SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 183

THE CONTROL OF COALMINE ROOF COLLAPSE USING CUTTABLE

FOAMED CONCRETE SUPPORT COLUMNS

J Visagie, J Latilla, D Neal, C Silver

Abstract

When MATLA COAL devised a method to allow even subsidence that eliminated the

ponding effect normally associated with the shortwall mining method and the creation of

chain pillars, a support medium was required to be installed in critical areas when mining

the pillars and creating crush pillars. Apart from the primary support function of the support

medium it had to be cuttable without damage to the shearer, contamination of the coal or

the unravelling of the support unit upon contact with the shearer.

A solution was found in the development and subsequent use of the Jumbopak that is a

large support unit (1,2 x 1,2 x 1,2m) that facilitated the building of packs to high mining

heights (4m) using an lhd.

To allow the use of the installed precast production capacity at DURASET of small

modular blocks (600x300x100mm) a duplicate to the Jumbopak was created by palletising

the small blocks in such a manner (named GrinCube) to retain the performance and features

of the large support units.

These units were installed using a non-weeping prestressing system to ensure active

performance and pack integrity during the cutting process.

The system proves that it is possible to use cellular lightweight concrete as a cuttable

support medium in pillar extraction applications.

1. Introduction

Matla Coal makes extensive use of the shortwall mining method. When undermining

sensitive areas like wetlands, vleis or streams, the impact that the total extraction mining

has on surface needs to be minimised. Undulations are left on surface by the chain pillars

when total extraction mining methods are employed. Methods to prevent the forming of

undulations had to be devised. The preferred solution was to make use of crush pillars and a

support system that will maintain roof stability during the creation of the crush pillars and

other remnants.

Numerical modelling was done to evaluate different geometries confirming the suitability

of the approach. In addition to the layout, a suitable support medium for roadways and

intersections was required to provide adequate support to a height of 3.9m.

Additional requirements of the support product were that the shearer could cut through it

without unravelling and not contaminate the coal product delivered to the power station.

Ideally the support should be able to be installed mechanically and only the final blocking

and prestressing are to be done manually.

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 184

2. Requirements of the Mining Method

Matla Coal employs the longwall mining method to extract coal at an average depth of 50m

from 4 seam at No 3 Mine. The dimensions of the panels are such that the section is

described as a shortwall (105m face length excluding chain and crush pillars or 143m

including all pillars). Primary development is by continuous miner (CM) and consists of

three 7m road accesses in both maingate and tailgate. Two rows of chain pillars of 14m x

38m are created with 7m bords between them.

Longwall mining is a total extraction method and at a mining height of 3,9m the surface

effects or subsidence after goafing is significant. When undermining a relatively sensitive

surface structure such as a wetland, undesirable ponding may occur between surface

undulations created by unmined chain pillars.

A mining method that minimises the ponding effect had to be devised. To achieve this, the

use of crush pillars was proposed. To create the crush pillars the first chain pillar and 78 %

of the second chain pillar need to be extracted while maintaining roof integrity. Different

methods of mining and support to create the crush pillars were evaluated using numerical

modelling.

The implementation of the layout in Figure 1 and the success of the mining process dictates

that the pack support must be cuttable to provide support for as long as possible while the

face advances.

GOAF

PANEL

SOLID

PA

NE

L 1

G

OA

F

BREAKER

LINE

CRUSH

PILLAR

CUTTABLE

PACK

SUPPORT

4.5m

3m

4.5m

2.8m

5m

FACE

SHIELDS

Fig. 1: Panel and pack support layout - Tailgate

3. Development of Suitable Support

3.1 Cellular Lightweight Concrete (CLC)

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 185

CLC is made by mixing a slurry consisting of cement, flyash and water before the slurry is

blended with a pre-manufactured foam. The foam is made using a hydrolysed protein

foaming agent that is processed through a foam generator to create a stable foam. This foam

is chemically inert and does not affect reinforcement that may be used in the CLC. Using a

folding action the slurry and foam are mixed resulting in evenly distributed cementitious

paste around the bubbles creating a lightweight cementitious matrix when the cement sets.

