Sedimentary environments and rock geomechanical behaviors

Sedimentary environments

Sedimentary environments are important for investigating rock physical, geological, and geomechanical behaviors (Peng and Zhang 1995).

Different from other mineral resources, coal-bearing formations mainly formed in different ancient coal accumulation sedimentary environments (Peng 1991). Its characters are controlled by the ancient geologic environment and its transition when the peat was piled up. Because of different sedimentary environments and sedimentary features, the thicknesses of the coal seam roof and floor formed under different environments changes greatly in both vertical and lateral directions (Peng 1993a). These lead to heterogeneity and discontinuity of coal bearing formations.

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During mining process of the coal seams, roof stability becomes worse in these weak formations. Roof caving, bottom heaving, and rock burst accidents often occur in the transition zone. Mainly happened in transition zone between the sandstone and mudstone of the roof. Investigation shows that one of the major reasons for the low production of the fully mechanized mining face in China was the difficulty caused by roof stability (Peng 1994). The non-productive time caused by the roof caving in the mining face occurs about 40% to 60% of the total non-productive time (Peng 1999a). There is statistical data from the world’s main coal producing countries about underground accident. The data show that about one third of underground casualty accidents mainly connect to improper control. The main area of improper control known are on roof stability or the earth pressure.

sedimentary environment illustration
common sedimentary environment

Why study sedimentary environments is important?

On the other hand, during mine development and production, in order to meet the requirements of transportation and ventilation, a safe, stable, and complete shaft and tunnel network system need to available. However, because the lithologies of the shaft and tunnel change significantly in the lateral direction, soft or weak formations can assemble. In this case, shaft and tunnel construction and maintenance become difficult. In the past, mine designers often considered through lateral direction approach. They think that the rocks distribute constantly in both thickness and lithology in the lateral direction. This design was not always true. Some cases can led to the failure of the original design.

Therefore, it is necessary to study rock sedimentary environments, to understand rock lithology distribution in the studied area, and to make the appropriate design according to geological conditions (Wang 2003, Sun and Wang 2000).

Facies are use to describe the formations making up the Earth’s crust. Sedimentary facies is the most interested facies with regard to fluid flow. Broadly divided into sandy facies, shaly facies, and carbonate facies. Sedimentary facies is related to the environment in which their sediments are deposited. In general, sedimentary environments include alluvial fan sediments, fluvial deposits, delta sediments deposits, lake deposits, barrier island deposits, and lagoon deposits.

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