STRESS CORROSION CRACKING OF REBAR ROOF BOLTS IN U.S. UNDERGROUND COAL MINES - A PRELIMINARY STUDY

Author

Gopi Bylapudi, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2014

Degree Name

Master of Science

Department

Mining Engineering

First Advisor

Spearing, A.J.S

Second Advisor

Mondal, Kanchan

Abstract

According to the National Institute of Occupational Safety and Health (NIOSH), about 100 million rock anchors were installed in the USA mining industry during 1999 (Dolinar, 2000). The rock bolt usage in US coal mining industry fell from 85 million in the year 1988 to 68 million by 2005 (Tadolini, 2006), and is assumed to be close to that number of rock anchors consumed currently since, the tonnage from underground is almost the same. Most underground coal mines have conditions such as moisture in the atmosphere, ground water with different chemical contents that are conducive for corrosion of rock anchors and ancillaries (such as plates), and the effects of this on the performance of the anchors had been researched in the US to an extent from the past research at Southern Illinois University Carbondale (SIUC). In addition to the general corrosion like pitting and crevice, stress corrosion adds to the process a potentially serious threat and results in material failure underground due to stress corrosion cracking (SCC) yet the effects are not fully understood in the USA. The results of this research therefore will have a positive and direct effect on rock related safety. During this research project in situ specific tests were conducted with bolts to try and determine the corrosion potential in a specific coal-mining region. The coal mining areas were divided into three regions and were named as East, Mid-West and West respectively. To enhance the value/importance of the field data collected from the mines, a metal mine and a salt mine (two non-coal mines) were included in the plan and the data analysis proved that the methodology developed for determining the corrosion potential underground is applicable to any underground mines. The Insitu studies include water samples collection and analysis and open circuit potential (OCP/Eoc) testing and analysis. Open Circuit Potential (OCP) data were recorded to estimate probability of active corrosion. Hypothetically, probability of active corrosion is lower if the actual OCP of roof bolts in the mine is less than the characteristic OCP of the steel grade, and vice versa. The effects of certain factors such as the roof condition, reference distance (distance between bolt and reference electrode) on the open circuit potential data during the measurements were studied to ensure its impact on the corrosion potential determination technique developed. The findings from this research helps standardize the corrosion potential determination methodology. The preliminary study of stress corrosion cracking of the subject test sample (Grade 60 rebar roof bolt) was conducted in this research work. The experimental study invloves testing a complete roof bolt in the mine simulated environment. The mine simulated environment in the test cell consists of the roof strata material collected from the mine site with continuous flow of water at slower and varaible flow rate (0 to 3 ml/minute) with pH in the range of 7.5 to 9.0. The results showed that stress corrosion could be very serious problem when it comes to long term mining applications. The stress corrosion test cell developed and tested was proved to be significant in conducting the long term stress corrosion tests. The strength results of the Grade 60 rebar roof bolt tested had a significant strength loss after 3 months of testing in the stress corrosion cell. Hence, more SCC studies are deemed necessary to evaluate the seriousness of the problem and if possible eliminate it.