Tensile stress field would appear in the proximity of arched vault after excavation , slacking of surrounding rock and subsequent dropping of blocks of rock from such area would possibly occur at any time in the context of extra - shallow depth . so i - steel arched support must be erected to provide against possible block dropping and be integrated into a ring with that of pilot tunnels on both sides 拱頂在核心土開挖后,在一定范圍內(nèi)出現(xiàn)拉應(yīng)力區(qū),在超淺埋情況下極容易掉塊和發(fā)生圍巖松馳,因此核心土必須邊開挖邊架設(shè)工字鋼拱架,使之與左右導(dǎo)洞拱頂工字鋼形成整體。