batteries was 3 pounds, and the dimensions were 6.5 by 4.5 by 6.5 inches. Two 6.5-volt mercury cells supplied power for the thermometer (figs. 8-9). Data The data collected for the two different resins are presented separately because of differences in bolt installation methods and in the temperature increases created. Resin A Normal bolting practices in the White Pine mine include bending all bolts before installation with resin A, reportedly because the gel time of the resin was too long (8). According to mine personnel, the unbent bolts tends to drop out of the hole. However, the bent bolts rubbed against the side of the hole during rotation, and the rubbing of the bolt created frictional heat. Because the added heat interfered with the study, the investigation was made using straight bolts. Some preliminary tests were made using bent bolts. When a full load of resin was in the hole, the frictional effects of the bent bolt could not be detected. However, when one-half load of resin was used, the effects were noticeable in the measured temperatures. Figure 5 shows best fit curves drawn using data obtained from four unbent bolts installed with a full load of resin and a like number installed with one-half load. Although the individual measurements are not shown, all measurements fell within ±0.5° F of the curves. The curves are shown with the computer predicted curves and with the curves established from the laboratory data because all are based on resin A and can be compared easily. Field data curves for resin A are also shown in figure 10 for ease in comparison with similar curves generated from resin B data. The As shown in figures 6 and 10, a maximum temperature increase of about 3.0° F would be expected with one-half load of resin. A full amount of the resin produced a maximum increase of almost 7.0° F. The temperature remains above 4.0° F from 2 minutes after installation until more than 10 minutes after installation. A high temperature measured when a full load of resin was used is in reasonable agreement with that measured in the laboratory, but the decrease after the peak is reached occurs more quickly. Possibly this results from the presence of the steel plate on the bolts installed in the mine. plate would serve as a heat sink and would also offer a much larger area for emission into the air. Greater temperature increases were measured for partial loads in the mine than in the laboratory. This phenomenon remains to be explained. However, if a 4.0° F temperature increase is selected as the minimum acceptable indicator for a bolt 4 feet long, it should be possible to determine whether or not a full amount of resin was used by measuring the temperature increase from 5 to 10 minutes after installation. consistent and proper installation procedures so that frictional heat does not enter into the measurements. This assumes Resin B The temperature increases with resin B are greater than those observed with resin A and are shown by the best fit curves in figure 10. Measurements from three bolts with a full amount of resin and three bolts with one-half loads were used to construct the curve. All data are within ±0.5° F of the curve shown. Temper ature increases of 5.0° F or more within 10 minutes of installation appear to indicate adequate resingrouting provided that neither the plate nor the bolt is spun against the back or roof (so that frictional heat does not influence the data). The temperature increases measured on bolts. in the field (with the full amount of resin) were greater than those predicted by the three-dimensional model. It is possible that the actual values for conductivity, specific heat, etc., for the materials involved are different from the average values used in the calculations, and that these variations contributed to the difference between the predicted and measured values. Bolt insertion into the resin, bolt rotation in the resin, and work done on the bolt by the bolting machine would all increase the measured temperature differences, but these effects are not in the three-dimensional mathematical model. Bolt-Anchorage Tests Pull tests were selected as being the most nearly suitable means of verifying bolthead temperature results. resin bolts may test the strength of the exposed steel rather than the holding capability of the resin anchor. However, this error will only occur when the bolt is well anchored, and pull tests represent all independent means of verifying or disproving results of the temperature study. A hydraulic jack was used to apply known loads to bolts installed with a full column and with one-half column of resin. These tests were made for each type of resin with the full and one-half column installation. To reduce the effect of changing roof conditions, the locations were within 10 feet of each other for the two columns of each resin even though the two resins were used in different sections of the mine. Resin around the bolt at each pull test location was allowed to cure about 2 hours. The purpose of the pull tests was to measure differences in anchorage with full and partial loads of resin. Figure 11 shows a test in progress. Each bolt was pulled with forces starting at 1/2 ton and increasing in 1/2-ton increments. pull on resin A was 12 tons; maximum pull on resin B was 11-1/2 tons. After application of the maximum force to each bolt, it was again tested at 1/2 ton. The difference between the reading obtained at this time and the reading obtained when the bolt first was pulled with a 1/2-ton force is considered to be the permanent set in the bolt. For example, resin A had a set of 0.027 inch when anchored with à full amount of resin and 0.037 inch when anchored with one-half the required amount. Resin B had a set of 0.054 inch when anchored with the required amount of resin. When anchored with one-half the required resin, the set was 0.125 inch. Figure 12 shows the values obtained for each resin with full and onehalf loads. As seen in the figure, the holding power for one-half load of resin A was consistently less than for a full load when forces were greater than 2-1/2 tons. Both full and one-half loads of resin B had about the same holding power for forces of about 6 tons. At forces of greater magnitude the bolt anchored with a full amount of resin exhibited significantly greater holding power (integrity). The pull tests indicate that bolts anchored with a full column of resin have a greater resistance to tensional forces than bolts anchored with a partial column (fig. 12). Measured temperature increases indicate that greater temperature differences develop on bolts anchored with a full column than on bolts anchored with a partial column (fig. 10). Because both the temperature differences and the pull tests can be correlated to the column of resin, it should be possible to relate temperature differences to the ability of resin-grouted bolts to withstand tensional forces. |