Cutting slope in shimajiri mudstone : I. Weathering and hardness (Department of Agricultural Eegineering)

びょう打ち銃と山中式土壌硬度計を使用して泥岩切土ノリ面の風化状態と硬度の関係について調査し, 以下の結果を得た。(1)未風化泥岩のピン貫入量及び硬度指数はそれぞれP=4∿6cm, Y=34∿37mmとなり, 泥岩表土ではそれぞれP=30∿40cm, Y=5∿25mmとなる。(2)ガリ侵食面ではP=3∿8cmとなり, 表層風化部が絶えず雨水によって洗掘されて泥岩が露頭している。(3)切土面は乾燥・湿潤を繰返し, P=15∿20cmの風化層を早期に形成する。深度25cm付近では含水比変化も小さく, 風化速度も緩慢となる。(4)非植生ノリ面ではP=10∿37cmが得られ, 風化土の崩落や小規模崩壊によってノリ面が絶えず後退している。(5)植生ノリ面ではP=10∿17cmが得られ, 植生の被覆効果によって表層及び内部の泥岩の乾燥化が抑制され, 風化速度も緩慢となる。(6)地すべり多発斜面の軟化泥岩の硬度はP=25∿30cm, Y=24∿26mmを示し, かなり強度を低下させている。Based on the idea that strength is the best index of weathering and implies a hardness when the degree of weathering is classified from the engineering point of view, this report deals with the relationship between the progressive state of weathering and hardness in the cutting slope of Shimajiri mudstone. Measurement of the hardness was conducted by using a pin penetration gun and a soil hardnessmeter of Yamanaka-shiki. Results obtained are as follows : 1. The pin penetration distance P is expressed in P=4-6cm, and the measured hardness Y in Y=34-37mm, respectively, in unweathered Shimajiri mudstone; and they have been P=30-40cm, and Y=5-25mm, respectively, in the soil layer from the mudstone. 2. P=3-8cm has been obtained in the face of a gully formed by erosion. 3. The cut plane in the mudstone repeats drying and wetting and forms soon a weathered layer with the hardness of P=15-20cm 4. In the slope without vegetation, P=10-37cm has been obtained, and the face of the slope keeps going back by falling of weathered soil in the mudstone and by small-scale collapse. 5. In the slope covered with vegtation P=10-17cm has been obtained. It is observed that drying of the mudstone is suppressed by the covering effect. 6. The softened mudstone has such hardness as P=25-30cm and Y=24-26mm, and its strength drops considerably in comparison with that of non-softened mudstone