Peng S., Zhang J., Engineering Geology for Underground Rocks

Подождите немного. Документ загружается.

References 25

Peng S (1998a) Establishment and implementation of geological support system of

high production and high efficiency in China coal mines. Symp 31

Young

scientists, China Sci Tech Soc, Coal Industry Press Beijing (in Chinese)

Peng S (1998b) Approach on mechanical properties of clastic rocks and rock mi-

cro-structure. Proc. 5

Ann Conf of Young Scientists, China Coal Society.

Coal Industry Press (in Chinese)

Peng S (1999) The state-of-art and the future work of engineering disasters in-

duced by coal mining at deep depth in China. Symp Engineering and Disaster,

Dept Material Science and Engineering, China NSF, Mt Jiuhua, Jianxi (in

Chinese)

Peng S, Qu H, Luo L, Wang L, Duan Y (2000) Complete stress-strain and perme-

ability experiments. J China Coal Society 25(2):113-116 (in Chinese)

Peng S, Wang X, Xiao J, Wang L, Du M (2001) Seismic detection of rockmass

damage and failure zone in tunnel. J China Univ Min Tech 30(1):23-26 (in

Chinese)

Peng S, Wang J (2001) Safe mining above confined aquifers. Coal Industry Press

Beijing (in Chinese)

Peng S, Meng Z (2002) Theory and practice of mining engineering geology. Geo-

logical Press (in Chinese)

Peng S, Sun L, Ma C (2002a) Comprehensive analyses of rockmass deformation

due to underground mining and their application. J China Min Tech Univ

(Natural Sci Ed) 31(1):80-83 (in Chinese)

Peng S, Ling B, Liu D (2002b) Application seismic CT detection technique into

roof coal caving comprehensive mechanical longwall mining. Chinese Rock

Mech Eng 21(12):1786-1790 (in Chinese)

Peng S, Wang S, Guo J, et al. (2002c) VSP log and its application in Dong #2

borehole of Huainan Coalfield. J China Coal Society 27(6):576-580 (in Chi-

nese)

Raymer LL, Hunt ER, Gardner JS (1980) An improved sonic transit time-to-

porosity transform. Paper P, Twenty-first Annual Logging Symp Lafayette

Louisiana

Schön JH (1996) Physical properties of rocks: fundamentals and principles of

petrophysics. Handbook of geophysical exploration – seismic exploration

(Helbig K, Treitel S, ed) Vol18, Pergamon

Terzaghi K (1943) Theory soil mechanics. John Wiley & Sons

Wang J, Peng S, Meng Z (2001) Permeability changes in complete stress-strain

test of rock samples. J Beijing Sci Tech Univ 23(6):489-491(in Chinese)

Wang S (1981) On the mechanism and process of slope deformation in an open pit

mine. Rock Mechanics Rock Engng 13(3):145-156

Weingarten JS, Perkins, TK (1992) Prediction of sand production in gas wells:

methods and Gulf of Mexico case studies. Paper SPE 24797 Washington DC,

4-7 October

Wyllie MRJ, Gregory AR, Gardner LW (1956) Elastic wave velocities in hetero-

geneous and porous media. Geophysics 21:41-70

Zhang J, Zhang Y, Liu T (1997) Rock mass permeability and coal mine water in-

rush. Geological Publ House Beijing (in Chinese)

26 1 Rock properties and mechanical behaviors

Zhang J, Standifird WB, Roegiers JC, Zhang Y (2007) Stress-dependent fluid flow

and permeability in fractured media: from lab experiments to engineering ap-

plications. Rock Mech Rock Engng 40(1):3-21

2 Sedimentary environments and geologic

structures

2.1 Sedimentary environments and rock geomechanical

behaviors

Sedimentary environments are important for investigating rock physical,

geological, and geomechanical behaviors (Peng and Zhang 1995). The

brief overview of sedimentary geology is introduced in this section.

Different from other mineral resources, coal-bearing formations mainly

formed in different ancient coal accumulation 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. 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 transi-

tion 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 stabil-

ity (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). Statistical data from the world’s main coal producing coun-

tries show that about one third of underground casualty accidents were

caused by improper control on roof stability or the earth pressure.

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 be established. How-

ever, because the lithologies of the shaft and tunnel change significantly in

the lateral direction, soft or weak formations can be encountered. In this

case, shaft and tunnel construction and maintenance become difficult. In

28 2 Sedimentary environments and geologic structures

the past, mine designers often considered that the rocks distribute con-

stantly in both thickness and lithology in the lateral direction. This design

was not always true and 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 de-

sign according to geological conditions (Wang 2003, Sun and Wang 2000).

Formations making up the Earth’s crust are described by the term, fa-

cies. 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 deposits, lake deposits, barrier island de-

posits, and lagoon deposits.

2.1.1 Alluvial fan sediments

Alluvial fan sediments are deposits of sediments in regions of high relief,

generally where streams issue from mountains onto a level plain. The fan

starts at the apex, the source of sediments from regions of higher relief, as

shown in Fig. 2.1. Sediment transport from the apex tends to follow the

steepest slope downward, and the sediments, therefore, spread out in a fan.

