Underground coal mine subsidence impacts forest ecosystem PDF

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Impacts Forest Ecosystem 1 CHAPTER EIGHT U NDERGROUND C OAL M INE S UBSIDENCE IMPACTS FOREST ECOSYSTEM R.S. Singh* and N. Tripathi Contents Abstract 0 1. Introduction 0 2. Impact of Subsidence movements 0 3. Safe limit of subsidence movement 0 4. Impacts on soil 0 5. Impact of subsidence on ground w...

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Impacts Forest Ecosystem

CHAPTER

EIGHT

U NDERGROUND C OAL M INE S UBSIDENCE IMPACTS FOREST ECOSYSTEM R.S. Singh* and N. Tripathi

Contents Abstract

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1.

Introduction

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2.

Impact of Subsidence movements

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3.

Safe limit of subsidence movement

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4.

Impacts on soil

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5.

Impact of subsidence on ground water

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6.

Materials and Methods

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7.

Results and Discussion

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8.

Conclusion

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Acknowledgements

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10.

References

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Central Institute of Mining and Fuel Research, Barwa Road, Dhanbad - 826 015 Jharkhand *e- mail: [email protected]  2010 Wide Publishing Project Environmental Clearance: Engineering and Management Aspects

All rights reserved.

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Underground Coal Mine Subsidence

Abstract The impact of underground coal mine subsidence in forest soil ecosystems of Singareni Coalfields area was studied with respect to physico-chemical characteristics, inorganic nutrients (plant available nutrients), fine root biomass, net nitrification and net nitrogen (N) -mineralization rates. Soil quality of the studied forest was slightly acidic in nature (6.78-6.80). The forest soil texture with respect to course sand content, was higher in fresh subsided site as compared to old subsided site, while the fine textured soil was maximum in old subsided site. Subsidence caused increased of 34.3%, 9.82% in soil moisture and water holding capacity (WHC) compared to unsubsided sites. After subsidence, organic carbon (OC), total N (TN) and phosphorus (TP) was found to increase by 41.7%, 0.45%, and 5.6% in subsided site, respectively. Mean plant available soil nutrients viz. nitrate-N ammonia-N and phosphate-P were increased in subsided site by 82.2%, 21.4% and 14.5%, respectively. On an average, subsidence resulted into a 1.84 fold increase in fine root biomass, 1.66-fold in net nitrification and 1.44-fold increase in net N-mineralization.

1.

INTRODUCTION

Coal is the world’s most abundant and important primary source of energy and plays a major role in world energy scenario. It contributes about 26% of the total global primary energy demand and is also a key input for the steel and other industries (http://scclmines.com/downloads/exploration.pdf.). Reserves of coal are spread worldwide throughout some 100 developed and developing countries, sufficient to meet global needs for the next 250 years. It is an important fossil fuel for generation of electricity and for other industrial purposes. India has the world’s third largest hard coal reserves, after the United States and China with an output of 328 million tones in 2001-2002. The contribution of public sector coal companies towards coal production in India is 95%, of which Coal India Limited accounts for 80% and Singareni Collieries Company Limited for 10%. Other private companies make up the balance (http://www.iea.org/ text base/nppdf/free/2000/coalindia 2002.pdf). Traditionally, coal mining is considered to be the most polluted industry. Mining of coal resources either by opencast or by underground process has serious insinuation for environmental security if proper management strategies are not adopted (Singh and Singh, 1998). Expansion of industrialization for human development needs massive energy generation, for which huge quantity of coal is extracted through mining, causing extensive landscape destruction (Singh and Zeng, 2008). Underground mining of coal began in the early 1800’s and continues to current day. Most mining is accomplished by direct human action utilizing heavy machinery to remove the material (http://ema.ohio.gov/Documents/SOHMPsec214.pdf). However, underground mining is progressively being abandoned due to problems

