The results of numerical modeling analysis strongly depend upon the quality input parameters used. It is clear that laboratory measured properties of rock material differ from the effective in situ properties which are usually developed for input data in numerical models. Several methods have been developed by researchers to derive equivalent in situ parameters from laboratory ones. The concept of a reduction factor to laboratory results has been introduced by a number of researchers and applied to several types of geological problems in mining engineering. This paper presents an overview of derivation of mechanical properties of rock mass from laboratory measured properties. Using for analysis of ground subsidence due to very shallow longwall mining at C1 coal seam in eastern part of Madanjoo mine in Paravadeh3 of the Tabas coal field in Iran simulated using FLAC. After running several models with back analysis, derivation values of reduction factor for elastic modulus of pre-failure was 1/30 and for bulk modulus.
Any underground excavation inevitably disturbs the ground and the original stress field, which in turn causes ground movement leading to surface settlement.One of the important features of mining beneath Coal Measure rocks is the surface subsidence produced after a longwall panel has been extracted. There is a long history of attempts to understand subsidence phenomena and to predict its effects. During the last few decades several new methods of subsidence prediction for inclined and steep seams have been developed world wide. These methods can be classified into the main following groups: a. Empirical models: These models have been designed on the basis of large number of field measurements. b. Physical models: Dimensional analysis is a technique by which all the variables involved in a physical model can be expressed in terms of mass, length and time. c. Analytical methods: can be classified to stochastic models and closed form solutions. d. Numerical modeling: Numerical modeling techniques are powerful tools for obtaining solutions to many engineering problems as well in subsidence engineering [1, 2]. Numerical modeling differ from other methods such as physical modeling techniques(profile functions and influence function) and empirical models in that they can take into account the mechanical properties of a rock system and analyse the failure and post failure behavior of the rock mass surrounding the mine openings in reasonable solution time. The finite method like the finite element is capable of dealing with the simulation of complicated problems in rock mechanics. During the last few 366 years both the finite difference and the finite element method have been successfully used for surface subsidence arising from longwall mining of flat and inclined seams.
The mine is situated in the Parvadeh coalfield, which is a basin between two major North-South trending fault systems, the Kalmard Fault and the Hidden Fault to the West and the Nayband Fault to the East (Figure1).