IRS

Laubscher [1] defined the IRS as the unconfined uniaxial compressive strength of the rock between fractures and joints. His definition accounts for the bias when estimating rock strength:

“It is important to note that the cores selected for testwork are invariably the strongest pieces of that rock and do not necessarily reflect the average values.”

Figure 1: Determination of average IRS where the rock mass contains weak and strong zones.

From the guidelines of Read & Stacey: [2]

As an example [1], a strong rock sample (UCS = 150 MPa) contains zones of weak rock (UCS = 30 MPa) over 45% of its total volume. The relative strength of the weak rock is 20% of the strong rock (30/150 × 100).

Using Figure 1, draw a horizontal line from point Y = 45 on the Y-axis until it intersects the 20% relative strength curve. Then draw a vertical line to the X-axis, which provides an equivalent IRS strength value of 37% of the stronger rock, i.e. 55 MPa.

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References

[1] (1,2)

D. H. Laubscher. A geomechanics classification system for the rating of rock mass in mine design. Journal of the South African Institute of Mining and Metallurgy, 90(10):257–273, 1990. doi:10.10520/AJA0038223X_1954.

[2]

John Read and Peter Stacey, editors. Guidelines for Open Pit Slope Design. CRC Press / Taylor & Francis, London / Boca Raton, 1st ed. edition, 2009. ISBN 9780415874410.