Impacts Forest Ecosystem

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of profitability. Presently 60% of the mine materials in the world are extracted by opencast method causing devastation of the ecosystem (Saleque, 2008). Two basically different methods are used for underground mining coal mining; longwall and room-and-pillar mining. In longwall mining all coal is recovered from the mined ponds; hence subsidence occurs at the surface almost immediately and it is planned for. Room-and-pillar mining leaves about half of the coal in the pillars or the floor strata under them fail, sometimes decades after mining. In room-andpillar mining, the subsidence is unplanned and, therefore, quite problematic. Underground coal mining, although not as surface mining, can alter the surface and impact agriculture ecosystem (Darmody et al, 1989). Surface alteration includes subsidence that creates depressions. Albeit, the underground mining does not disrupt the soil and geologic overburden as radically as surface mining, but it can create significant disturbance of surface soils if subsidence occurs. Subsidence has deleterious effects on man-made structures. Nevertheless, because the subsidence from longwall mining is predictable and short-term, damage to structures can be reduced. Damage is most severe to structures that span the edge of subsided troughs (Boscardin, 1992). Structures toward the center of the subsidence trough are generally less prone to damage because they are let down more uniformly after the dynamic subsidence wave passes. Repairs can begin soon after mining because most of the subsidence occurs within a few days after undermining, and the surface typically within three to six months (Mehnert et al., 1992). According to Darmody (http://www.mcrcc.osmre.gov/PDF/Forums/Prime.Farmland 201998/4d.pdf), the strength of a rock mass changes with time, therefore the subsidence in a coal mine is also time dependent. It is found that the subsidence increases with time, however, the subsidence attains a maximum value which remains almost constant with the increase in time. The underground mines generally have less visible impacts on the environment than opencast mines. There is less disturbance of the ground surface but it can affect the water by contaminating with acids and metals and by intercepting aquifers. Underground mines not only impact groundwater hydrology, they are prone to subsidence. Subsidence occurs when the ground above the mine sinks because the roof of the mine either shifts or collapses. Mine subsidence can be defined as movement of the ground surfaces as a result of readjustments of the overburden due to collapse or failure of underground mine workings. Surface subsidence features usually take the form of either sinkholes or troughs. Subsidence can alter ground slopes to such an extent that roads, water and gas lines and buildings are damaged (Op. cit. Office of Technology Assessment, 1979). Subsidence can cause loss of productive land (Guither, 1986), damage to

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Underground Coal Mine Subsidence

underground pipelines (Hucka et al., 1986) and above-ground structures (Kaneshige, 1971), decreased stability of slopes and escarpments (Shea-Albin, 1992; Slaughter et al., 1995), contamination of groundwater by acid drainage (Emrich and Merritt, 1969), and dewatering of streams (Cifelli and Rauch 1986; Dixon and Rauch, 1990) and groundwater supplies (Stoner, 1983; Matetic and Trevits, 1992). Mining causes drastic disturbances in soil properties. Typical activities during the construction and mining phase include ground clearing (removal of vegetative cover and topsoil), drilling, blasting, trenching, excavation, and vehicular and pedestrian traffic. Mining activities usually cause catastrophic and extensive environmental changes, and eventually cause major damage to the whole ecosystem. During surface mining, 2-11 times more land is damaged than with underground mining (Li, 2006). The direct effects of mining activities can be an unsightly landscape, loss of cultivated land, forest and pasture land, and the overall loss of production. The indirect effects can be multiple, such as soil erosion, water and air pollution, toxicity, geoenvironmental disasters, loss of biodiversity, and ultimately loss of economic wealth (Wong, 2003; Xia and Cai, 2002).

2.

IMPACT OF SUBSIDENCE MOVEMENTS

Damage to different surface features and structures due to underground coal mining has been a serious problem and becoming more widespread as the demand for coal increases. It is, also increasing due to population explosion in and around the mining leasehold area. Damage from subsidence movements can be caused by change in surface slope, differential vertical displacement and horizontal strains. Further, mining may restore the original slope or close tensile fractures thereby rendering some remedial measures unnecessary or even harmful to the environment. Compressive strains with concave curvature of the ground surface results in crushing, over-thrusting and horizontal openings in brick-wall. Tensile strain accompanying convex curvature of the ground resulting in fractures tapering from the ground upwards. Similarly, bridges, roads etc may experience movement towards or away from each other depending upon the nature of ground strains. Differential vertical ground movement can adversely affect surface drainage, tall structures, factory plant and machinery. Sewerage pipes may be broken or cracked due to vertical movement which results in malfunctioning of the system. Similarly, water mains breakages commonly occur at intersection points where service pipes tap into the mains. Tilting of factory machines and plants results in inoperative equipment (CIMFR, 2007). Subsidence trough can give rise to potential formation of small pond and flooding on the surface of the land resulting in increase in surface water. The crack at the edge of the panel causes damage to the plant root system and tilting of the trees and sometime fall of the tree (Tripathi et al., 2009)

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Impacts Forest Ecosystem

3.

