Laboratory Tests

Rock strength is measured by laboratory testing. Strengths are very different depending on the stress field applied to the rock. All rocks and soils are very much stronger in compression than in tension.

The two common laboratory tests to determine the compressive strength of rock are:

• Uniaxial Unconfined Compression Test - A cylindrical rock core is loaded axially until it fails.
• Triaxial Confined Compression Test - A cylindical rock core is placed in a cell, subjected to all around (confining) pressure by hydraulic oil acting through a thin impermeable membrane, and loaded axially to failure.

There are a variety of tests to determine the tensile strength of rock:

• Direct Pull Test - A cylindrical rock core sample is anchored at both ends and stretched.
• Brazilian Test - A relatively thin disk is load across the diameter until it splits.
• Beam Flexure Test - A thin slab of rock is loaded vertically when supported at three or four points along it's length.

The Hoek cell is a specially designed triaxial cell that can be used with regular hydraulic jacks. This makes it highly portable and suitable for field i operation. The hydraulic pressure is applied radially through a thick membrane. The axial pressure is applied directly to the sample endcaps.

All the tests mentioned so far are used to determine the properties of intact rock. Such properties are rarely representative of rock mass behaviour because of the presence of numerous discontinuities (fractures, joints, faults, etc) in virtually all rocks close to the Earth's surface.

The portable field shear box is used to directly measure the shear resistance of joint surfaces. A block with a natural fracture is cemented into a split-box mould with the fracture surfaces in contact. When the cement or plaster is cured, normal and shear loads are applied using hydraulic jacks. The displacement is meassured with a dial-gauge. The shear-box provides information on the shear strength and shear stiffness of jointed rock masses.