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  1. RELATIONSHIP BETWEEN THE GROUND AND DAMAGE
  1. SHALLOW WATER TABLE AND SOFT ALLUVIAL CLAY -- IMPORTANCE OF RESEARCH AND ANNOUNCEMENT

Land Originally the Seafloor with Deposition of Clay

It was the earthquake vibration which caused damage to many buildings in the Hanshin Earthquake, meaning that the strength in the foundation strata was hugely influential. The ground in Nagata-ward and Nada-ward, where the damage was immense, is a young alluvial clay stratum formed during the Jomon transgression. The stratum was composed of clay deposited on the seafloor about 6,000 years ago, when the sea level rose so high that the sea invaded to reach the foot of the mountains. The foundation stratum is soft with an N value (standard penetration test) of 10 or lower and the ground is soft like a rice field. It is quite presumable, therefore, that the earthquake vibration was amplified to the extent of making the tremor far stronger than the surrounding stable ground experienced, causing huge damage. The relation between the thickness of the clay stratum and the degree of the damage was very close in Nagata; the thicker the clay stratum was, the bigger the damage was.

Severe Damage in the Areas with a Shallow Water Table

It is not only the strength of the foundation strata but also the ground water level that has a lot to do with the extent of the earthquake damage. The ground water level was two meters in depth or less in most of the areas which suffered severe damage. The area with a two-meter-depth ground water level almost exactly coincided with the "Earthquake Belt", going through Nagata, Suma, Nada and Higashinada, the areas which experienced a degree seven of seismic intensity. When limited to the areas with a ground water level of less than 1.5 meter in depth, it shows almost the same area as that of the quake-shattered areas, Nagata and Nada, where the seismic intensity was over degree seven and more than half of the wooden housing collapsed (Tainosho, 1997).

When the ground water level is shallow, the stratum is saturated with water and, in an earthquake of degree five or more on the seismic intensity scale, pore water pressure between sand grains exceedingly increases, causing an extreme fall in the strength in the foundation strata, further causing liquefaction. Thus, buildings easily collapse on the soft ground.

Land with a shallow ground water level requires in-depth investigation and careful consideration to form foundations for buildings. It is desirable for the governmental bodies to carry out ground improvement works to the shallow ground water level in a wide area.

(TAINOSHO Yoshiaki)