CRACK FORMATION AND ERODIBILITY ON THE ARTIFICIAL CUTTING SLOPE COMPOSED OF “KANTO LOAM”, IN TSUKUBA UPLAND, IBARAKI PREFECTURE, JAPAN

Abstract

There are a lot of artificial cutting slopes composed of various beds, that is, volcanic ash, clayey, sandy beds and so on, and such slopes have frequently marked relief or irregularity. It is considered that the relief indicates the difference in the properties of each bed. In Tsukuba upland, many cutting slopes have been artificially formed accompanied with the expansion of residential areas, and a few number of crack horizons are usually observed on the slope as is often the case with other places in Kanto Regions, covered with so called “Kanto Loam” (tephra) deposited in late Pleistocene. The authors analyzed physical, mechanical properties, and mineral constituents of “Kanto Loam” composing cutting slope in order to understand the crack forming mechanism and erodibility of the slope located near Tsukuba Academic city. The results obtained are as follows: (1) The relief or irregularity of the slope is greatly concerned with crack development (Fig. 2). It does not always correspond to mechanical properties estimated from soil hardness (P) and plasticity index (PI), but has close relations with ability of crack formation caused by physical, chemical properties and mineral constituent (Table 1, Photo. 2). (2) The beds 2 and 7 (Photo. 1, Fig. 2), having the ability of great shrinkage upon dry-ing, show the remarkable crack formation, while the bed 3 has the opposite chracteristics (Fig. 3). (3) The crack formation in the bed 7 is attributed to the great amount of clay content. In the “Kanto Loam”, the existence of imogolite, in addition to clay content, seems to play the important role in this phenomenon (2, 4 in Photo. 2). (4) The crack formation in the beds 2 and 7 increases the susceptibility to soil fall, and excerts the important influence upon relief formation on cutting slope composed of “Kanto Loam”.