A new contact model to improve the simulated ratio of unconfined compressive strength to tensile strength in bonded particle models

• A new contact model for BPM was developed and implemented in PFC3D.
• The new contact model can properly consider the contribution of moments to contact stresses.
• The new contact model can properly consider the condition at which the contact fails.
• The UCS/T ratios from simulations using the new contact model are in good agreement with experimental values.

The bonded particle model (BPM) has been increasingly used to simulate and analyze the mechanical behavior of rocks. However, traditional BPMs that use symmetric particles bear the intrinsic limitation of severely overestimating the tensile strength of simulated rock and leading to unrealistically low unconfined compressive strength to tensile strength (UCS/T) ratio. Researchers have proposed different methods to increase the UCS/T ratio in BPM simulations; but they all have limitations. In this paper, a new contact model which properly considers the contribution of moments to normal and shear contact stresses and the condition at which the contact fails was proposed and implemented in the three-dimensional Particle Flow Code (PFC3D). The new contact model does not impose significant additional calculation burden. Detailed parametric studies were performed to evaluate the effect of different microscopic parameters on the UCS/T ratio of BPM specimens. The results indicate that the moment contribution, cohesion and tensile strength are three key factors affecting the UCS/T ratio. When the value of the moment contribution parameters is smaller than 0.5, a wide range of UCS/T ratios can be achieved by changing the cohesion and/or tensile strength values. Finally, the new contact model was used to simulate two widely studied rocks, Lac Du Bonnet (LDB) granite and Carrara marble, and the obtained UCS/T ratios were in good agreement with the measured values from laboratory experiments.