame with servo-hydraulic actuator powered by servo-intensifiers under digital electronic control.
The currently available cell configurations, matching rock mechanics, test frames, acoustic transducers, transducer pulser and supporting data acquisition electronics are described in individual technical information data sheets.