A BOOKLET ON. T Rangasamy, A R Leach and A P Cook. Facilitating safety and health research in the South African mining industry

Transcription

1 A BOOKLET ON THE HYDRAULIC DESIGN OF COAL BARRIER PILLARS T Rangasamy, A R Leach and A P Cook Facilitating safety and health research in the South African mining industry

2 A BOOKLET ON THE HYDRAULIC DESIGN OF COAL BARRIER PILLARS T Rangasamy, A R Leach and A P Cook Itasca Africa (Pty) Ltd. Facilitating safety and health research in the South African mining industry December 2001

3 Published by The Safety in Mines Research Advisory Committee (SIMRAC) Braamfontein Centre, 23 Jorissen Street, Braamfontein 2001, South Africa This publication is copyright under the Berne Convention. In terms of the Copyright Act No.98 of 1978, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording or by any information, storage and retrieval systems, without permission from SIMRAC. ISBN

4 CONTENTS Page 1 INTRODUCTION 1 2 COAL MINE WATER STORAGE 1 3 MECHANISMS FOR WATER FLOW 2 4 CLASSIFICATION OF FLOW REGIMES 2 5 REQUIREMENTS AND USE OF THE DESIGN CHARTS Basic information required Design chart operating ranges Correction factor for length of barrier pillar Operation of design charts Determining minimum hydraulic barrier width new barriers Determining the maximum tolerable water head existing barriers 10 DESIGN CHARTS 13 EXAMPLE APPLICATIONS 21

5 1. INTRODUCTION In addition to providing local and regional mechanical stability to underground collieries, coal barrier pillars are used as hydraulic curtains for the management of water flow. Barrier pillars are left to provide mechanical support to the roof, considering factors such as positions of mine and section boundaries, legislative constraints, quality of coal, geological and rock engineering constraints, life of mine strategy and the production and ventilation requirements of the mine. Barrier pillars are not necessarily planned or designed to fulfill a water retaining/management function. Currently, no adequate design guidelines or procedures exist either locally or internationally that govern the design of barrier pillars for providing hydraulic stability for a mine or section. This booklet, prepared as an output of the SIMRAC COL702 Project, provides charts for the design of barrier pillars for the sole purpose of providing hydraulic stability in coal mines. It must be emphasized that the mechanical stability of the barrier pillar must first be established before attempting to design a barrier pillar for water management. The charts provide the user with an ability to determine either the minimum barrier pillar width, maximum water head or minimum pumping rate required to manage or reduce the risk of sudden inrushes of water into the mine. This booklet is for use by personnel on coal mines who either directly or indirectly are involved in water management, planning or design on the mines. Further information on the methodologies adopted to produce these design charts can be found in the COL702 final report. 2. COAL MINE WATER STORAGE Water reporting underground from inflow of natural groundwater (largest contributor) or service water (water used to suppress dust and for cleaning) is stored primarily in compartments, dams or ponds. These water reservoirs are usually bound by coal barrier pillars. Leakage of water through and around these pillars is inevitable. Although water leakage cannot be avoided, the rate and hence the quantity of water reporting on the dry side of the barrier pillar can be managed through engineered design. Restrictions can be placed on either the minimum required barrier pillar widths, maximum allowed reservoir water head or the rate of compartment and/or 1

6 roadway dewatering, depending on mining depth below surface and the geotechnical environment within which flow occurs. 3. MECHANISMS FOR WATER FLOW The porosity and hence the intrinsic permeability of the rock mass in and around barrier pillars is extremely low. Water ingress is dominated by flow along rock mass discontinuities such as bedding planes, joints, stress fractures, faults, dykes and cleats. The flow path from a water bearing area to a dry area is thus dependent on the location, persistence and hydraulic condition of these discontinuities. The hydraulic conductivity of discontinuities is assumed to obey Darcy s Law for laminar flow. q = k j a 3 p l where 1 k j is the joint permeability factor (whose theoretical value is ) 12µ µis the dynamic viscosity of the fluid a is the contact hydraulic aperture, l is the length of the cont act, p is the water pressure between contact endpoin ts, and q is the rate of discharge (flow rate in m 3 ) sec. m The path and rate of water flow from barrier pillar bound reservoirs will thus vary considerably from mine to mine. 4. CLASSIFICATION OF FLOW REGIMES Water flow through and around coal bound barrier pillars can be classified according to the rock type hosting the discontinuities that allow water leakage. Discharge of water onto the dry side of a barrier pillar from a flooded or partially flooded area will normally be a combination of water seeping from the roof, coal or floor. The geohydrological condition of the immediate roof, coal and floor will determine the type of flow regime pertinent to a particular mine. The bulk of local underground collieries can be classed into the seven flow regimes depicted in Figures 1a to 1g. 2

