Non-core boring with tri-corn bit has advantages in comparison with core-boring in boring operation and it becomes to be fairly certain to estimate the rock strength by measuring the cutting speed, thrust, torque and amount of revolution in non-core boring in situ.
It has been clear in laboratory test that there are three methods to estimate the compressive strength of rock by non-core boring.
(1) To correlate it to the cutting speed and the multiplied product of the amount of revolution and thrust.
(2) To correlate it to the boring power and the cutting speed.
(3) To use the cutting coefficient α defined by pretest as
α; coefficient of boring ability
σc; compressive strength (kg/cm²)
R; cutting speed (cm/min)
N; amount of revolution (r. p. m.)
W; thrust (kg)
In order to compare the results of laboratory test and the actual ones in field, both core boring and noncore boring were performed at the same point. The actual compressive strength was examined using the coreboring samples. Compressive strength of rock in non-core boring was also estimated by the three methods, and was compared to the former. Judging from the test results, it is clarified that the actual strengthes are similar to those estimated using the method (1) and (3), and in the case of the method (2) the estimated strengthes are commonly larger than the actual ones.