Prevention of Manhole Floating

Earthquake is a matter of life and death in Japan.
Sewerage is no exception.
Underground structures are susceptible to liquefaction especially if they are in sand with high water table.
Manholes are the weakest1).

Three solutions are proposed in the national guideline for new manhole installation2).
They include 90% compact fill with sand, filling with soil cement, or filling with gravel.

For the existing manholes, there are several proposed technologies.
Here is an example with experimental demonstration3).
It uses metal weight to cope with buoyancy.
Metal weight is put in the frame fastened around a manhole.

In the experiment, without the weight, manhole came up out of ground.
With the weight, the manhole stayed in the ground.
The videos are on this site; http://www.fmhk.jp/product1.php?p=2.

1)http://ja.wikipedia.org/wiki/%E3%83%95%E3%82%A1%E3%82%A4%E3%83%AB:%E5%B0%8F%E5%8D%83%E8%B0%B7%E5%B8%82%E3%81%AE%E6%B6%B2%E7%8A%B6%E5%8C%96%E7%8F%BE%E8%B1%A1.jpg
2)http://www.mlit.go.jp/common/000026846.pdf
3)http://www.fmhk.jp/