By correlating standardized rock strength and elastic constant properties to historical linear cutting test data for 3 different sized disc-cutters in 10 different hard-rocks, a rock cuttability index (RCI) is developed. This RCI is at least comparable and shows evidence of being superior to the unconfined compressive strength (UCS) for indicating a rocks resistance to disc based excavation. The results from 3 different sized cutterhead tests in 5 rock samples were then examined to determine the usefulness of the RCI. By utilizing the RCI or the UCS to determine the resistance to excavation of the rock, a linear relationship was discovered between the horsepower to cutterhead diameter ratio and the excavation efficiency of the cutterhead. This relationship allows a contractor to accurately estimate machine advance rates from a small sample of core.
BACKGROUND
1.1 Disc Cutters
While all indentive type tools create shear fractures, study of the cutter/rock interaction has shown that disc cutters create significantly more tensile fractures than either button-bit, or multi-disc cutters. Tensile fractures tend to create larger chips than shear fractures. This minimizes the amount of fracture surface created, and maximizes the volume of rock excavated for a given amount of energy. Additionally most rock has a compressive strength to tensile strength ratio of about 10, and this ratio increases as the overall strength of the rock increases. These are the two main factors in explaining the superior efficiency of the disc cutter over other types of mechanical hard-rock excavation tools.
These large diameter discs (12-inch diameter) have been installed on small diameter cutterheads but the large size of the disc mounting system creates problems for the designer. It is difficult to balance the cutterhead and to achieve proper spacing, resulting in significant reductions in efficiency for cutterhead diameters smaller than 6-8 feet. These problems have resulted in the use of multi-disc or keri' cutters on smaller diameter cutterheads. The development of small discs in the 5 to 6 inch diameter range early in the 1990s, permits the use of highly efficient single disc based excavation for small diameter cutterheads. Cutterheads as small as 13 inches have been tested in the laboratory and several single disc equipped cutterheads have been built and are successfully completing jobs in the 2 to 6-foot, diameter category
