The effects of roof and floor interface slip on coal pillar behavior 

A.T. Iannacchione 

Bureau of Mines, Pittsburgh Research Center, Pittsburgh, Pa. 

ABSTRACT: Designing coal pillars to provide resistance 

against overburden and gob loads has long been an aim of 

rock mechanics engineers. This requirement has become more 

imperative as greater overburdens are encountered and when 

mining in stiff coal-bearing strata. Current design procedures rely on theories of coal pillar behavior that take 

into consideration a common hypothesis. This hypothesis 

states that the elastic core is surrounded by an inelastic 

yield zone. The distribution of stress at low-to-moderate 

pillar loads has been effectively defined by this hypothesis. However, it suffers greatly when applied to large 

width-to-height (w/h >lo) coal pillars under considerable 

overburden (>500 m) . In these situations, the hypothesis 

says the elastic core can achieve unrealistic stress states 

giving the pillars extremely high calculated strength. A 

growing body of field studies has shown this is not the 

case. It has become clear that same other mechanism must be 

involved. It is the purpose of this paper to discuss the 

importance of an interface slip mechanism between the coalbed and the surrounding strata in controlling the extent and 

pattern of stresses and deformations in a coal pillar. 

1 INTRODUCTION 

Over the years empirical methods for designing coal pillars (~ieniawski, 1984) have incorporated a size effect 

through the concept of the in situ coal strength and a shape 

effect through a w/h ratio. ~mpirical design methods have 

been very useful for design purposes in areas where extensive mining has allowed for calibration (overburdens <300 m 

and w/h <lo). However, these methods are not based on any 

mechanical model for coal behavior and may not be appropriate in designing coal pillars under considerable overburdens with large w/h ratios. 

One highly accepted theory for coal pillar behavior was 

proposed by Wilson (1972). He postulated that the coal followed a linear Mohr-Coulomb failure criterion. This theory 

analyzed pillars with an elastic core surrounded by an inelastic yield zone. The extent of the yield zone is depen- 
2 THE COALBED INTERFACE SLIP MECHANISM 

dent upon the coal strength, overburden, and the condition 

of the roof and floor rocks. According to the theory, the 

peak stress should be encountered at the boundary between 

the yield zone and the core. The ultimate strength of the 

pillar is surpassed just prior to the yielding of the entire 

core. The assumption of a linear failure criterion for the 

core and an exponential stress gradient for the yield zone, 

allows coal pillars with strong roof and floor rock to have 

The interface between coalbeds and surrounding roof rocks 

almost always represents a sharp change in lithology. his 

contact surface is generally smooth and is often polished. 

In a few locations, the contact has been the location of 

premining lateral displacements and could be considered a 

bedding plane fault. Given the nature of the interface, it 

unrealistic strength. 

Barron (1984) expanded 

Wilson's hypothesis to incorporate a second failure mechanism 

for coal. At high confining 

pressures, the model assumes 

pseudo-ductile behavior, or 

deformation without change in 

stress, as the failure mechanism. Therefore, the Barron 

model assumes a non-linear 

failure criterion for the material in the elastic pillar 

core. This has the advantage 

of limiting the ultimate pillar 

strength as overburden and w/h 

of the pillars increase. The 

theory is now being tested with 

Bureau field data and is being 

implemented into a computer 

code. 

The above methods tend to 

predict a wide range of 

strengths for large pillars 

180 - 

160 -- 

a field data 

5 120 -- 0) 

t 

f! -- 100 -- m 

L 

O 

= 80 -- . - 

CT 

60 -- 

0 10 20 30 40 50 

Width of a 1.8 m thick pillar 

(Figure 1). This figure shows Fi~ure 1. Pillar strengths 

the ultimate strengths of calculated by different deactual pillars of different sign methods (coal 

sizes within the same mine. str. (a,) =6.2 MPa and overThe field data was collected burden=564 m) . with new instrumentation 

