Time dependent deformation behaviour of the hangingwall at the Kiirunavaara mine and its effect on the railway

Two types of deformation zones have been identified on the hangingwall at the Kiirunavaara mine - continuous and discontinuous. The continuous deformation zone is characterized by a smooth lowering of ground surface accompanied by horizontal displacement moving in the direction of the mine to the cavity created by the mine extraction. The discontinuous deformation zone is characterized by large displacement over limited surface areas forming chimneys, steps and large tension cracks. Since the deformation of the hangingwall is time dependent, the time-displacement behaviour was independently analyzed in two directions - X (perpendicular to the ore body) and Z (vertical) using surveying data from a network of stations from twelve surveying profiles along the hangingwall. These data cover a period of time of 13 years starting from 1996 and ending in 2008. The movements along the Y-direction (parallel to the ore body) are smaller and erratic therefore they were not considered. In addition, extension strain and slope were calculated for some areas where the limit between deformation zones was not clear. Curves of time versus displacement were analyzed. In the curves three stages were identified -regressive, progressive and steady state. During the regressive stage only continuous deformation is observed in the field using monitoring systems. The progressive stage is an intermediate stage between the regressive and steady state where the first cracks appear and develop. The limit between the first two stages is defined by the on-set of failure point in the curves. The point of failure (critical vertical displacement, CVD, or critical horizontal displacement, CHD) in the hangingwall was defined using the change of stage between the progressive and steady state stages. This point also defined the limit between the continuous and discontinuous deformations zones. Although the hangingwall subsidence is extending with deeper mining levels, the results show a general tendency of decreasing rate of movement due to the increasing level of caved rock that confines and constrains the movement. This fact means that it will take a longer period of time for a point farther from the mine to reach the same displacement magnitude compared to the points closer to the mine. Today, the railway is located in the outer limit of the continuous deformation zone and therefore in the regressive stage. The ground in this area is experiencing about 2 to 5 cm of vertical and horizontal accumulated displacement. To predict further displacements and rates of movements, polynomial functions were fitted to the timedisplacement data during the regressive and progressive stages. The most representative functions for each surveying line were selected to do the forecast for nine stations located close to the railway. The result indicates that the vertical and horizontal displacements could increase to about 10 to 30 cm in the following 4 years (until 2012). This prediction was also extended to cover the whole regressive stage until the year 2018 (see Appendix C). This is a conservative forecast because it was estimated with data from points closer to the mine. No large movement or catastrophic collapse is likely to occur during the regressive phase. However, the deformation may concentrate at large geological structures and its effect needs to be evaluated to adjust the forecast. There is a documented case by Herdocia (1991) where a regional fault in Gränsgesberg mine was reactivated, thus changing the overall deformation behaviour of the hangingwall.Godkänd; 2010; 20091217 (tomvil