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The deep weathered region of the granitic rocks in the western part of Hiroshima-city coincide with the dessected pediment geomorphologically which was studied in detail by Akagi Y. (1962). In this paper, the author has made clear the following points from an analysis of p-wave velocity, N-value profile and geomorphological development.
(1) The deep weathered zone in this region is composed of two mirage layers which is given a name respectively “the upper weathered zone” and “the lower weathered zone” for a distribution of P-wave velocity. Each zone is characterized by the constant rate of increase for P-wave velocity and N-value.
(2) The primary sheeting joint (P·S·J) and the secondary sheeting joint (S·S·J) descrived by previous paper, Hashikawa, et al (1974) develope respectively in the lower weathered zone and the upper weathered zone.
(3) The P·S·J develope almost parallel to the horizontal or slightly inclined land surface of pediment and to the unweathered bed rocks. The other hand the S·S·Jis almost parallel to the convex land surface of the dessected pediment.
(4) The structural development history of the weathered zone is divided to 5 stages in Fig. 9.

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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.

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