Subsidence and Stability Investigation of an Illinois

Coal Mine

Siavash Zamiran1, Abdolreza Osouli2

1PhD Candidate, Instructor, Dept. of Civil Engineering, Southern Illinois University Carbondale,

Carbondale, Illinois, USA, Email: zamiran@siu.edu, zamirans@gmail.com, Website: www.zamiran.net,

Phone: +1 (618) 334-4572 2PhD, PE, Assistant Professor, Dept. of Civil Engineering, Southern Illinois University Edwardsville,

Edwardsville, Illinois, USA, Email: aosouli@siue.edu, Phone: +1 (618) 650-2816 (Corresponding Author)

13th International Congress on Rock Mechanics. Montréal, QC, 2015 (ISRM 2015)

Postprint version

Zamiran, S., & Osouli, A. (2015). Subsidence and Stability Investigation of an Illinois

Coal Mine . In 13th International Congress on Rock Mechanics. Montréal, QC.

ABSTRACT

One of the most concerns of room and pillar mining system is subsidence of the ground surface due

to coal mining particularly in room and pillar mines. Subsidence of ground surface is a result of floor, pillar,

or roof failures of the mine. Any deterioration of the three elements is alarming for the mining industry. At

the same, due to many difficulties associated with surface disposal facilities of coal refuse, some mining

companies are trying to use underground space as a disposal storage place. In this study, stability and

subsidence analyses of an underground coal mine located in the Illinois Basin are conducted. This room and

pillar mine was abandoned and later on was used as a storage place to accommodate the coal fine refuse with

slurry backfilling method. The geological stratigrophy and geomechanical properties of the mine floor and

roof were collected. Floor and pillar stability analyses were conducted for areas with various extraction

ratios. The stability of the mine was evaluated both prior and after slurry backfilling. The stability analysis

of the mine due to moisture exposure of the floor and pillars after slurry backfilling are discussed. The

subsidence potential due to mine failure is also evaluated.

KEYWORDS

Subsidence, Illinois Coal Mine, Floor stability, Pillar stability, Slurry backfilling

INTRODUCTION

A typical lithology of coal seams in the Illinois Basin consists of weak underclay stratum.

The immediate weak underclay causes frequent floor instability problems in Illinois underground coal

mines. There are two different states in assessing floor stability of the mines where slurry backfilling is

planned. The first state is during the mining operation when weak underclay layer squeezes under pillar

stress and results in punching or bearing capacity failure. The second state is after slurry backfilling. During

this state, the slurry material soaks non-durable floor layers and causes a reduction of floor strength. Pillar

stability conditions should also be taken into account during both the operational period and after the slurry

backfilling.

The assessment methods of floor stability of the Illinois coal mines generally were developed

based on classical bearing capacity theories developed by Terzaghi (1967), Meyerhof (1951) and a more

recent one Vesic (1975). The bearing capacity methods which are used for floor stability analyses of Illinois

coal mines should consider multiple-layered system to accommodate different floor layers.

Several investigators including Button (1953), Brown and Meyerhof (1951), Vesic (1975), and Kulhawy et

al. (1983) developed methods to estimate bearing capacity of layered soils and rocks. Speck (1979)

developed Vesic’s approach further to include mutli-layer floor system. Speck’s approach considers both the

weak non-durable and durable layer in the floor. Chugh and Pytel (1992) provided new approaches for

measurement of floor strength based on plate tests and updated the Vesic’s approach in calculating the floor

strength. Furthermore, Gadde (2009) suggested new strength correlations for durable and non-durable

Illinoisan floors based on moisture content of weak underclay layer.

In weak materials prone to moisture exposure, the stability analyses should take into account

the strength properties of the material in soaked conditions. Particularly in the Illinois Basin, soaking reduces

the strength properties of immediate floor. In order to evaluate the effect of soaking, Marino and Choi

(1999) evaluated bearing capacity of Illinois coal mines using finite element numerical simulation method.

In a related study, Marino and Osouli (2012) modified the classic bearing capacity equations to incorporate

the softening and soaking of weak layer in mine floor stability analyses.