CLC can be formulated to exhibit specific characteristics. In conjunction with the density

(or support product weight) that can be varied a strong relationship between the density and

compressive strength of the CLC exists. Fig 2 shows the typical compressive strength

versus density relationship.

Fig. 2: Typical CLC Strength vs Density Envelope

CLC exhibits virtually no creep or shrinkage and is therefore a stable support medium that

will not lose load after being pre-stressed as timber support will do under the same

conditions.

3.2 Durapak®

Durapak®

is a precast support unit that was developed during the early 1990s using CLC as

the primary material in its manufacture. It is produced in block form with dimensions 600 x

300 x 100 mm.

Figure 4 shows the typical load versus deformation of Durapak®

60-50 tested in different

pack configurations and at height to width ratio of 2:1.

Of note is the exceptional stiffness of the CLC pack. The test packs were not pre-stressed

before the test and full load was reached before 25mm of deformation. The accurate

CELLULAR LIGHTWEIGHT CONCRETE

Strength vs Density Envelope

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

27.5

400 600 800 1000 1200 1400 1600

Density (kg/m^3)

Stre

ng

th

(M

pa

)

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 186

manufacturing of the blocks ensures flat mating surfaces that contribute to the high initial

stiffness.

Durapak 60-50

60x60

-100

100

300

500

700

900

1100

1300

1500

0 100 200 300 400 500 600

Displacement [mm]

Lo

ad

[kN

]

Durapak 60-50 60x60

Fig. 3: Durapak®

test results

Fig. 4: CLC packs installed

The post yield performance of the Durapak®

was not a design consideration in this

application as the pack will not be subjected to significant closure as would be the case in

the gold mining industry for which the product was initially developed and is currently

used.

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 187

However, the other attributes (high initial stiffness, engineered yield load, accurate

dimensions of the blocks) made the use of Durapak®

in high mining width applications the

preferred solution. Mechanised cutting equipment (continuous miner or long wall shearer)

coming into contact with an installed pack, will cut into the CLC pack without causing total

unravelling and destruction of such pack. The packs could therefore be built in any location

and provide support until the last moment when the pack has been mined away by the

cutters.

Matla Coal pioneered the use of Durapak®

in the application described above at Matla 3

Mine in 2002 when they encountered difficult roof conditions in the tailgate of the

shortwall.

3.3 Jumbopak

The construction of a suitable pack for the 4m mining height using the Durapak®

blocks is a

labour intensive exercise. The search for a more efficient construction method led to the

development of the Jumbopak. The Jumbopak is a lightly reinforced CLC cube measuring

1,2 x 1,2 x 1,2m. Figure 5 shows the slots moulded into the bottom of the cube to allow the

LHD mounted with forklift forks to handle the 1,3ton cube when building the support unit.

The 1,2m height was selected to allow 3 units to be stacked mechanically to 3,6m. The gap

to roof height was left to accommodate roofbolts that may protrude from the roof. This gap

will allow the use of Durapak®

as filling material and a non-weeping pre-stress bag to

provide the required preload.

Fig. 5: Jumbopak tested at the CSIR

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 188

Figure 6 shows the load deformation curve for the Jumbopak. The 150mm high forklift

slots provide allocated yield zones and once these zones have been crushed an increase in

load is experienced after about 80mm deformation to achieve the peak load of 1800kN.

JUMBOPAK MINE SUPPORT

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 50 100 150 200 250 300 350 400

Displacement (mm)

Load (kN

)

Fig. 6: Jumbopak test results

3.4 The GrinCube

To make use of the installed precast capacity of the Duraset factory to produce Durapak®

and Grinpak¹ while still satisfying the MATLA COAL demand for Jumbopaks the

GrinCube concept was created. This concept meant that the 600 x 300 x 100 mm precast

blocks were palletised in such a way that the features, performance and dimensions of the

Jumbopak were duplicated as described in paragraph 3.3.

By palletising the modular blocks in a 1,2 x 1,2 x 1,2 m cube at the factory in such a

manner that apertures are left in the completed pallet for the forks of the forklift or LHD,

the Jumbopak duplicate is created. The mechanical handling and stacking of the cubes

eliminate the manual de-stacking and pack building exercise when using the small blocks.

