FORENSICS AT COLLAPSED PHOSPHATE UNDERGROUND MINE: NUMERICAL APPROACH

Mining and Petroleum Engineering Department, Faculty of Engineering, Al Azhar University,

Nasr City, Cairo Egypt

A dry self-supported Room & Pillar phosphate mine at Zug ElBahr area - Egypt flooded

on winter of the 1994. Given that only random timber are in place, the Marly immediate

roof deteriorated and finally collapsed. A previous mineralogical SEM study indicated

that the Marly roof contains nearly 70% of the swelling Ca Montmorillonite. A 22%

volume increase occurred at no-load point of Odometer experiment pointing to swelling

as possible cause of roof failure. Such scenario is highly possible as most of the Eastern

Egyptian UG phosphate mines are developed in equal fashion at similar geological

setting. The geotechnical responses of the mine roof to assess future roof support

requirements and action plans are numerically assessed. A FLAC2D mine model is

developed to replicate mining, swelling and act of gravity leading to roof collapse. The

model incorporates plastic behavioral regime for phosphate and its country rocks. The

model quantitively showed that the mine was initially self-support before flooding.

Model qualitatively was able to replicate roof swell and failure regime modes. The roof

reaction curves before and after flooding was developed to assess required support

capacities needed. A regional support system rather than random tempers is

recommended. Surface flood preventive measures are foreseen as prime solution while

proposed regional support system is sought at flood-vulnerable areas.