In parallel with floor stability investigation in room and pillar coal mines, investigations were

also conducted for assessing pillar stability and subsidence potential at this mine. Mark and Bieniawski

(Mark and Chase, 1997) method was used to evaluate pillar stability against crushing. Ground movements

due to pillar failures can propagate to ground surface and result in subsidence. For subsidence analyses,

several empirical correlations were suggested. Some methods are based on profile function method such as

King and Whetton (1957) and National Coal Board (NCB, 1963) and some are based on influence function

method such as Knothe (1957), Zenc (1969) and Kochmanski (1959). After considering both the profile and

the influence function method for subsidence estimation, profile function method was selected for subsidence

analyses.

CASE STUDY

The studied mine is a room and pillar underground coal mine, which is located in Southern Illinois.

The studied area was mined in the past 20 to 30 years and then backfilled with slurry. The coal seam is

at depth of 260 m (850 ft), and the overburden layers include shale, sandstone, limestone, and siltstone. A

thin non-durable layer is found immediately below the coal seam. The non-durable layer mostly consisted of

claystone. The average coal seam height is about 1.4 m (4.5 ft). Figure 1 shows a typical borehole stratum

within nearly 9 m (30 ft) above and below the coal seam. The extraction ratio (e) varies in different parts of

the mine. For this study, two different zones with maximum and average extraction ratio (i.e. 56% and 49%)

were selected.

FLOOR STABILITY ANALYSIS

The floor stability investigation of the coal mine was conducted using Vesic-Speck and Kulhawy

et al. bearing capacity approaches. There are several assumption made for these analyses. First, the floor

is assumed to be a thin immediate non-durable layer over on an infinite durable stratum. The

second assumption is that the mechanism of failure is controlled by undrained behavior. Finally, the

superposition effect of stress from adjacent pillars is neglected in calculation of bearing capacity of floor.

This means that the bearing capacity of an individual pillar is considered and the effect of the neighboring

pillars on the bearing capacity is ignored. The ultimate bearing capacity of the floor based on Vesic-Speck

approach is derived by Equation 1.

Figure 1- Stratigraphy based on boring log in proximity to analyzed area of the mine

Depth of

Cover (m)Lithology Thickness (m)

261.00

261.37 Sandstone 0.37

263.72

266.13

266.31 Ironstone 0.18

269.59

269.68 Shale 0.09

270.05 Claystone 0.37

Shale 2.35

Coal 2.34

265.15Limestone 1.43

Shale 0.98

267.25Shale 0.94

Siltstone > 5

qu=Rc1 Nm (1)

where R is a reduction factor proposed by Speck (1979) and can be assigned as 0.6 for claystone floor

based on experimental studies by Speck on in-situ plate test results. The parameter c1 is undrained shear

strength of non-durable upper layer. Nm is bearing capacity factor which has been modified by Vesic for

multi-layered floor system:

𝑁𝑚 =𝐾𝑁𝑐

∗(𝑁𝑐∗+𝛽−1)[(𝐾+1)𝑁𝑐

∗2+(1+𝐾𝛽)𝑁𝑐

∗+𝛽−1]

{[𝐾(𝐾+1)𝑁𝑐∗+𝐾+𝛽−1][(𝑁𝑐

∗+𝛽)𝑁𝑐∗+𝛽−1]}−[(𝐾𝑁𝑐

∗+𝛽−1)(𝑁𝑐∗+1)]

(2)

where K is the proportion of undrained shear strength of durable layer to the non-durable layer. The

parameter N*c is derived by Equation 3. The parameter sc is the shape factor and is estimated by Equation 4.

Nc is the classical bearing capacity factor which is 5.14 for undrained condition with friction angle of zero.

Nc*=scNc (3)

sc=1+W

LNc (4)

where W is pillar width and L is pillar length. The parameter β is punching index and can be calculated by

Equation 5.

β=WL

2h1(W+L) (5)

where h1 is the thickness of non-durable layer. Alternatively, in Kulhawy et al (1983) approach, bearing

capacity factor, (i.e. Nm) can be directly estimated based on Figure 2. To evaluate the floor stability, a safety

factor is calculated based on the bearing capacity and the overburden stress from the tributary area (see

Equation 6):

SP= 1.1H(W+B)(L+B)

WL (6)

where SP is overburden stress in psi, H is depth of coal seam (ft), and B is entry width (ft).

