“Galurgiya” Company:

B. A. Borzakovskiy, Head of Laboratory, Candidate of Engineering Sciences, e-mail: mail@gallurgy.ru

A. Ya. Grinberg, Chief Executive Officer Candidate of Engineering Sciences

B. N. Tolmachev, Project Manager

In October, 2006 water rushed into the mine of the First Berezniki potassium production mine group and flooding began. Three beds were being developed in the mine field: two lower sylvinite ones and upper carnallite one. Chamber-type development system. Salt beds of 70 to 110 m total thickness were situated above the beds being developed. The mine field is situated within the doming. A part of urban development and industrial features are situated above the goaf. Water rushed into the central part of the mine field at the doming slope. Excavations were flooded first in the lower, then in the upper part of the mine field. Mainly soil and excavation walls were being leached while the lower part was being flooded, the excavation walls and roofing while the upper part was being flooded. Average initial salinity of waters entering the goaf was determined from dissolved salt volume which was calculated from the volume of caved insoluble rocks in the collapse. Calculations show that the initial water saturation degree was: 46 % for NaCl, 12 % for KCl, 2 % for MgCl₂. In July, 2007 a collapse was formed above the inrush place and it expanded gradually. In December, 2008 the mine was flooded completely. The flooding volume was about 84 mln m³. When flooding finished, the collapse had size of 340×420 m, area of 10.8 ha. Subsequent survey showed the collapse was 83 m deep, the collapse sink-hole had volume of 4.02 mln m³. In November, 2010 the second collapse occurred. The second collapse was formed 770 m from the first. The prerequisites for formation of the second collapse were: additional excavations in the upper part of pillars, small backfill volume, water motion at excavation roofing, presence of conditions for heavy MgCl₂-bearing solution flowing into lower excavations. Initially the collapse was 10×14 m, in expanded and in April, 2011 it increased to 60×90 m. The collapse was 95 m deep, the flanks inclined by 78 to 81°, the volume about 420 ths m³. The collapse was situated within a railway station so it was decided to have it backfilled. Under conditions of expanding collapse, a safe high-performance backfill process was developed. A weakly compressible material, gravel/sand mix was used as a backfill. The backfilling took 150 days. Backfill weight: 900 ths t. Due to the bulk of brine in the watersealed mine not being saturated with MgCl₂, accelerated falling is likely at mine field areas where roofing may collapse to the carnallite bed.