The largest boulders or pebbles are deposited near the apex. Downslope,

the fan channel splits up into a number of smaller channels. This reduces

the velocity of the water flow, and capacity for carrying sediment is low-

ered. Therefore, sediments become finer-grained down-slope, even if there

is no reduction in steepness of the slope. Further downslope it becomes

braided streams and lake deposit. In front of the apex the large faulting

sedimentary basin usually are formed. This phenomenon is quite common

in the Mesozoic coal-bearing strata in the eastern China.

2.1 Sedimentary environments and rock geomechanical behaviors 29

Fig. 2.1. Schematic sediments of an alluvial fan.

Near the apex the sediments are primarily consisted of conglomerates,

and their main components are coarse gravels and boulders. The gravels

are filled with clay, silt, and sand. The conglomerate layer distributes in

strips and is parallel to the direction of the water flow.

Downslope to the middle fan, the sedimentary rocks are mainly com-

prised of sandstone with some gravels. In the fan tail the sediments are

much finer. The sedimentary rocks are consisted of sandstones, siltstones,

claystones, and coal seams

(Meng et al. 1999). The main geomechanical

features of the alluvial fan sediments are as follows:

x The rocks are blocky with great thickness and high strength.

x The sediment lithology is complex with coarse granularity, and the

rocks are easily weathered.

x On the end of an alluvial fan, the coarse rocks have high porosity

and it may be a good aquifer.

2.1.2 River sediments

Fluvial deposits are sediments deposited as the result of rivers. Fluvial

sediments include deposits of braided streams, meandering rivers, and an-

astomosing streams (Peng 1990). Figure 2.2 shows the fluvial facies of

meandering rivers. Braided streams have branched channels because the

river channel is not very stable. This usually occurs with steeper stream

gradients and an abundant supply of sediments. Braided streams favor the

30 2 Sedimentary environments and geologic structures

deposition of coarse sediments containing coarse sand and gravel with lit-

tle clay and silt. Meandering rivers move in loops, with the greatest veloc-

ity at the outer bank where erosion occurs and lower velocity at the inner

bank where deposition occurs. The fining-upward sequence from sand to

silt to clay is typical of meandering river facies. This sequence is the result

of the water velocity decreasing as the river, over a given spot, migrates

from the outer bank to the inner bank. If the spot is no longer in the river

but in the flood plain, then only clay and silt from nearly stagnant water

are deposited. An anastomosing stream is defined as a branching, interlac-

ing stream having a netlike appearance. Along the U.S. Gulf Coast is a

typical example of bayous and slough in regions of very low stream gradi-

ent and with subsidence.

Fig. 2.2. Schematic representation of meandering rivers of fluvial facies.

Meandering river sediments vary from coarse to fine grains upwards, as

shown in Fig. 2.3. During the basin deposit process, thick sediments may

form due to the continuous development of the meandering rivers. In the

meandering river sedimentary system, channel sands act as the skeleton in

the rockmass. Generally, it forms laterally many strips of sandstones sur-

rounded by the flood basin deposits. In the lateral direction some layers in

the strata were thickened, thinned, or even disappeared. Vertically, the

sandstones array or overlap each other in lens shape, and lithology varied

cyclically. Therefore, in the meandering river deposit system, fine-grained

sediments always surround sand sediments. These made rock mechanical

2.1 Sedimentary environments and rock geomechanical behaviors 31

properties anisotropic and heterogeneous (Peng 1996b).

The rocks in fluvial deposits are sandstone, siltstones, shales, mud-

stones, and claystones and have the following geomechanical behaviors:

x The rocks are stratified layers and interbedded and alternated with

soft and hard layers.

x The sandstones are weak in weathering resistance, and the strength

changes gradually from the bottom to the top.

x The strength of the sandstone in which the sandstone becomes

thinner and near the dead-end is the lowest.

Fig. 2.3. Schematic representation of different types of fluvial facies.

2.1.3 Delta sediments

Deltas form where rivers carrying a large supply of sediments empty into a

sea coast where the sediments cannot be transported away as fast as it is

deposited

(Peng 1996a). Thus, deltas lie in the transit region between the

fluvial and marine environments. The Mississippi River Delta is a good

example.

The rocks in delta deposits have the following geomechanical behaviors

(Peng 1994):

x The rocks are stratified with significant variations of lithology and

thickness in the lateral direction.

x The rocks are weak in weathering resistance, and the rock strength

32 2 Sedimentary environments and geologic structures

increases from the bottom to the top as the granularity increases.

2.1.4 Lake sediments

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 ex-

tends 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:

x The rocks have alternately soft and hard layers deposited. Periodic

changes of the lake level generate cyclical soft and hard strata.

x The rock layers are continuous with little change in thickness and

have low strength.

x Most strata are impermeable layers.

2.1.5 Barrier island sediments

Barrier islands or spits are long, narrow, offshore deposits of sand or sedi-

ments 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 bar-

rier islands. In addition, rivers washed sediments from the mainland that

settled behind the islands and helped build them up. The sediments of bar-

rier islands are well-sorted, generally medium to fine grained sandstones

with silicate cementation. These sandstones have very high strength.

2.1 Sedimentary environments and rock geomechanical behaviors 33

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.

2.1.6 Lagoon and tidal lagoon sediments

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 lit-

toral 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 suffi-

cient 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 la-

goon is a wide and flat region around the lagoon and depends on the dif-

ference 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.

34 2 Sedimentary environments and geologic structures

Fig. 2.4. Schematic representation of Lagoon sediments.

Fig. 2.5. Conceptual representation of Bolinas lagoon and barrier island/spit.