(Heasley, 1989) and in locations where pillars have either 

achieved or are close to achieving their ultimate strength 

(Iannacchione, 1988; Heasley and Barron, 1988; and ~ampoii 

et al., 1990). Figure 2 shows stress gradients in response 

to low, moderate and high load increments after development 

of two of these pillars. The ultimate strength (development 

+ change in stress) of these pillars was measured between 55 

and 65 MPa. These results indicate that large coal pillars 

may indeed fail at similar average stress values. 
is reasonable to 

assume the coalbed 

interface will have 

material properties, 

such as cohesion (C,) 

concentratedinthe bigure 2. Vertical stressgradients 

coal pillars, slip of two abutment pillars at low, modwill Occur when the erate and high loads. frictional resistance 

of the interface is 

K ~'i 

~-.3-*+--3- 

high 

20 -e----,-- --a --- moderate -e---e--- ----- * ----- ------- 

t; 0 .I e-- low C 

and friction angle 

() less than the 

coal. Therefore, as 

mine openings are 

created and stress is 

overcome. 

L 

6 

- -a----+-+-4 

------ 8 ------------ e-----6-----6 

I 

0 0 2 4 6 8 10 12 14 16 18 

Distance from edqe of pillar, m 

Once a portion of the interface slips, the horizontal 

stress component along the interface is also decreased, 

thereby reducing pillar confinement. This lowers the pillars ability to support vertical stress. Vertical loads are 

therefore transferred farther into the pillar core which may 

in turn induce farther slip along other portions of the 

interface. Eventually, the frictional resistance of the 

entire interface may be overcome, causing considerable 

lateral pillar movement into the mine opening. This movement may be gradual or violent depending on the material 

properties of the strata and the interface, the stress 

levels within the coal and the loading rate applied to the 

coal. 

A mechanism such as this has been suggested by Babcock and 

Bickel (1984) as a possible explanation for coal bursts. 

They proposed that the sudden release in constraint between 

the coal pillar and the surrounding strata due to slippage 

between the coalbed and roof rock could produce changes in 

the stress state of the pillar, initiating a burst. 

3 INFLUENCE OF MATERIAL PROPERTIES ON INTERFACE BEHAVIOR 

Although no direct measurements of the material properties 

of coalbed interfaces have been nade, there exists a considerable amount of information concerning the properties of 

other discontinuities. These properties, C, and $,, are influenced by surface roughness, normal stress, moisture content, width and. inf ill of fissures, and degree of weathering. Barton (1976) has indicated that the majority of unweathered rock surfaces have +, ranging from 25' to 35' at 

medium stress levels, while Farmer (1983) reported @, of 10" 

to 20' for typical discontinuity with clay infill. In situ 

coal has been suggested to have + of 22' to 27' (Mark, 1988) 

and a cohesion of 0.07 to 1.38 MPa (Barron, 1984) . Therefore, the coalbed interface +, could range from a smooth 

joint surface (@,=25' ) to a fault gouge (lo ' ) . C, could 

range from approximately 1.38 MPa for unpolished surfaces to 

0 MPa for wet polished contacts. 

~etermination of the potential for slip at various stress 

levels within a coal pillar must take into consideration the 
varying state of stress along the interface. Because of the 

complex nature of stress profiles across the inelastic and 

elastic zones of coal pillars, a detailed analysis of the 

influence of coalbed interfaces on pillar behavior requires 

the use of numerical procedures. 

I I 

4 PILLAR MODELS WITHOUT 

INTERFACE SLIP 

A numerical code called FLAc' 

(Itasca, 1989) was chosen to 

evaluate the effect of interfaces on the behavior of coal 

pillars. This code is an 

explicit large-strain finite 

difference code with a model 

which assumes that the rock 

mass obeys a Mohr-Coulomb 

elasto-plastic model where 

cohesion, friction and dilatancy angle may be controlled as 

functions of plastic strain. 

This program's strain softening 

capability has the advantage of 

solving problems containing 

extensively yielded material 

subjected top large increments 

of load changes. 

previous uti-lization of the 

strain softening procedure 

(Iannacchione , 1989) showed 

reasonably good results in simulating coal cube and pillar 

behaviors at low and moderate 

very high loads 

high loads 

0 2 4 6 

Distance from rib of pillar, m 

loo-- 

Figure 3. Vertical stress 

profiles across pillars at 

increasing load increments. 

