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  1. RELATIONSHIP BETWEEN THE GROUND AND DAMAGE
  1. ARTIFICIAL VALLEY FILLS -- DAMAGE OF THE ARTIFICIAL LANDFORM

Schools on the Valley Fills

Huge damage was seen on the Osaka Group, which was formed about one million years ago and had been regarded as stable ground. Investigation into the damaged areas discovered that the ground was artificial valley fills in the Osaka Group.

Out of 122 schools in Kobe city, 21 were reconstructed, 10 needed major repairing and 35 needed medium scale repairing (Kobe City's Education Board, 1996). In and around Nagata ward, 51 schools were built on soft landfills, or an artificial landforms. Six out of seven schools built on valley fills in the hill area, in particular, suffered devastating damage. On top of that, there is another serious issue that about 35% of official buildings such as schools and hospitals are built on artificial landform such as reservoir fills (Figure 1).

Figure1

Landslide in the Valley Fills

Valley fills easily slide -- the ground of valley-fills was soft and, as it used to be a valley, it served as a drainage basin, the ground water level was shallow, the earthquake vibration was amplified, the pore water pressure increased, and the strength in the foundation strata was excessively weakened. The inclined ground water level was another contributor. As a result, the upstream area of the valley fills were pulled down, causing cracks to open, and compressive deformations to occur downstream. There are many artificial landforms such as leveled hills and valley fills which have been made in the Hanshin area in need of flatland for residential use (Figure 2). To make the situation worse, such artificial land formation works have not been operated with adequate consideration, which amplified the damage. In other words, it was a man-made disaster.

Figure2

(TAINOSHO Yoshiaki)