The stability analyses of the mine for after slurry backfilling follows the same procedure as above.

However, after slurry backfilling, the slurry permeates to non-durable floor materials and reduces the strength

of the layers. The strength reduction factor for shale due to soaking was determined at 0.58 using axial point

load tests on shale samples from Southern Illinois mines. The reduction factor for underclay is assumed at

0.05 based on previous experience of authors.

In the selected borehole near the study area, the non-durable weak zone consists of underclay over

shale layers. The thicknesses of shale and underclay layers are 10 cm (0.3 ft) and 36 cm (1.2 ft), respectively.

Due to different strength reduction in in shale and underclay after soaking, a weighted average reduction

factor based on the thicknesses of layers was estimated for the non-durable weak zone.

Figure 3 shows the safety factor of floor based on Vesic (1975) and Kulhawy et al. (1983) for the

operational period and after slurry backfilling for areas with maximum and average extraction ratio. The

factor of safeties for all of the analysis in the Vesic approach are approximately 85 percent of the factor of

safeties from the Kulhawy et al. method. It is observed that the areas with larger extraction ratios will have

up to about 20 percent less safety factor comparing to areas with lower extraction ratios. Slurry backfilling

results in nearly 55 and 58 percent reduced safety factor for both extraction ratios based on Vesic, and

Kulhawy et al. approaches, respectively. Both methods show with a 7 percent increase in the extraction ratio;

the floor safety factors will be about 20 percent reduced for both operational and backfilling periods. Since

a safety factor of 1.5 is typically used in design, the floor stability analysis shows that the safety factors are

adequate for the slurry backfilling period except for areas with very high extraction ratio.

Figure 2 - Evaluation of modified bearing capacity based on Kulhawy et al. (1983) (Continues line for

Lp/Wp = 1 and dashed line for Lp/Wp ≥ 5)

Wp/h

1

Figure 3 - Factor of safety for floor in operational period and after backfilling period

PILLAR STABILITY ANALYSIS

Pillar strength is evaluated based on the Mark-Bieniawski approach (Mark and Chase, 1997) using

Equation 7:

σp=σ1(0.64+0.54 (W

h) -0.18 (

W2

hL) (7)

where σP is pillar strength (psi), σ1 is in-situ coal strength (psi), and h is coal seam height. According

to several case histories conducted by Mark and Barton (1997) in-situ coal strength (Sp) is assumed at 6200

kPa (900 psi) for pillar stability analysis.

After slurry backfilling, the strength of pillars are also reduced due to coal soaking. According to

laboratory tests conducted on Illinois coal samples by the authors, coal strength will be reduced to 52% after

soaking. Due to low hydraulic conductivity of coal, soaking of the pillar is a time dependent phenomenon.

To estimate the soaked depth of coal pillars with respect to time, a series of numerical modeling in

FLAC software (Itasca, 2013) was conducted on a typical 12 m (40 ft) square pillar of the studied mine.

The hydraulic conductivity of coal is selected equal to 0.045 m/day (Harlow and Lecain, 1993). Based on

the numerical results, the correlation of soaked depth with time is shown in Figure 4. According to the

analyses results, after 1000 days approximately all of the pillar would be soaked. It is worth noting that

excessive pillar loads will make moisture penetration even more difficult.

To evaluate the factor of safety of pillars, the equivalent pillar strength was calculated based on the

weighted average of strengths in soaked and unsoaked areas of the pillar. It is worth noting that the stability

analyses were conducted on the most critical pillars. Figure 5 presents the factor of safety of pillars for

maximum and average extraction ratios. The factor of safety before slurry backfilling is 2.09 and 1.51 for

areas with extraction ratios of 49 percent and 56 percent, respectively. According to Zipf (2001), acceptable

range of factor of safety for pillars are from 1.1 to 2 and is dependent on tolerable risk of failure.

