In order to eliminate fall of ground accidents, Safety in Mines Research Advisory Committee (SIMRAC) initiated a research programme to investigate the roof support systems that are currently being used in South African collieries. The aim of this programme was to gain an understanding of the fundamental mechanisms of roof support systems and to develop guidelines and design methodologies for their improvement.
SIM 020205 investigated all of the currently available elements of roof bolt support, along with their related installation machinery. These were tested underground in three different rock types, namely sandstone, shale, and coal. The current project, SIM020205b is aimed at:
- Quantification of the effect of parameters that were identified by SIM 020205 as important in roof support system performance.
- Development of guidelines for testing procedures to achieve the maximum support performances as recommended by SIM 020205.
- An improved roof bolt installation technique.
- Development of new resin quality control and testing procedures.
The findings of these two projects will also be incorporated into a booklet, entitled “roof bolting guidelines for South African collieries”.
SIM 020205 highlighted hot and cold rolling of steel as an important parameter in controlling the performance of a support system. In the current project, it has been established that the hot rolling process of bolt manufacture is preferential for use in the coal mining industry. This is because hot rolling produces a more consistent product in terms of the bolt’s dimensions, i.e. core diameter and rib height. From underground short encapsulation pull tests, it is found that hot rolling gives a better performance in terms of grip factor and stiffness when compared to bolts manufactured using the cold rolled process.
“Finger gloving” is one of the most discussed topics of roof bolting. “Finger gloving” occurs when the Mylar cartridge wrapper remains intact around the hardened resin. This prevents the resin from completely bonding to the rock. In order to determine the severity of this phenomenon, a series of test installations were carried out in Perspex tubes to observe the quality of resin mixing when using different installation methods. These observations revealed that “finger gloving” occurred, to a greater or lesser extent, in almost all installations. It was found that more “finger gloving” was likely to occur if the bolt was pushed through the resin capsule before spinning took place. Whilst underground SEPT identified a slight difference in the performance of bolts installed in this method, indicating a higher percentage of finger gloving, the difference was not great enough to be alarming and still comfortably passed recommended support requirements. It is therefore concluded that “finger gloving” is present in most installations but has little effect on the performance of the system.
A new support design methodology, using “height of fracturing in the roof”, was developed and summarised in the final report of SIM020205. This new methodology highlighted the importance of the shear strength of roof bolts that are currently being used in South African collieries. Laboratory shear tests were therefore performed on full column resin roofbolts, indicating that the shear strength of roofbolts is approximately 89 per cent of the ultimate tensile strength of the bolt, which is significantly greater than the previously assumed 50 per cent of tensile strength. It is established that the shear strength of 20 mm roof bolts currently being used by the steel supplied by ISCOR is approximately 213 kN.
SIM 020205 identified roofbolt installation as an area, which must be investigated. Four different types of installation were identified, namely:
- Flat end bolt spun through resin;
- Flat end bolt pushed through resin, then spun;
- Angle end bolt spun through resin;
- Angle end bolt pushed through resin, then spun.
Visual observations on the quality of resin mixing in each method were carried out. It was observed that when bolts, either flat or angle end, were pushed through the resin capsule before spinning, then a displacement of the catalyst took place. The catalyst is less viscous than the resin in a resin capsule and so when a bolt is pushed through the capsule, the less viscous material is displaced to the bottom of the hole. This effect is increased by gravitational effects in vertical holes. Mixing of resin and catalyst cannot take place properly therefore reducing the effectiveness of the support.
In addition, underground short encapsulation pull tests were conducted to quantify the effects of the different methods. The results showed that whilst differences in grip factor were minimal, large differences were found when comparing the stiffness of a system. In tests done for both 15-second and 30-second resins, grip factors for an angled bolt pushed through the resin before spinning were the poorest for both resin types. The other three installation methods performed identically well. In terms of stiffness, the flat end bolts outperformed the angle end bolts, with a flat end bolt spun through the resin capsule giving the best performance. It is therefore concluded that the best installation method to use is a flat end bolt, spun through the resin in order to achieve a stiffer roof support system.
Pull tests were also conducted on resins of various ages to determine the effect of expiry upon performance. It was established that performance of the system is reduced as the time increases past the expiry date. A resin, which had expired by 6 months, gave 33 per cent poorer performance than a new resin.
In order to determine a resin testing procedure, a series of tests were conducted using the testing facilities at Anglo Coal Rock Engineering Laboratory. Resins of different ages were tested. The results indicated that the older the resin, the lower the torque achieved. This indicates that a torque test could be a quick and easy method of determining expired resin.
Additional tests were conducted to determine whether resin gel time could be used to determine expiry of a resin but the results proved inconclusive.
The effects of over or under spinning of resin were observed and the effects quantified. It was established that under spinning of a 15-second spin to stall resin still provided results within the acceptable limits of performance. For a 30-second resin, over spinning resulted in reduced performance and it was concluded that a 30-second resin should not be spun for more than 20 seconds no matter how slow the spinning speed.
It is stated that laboratory testing is not appropriate to determine the performance of a roof bolting system to be used in an uncontrolled environment. It is therefore suggested that the performance and the quality control tests of roof bolt systems should be conducted underground in the environment where the roof bolts will be used. A simplified short encapsulated testing procedure is suggested, which eliminates the time consuming measurements which are taken in detailed SEPT. It is suggested that the measurements taken from the bolt, resin capsule and hole diameter can be eliminated without jeopardising the quality of test results.