The authors engaged in trial researches to grasp correctly weathering process through inspection intoforest physiogomy in Chichibu mountainous district, from the view point of both forest pedology and plantecology. They considered that weathering evidences would be represented by the soil cover thickness ofgrounds. Generally speaking, a tree keeps its habitat in soil cover, the thickness of which is determined byland shapes, parent rocks and the duration term of weathering process etc. Accordingly it is not so difficultto estimate soil cover thickness by the prevailing species of trees in a district, if the root length of a tree correlatesto soil cover thickness. Pinus densiflora, for instance, grows up on sterile lands, such as summits, ridges and steep slopes of mountains, or barren grounds in the relics of landslides and slope failures, becauseit sustains a thinner soil cover and needs more sufficient insolation, while Criptomeria japonica and Chamaecyparis obutusa require a wellfed soil, sufficient water supply and suitable drainage condition, thereforethey grow up often on a mountain foot and a tablus slope, where weathering process into such a deeper layeras the afforestation of both kind of the trees is possible. Quercus serrata, Acer Mono and Prunus jamasakuraluxuriate in an intermediate zone between the above-mentioned two extreme plant communities, bacause soildevelopes to an average grade in this plant zone which occupies an extensive area as a natural forest, consistingof deciduous and latifoliate trees.
The authours could estimate approximately the thickness of soil cover of places, where each kind of theforegoing plant community was found, because they witnessed the fact that the penetrating depth of a mainroot corresponds to 1/5-1/7 of a trunk height. In order to prove the above-mentioned fiied observation, theauthors undertook several field experiments, such as test pits, boring works and seismic prospectings on the37 locations in Chichibu mountainous district, arriving at such conclusion that the thickness of soil cover (=y) interrelates to the ratio of σ/Vp (=x) as an exponential function, that is,
here-y=a·Ebcx…(1)σ(kg/cm²)…compressive strength of a fresh rock. Vp (km/sec)…velocity of P-wave of the second speed layer.x… ratio of a: Vp, y… thickness of soil cover or weathering stratum.a, b, c… constnt.
The constants have been determined through calculation by the method of least square, as follows: y=1.758×10^0-.0817x…(2)«thickness of weathering earth»y=7.007×10^-0.0626x…(3)«depth down to the fresh rock base»
Finally the authors have reviewed the utility of the formula and the graph-3, in regard to application to engineering geology.