Rock Shear Box Apparatus

The Rock Shear Box measures the shear strength of rock, above all the strength along a discontinuity such as a joint, a bedding plane or a fault surface. It is the equipment behind the direct shear test on rock discontinuities described in ASTM D5607, in which the specimen is sheared while the normal force on it is held constant.

The design was originally developed at Imperial College, London, by Professor E. Hoek, as a simple and practical way to determine the strength and slope stability of rock both in the field and in the laboratory.

Joints are usually the weakest path through a rock mass, so their shear strength often decides whether a rock slope, a foundation or the wall of an excavation stays stable. Testing a sample of the joint itself gives the designer a measured value from the actual site instead of an estimate.

Both versions run the same test on the same frame. They differ in how the readings are taken: the NG-ShearBox/A is read by eye from digital gauges, while the NG-ShearBox/D records loads and displacements electrically into an external data logger.

Each version comes with two mould formers, two hand operated pumps for the lateral and vertical load with Bourdon gauges, and the pressure maintainer on the vertical load pump. The /D adds potentiometric transducers in place of the digital gauges and two pressure transducers for the loads.

Choose the /D when the test has to produce a recorded curve rather than a set of readings written down by the operator, and order the multipurpose data logger with it. The /A suits programs where the operator logs readings by hand at set intervals.

Let us know whether you read or log the test and how large your samples are through a brief request, and we will match the version and the logger to your program.

The shear box takes samples up to 115 × 125 mm (4.5 × 4.9″), or cores up to Ø102 mm (4″) in diameter. That covers a block of jointed rock trimmed around the discontinuity as well as a drill core with a joint running across it.

The specimen is set in the two halves of the box so that the plane to be tested lies on the line between them. Two mould formers come with both versions for this step, so that each half of the specimen is held firmly while the joint itself stays free to slide.

Because the box accepts core as well as blocks, a joint intersected by a drilling program can be tested without first cutting it into a block, which keeps the natural surface intact. Coring and cutting equipment for the steps before testing is in our rock specimen preparation range.

The box is built in two halves, and each half has its own loading. The lower half is connected to a ram that applies the normal load, pressing the two faces of the joint together. The upper half is connected to two rams that drive it across the lower half and shear the specimen along the plane between them.

Both loads come from hand operated pumps, one for the lateral load and one for the vertical load, so the operator controls the rate of loading directly at the pump.

The two shear rams give a reversible shearing action. The specimen can be sheared in one direction and then driven back, which lets a laboratory follow the joint past its peak strength and study how the surface behaves once it has already been sheared. That second stage is where the residual strength of the joint is found.

A rough joint rides up over its own asperities as it shears, so the specimen dilates and pushes the normal ram back. Left uncompensated, that change in volume would change the normal load partway through the test.

The normal loading system of the Rock Shear Box carries an adjustable low friction pressure maintainer for exactly this reason. It absorbs the volume change of the specimen while shearing is under way and keeps the vertical stress constant through the test, so it cannot drift as the specimen dilates.

Constant normal force is the boundary condition the direct shear method is built on, and it is what makes the result a shear strength at one known normal stress. On both the /A and /D versions the maintainer sits on the vertical load pump, so it works the same way whether the test is read by gauge or logged.

Yes. The frame is small enough to be carried to a site, and that is the point of the design: the same box is used in the field and on the bench.

The frame alone measures 46 × 25 × 60 cm (18.1 × 9.8 × 23.6″), and the apparatus weighs about 45 kg (99 lbs). Loads come from hand operated pumps and are read on Bourdon tube gauges, so a field crew can run the test beside the outcrop or the core store.

Testing on site shortens the time between sampling and testing. A joint sample can be sheared before transport has a chance to disturb it, and the results can guide the next round of sampling while the crew is still there. Our full line of field and laboratory rock equipment is in the rock mechanics section.

Five displacement readings are taken on both versions, four vertical and one horizontal. The horizontal reading follows the shear displacement of the upper half over the lower half, and the four vertical readings follow the normal movement of the specimen as the joint opens or closes.

On the NG-ShearBox/A these are digital gauges with a 25 mm range read to 0.001 mm. On the NG-ShearBox/D they are potentiometric transducers with 25 mm travel, which feed the external data logger.

Four vertical points instead of one show whether the box tilts as well as how much the joint dilates. Dilation measured this way, plotted against shear displacement, shows how much of the strength comes from the roughness of the surface and how quickly that roughness is worn down as shearing continues.

The Bourdon gauges on the shear and normal load pumps read up to 50 kN in 1 kN divisions. Both versions carry them, so the load can always be read at the pump, with or without a logger.

On the NG-ShearBox/D, two pressure transducers take the same load values directly into an external data logger. Load and displacement are then recorded on one time base and can be plotted against each other without transcribing readings.

Stress on the joint is the load divided by the area of the shear plane, and that area differs from one specimen to the next. The operator therefore measures it on every specimen before the test, so the same gauge reading can be turned into a stress for a small core or a full-size block.

Each test gives the shear stress on the joint against the shear displacement at one constant normal stress. From that curve the laboratory reads the peak shear strength and, after further displacement, the lower residual strength of the sheared surface.

Testing several specimens of the same joint set at different normal loads gives a series of points on a plot of shear strength against normal stress. The line or curve through them is the strength envelope, and it yields the friction angle and cohesion that slope stability calculations use for the discontinuity.

Peak values apply to a slope that has not yet moved; residual values apply to one that already has, or to a joint that has slipped before. Equipment for the intact rock side of the same investigation is in our rock strength testing range.

When the question is the strength of the intact rock rather than the strength of a joint. The Rock Shear Box loads a specimen along a discontinuity and measures how it slides, while a point load tester breaks a piece of intact rock between two cones and gives a strength index.

The two answer different design questions, and a site investigation often needs both. In a jointed rock mass the joints usually govern the stability of a slope or an excavation face, while the intact strength governs how hard the rock is to excavate and how much load massive rock can carry.

Both suit field work. The point load tester gives its index with almost no specimen preparation, and the shear box runs on hand pumps, so a field program can collect intact and joint strength from the same exposure in the same visit.