Values above curves represent average stress. 

confinements (or low to moderate applied pressure). Figure 3 shows a realistic development of a yield zone, peak stress and elastic core during 

incremental loading of a modeled pillar. 

The strain softening model was found to be inaccurate at 

high confinements (or high applied pressure), due to the 

modeled pillar's extremely high peak stress values (figure 

3). Peak stress levels within abutment pillars with high 

w/h ratios rarely exceed 70 MPa , whereas, the peak stresses 

calculated from a modeled pillar (w/h=6) are much greater. 

0 

a 

I 

50 -- 

. . .. 

- -- moderate loads 

. ' low loads ; !' 't,,. 

5 PI'LLAR MODELS WITH INTERFACE SLIP 

49 MPa 

The effect of the interface slip mechanism on the behavior 

of coal pillars was modeled by utilizing the strain softening model with an interface at the coalbed boundary. In 

this model, 18.3 m wide by 3.1 m high pillars are separated 

'This does not imply endorsement of the program by the 

US. Bureau of Mines. 
by 5.5 m wide entries 

(figure 4) . The superand subadjacent strata are 

an elastic homogeneous 

material (roof - E=34.5 

Gpa and v=0.3; coal - 

p2.1 GPa and v=0.3). The 

model grid was fixed vertically along the bottom 

and horizontally along 

both sides. The top row 

of the grid is free in the 

vertical direction, allowing application of a downward pressure in 3.45 MPa 

increments. After each 

increment of applied pressure, the model was 

allowed to reach total 

equilibrium. The explicit 

formulation of the code 

requires that a model be 

stepped through small time 

periods until it reaches 

equilibrium. The equilibrium state was monitored 

by observing horizontal 

velocities of the pillar as 

adjacent opening. Once the 

applied pressure to the top 

the model stepped again to 

6 EFFECTS OF INTERFACE ON 

PILLAR STRESS 

The influence of the 

interface slip mechanism 

on pillar behavior is best 

illustrated by observing 

stress profiles during 

loading and by measuring 

the ultimate strength of 

the pillars. As stated, 

@, and C, for a coalbed 

discontinuity most likely 

range from 5' to 20' and 0 

to 1.03 MPa. Therefore, a 

series of models were run 

Figure I. Model for simulating 

pillar behavior with interface 

slip mechanism. 

the coal llslipped" into the 

pillar stopped moving, the 

of the model was increased and 

equilibrium (figure 5) . I 

13.8 MPa 

applied 

pressure 

KEY 

0 20,000 30,000 40,000 50,000 

arbitrary model time increments 

Figure 5. Use of velocity monitoring to determine equilibrium 

state after the incremental increase in applied pressure. Key 

shows the location of obsewation point. 

17.3 MPa 

applied 

pressure 

using various combinations 

of these interface material properties. 

Examination of the vertical stress profiles with a @, = 

15" (Figure 6) indicates that the interface slip mechanism 

tends to control the rate at which the inelastic yield zone 

develops in response to increasing increments of applied 

pressure. In this way, the modeled pillar can have different shaped vertical stress profiles (width of yield zone 

\ :;zieZ: pressure p\ '"\~/ 

I I 1 T 1 
and magnitude of peak stress) base solely upon the interface 

material properties. 

Analysis of stress profiles at different combinations of 

interface material properties continued until the modeled 

pillar reached its ultimate strength. In the model, the 

ultimate strength of the pillars occurred when all but the 

innermost zones within the coal pillar had undergone plastic 

deformation or when the coal elements would not come to rest 

(vel=O) after incremental application of applied pressure. 

These average vertical stress conditions at the pillar's 

ultimate strength were analyzed in relation to the interface 

material properties (@, and C,). Figure 7 shows that different @,Is and C,Is produce different vertical stress profiles which in turn alter the 

load bearing capacity of the 

pillar. his graph indicates 

that @, of >20° and C, of >1.38 1570.34 MPa I) MPa simulate pillar strengths 

comparable to a model without 15m.69 MPa I) 

interface slip. It also shows 

that a slightly slickensided 

interface (@, of 15' and C, of 

0.69 MPa) could reduce pillar 

strengths by approximately 50%. 

