ABSTRACT

Investigation concerned with numerical modeling of geodynamic processes within Polish copper mines districts affected with a thick salt rock stratum presence are presented. The adequate parameters of the developed numerical model of rock mass behavior were determined using laboratory tests and back calculation procedure based on FEA. Particular attention was paid to estimating the salt-rock time-dependent behavior and its appropriate rheological parameters. Geological and geomechanical analyses permitted formulating 3d FDM based model utilizing the thick composite plate analogue, which represents sufficiently well the real geological structure and load floor and immediate roof strata with the load equal to their residual strength. The rocksalt presence above the exploited copper ore seam introduces significantly worse safety conditions as compared with salt less rock mass. This is because of the relatively low saltrock strength parameters (particularly tensile strength) as well as great thickness of salt deposit.

1 INTRODUCTION

The perspective of copper ore development in the northern part of the Polkowise- Sieroszowise mine in Poland, introduces into the local mining practice the new, extremely important parameter such as time factor. This is due to thick saltrock deposit presence in the rock mass surrounding the prospective mine excavations at the depth of approximately 1,500m below the ground (Figure 1). The time factor links the mentioned upper rheological layer behavior and the stability beneath located hard rock mine workings, since:

  • due to stress relaxation, generally considered as a positive phenomenon, this layer is able to relieve stress concentrations within its own and the adjacent rock volume, due to the associated load transfer process the layer may develop increasing rock deformations within neighboring areas;

  • the stiff dolomitic- anhydritic rock beam which constitutes the immediate roof of mine function due to creep processes within the upper rheological deposit; assuming boundary conditions to be fixed in time domain (e.g.) face stopping), one may expect that after an ample time period, the salt deposit's entire weight should be the only load acting on the lower hard rock layer.

2 PARAMETERS OF ROCKSALT DEPOSIT

The numerical analyses presented below are based on the linear Burger's model (Figure 3). Laboratory investigations and fields measurements (Pytel, 1999; Kortas, 2004) prove that saltrock, after a sufficiently lengthy period of time, behaves as a kind of liquid deforming linearly with increasing time.

(Figure in full paper)

Figure 1: Geotechnical diversity of overburden strata within the area of copper bearing deposit (Central Zone – Zechstein formation of stiff dolomite-anhydrite strata of 160–220 m of thickness overlaid by 200–400 m Triassic sandstone; Northern Zone – thickness of dolomite-anhydrite structure of about 32–90 m with presence of rock salt deposit and thick Triassic sandstone; Southern Zone – glacial deposits directly on Zechstein formation, no Triassic sandstone) (Monograph, 1996). Thus, such strain function characteristics permit assuming the Burger's model as a basis for numerical analysis of the loaded saltrock behavior in following parts of the paper (Figure 2).

(Figure in full paper)

In practice, viscosity of saltrock mass manifests itself by increasing salt mass manifests itself.

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