It should also be noted that after slurry backfilling, horizontal confinement stresses from slurry

material and hydrostatic pressures are applied to the pillars. The effect of these confinement pressures, which

act as a support and increase the stability of pillars, is not considered in the Mark-Bieniawski approach. Also,

0

0.5

1

1.5

2

2.5

3

Vesic (1975) Kulhawy et al. (1983)

Fac

tor

of

safe

ty

Operational period (e=56%)

Operational period (e=49%)

After backfilling (e=56%)

After backfilling (e=49%)

the amount of permeability is selected without considering the effects of joints and the discontinuity that

may be present in the coal seam. Conducting a permeability test on a representative sample is suggested.

Therefore, the presented results are conservative and are considered for the extreme scenario.

Figure 4 - Correlation of soaked depth of coal pillars with time

Figure 5 - Factor of safety of pillars versus time

SUBSIDENCE EVALUATION

Subsidence evaluation of room and pillar mines is developed based on empirical correlations. In

this study, the profile function method is used to determine total subsidence of ground surface due to failure

of a panel in the mine area, which is shown in Figure 6. The studied group panel is shown as hatched zone

in the figure. NCB (1963) method, as shown in Equation 8, is used for subsidence evaluation.

S(x)=1

2Smax [1- tanh (

cx

D)] (8)

where S(x) is the subsidence from a reference point, Smax is the maximum subsidence of the panel, x is the

distance of the studied point to the reference point, D is the distance from the inflection point to a point where

Smax occurs and c is a constant which is set as 0.75. Smax can be derived from Equation 9.

1

2

3

4

5

6

7

0.01 0.1 1 10 100 1000 10000

So

aked

dep

th (

m)

Time (day)

0.50

0.70

0.90

1.10

1.30

1.50

1.70

1.90

2.10

2.30

0.1 1 10 100 1000 10000

Fac

tor of

safe

ty

Time (day)

e= 49% e= 56%

Smax= a . h . e (9)

where a is a constant which can be derived from percent of hardrock in the overburden, and the proportion

of panel width to overburden depth. Also, h is the seam height and e is the extraction ratio. In order to

evaluate the total subsidence of group panels, the superposition principal was utilized. As shown in Figure

7, the total potential subsidence of the group panels in the cross section A-A (see Figure 6) is approximately

60 cm (2 ft) for overall extraction ratio of 56 percent. It is noteworthy that the maximum subsidence is

resulted if all panels fail due to pillar or floor stability problem. However, because of sufficient factor of

safety for floor and pillar stability before and after slurry backfilling, the actual subsidence is expected to be

considerably less than 60 cm (2 ft). The subsidence due to the failure of Panel A, shown in Figure 6, is also

considered and presented in Figure 7. The subsidence due to the failure of Panel A is estimated at 35 cm

which is about 60 percent of maximum potential subsidence due to the failure of all panels in cross section

A-A. Given the fact that the extraction ratio practiced in this mine is less than 60%, the probability of having

any massive failure inside each panel is very low. Therefore, the risk of subsidence due to the slurry

backfilling appears to be minimal for the studied mine.

Figure 6 - Selected panels for subsidence investigation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000 2500 3000 3500

Su

bsi

den

ce (m

)

Distance (m)

Group panel

Single panel

Figure 7- Subsidence profile due to failure of eight panels along cross section A-A (continues line) and

single Panel A (dashed line)

CONCLUSION

In this study, pillar and floor stability of a coal mine in the Illinois Basin was evaluated during mine

operational period and after slurry backfilling. The floor stability was evaluated based on Vesic and Kulhawy

et al. approach. The Vesic approach yielded less factor of safety comparing to Kulhawy et al. approach. Floor

safety factor was decreased by almost 50% after slurry backfilling. The geologic setting and thickness of the

weak floor layer play important role in determining the floor safety factor. A numerical modeling in FLAC

was conducted to evaluate the permeation propagation in coal pillar which results in strength reduction

during the soaking of coal pillars. Pillar stability analysis indicated 75 percent reduction in factor of safety

after the slurry backfilling. Eventually, subsidence analysis shows that there is a potential of 60 cm (2 ft)

subsidence at this site if several panels fail at the same time, however, due to the depth of the mine,

the subsidence risk is very low.