7 Figure 1a Figure 1b Bedded Sandstone Roof Impermeable Sandstone Roof Joints Bedded Sandstone Floor Impermeable Sandstone Floor Leakage of water occurs through the coal seam and discrete vertical joints in bedded sandstone roof and floor strata Go to design chart 5a Leakage of water occurs through the coal seam only The roof and floor strata are massive and impermeable Go to design chart 5b Figure 1c Figure 1d Bedded Sandstone Roof Laminated Siltstone Roof Joints Damaged floor Damaged roof Damaged floor Laminated Siltstone Floor Laminated Siltstone Floor Leakage of water occurs through the coal seam and discrete vertical joints in bedded sandstone roof and soft, damaged, laminated siltstone/gritstone floor Go to design chart 5c Leakage of water occurs through the coal seam and soft, damaged, laminated siltstone/gritstone roof and floor Go to design chart 5d 3

8 Figure 1e Figure 1f Bedded Sandstone Roof Impermeable Sandstone Roof Joints Joints Impermeable Sandstone Floor Bedded Sandstone Floor Leakage of water occurs through the coal seam and discrete vertical joints in bedded sandstone roof. The floor is massive and impermeable. Go to design chart 5e Leakage of water occurs through the coal seam and discrete vertical joints in bedded sandstone floor. The roof is massive and impermeable. Go to design chart 5f Figure 1g Laminated Siltstone Roof Damaged roof Joints Bedded Sandstone Floor Leakage of water occurs through the coal seam and soft, damaged, laminated siltstone/gritstone roof and discrete vertical joints in bedded sandstone floor. Go to design chart 5g 4

9 5. REQUIREMENTS AND USE OF THE DESIGN CHARTS 5.1 Basic Information Required Basic generic and mine specific information is required to enable use of the hydraulic design charts. The flow chart illustrated in Figure 2 depicts the inputs required to determine the optimum hydraulic pillar width for planned barriers and the maximum tolerable head for existing barriers. Design for New Water Barrier Pillars Design for Existing Water Barrier Pillars Generic Information Reservoir floor depth below surface (m) Maximum compartment head (m) Acceptable Rate of Leakage (ML/day) Reservoir floor depth below surface (m) Barrier pillar width (m) Acceptable Rate of Leakage (ML/day) Mine Specific Information Classify the Geotechnical Flow Regime (Figures 1a to 1g) Classify the Geotechnical Flow Regime (Figures 1a to 1g) Outputs Determine minimum hydraulic pillar width Determine maximum water head If pillar widthhydraulic > pillar widthmechanical : use hydraulic width If pillar widthhydraulic < pillar widthmechanical : use mechanical width Figure 2: Flow chart illustrating the information required to determine the minimum hydraulic barrier width for new water reservoirs and the maximum tolerable head for existing barrier pillar bound water reservoirs 5

10 5.2 Design Chart Operating Ranges Operating ranges for the design charts have been determined from local industry wide surveys of collieries that store water underground. These ranges are shown in Table 1. Table 1: Operating ranges for geotechnically classified hydraulic design charts Operating range Reservoir floor depth below surface (m) Seam heights (m) Barrier pillar widths (m) Water head (m) Reservoir dewatering (ML/day) Operating conditions for the design charts exclude: (1) Water seepage associated with seals (observations indicate that seepage associated with seals could be significant). (2) Geotechnical conditions different from the seven most common described in Section 4. (3) Water seepage associated with geologically anomalous features (faults,dykes). (4) Significant water seepage beyond 5m into the roof and floor. 5.3 Correction Factor for Length of Barrier Pillar The design charts applicable to the seven geotechnical flow regimes defined have been normalized for a 1km long barrier pillar. This normalization has an influence on the rate of water leakage or rate of roadway pumping determined from the charts. For a specific length of barrier pillar applicable to your mine the actual rate of water leakage or roadway pumping rate required can be determined from the following simple correction factor. where L Lk actual = Lk chart 1000 Lkactualis the actual rate (ML/day) of water leakage or pumping rate Lkchart is the rate (ML/day) of water leakage or pumping rate determined from the design charts (Figures 5a-5g), and 6