SAFE LIMIT OF SUBSIDENCE MOVEMENT

Different surface features and structures can sustain certain magnitude of ground movement which are called safe limit of ground movements. The safe limit of maximum permissible subsidence movements to different surface features and structures for Indian geo-mining conditions (Anon., 1991) are given below: Railway line of jointed construction Railway line of welded construction Buildings or compression=60 mm Water bodies Surface topography Forest cover (Slight impact)

: Strain=3mm/m : Limiting operating gradient=1 in 100 : No movement permitted : Total elongation : Tensile strain=4.5 mm/m : Strain=3 mm/m : Strain=20 mm/m

As per Ministry of Environment and Forest, Government of India guidelines, the maximum permissible tensile stain and width of surface cracks in forest land are 20 mm/m and 300 mm, respectively.

4.

IMPACTS ON SOIL

The coal extraction process drastically alters the physical and biological nature of the mined area. The principal surface impact of underground coal mining is subsidence (Booth, 1990). In general, soil physical properties are sensitive to mining subsidence and they become worse from the top to the centre of the subsidence trough. Nevertheless, the soil chemical properties except for electrical conductivity are not so sensitive to mining subsidence and might be changed after subsidence process. The studies show that the bottom of prone land accumulates nutrients and salt. Thus, the most important impact of mining subsidence on soil chemical properties can be seen in soil electrical conductivity reflecting high salt content, which might occur after the subsidence process. The soil biomass C in newly subsided land has also shown a significant tendency in the old subsided land and subsiding land. Mining subsidence causes decreased stability of slopes and escarpments, contamination of ground water by acid drainage, increased sedimentation, bank instability and loss, creation or alteration of riffle and pool sequences, changes to flood behavior, increased rates of erosion with associated turbidity impacts and deterioration of water quality due to a reduction in dissolved oxygen and to increased

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Underground Coal Mine Subsidence

salinity, iron oxides, manganese, and electrical conductivity (Booth and Bertsch, 1999; Sidle et al., 2000; DLWC, 2001; Gill 2000; Stout, 2003) resulting into death of fringing vegetation (NSW, 2007). The occurrence of iron precipitate and ironoxidizing bacteria is particularly evident in rivers where surface cracking has occurred. Much of the impact of subsidence on soils and landspcape is related to the pre-mining surface topography. Landscapes with erosive soils on long slopes may be subject to increased erosion potential because of slope increase or displacement of erosion control structures. The effect of longwall mine subsidence on soil has been studied for two years over two mines at Queensland in Australia (http:/ www.acarp.com.au) above two physical properties of soil. Large cracks that develop at the soil surface after subsidence can pose a hazard and may alter soil water movement. Most subsidence cracks are small and are quickly obscured by normal cultivation. Larger cracks are generally backfilled or graded to prevent them from posing a hazard to foot or wheel traffic. Along the panel edge, cracks remain open after the dynamic subsidence wave passes. This may allow surface water to infiltrate more easily and may increase the hydraulic conductivity of some soil horizons. These changes are in a very small portion of the mined area and may revert to the original conditions with time (Seils et al., 1992). Subsidence effects on agriculture land have been documented in Illinois (Darmody et al., 1989; Guither, 1986; Guither et al., 1985; Guither and Neff, 1983), the United Kingdom (Selman, 1986), India (Kundu and Ghose, 1994), China (Hu and Gu, 1995), South Africa (van der Merwe, 1992), and Australia (Holla and Bailey, 1990). These effects include soil erosion, disruption of surface and subsurface drainage and reduction of crop yields. Hu et al. (1997) observed that the physical properties of soil sensitive to mining subsidence were bulk density, moisture content and hydraulic conductivity, and they showed worsening from the top to the centre of the subsidence trough. The soil biomass C in newly subsided area showed a decreasing trend from the top to the centre of the subsidence trough, but no obvious trend was observed in the old subsidence areas. Based on the soil analysis of the subsided land, soil erosion was identified as a serious problem, most severe in the middle of the prone land. In low areas with high water tables, ponding is a particular problem. In some situations ponding might be viewed in a positive way because it creates wetlands beneficial to wildlife, but negatively when it reduces net returns to a food or fiber producer. In Southern Illinois landscape, subsidence from underground longwall coal mining creates wet or ponded areas that delay and disrupts farming practices, causes low seed germination, and reduces crop growth and grain

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yields. Darmody et al (1989) found a 4.7% average reduction in overall corn yields on subsidence affected land in southern affected land in southern Illinois.

