Triaxial or Biaxial Hoek Cell

The ESS Triaxial Hoek cell is made up of eight pieces two end caps, two confining rings, one cylindrical body and two spherical platens. Inside the cell its has a flexible urethane membrane that is either plain or strain gauged depending on preference.

The unique feature of the ESS Hoek cell is that unlike other manufacturers ours are modular and not dedicated to one Hoek cell per diameter leading to unnecessary purchase of multiple Hoek cells that can be very costly. This feature enables one to re-use the Cell body saving on storage space, cost and wastage.

The ESS Hoek cell features a multi-core diameter capability within each unit is achieved by obtaining a changeover kit that includes different dedicated sized confining rings and membrane. The Hoek cell is robust and does not require disassembly to insert or remove the core sample or to conduct a multi test program. The Hoek cell is electroplated to prevent corrosion. The maximum safe working pressure of the larger unit is 30 MPa and the two smaller sizes 40 MPa respectively. The cell is supplied with a membrane fitted as well as a quick connect hydraulic fitting and air bleed valve. Spare membranes are readily available and sizes are listed below. Should one purchase a unit for a specific size but later need to test a different size its as simple as ordering a changeover kit for the new size.

ESS has the widest range of sizes available

Conveniently embedded Orthogonal strain gauges in the confining membrane to measure poissons ratio. Strain gauges are protected waterproof and re-usable. When opting for the instrumented membrane an ESS digitiser is suppled to read via a portable handheld reader, datalogger or third party data acquisition devices.

ASTM compliant

32 Existing Membrane sizes available

Changeover kit options available

Unit 3 – core diameter 40 to 70 mm

38mm, 40mm, 43mm, 45mm, 47mm, 51mm, 52mm, 55mm, 57mm, 61mm, 62mm, 63.5mm and 70mm.

Unit 2 – core diameter 70 to 110 mm.

70mm, 72mm, 76mm, 82mm, 92mm, 93.5mm, 94.5mm, 96/97mm, 100/101mm, 107/108mm and 110/111mm.

Unit 1 – core diameter 110 to 145 mm

110, 120mm, 121mm, 128mm and 144mm

Custom size mould manufacture done at additional cost

Description

TEST PROCEDURE

ASTM require a minimum length to diameter ratio of 2

The ends of the rock specimen must be ground flat and parallel to 0.025 mm to 0.012 mm, depending on the diameter of the sample. The cylindrical rock sample is inserted into the Hoek cell within the confining membrane. The two spherical seats are positioned so that the rock core lies centrally in the triaxial loading frame press or chamber. The spherical platen will self centralise and confine the sample evenly through the loading process.

To hold the rock core in place one must apply a small confining pressure, the cell with its spherical seats is placed in a loading frame and a small axial load is also applied to keep everything in position. To determine Poisson’s ratio, a series of two orthogonal strain gauges are embedded in the membrane. Up to 6 sets of double orthogonal gauges are embedded for ample redundancy to determine poissons ratio. The strain gauges are read during the test via a hand held portable reader or datalogger. Third party devices that read RS232 serial output can be used to read the gauges through the ESS digitiser. The triaxial test may then be run after adjustment of the confining pressure to the required value.

Optional – The pressurisation system comprises of a hand pump, pressure gauge and high pressure hose with quick connect fitting. The pressure gauge is supplied with a calibration chart. When ordered together, the Hoek and pressurisation system are supplied in a rugged, reusable transport case. The Hoek cell is generally manufactured and supplied to operate with a core of particular diameter. The design is such that three basic units can be easily adapted to accommodate core diameters within the most commonly required ranges.

CORE DIAMETERS

Unit 1 110-150mm diameter (0-30 MPa) 50kg

Unit 2 70-110mm diameter (0-40 MPa) 40kg

Unit 3 40-70mm diameter (0-40 MPa) 30kg

CORE LENGTHS – Minimum 200mm

CELL TYPE – Hoek

PRESSURISATION SYSTEM – Enerpac hand pump with gauge and 1.5m hose

BOX/TRAVEL CASE DIMENSIONS – 86 x 43 x 40cm

FITTINGS – ¼” BSP

MEMBRANES WITH EMBEDDED STRAIN GAUGES

6 x two orthogonal 120ohm Strain gauges

12 Channel digitiser with serial RS 232 output

Breakout box power supply

APPLICATIONS

Hoek Cells are designed for Testing Triaxial strengths of Rock or Concrete samples up to a Pressure of 40MPa. These are designed to accept samples of EX, AX, AQ, BX, BQ, NQ, HQ, HQ3, PQ, PQ3 and so on. Tests carried out on a series of samples under different confining pressures allow the user to determine:
• Strength & Elastic properties
• Shear strength at different confining pressures
• Angle of Shearing Resistance & Cohesion
• Modulus of Elasticity & Poisson’s Ratio