11 L is the length of pillar (m) applicable to your mine 5.4 Operation of Design Charts The primary use of the design charts is to determine the minimum hydraulic pillar width for new pillars and the maximum manageable water head for existing compartments (barrier pillar width established). The versatility of the design charts also allows for their use in water management feasibility planning. These additional uses are demonstrated in the EXAMPLE APPLICATIONS Determining minimum hydraulic barrier width NEW BARRIERS The steps outlined below must be read in conjunction with the design chart depicted in Figure 3. Step 1: Choose the geotechnical flow regime applicable to the roof, coal and floor strata at the position of the proposed barrier pillar from Figures 1a to 1g. Step 2: Go to the appropriate design chart depicted from Figures 5a to 5g. Confirm the choice of design chart by matching the boxed schematic of the geotechnical flow regime with that chosen from Figures 1a to 1g. Step 3: Mark the floor depth of the proposed barrier pillar on the design chart x-axis titled: Depth below surface (m). Step 4: Draw a vertical line from the marked depth position to a line depicting the planned rate of water pumping (or acceptable leakage rate) from the roadway adjacent to the barrier. Step 5: Draw a horizontal line from the rate of leakage (or pumping rate position) all the way across to the right vertical boundary of the graph titled: Barrier pillar width as a function of water head. Step 6: Choose an appropriate planned maximum water head for the compartment. The intersection of the line projected vertically from the chosen water head to the drawn horizontal line indicates the minimum required hydraulic barrier width. Remember: If the hydraulic width determined is less than the mechanical width (designed by Rock Engineer) USE MECHANICAL WIDTH If hydraulic width determined is greater than the mechanical width (designed by Rock Engineer) USE HYDRAULIC WIDTH 7

12 Correct the rate of leakage (SECTION 5.3) for the length of pillar being planned. 8

13 Figure 3: Determining the minimum hydraulic width of barrier pillars for the design of new compartments Hydraulic Design Chart for Barrier Pillars (Flow Regime = Coal bound + bedded sandstone roof + bedded sandstone floor) Approximate time to knee <10hrs High <20hrs <3days <8days Very low >8days Approximate time to knee High Very Rate of water Rate of leakage water leakage into into adjacent roadways 5ML/day 5ML/day 10ML/day 20ML/day < 10 hrs < 20 hrs < 3 days < 8 days > 8 days 3ML/day Barrier Barrier pillar pillar width width as as a function of water of water head head m Step 5: Horizontal line to boundary of graph 10m 15m 20m 25m 30m > 50m (residual flow) Step 4: Vertical line to rate of pumping Decreasing Rate of Leakage 1.5ML/day 1.5ML/day High Flow number Step 6: Intersection of planned head indicates hydraulic barrier width Geotechnical Flow Regime Bedded Sandstone Roof 20ML/mth month 5.0 Joints 2.5 Very low Bedded Sandstone Floor Depth below surface (m) Water head above seam floor of compartment (m) Step 3: Mark depth below surface Step 1 + 2: Choose and confirm geotechnical flow regime (Figures 1a to 1g) 9

14 5.4.2 Determining the maximum tolerable water head EXISTING BARRIERS The steps outlined below must be read in conjunction with the design chart depicted in Figure 4. Step 1: Choose the geotechnical flow regime applicable to the roof, coal and floor strata at the position of the proposed barrier pillar from Figures 1a to 1g. Step 2: Go to the appropriate design chart depicted from Figures 5a to 5g. Confirm the choice of design chart by matching the boxed schematic of the geotechnical flow regime with that chosen from Figures 1a to 1g. Step 3: Mark the floor depth of the proposed barrier pillar on the design chart x-axis titled: Depth below surface (m). Step 4: Draw a vertical line from the marked depth position to a line depicting the rate of leakage (or roadway water pumping capacity) from the water reservoir to the roadway adjacent to the barrier. Step 5: Draw a horizontal line from the rate of leakage (or pumping rate position) across to the line representing the existing barrier pillar width. Step 6: Subtending this line vertically down to the x-axis indicates the maximum tolerable reservoir head allowed for an acceptable rate of water leakage. Remember: It is assumed that the mechanical design of the barrier pillar has been established. Correct the rate of leakage (SECTION 5.3) for the existing length of pillar. 10

