Abstract

Rockbolts have been widely applied as permanent and temporary support systems to stabilize openings in tunneling and mining engineering. The presence of rockbolts improves the rock mass behavior by increasing bond strength between the rockbolt and drill hole. A friction rockbolt system, referred to as Starbolt, has been suggested to achieve instant bond strength and rapid installation of multiple rockbolts. The Starbolt system consists of a star-shaped, cold drawn steel tube and blast ignition system. The blast ignition system is able to use the thermite reaction of metal powder and detonation cords to inflate the drawn tube rapidly. In order to successfully install the star-shaped steel tube into drill holes, it is necessary to avoid the burst of the steel tube during inflation by vapor-gas production or detonation of the explosive cord.

This study suggested an approach for selecting the materials of the steel tube, which is suitable for the frictional steel tube rockbolts. Dynamic direct tensile tests of five steel tube specimens were conducted using a modified split Hopkinson tension bar to investigate the dynamic failure strengths and deformation. The Johnson-Cook model was used to analyze the dynamic inflating characteristic of steel materials. It was concluded that the STKM11A material, along with the other steel materials, is selected for the material of the steel tube rockbolt. The STKM11A steel tube rockbolts were installed in the thick steel tube to perform the pullout tests. The obtained load and displacement curves of the bolts were analyzed through commercial software, which was able to incorporate the rockbolt models in order to compare with the other types of rockbolts, such as Swellex and anchor bolts.

1.
Introduction

In rock engineering, rockbolts have been widely utilized to improve the stability and maintain the rock-bearing capacity near the underground openings or slope for many years. Frictional rockbolts, which interact with borehole wall by mechanical friction, are widely used as a passive bolt for preventing large rock deformations (Cao et al., 2013). The most widely used frictional rockbolts are the Split Set (International Rollforms, Inc.) and Swellex (Atlas Copco), which are mostly used in civil engineering and mining applications (Grasselli, 2005). However, their pullout loading capacity is relatively low. For example, a typical Split Set bolt may not be able to withstand loads higher than about 50kN (Li, 2010).

A dynamic-expansion rockbolt system, called Starbolt, has been suggested to provide an instant reinforcement and rapid installation of multiple rockbolts in openings without heavy apparatus (e.g. water pump) (Jeong et al., 2015). The Starbolt system consists of a star-shaped cold drawn steel tube and a dynamic expansion blast system. The blast ignition system should use a mixture of metal powder and crystallized water or electric detonator and detonation cord. The thermite reaction of metal powder was applied to vaporize the crystallized water. After the expansion agent is ignited in the steel tube, vapor gases are generated, and the inside of the tube is pressurized. This reaction leads to expansion of the steel tube in the borehole as shown in Fig. 1. In previous study (Jeong et al., 2015), the Starbolt showed that ultimate pullout bearing loads of 114 and 91 kN, which are reasonable requirements for applying rockbolt to underground support and rock bursts.

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