Synthetic Rock Mass (SRM)

Provider: ITASCA


The main focus of early years of the original LOP Project (2005 to 2014) was on preparation of the book Guidelines for Open Pit Slope Design and delivery of research tasks that included Siromodel (also known as the OPS or Open Pit Simulator software), JointStats, Stepsim4, and the development of the Synthetic Rock Mass (SRM) method for representing the strength of the jointed rock mass.  The SRM work was performed via a research contract with the Itasca Consulting Group, Inc. (Itasca) under the direction of Dr Peter Cundall [Pierce, M., Cundall, P., and Potyondy, D. (2007). “A synthetic rock mass model for jointed rock”. 1st Canada-U.S. Rock Mechanics Symposium, Vancouver, B.C., May 27-31, 2007, pp. 341-349]. The SRM studies culminated with the development of a new slope stability code, Slope Model, that was specifically targeted at improving the resolution and speed of PFC3D SRM analyses using a special purpose code based on a simplified lattice that runs 10 times faster than PFC3D. The most significant outcomes of this research work are:

  • given reliable intact strength and jointing data, using the SRM methodology it is possible to construct an “equivalent material” envelope that honours the strength of the intact rock and joint fabric within the rock bridges that may occur along a candidate failure surface in a closely jointed rock mass; and
  • because the characterisation tests are done for different sample sizes (ranging between 5 m and 80 m cubes in the test cases) and orientations, using the SRM methodology it is possible to demonstrate the effect of scale and defect orientation on the strength of the rock mass.

In fact, an SRM sample consists of a bonded-particle assembly. The intact strength, determined by the bond strengths, is calibrated with simulated UCS tests. The discontinuities are added (as weak bonds) from joint segments from a generated DFN (Discrete Fracture Network) derived from observed joint statistics. Then, UCS, triaxial and tensile tests are performed on the resulting rock-mass sample. Development of the SRM within the LOP Project has provided a means of establishing a constitutive material model (strength envelope) that is not reliant on Mohr-Coulomb or Hoek-Brown criteria and a strength envelope from which the Hoek-Brown parameters can be derived (i.e. it provides a means of calibrating the Hoek-Brown strength envelope). The SRM development work also showed, however, that hardware limitations made it difficult to perform 3D SRM simulations using PFC3D at the same resolution as in 2D.  This resulted in work specifically targeted at improving the resolution and speed of full 3D using a special purpose code based on a simplified lattice that will run 10 times faster than PFC3D and will be able to handle much larger models.  The new code, Slope Model, written for the LOP project, embodies the SRM concept so that a DFN may be imported, with failure involving both movement on the faults and joints that intersect the rock mass and breakage through the intervening rock bridges.

Chapters 5, 7 and 10 of the Open Pit Slope Design Guidelines book contain more information about the outcomes of this research work.  The SRM method is also outlined and endorsed in Dr. Evert Hoek’s keynote address for the November SS09 conference in Santiago.