15 Figure 4: Determining the maximum tolerable compartment water head for existing barrier pillar bound reservoirs Approximate time to knee <10hrs High <20hrs <3days <8days Very low >8days Approximate time to knee High Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days 3ML/day Hydraulic Design Chart for Barrier Pillars (Flow Regime = Coal bound + bedded sandstone roof + bedded sandstone floor) Rate of water leakage into adjacent roadways 5ML/day 10ML/day Rate of water leakage into adjacent 10ML/day 20ML/day Step 5: Horizontal line to the existing barrier width m 10m 15m 20m 25m 30m > 50m (residual flow) Step 4: Vertical line to rate of leakage Decreasing Rate of Leakage 1.5ML/day High Flow number Step 6: Project line vertically down to determine maximum water head Geotechnical Flow Regime Bedded Sandstone Roof 20ML/mth 20ML/ month 5.0 Joints 2.5 Very low Bedded Sandstone Floor Depth below surface (m) Water head above seam floor of compartment (m) Step 3: Mark depth below surface Step 1 + 2: Choose and confirm geotechnical flow regime (Figures 1a to 1g) 11

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17 DESIGN CHARTS 13

18 Approximate time to knee <10hrs High <20hrs <3days <8days Very low >8days Approximate time to knee High Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days 5ML/day Hydraulic Design Chart for Barrier Pillars (FIGURE 5a = Coal bound + bedded sandstone roof + bedded sandstone floor) Rate of water leakage into adjacent roadways Rate of water leakage into adjacent 10ML/day 20ML/day m 10m 15m 20m 25m 3ML/day m 20.0 > 50m (residual flow) 17.5 Decreasing Rate of Leakage 1.5ML/day High Flow number Geotechnical Flow Regime Bedded Sandstone Roof 20ML/ month 5.0 Joints 2.5 Very low Bedded Sandstone Floor Depth below surface (m) Water head above seam floor of compartment (m) 14

19 Approximate time to knee <10hrs High <20hrs <3days <8days High Very low >8days Approximate time to knee Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days 5ML/day 10ML/day Hydraulic Design Chart for Barrier Pillars (FIGURE 5b = Coal bound only) Rate of water leakage into adjacent roadways Rate of water leakage into adjacent roadways 20ML/day Geotechnical Flow Regime Impermeable Sandstone Roof 5m 3ML/day 22.5 Impermeable Sandstone Floor Decreasing Rate of Leakage 1.5ML/day High Flow number m 15m 20ML/ month m 300 Very low Depth below surface (m) Water head above seam floor of compartment (m) 25m 30m > 50m (residual flow) 15

20 Approximate time to knee <10hrs High <20hrs <3days <8days High Very low >8days Approximate time to knee Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days Hydraulic Design Chart for Barrier Pillars (FIGURE 5c = Coal bound + bedded sandstone roof + laminated siltstone/gritstone floor) Rate of water leakage into adjacent roadways 5ML/day Rate of water leakage into adjacent 10ML/day 20ML/day m 10m 15m 20m 25m 30m 3ML/day 22.5 > 50m (residual flow) Decreasing Rate of Leakage 1.5ML/day High Flow number Geotechnical Flow Regime Bedded Sandstone Roof Joints 20ML/ month 5.0 Damaged floor 2.5 Laminated Siltstone Floor Very low Depth below surface (m) Water head above seam floor of compartment (m) 16

21 Approximate time to knee <10hrs High <20hrs <3days <8days High Very low >8days Approximate time to knee Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days 3ML/day 5ML/day Hydraulic Design Chart for Barrier Pillars (FIGURE 5d = Coal bound + laminated siltstone/gritstone roof and floor) Rate of water leakage into adjacent roadways Rate of water leakage into adjacent 10ML/day 20ML/day m 10m 15m 20m 25m 30m > 50m (residual flow) Decreasing Rate of Leakage 1.5ML/day High Flow number Geotechnical Flow Regime Laminated Siltstone Roof 20ML/ month 5.0 Damaged roof Damaged floor Very low 2.5 Laminated Siltstone Floor Depth below surface (m) Water head above seam floor of compartment (m) 17

22 Approximate time to knee <10hrs High <20hrs <3days <8days High Very low >8days Approximate time to knee Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days Hydraulic Design Chart for Barrier Pillars (FIGURE 5e = Coal bound + bedded sandstone roof + massive impermeable floor) Rate of water leakage into adjacent roadways 5ML/day Rate of water leakage into adjacent 10ML/day 20ML/day m 10m 15m 20m 3ML/day m 17.5 Decreasing Rate of Leakage 1.5ML/day High Flow number m Geotechnical Flow Regime Bedded Sandstone Roof > 50m (residual flow) 20ML/ month 5.0 Joints 2.5 Very low Impermeable Sandstone Floor Depth below surface (m) Water head above seam floor of compartment (m) 18

