Liquefaction seen over wide areas along Tokyo Bay

[Photo: Manholes sit pushed up from the ground from the recent earthquake in Urayasu, Japan.] 

Liquefaction was reported in many areas along Tokyo Bay from Chiba to Kanagawa prefectures after the March 11 earthquake, pushing up manholes and wrecking homes and roads.

The watery sand that gushed from the ground during the earthquake formed piles up to 30 centimeters deep on reclaimed land stretching more than 30 kilometers from Mihama Ward, Chiba, to Shin-Kiba in Koto Ward, Tokyo.

The liquefaction left houses and power poles tilting in many areas.

Liquefaction occurs when saturated sandy ground, such as that found in reclaimed land and marshes, is loosened by a strong earthquake. The unconsolidated sand becomes like muddy water. This muddy water gushes up through cracks and opening in sidewalks and roads, and then drains away to leave the sand on the ground.

The latest earthquake caused ground subsidence of up to 50 centimeters, which destroyed underground water and sewerage pipes.

Reclaimed land usually stabilizes with time. Some areas along Tokyo Bay were reclaimed during the Edo period (1603-1867) and before World War II. However, the areas hit hardest by liquefaction during the March 11 earthquake had been reclaimed after the war with sediment taken from the seabed. In those areas, digging down a few meters will reveal a saturated sandy layer.

However, liquefaction did not affect places such as the Makuhari Messe complex in Mihama Ward, Chiba Prefecture, because the land had been drained or cement was mixed in the soil before construction.

The severity of the shaking during the quake also affected the extent of the damage caused.

The earthquake was felt less strongly on reclaimed land from east of Chiba city to Kisarazu in eastern Chiba Prefecture, as well as from Kawasaki to Yokohama, where liquefaction was not a serious problem.

The shaking continued for quite a long time, and the ground where the liquefaction was most severe was still slowly shaking even after liquefaction occurred, expanding the scope of damage.

Many residents are torn over whether to rebuild their damaged houses.

Foundations can be fixed by jacking up a house and doing the repairs. But with the lingering threat of aftershocks, the land could be vulnerable to more damage. Leaving a house as it is, however, could leave it beyond salvaging if liquefaction strikes again.