Impacts of gloving and un-mixed resin in fully encapsulated roof bolts on geotechnical design assumptions and strata control in coal mines

Abstract

Modern strata control practices utilising resin encapsulated roof bolts has been adopted in high productivity coal mining operations as the primary means of reinforcement. The geotechnical design of safe and fit-for-purpose underground excavations relies on many design assumptions regarding the effectiveness and load transfer characteristics of the installed reinforcement. The effectiveness of the installed roof bolts can be compromised by glove fingering (gloving) of the resin cartridge and un-mixing of the resin mastic, resulting in a reduction in the load transfer characteristics of the roof bolts, and ultimately, compromising the design assumptions used. Following a routine installed bolt quality audit and some small roof falls containing gloved bolts across a range of mine sites involving many geological and geotechnical conditions, a work programme was initiated to determine the extent of the gloving and un-mixing problem and to develop an understanding of mechanisms involved. Results have shown that gloving and un-mixing is a systematic and widespread phenomena, occurring across the range of resin and/or bolt manufacturers, and in a variety of roof types. Gloving was found in bolts installed using either hand held pneumatic or continuous miner-mounted hydraulic bolting rigs, under face conditions by operators, and under controlled manufacturers “best practice” conditions. Research has been undertaken using recovered bolts from various mine sites, as well as test bench trials and the quantification of the loading characteristics of gloved bolts using strain-gauged roof bolts. To understand the impacts of gloved bolts and un-mixed resin on the geotechnical design assumptions used a FLAC 2D numerical simulation was undertaken. The results provided an insight into the way the roof bolts interact, the resultant failure mechanisms/pathways and provided quantification of deformation levels and reinforcement requirements. The results being incorporated into the strata management plans.