Predicting the penetration rate of percussive blasthole drills using coarseness index and median particle size
Percussion drilling equipment is widely used in quarries and construction sites. Having some prior knowledge of the potential performance of the selected rock drilling equipment is very important in rock excavation projects for planning and cost estimation purposes. Many investigators have tried to correlate drillability and various mechanical rock properties. However, to date there is no correlation between drillability and the coarseness index (CI) or median particle size (MPS) value. In this study, the relations between the penetration rate of percussive drills and both CI and MPS values were investigated. CI is a
non-dimensional number, being the sum of cumulative weight percentages that are oversize in a particular sizegraded
sample of debris. In the calculation of CI, a drillcutting sample is sized in a set of sieves and the cumulative weight percentage of each size fraction is determined. The sum of cumulative weight percentages is defined as CI. There are several methods of recording the results of sieve analysis and extracting an average size of particle distribution. The plot of cumulative undersize (or
oversize) versus particle size on a logarithmic scale is the most common method. MPS is obtained from this plot and defined as the particle for which the quantity of particles equals 50% of the total of the cumulative distribution.
Hydraulic top hammers, pneumatic top hammers, and down-the-hole drills were observed in several rock types at quarries and highway construction sites for the performance data. Drill rig type, bit type and diameter, hole length, feed pressure, rotation pressure, blow pressure, air pressure, net drilling time, etc. were recorded during performance studies. Four cases using hydraulic top hammer drills, one case using pneumatic top hammer drills, and three cases using down-the-hole drills were studied. Net penetration rates were then calculated from performance data. During the performance studies, drill cuttings from each drill hole were collected for the determination of CI and MPS values. In the sieve analysis, two different sieve series
were used to suit drill cuttings. The first series was selected
as 16.00 mm, 8.00 mm, 4.00 mm, 2.00 mm, 1.00 mm, and 0.50 mm, and the second series as 8.00 mm, 4.00 mm, 2.00 mm, 1.00 mm, 0.50 mm, and 0.25 mm. Collected drill cuttings were reduced using the quartering method. The sieves were stacked on a sieve shaker and a sieving time of five minutes per sample was adopted. After each sieving was completed, the particles remaining on each sieve were weighed. Percentage weights, cumulative weights percentage that are oversize and undersize of the particles, were calculated from the results of the sieve analysis for each sample. The sum of cumulative weight percentages oversize was taken as the CI value. The graphs of cumulative undersize vs particle size on a logarithmic scale were plotted for each sample and MPS values were extracted from
these graphs. The penetration rates of the drills, CI values,
and MPS values were analyzed using the least square regression method. The equation of the best-fit line, the 95% confidence limits, and the correlation coefficient (r) were determined for each regression. It was found that there are usually strong linear relationships between penetration rates and CI values, and penetration rates and MPS values, i.e. the higher the CI and MPS values, the higher the penetration rate. A plot indicating the relation between penetration rate and CI value for a hydraulic top hammer drill (case 1) is shown in the figure.
It was shown that CI and MPS values strongly correlate with penetration rates of percussive drills. Therefore, CI and MPS may be a representative measure of penetration rate of percussive drills.
Percussive drills, Penetration rate, Coarseness index, Median particle size, Statistical analysis.