Sedimentary Characteristics and Strata Lithology

Sedimentary rocks differ according to their sedimentary environments. The mechanical properties of the sedimentary strata depend on the sediment facies haracteristics. For instance, in mining engineering, the distribution of facies of sedimentary rocks controls the integrated quality of seam roof stability (Peng 1993b). Therefore, a full understanding of the lithology and thickness of the sedimentary rocks prior to coal mining is important for strata control and management as well as for roof stability of the mining panel. The following case shows the effects of lithologic variations on rock mechanical properties (Peng 1995, Peng and Li 1996).

The Permian coal seams in Huainan Coalfield of Anhui Province, China were deposited in delta-plain environments (Peng et al. 2002). The major coal seams, Seam 13-1 and Seam 11-2, were formed in a braided river system in the compound delta plain of late Permian.

The roof strata were formed in the abandoned phase when the shoreline pushed landwards, and the strata are mainly composed of mudstone, silty mudstone, siltstone, and medium- and fine-grained sandstones (Meng et al. 2000). For example, in Xinji coal mine of Huainan Coalfield the immediate roof of Seam13-1 consists of mudstone or sandy mudstone with horizontal bedding, and it sometimes transits into siltstone. The thickness of the immediate roof varies from 0 to 12 m and is instable, varying greatly from east to west (Peng and Meng 2002). Along the east-west direction, the immediate roof alternates between having thick and thin rock layers. These layers extend along the north-south direction of the roof (Figs. 2.6 and 2.7). The sedimentary environment of the formations belonged to the flooding plain facies, which made the immediate roof come in contact with main roof of the sandstone by flooding erosion. The immediate roof was partially scoured, thus the coal seam and the main roof touch directly in some areas, as shown in Fig. 2.7.

The plan view of the contours of the thicknesses of the immediate roof in the coal seam 13-1 in Xinji coal mine, Huainan Coalfield, China. The solid dots are boreholes with hole names marked, and the unit in the figure is in meters. The main roof of Seam 13-1 consists of fine- or medium-grained sandstone cemented by siliceous or calcic minerals. Thicker in the west and thinner in the southeast, the thickness of the roof varies from 0 to 24 m, with greater variation occurring along the east-west direction. The sand stone is distributed anastomosingly in the plan view, while in crosssectional view it is in lentiform, i.e. the sandstone thickness varies greatly (Fig. 2.7). Upwards, at shallower depths, it usually transits into interbedding of sandstone and mudstone, and further upwards a very thin layer of coal seam is frequently found. These characteristic show that the main roof of sandstone of Seam 13-1 was created by an anastomosing river deposit (Peng and Meng 2002).

The lithology and mechanical properties of sedimentary rocks are different and controlled by their sedimentary environments and facies. In the vertical direction, lithology varies cyclically, and in the lateral direction some layers in the strata are thickened, thinned, or even disappear. These variations generate obvious spatial differences in mechanical properties of the rocks. The roof stability in tunnels and mining panels are directly dependent on the sedimentary environments of the rocks, because roof stability depends on mechanical properties of the rocks. For instance, a stronger washing action of the main roof sandstone on its underlying rock creates a more apparent and smoother interface. This is important since when min ing under this condition the rocks below the main roof are more likely to collapse. When the thicknesses of sandstone and mudstone in the roof vary or when lithology changes suddenly, formations usually are in a weak rock zone (Peng and Meng 1999a). This corresponds to a risky zone in the roof.

Lake sediments are deposited in a lake accumulated on the lake shore and on lake floor. They are deposited in a terrestrial environment and contain organic and inorganic particles, microfossils such as pollen and algae, and macrofossils such as leaves and seeds. Deposit speed in lake environment is faster than that in marine environment, because of a smaller wave in the lake.

Lakeshore deposits are generally well-sorted sands. The sediments load of a stream entering a lake will be dropped as the stream’s velocity and transporting ability suddenly decrease. The resulting deposit, which extends outward into the lake, is a delta (Murck and Skinner 1999). Inclined, generally well-sorted layers on the front of a delta pass downward and outward into thinner, finer, evenly laminated layers on the lake floor.

Most lake sediments are layered, in which the layers/strata are defined by color variations. In the deeper parts of the lake, the sedimentary layers are very thin, and deeper-water sediments are fine-grained while those in shallow water are coarse. The strata in lake deposits have the following geomechanical behaviors:

- The rocks have alternately soft and hard layers deposited. Periodic changes of the lake level generate cyclical soft and hard strata.

- The rock layers are continuous with little change in thickness and have low strength.

- Most strata are impermeable layers.

Barrier islands or spits are long, narrow, offshore deposits of sand or sediments that parallel the coast line. The islands are separated from the main land by a shallow sound, bay or lagoon. Barrier islands are often found in chains along the coast line and are separated from each other by narrow tidal inlets. The rising waters carried sediments from those beach ridges and deposited them along shallow areas just off the new coast lines. Waves and currents continued to bring in sediments that built up, forming the barrier islands. In addition, rivers washed sediments from the mainland that settled behind the islands and helped build them up. The sediments of barrier islands are well-sorted, generally medium to fine grained sandstones with silicate cementation. These sandstones have very high strength.

Some strata of barrier island sediments can extend for 160 km or more. They are very hard strata. If a coal seam roof is this kind of strata for longwall caving mining, it is very easy to form a large area of un-caving strata; therefore it is likely to have rock burst.

Lagoon, tidal lagoon, and barrier islands or spits are sedimentary elements that parallel to the coast line, as shown in Fig. 2.4. Figure 2.5 shows the littoral sediment of the lagoon and barrier spit of Bolinas Bay of California (Danmeier and Williams 2005). Lagoon environment belongs to a shallow basin that are separated from the ocean by barrier islands or barrier spits and jointed with the ocean through tidal inlets. In the places where tide develops, a lagoon is a shal low depression full of water even in the period of low tide. If there is sufficient sediment supply, a coastal lagoon can gradually develop into a tidal lagoon or swamp. Therefore, lagoon deposit is closely associated with tidal lagoon and swamp deposits. They transit vertically and are contiguous horizontally.

Lagoon sediments generally are laminated fine-grained sediments, such as clay and silt. In humid and semi-humid regions where coal measures form, these fine sediments often are rich in organic substances. Tidal lagoon is a wide and flat region around the lagoon and depends on the difference of the low and high tides and the ground slope. Near low tide line in the intertidal zone, due to the strong hydraulic activity, flat sand deposits can be formed and developed to be large slaty or sphenoid crossbeddings. Near high tide line, the sediments are mostly mud and silt with horizontal lamination and current lamination.

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