23 Approximate time to knee <10hrs High <20hrs <3days <8days High Very low >8days Approximate time to knee Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days Hydraulic Design Chart for Barrier Pillars (FIGURE 5f = Coal bound + massive impermeable roof + bedded sandstone floor) Rate of water leakage into adjacent roadways 5ML/day Rate of water leakage into adjacent 10ML/day 20ML/day m 10m 15m 20m 3ML/day m 17.5 Decreasing Rate of Leakage 1.5ML/day High Flow number m Geotechnical Flow Regime Impermeable Sandstone Roof > 50m (residual flow) 20ML/ month 5.0 Joints Very low 2.5 Bedded Sandstone Floor Depth below surface (m) Water head above seam floor of compartment (m) 19

24 Approximate time to knee <10hrs High <20hrs <3days <8days High Very low >8days Approximate time to knee Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days Hydraulic Design Chart for Barrier Pillars (FIGURE 5g = Coal bound + laminated siltstone/gritstone roof + bedded sandstone floor) Rate of water leakage into adjacent roadways 5ML/day Rate of water leakage into adjacent 10ML/day 20ML/day m 10m 15m 20m 25m 30m 3ML/day 22.5 > 50m (residual flow) Decreasing Rate of Leakage 1.5ML/day High Flow number Geotechnical Flow Regime Laminated Siltstone Roof 20ML/ month 5.0 Damaged roof Joints 2.5 Bedded Sandstone Floor Very low Depth below surface (m) Water head above seam floor of compartment (m) 20

25 EXAMPLE APPLICATIONS 21

26 Approximate time to knee <10hrs High <20hrs <3days High <8days Very low >8days Approximate time to knee Very < 10 hrs < 20 hrs < 3 days < 8 days > 8 days Hydraulic Design Chart for Barrier Pillars (FIGURE 5c = Coal bound + bedded sandstone roof + laminated siltstone/gritstone floor) Rate of water leakage into adjacent roadways 5ML/day Rate of water leakage into adjacent 10ML/day 20ML/day Example 1 5m 10m 15m 20m 25m 30m A coal mine 100m below surface plans to leave a 30m wide, 1km long barrier pillar along a goafed water compartment. The roof strata consists of well bedded competent sandstones and the immediate floor strata is laminated siltstones. For a maximum allowed water head of 20m, what rate of roadway dewatering can be expected? 3ML/day > 50m (residual flow) Path to solution Steps to follow Decreasing Rate of Leakage 1.5ML/day 20ML/ month 300 Very low Depth below surface (m) High Solution Flow number Bedded Sandstone Roof Laminated Siltstone Floor Water head above seam floor of compartment (m) Geotechnical Flow Regime Joints Damaged floor Confirm correct design chart for bedded roof and laminated floor strata (Figure 1c chosen thus design chart Figure 5c to be used Solution: The rate of leakage into the roadway adjacent to the pillar will be 10 ML/day. To prevent knee in the roadway, a pumping cycle of hours needs to be in place. 22

27 Approximate time to knee <10hrs High <20hrs <3days High <8days Very low >8days Approximate time to knee Very Decreasing Rate of Leakage < 10 hrs < 20 hrs < 3 days < 8 days > 8 days 3ML/day Project to 5ML/day line 1.5ML/day 20ML/ month 5ML/day 300 Hydraulic Design Chart for Barrier Pillars (FIGURE 5a = Coal bound + bedded sandstone roof + bedded sandstone floor) Rate of Rate water of water leakage into adjacent roadways ML/day Very low ML/day for a 600m long pillar = 22.5 Very 8.3 highml/day Depth below surface (m) High Flow number Apply correction factor i.e. 5ML/day for 1km long pillar Example 2 Barrier Barrier pillar width as a a function of water of head water head 5m 10m 3 15m Water head above seam floor of compartment (m) 4 Solution 20m 25m 30m Geotechnical Flow Regime Bedded Sandstone Roof Bedded Sandstone Floor > 50m (residual flow) Joints A coal mine 200m below surface plans to leave a 20m wide, 600m long barrier pillar along a goafed water compartment. The roof and floor strata consists of well bedded competent sandstones. What maximum compartment water head is required to prevent water leakage greater than 5ML/day along the length of the barrier. Path to solution 1 Steps to follow Confirm correct design chart for bedded sandstone roof and floor strata (Figure 1a chosen thus design chart Figure 5a to be used Solution: A maximum compartment water head of 23m will prevent leakage greater than 5ML/day for a 600m long barrier pillar. 23

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29 This booklet was completed as part of SIMRAC project COL 702 ITASCA Africa (Pty) Ltd