For purposes of constructing the pack, three GrinCube are stacked vertically. A non-

weeping pre-stress bag is placed in position before the final layer of individual blocks is

placed into position filling the remaining space to roof height.

The prestress bag is filled with a quick set grout to a pressure of 4 bar providing a preload

of 58 tons.

Note 1: The Grinpak was developed as a cost effective alternative to the Durapak®

The

Grinpak combine timber and CLC in such a way as to retain the good attributes of CLC and

reduce cost. The product uses the same block dimensions as Durapak®

.

The load performance of the Grinpak block to be used as a cuttable support by MATLA

COAL in the GrinCube format had to be verified. The physical size of the press at

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 189

DURASET necessitated the use of a scaled down version from the 1,2m x 1,2m pack

footprint. The largest pack size that could be accommodated in the press using Grinpak

blocks is a 900mm x 900mm footprint and 1,8m pack height. The peak load result for the

larger footprint will then be calculated. This was considered a safe approach as the

confinement experienced by the inner blocks used in the larger footprint pack will provide a

higher peak load than the calculated value.

Fig 7: Grinpak block test format in the press

As can be seen in Figure 8 the peak load of 1779kN was achieved at 25mm displacement.

Using the results the calculated peak load for a 1,2m x 1,2m footprint will be 3163 kN.

When applying the same factor, to account for the height effect, as proposed by **Latilla

and van Wijk, 2003 namely 1.74, the peak load is similar to that of Durapak®

at 1818kN.

Comparing the height to width ratio effect published by **Erasmus and Smit, 1999 this

approach provides a conservative estimate of the peak load suitable for numerical

modelling.

The result confirmed that the Grinpak block was a suitable replacement for of Durapak®

in

this application.

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 190

GRINPAK BLOCK 60-9090 X 90 180

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280

Displacement [mm]

Lo

ad

[kN

]

Fig 8: Grinpak block Test Graph (900 x 900 x 1800 mm pack)

4. Underground installations

The quality of installation depends largely on the quality of the floor preparation where the

first GrinCube is to be placed. Failing this will cause the pack to be built off the vertical

that can lead to buckling or premature failure.

Proper pre-stressing is of utmost importance as poor pre-stressing will allow excessive roof

movement before load take-up and will cause the shearer to push the whole pack over as

soon as it comes into contact with the pack. (Figure 9)

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 191

Fig. 9: Poorly pre-stressed pack destroyed on contact with shearer

Figure 10: Installed packs

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 192

Fig. 11: Shearer cutting into an installed pack

Matla Coal plans to continue with this system of creating crush pillars through the partial

extraction of the tailgate chain pillars until the sensitive vleiland has been completely

undermined and all surface activities returned to normal. At current mining rates the project

will come to an end at the end of 2008.

5. Conclusion

CLC is the material of choice for the manufacture of support units. The properties of high

initial stiffness, non-combustibility, dimensional stability and no detrimental effect on

cutting equipment allows the manufacture of support units for special applications as found

at Matla Coal. The support units are made in a pre-cast environment but packaged to suit

the mining conditions and installed by mechanical means.

In addition, by being successful in the cutting of CLC support units, Matla Coal proved that

it is possible to use CLC for the partial extraction of chain pillars. Projects like pillar

extraction can be initiated with a degree of confidence as support designed for the specific

application will lead to alternative and safer means of pillar recovery.

6. Acknowledgements

We would like to express our sincerest appreciation to the management and personnel of

Matla Coal for assisting us in writing this paper and allowing us to present and publish it.

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 193

7. References

Latilla, J 2007 Crush Pillar Design for the 3 Mine, 4 Seam Shortwall

Van der Merwe, JN and Madden, BJ 2002 Rock Engineering for Underground Coal Mining

Latilla, J and van Wijk, JJ 2003 Shortwall facebreak beneath a dolerite sill – learning points

from a recent event at Matla Coal mine, ISRM 2003

Erasmus, PN and Smit ,J 1999 Assessment of precast cellular lightweight concrete (CLC)

support structures, Proceedings International Symposium Rock Support and Reinforcement

Practice in Mining, Kalgoorlie

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

J Visagie, J Latilla, D Neal, C Silver

______________________________________________________________________________________

Page 194