5.

IMPACT OF SUBSIDENCE ON GROUND WATER

Subsidence depressions caused due to U/G coal mining is accompanied by rock fracturing, dilations of joints and separation along bedding planes. Rock movements occur above the mine workings and at an angle projected away from the mined-out area. Mine induced fracturing within the angle can result in hydrological impacts, i.e. disruption of surface and underground water bodies (aquifer), contamination of aquifer, beyond the margins of the mine workings. Walker (1988) correlated the subsidence movements with fluctuation in piezometric levels in 10 shallow observation well lying above and around 4 longwall panels in the northern Appalachian Coalfields. He reported that water level decrease in the well is greater when the ground surrounding the well is in tension. The rate of recovery is greatest when the ground was subjected to maximum compressive strain. The fluctuation of water levels appears to be a function of both the position of the well relative to the layout of the panel and the proximity of the mining. A well is unaffected by mining of a preceding panel unless it was positioned within the angle of draw for the panel. Well located over the centre of the panel exhibit the greatest fluctuation and head loss. Further, wells located in stream valley, exhibited a lesser response to mining. Nine of the ten wells investigated recovered to their premining level after mining was completed. Booth et al, (1998) also made similar observation after conducting seven years of study over longwall mining in Illinois. Moebs and Barton (1985) reported complete loss of water after mining on four shallow wells directly above the panel. While Liu (1981) stated that beyond an influence angle of 20-260 from the panel edges the effect on well water were minor or none at all. Cifelli and Rauch (1986) also reported that water wells located within an influence angle of 200 from the panel edge of the opening were affected by underground mining. Singh and Singh (1998) reported 6.5m water depletion level in the unconfined aquifer due to longwall mining in Kamptee coalfield, India. In this paper an attempt has been made to quantify the impact of underground coal mine subsidence on changes in soil physico-chemical characteristics, nitrogen transformation rate and hydrological status of SCCL coal mines of India.

6.

MATERIALS AND METHODS Location

The study mine sites spread over an area of 2,85 km2 are located in Singareni Coalfields of Singareni Collieries Company Ltd. (SCCL), Kothagudem (Andhra

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Underground Coal Mine Subsidence

Pradesh, India) at 17030’N latitude and 80040’E longitude. The topography is almost undulating plain terrain to gently sloping towards the river Godavari in the southeast with the average elevation varying from 119 to 157 m above mean sea level. There is no effective drainage developed in this are due to sandy soil cover and number of faults and fractures. Climate The climate is seasonally tropical and divisible into 3 distinct seasons, namely, rainy (mid June to October, winter (November to February) and hot summer (March to mid June). According to the mean monthly rainfall data (1996-2006) of Kothagudem, the annual rainfall of the area is 1084 mm, being maximum in July (300.31) and minimum in Janurary (7.82mm). The study site receives about 86.8% of rainfall during S-W monsoon and 7.4% during N-E monsoon season. The mean air temperature varies from as low as 11.420C in December to as high as 46.60C during May. The predominant wind direction is southeast to west. The relative humidity of the area fluctuates from 49% during February to 73.21% during April (CIMFR, 2007). Land use pattern Within 10 km radius from the edge of mine site 23,004 ha area is covered by Ramvaram Reserve Forest. An area of 2781 ha is barren and uncultivated land, while 2377 ha area is fallow land and 1757 ha of land is put to non-agricultural uses. The net area sown is 14,005 ha and 535 ha during Kharif (July to October) and Rabi (November to February) seasons, respectively. Forest Vegetation of this region has been classified as Southern Tropical Dry Deciduous Forest (Champion and Seth, 1968). The total forest area is spread over an area of 748,882 ha, constituting nearly 46.72% of the total geographical area. The density of forest ranges from 45-65stems 100m2. The forest is dominated by Tectona grandis. The other co-dominant species are Holarrhena antidysentrica, Hardwikis binnata, Cho...