ACKNOWLEDGEMENT

The authors would like to acknowledge Itasca Consulting Group for providing the software

necessary for this study through Itasca Education Partnership program.

REFERENCES

Button, S. J. (1953). The bearing capacity of footings on a two-layer cohesive subsoil. Proc. of the 3rd Int.

Conf. on Soil Mechanics and Foundation Engineering, Zurich, 1, 332-335.

Chugh, Y.P., and Pytel, W.M. (1992). Design of Partial Extraction Coal Mine Layouts for Weak Floor Strata

Conditions. U.S. Bureau of Mines IC 9315, 32-49.

Kulhawy, F.H., O'Rourke, T.D., Stewart, J.P., and Beech, J.F. (1983). Transmission Line Structure

Foundations for Uplift-compression Loading, Load Test Summaries: Appendix to EPRI Final

Report EL-2870. Cornell University, Geotechnical Engineering Group, Electric Power Research

Institute

Gadde, M. (2009). Weak floor stability in the Illinois basin underground coal mines (Doctoral dissertation).

West Virginia University

Harlow, G. and Lecain, G.D. (1993). Hydraulic Characteristics of, and Ground-Water Flow in, Coal-Bearing

Rocks of Southwestern Virginia, United States Geological Survey Water-Supply. Paper 2388.

Hoffman, H. (1964). The effects of direction of working and rate of advance on the scale-deformation of a

self-loaded stratified model of a large body of ground. Proc. Int. Conf. Strata Control, New York,

N.Y., pp. 397-411.

Itasca consulting group, Inc. (2013). FLAC Version 7.0, Minneapolis.

King, H.J. and Whetton, J.T. (1957). Mechanics of mine subsidence. Proc. Eur. Congr. Ground

Movement, Leeds, pp. 27 36.

Knothe, S. (1957). Observations of surface movements under influence of mining and their

theoretical interpretation. Proc. Eur. Congr. Ground Movement, Leeds, pp. 210-218.

Kochmanski, T. (1959). Integral - Theorie der Bodenbewegungen unter Einwirkung des Abbaus.

Freiberger Forschungsh., Al18: 36-58.

Marino, G.G., and Choi, S. (1999). Softening Effects on Bearing Capacity of Mine Floors. Journal

of Geotechnical and Geoenvironmental Engineering, Vol. 125, No. 12, 1078-1089.

Marino, G.G. and Osouli, A. (2012). Influence of Softening on Mine Floor-Bearing Capacity: Case History.

J. Geotech. Geoenviron. Eng. 2012.138:1284-1297.

Mark, C. and Barton, T.M. (1997). Pillar design and coal strength, National Institute for Occupational Safety

and Health

Mark, C. and Chase, F.E. (1997). Analysis of Retreat Mining Pillar Stability (ARMPS). In Proceedings of

New Technology or Ground Control in Retreat Mining. Information Circular IC 9446. Washington,

DC: U.S. Bureau of Mines. PP. 17-30

Meyerhof, G. G. (1951). The ultimate bearing capacity of foundations, Geotechnique, 2: 301-32.

National Coal Board (1963). Principles of subsidence engineering. Natl. Coal Board, Prod. Dept., Inf. Bull.,

61/231, 16 pp.

Speck, R. (1979). A Comparative Evaluation of Geologic Factors Influencing Floor Stability in Two

Illinois Coal Mines. Ph.D. Thesis submitted to the University of Missouri, Rolla, 265 p.

Terzaghi, K. and Peck, R. B. (1967). Soil Mechanics in Engineering Practice, 2nd ed., Wiley, New York.

Vesic, A.S. (1975). Bearing Capacity of Shallow Foundations. Foundation Engineering

Handbook, Winterkorn, H.F., and Fang, H., Eds., Van Nostrand Reinhold, Co., 121-147.

Zenc, M. (1969). Comparison of Bal's and Knothe's methods of calculating surface movements due

to underground mining. Int. J. Rock Mech. Min. Sci., 6: 159-190.

Zipf, R.K. (2001) Toward Pillar Design to Prevent Collapse of Room-and-Pillar Mines. 108th Annual Exhibit

and Meeting, Society for Mining, Metallurgy and Exploration, Denver.