Handy Geotechnical Instruments, Inc

Rock Borehole Shear Test

For rapid in situ measurements of the cohesion and friction angle of rocks including limestone, sandstone, shale, coal, weathered granite, basalt, etc.

The Rock Borehole Shear Test was developed as a spin-off from the Borehole Shear Test described on another page.

Major advantages are speed and the ability to test rocks that are too weak or fractured to core. Test removes bias to the unsafe side caused by testing only surviving sections of rock cores.

RBST Consolie

Fully self-contained RBST console with pump, selector valves and gauges.

Shearhead Schematic

Shear head schematic. Push plates are held parallel during the opening phase. The intricate design was accomplished by master mechanical engineer and gentleman farmer Bruce Roorda.


Facts about the RBST:

  • Shear plates rotate to engage uneven rock surfaces; carbide teeth bite into the rock.
  • Works in 75 mm (3 in.) cored or pneumatically drilled holes.
  • A Mohr-Coulomb failure envelope usually within 20-30 minutes after a hole is drilled.
  • Maximum recommended stresses are 12,000 psi normal and 5,000 psi shear (80 and 35 MPa).
  • Residual strength envelopes are easily obtained by continuing shearing after each break, and correlate with strengths along fractures and bedding planes.

The RBST was developed at Iowa State University for the U.S. Bureau of Mines. It is used for mine safety, tunnel and wall design In the U.S., Japan, Korea, India...

Extraordinary precision is obtained by rotating the shear head 45o for each data point so each is at the same depth.

Shear Stress

Sandstone roof rock in a coal mine in Utah. Courtesy U.S. Bureau of Mines.

Rock Borehole 1 Rock Borehole 2 Rock Borehole 3

Inserting the shear head, here into a horizontal boring; measuring pulling force; writing down and on-site plotting of data.

Shear Strenght Chart

Depth 30 ft in fly ash pond. Data courtesy of In-Situ Testing L.C., Fairfax VA.

Shear Stress Four Tests

Four tests (16 data points) in Ohio coal layer reveal a sharp change to plastic behavior.

© 2019 Handy Geotechnical Instruments, Inc.