Unfortunately, the use of a 

linear Mohr-Coulomb failure 

criterion with strain softening 

resulted in an extremely large 

inelastic zone with a high peak 

pillar stress. The magnitudes 

of these yield zones and peak 

stresses do not correlate well Figure 6. Influence of difwith field observations. A ferent interface properties 

properly designed nonlinear on vertical stress gradiMohr or Mohr-Coulomb (i.e. Hoek ents with similar load conand Brown model) failure crite- ditions. 

rion could reduce the size of 

the yield zone and the magnitude of pillar core stresses 

resulting in a more accurate simulation. 

7 EFFECTS OF INTERFACE ON PILLAR DEFORMATION 

It is useful to examine the manner in which the modeled 

grid deforms in responses to increased increments of load 

and compare it with pillar deformations observed in the 

field. Magnitudes of deformation produced by the model are 

unrealistic compared with field data. However, the relative 

magnitudes of the deformations associated with the inelastic 

and elastic coal zones appear realistic. More important, 

the shape of the modeled pillar deformation appears to 

closely represent the pattern of deformations observed 

underground. Models run without an interface show a rotational distortion of the elements of the pillar close to the 

interface (Figure 8b). When the interface slip is allowed, 

the modeled coal pillar tends to lvflowlq laterally into the 

mine opening (Figure 8c). The penetration of elements is 
caused either by the relatively 

low stiffness assigned to the 

interface or displacement of 

elements beyond the interface. 

The uniform lateral deformation 

across a vertical section of 

coal, appears to more closely 

represent observed conditions, 

especially at high load conditions. 

8 SUMMARY AND CONCLUSIONS 

Using data gathered from 

recent field studies and numerical models, the effect of a 

coalbed interface slip mechanism on pillar behavior was 

investigated. The important 

characteristics of the interface slip mechanism are summarized below: 

1) The coal-roof rock interface 

ranges from a sharp break in 

lithology to a polished, slickensided surface. 

2) Although no direct measurements exist concerning the 

material properties of the 

coalbed interface discontinuity, the @, should range from 

10' to 20' and the C, should 

range from 0 to 1.03 MPa. 

3) Interface slip tends to control the rate at which the 

inelastic yield zone develops 

in response to increasing 

increments of applied pressure. 

4) Lower interface frictional 

properties lower the ultimate 

strength of the coal pillar. 

5) As the material properties 

of the interface approach that 

of the yielded coal, its effect 

on pillar strength diminished. 

6) At moderate frictional 

properties (@, = 15' and C, = 

0.69 MPa), the ultimate pillar 

strength is reduce by 50%. 

7) At low frictional properties (a, <lo0 ; C, = 0) , the ultimate 

pillar strength may be reduced 

by >80%. 

8) The interface slip mechanism 

allows the coal pillar to move 

p~llar without interface 180,---- 

I I KEY f I 

c 0 MPa - 

--- C =0.34MPa 

-- 6 

C = 0.69 MPa - 6 

C = 1 .O3MPa 6 

Interface friction angle 

Figure 7. Graph showing the 

relationship between @, , C, 

and ultimate strength of 

pillars with interface slip 

mechanism. 

~igure 8. Deformation of 

pillar represented by distortion of grid. [Note: a) 

no applied pressure, b) 

large applied pressure 

without interface, c) large 

applied pressure with 

interface] 
almost uniformly into the mine opening. 

Conclusions drawn from this investigation are-- 

1) The effects of interface slip should be considered in 

designing large coal pillars. Failure do so may result in 

overestimating pillar strength. 

2) The addition of the interface slip mechanism with a 

linear failure criterion/strain softening model still does 

not correctly reproduce the behavior of observed coal pillars. A properly designed nonlinear Mohr or Mohr-Coulomb 

failure criterion that delays the onset of plastic deformation in the low confinement zones and hastens plastic deformation in the high confinement zones needs to be incorporated into pillar behavior mechanisms. 

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