Numerical modeling of threadbar under dynamic loading

Abstract

The prevention or mitigation of the effects of rockburst in rock masses subjected to high levels of stress, is one of the most challenging problems for the field of study of rock mechanics. Although the use of bolts is a main requirement when designing / installing a reinforcement system for underground mines, the design and selection guidelines of these that are used today are limited, since the choice of an element or another is often based on empirical knowledge and field observations. In this context, the use of laboratory scale test to represent in-situ conditions has become a useful tool to quantify the deformation and energy absorption of support elements and systems, providing the possibility for a comparative analysis of the performance between different types of fortification and retention elements, as well as their behavior under static and / or dynamic load. However, it is observed that, for practical reasons, most laboratory tests involve a high cost in preparation and validation. The challenge then is focused on how to obtain reliable results that can be used in the design of fortification systems for underground excavations. Numeric modeling appears as an alternative that, in addition to complementing laboratory results, it can be used to explain the process of deformation and energy absorption of tested support elements. An FDM numerical model is proposed considering that is necessary to implement different boundary shapes, different kinds of boundary conditions, and regions containing a number of different materials in order to represent the explicit dynamic response of reinforcement elements tested in laboratory. The main result is the accurately representation of the dynamic response of grout and threadbar (bolt) by varying the test conditions. Grout results are presented in terms of 𝜎�1 Vs 𝜎�3 taking values between 40-60 and 3-11 MPa respectively and 𝜏� Vs 𝜎�𝑛� between 16-21 and 11-23 MPa. Obtained results are compared with appropriate failure envelopes showing good agreement. Bolt results are mainly presented in terms of load capacity Vs displacement and absorbed energy Vs displacement. Results from initial model was compared and calibrated with information extracted from the existing literature and available laboratory results. After parametric analysis the relation between absorbed energy, displacement and characteristics of the bolt was established.