液状化によるマンホールの最大浮上量の推定法

Abstract

大地震時のマンホールの浮上りは，埋戻し土の液状化に伴う過剰間隙水圧の上昇によりマンホール底面に作用する上向きの力が，マンホールの自重，側面に作用する摩擦力および下水管接続部の抵抗力の合力を上回ったとき生じる．ただし，いったん浮上がった後沈まないためには，マンホール直下に埋戻し土が回り込まなければならない．したがって，浮上過程が非排水条件だと仮定すれば，マンホール直下に回り込んだ土の体積分，すなわち浮上したマンホール本体の体積分の沈下が埋戻し領域に発生すると考えてよい．ただし，間隙水圧消散に伴う埋戻し土自体の圧縮による沈下は相対的に無視しうるほど小さい(εv縲・%)とする．

Uplifting of sewage manholes is one of the typical and striking damage pattern observed in the area being hit by large earthquakes. A simplified method to estimate the maximum uplift displacement of a manhole and settlements of backfill soil under liquefaction is derived based on the mechanism of uplift of a manhole under undrained condition of backfill soil. The method is capable of evaluating effectiveness of countermeasures against uplift by considering excess pore water pressure ratio and/or unit weight of backfill soil. In the present study, the applicability for the case without countermeasures is investigated through comparison with experimental results. Results show that measured uplift displacements and settlements are within the range predicted by the proposed method.