ISSUE DATE: 03/16/09 

PROGRAM INFORMATION BULLETIN NO. P09-03 

FROM: MARK E. SKILES 

Director of Technical Support 

KEVIN G. STRICKLIN 

Administrator for 

Coal Mine Safety and Health 

SUBJECT: General Guidelines for the Use of Numerical Modeling to Evaluate 

Ground Control Aspects of Proposed Coal Mining Plans 

Scope

Coal mine operators, miners and miners' representatives, and Mine Safety and Health 

Administration (MSHA) enforcement personnel should receive this bulletin. 

Purpose

The purpose of this program information bulletin (PIB) is to provide the mining 

community with general guidelines when utilizing numerical modeling to evaluate 

ground control aspects of proposed mining plans. Assessing the stability of mined 

areas and the compatibility of mining plans with existing conditions are essential 

elements in assuring a safe working environment at a given mine site. 

Background

Effective mine design has long been recognized as an essential element in establishing 

safe and productive mining operations. Over the years, numerous empirical and 

analytical techniques (e.g. Analysis of Retreat Mining Pillar Stability (ARMPS) and 

Analysis of Longwall Pillar Stability (ALPS) computer programs) have been developed 

to analyze pillar stability. These methods can provide a reasonable estimate of pillar 

strength and stability under specific conditions and relatively simple mining 

geometries. In practice, however, situations often arise where areas of concern contain 
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pillar configurations with varying entry and crosscut orientations and widths in 

addition to differing pillar dimensions. Additional factors such as non-uniform pillar 

lines, remnant stumps scattered throughout irregularly shaped gobs and multiple seam 

mining can further complicate an analysis. In such instances, application of empirical 

and analytical methods to evaluate ground stability is difficult. In order to evaluate 

mining configurations and sequences not easily treated by simplified empirical or 

analytical methods, numerical modeling methods (i.e. boundary element, finite 

element) can be employed. 

Information

Simulation Process

The following is an eight-step process developed by MSHA, Technical Support, Roof 

Control Division for the simulation of underground mining systems1. While it is 

specifically directed to numerical modeling applications, it can also be used in 

conjunction with empirical or other analytical methods. 

1. Observe Underground Areas - This is an essential first step in solving ground 

control problems regardless of the methodology employed. Mine conditions 

should be categorized in a number of areas where differing pillar sizes, panel 

configurations and overburden levels are found. A deterioration index rating 

system, discussed later in this PIB, can aid in the description of in-mine ground 

conditions. 

2. Estimate Model Parameters - Coal, rock and gob properties must be established 

consistent with the requirements of a particular numerical method. Ideally, 

those properties will be based on coal and rock tests of the specific mine site. In 

the absence of that data, published properties of adjacent or same seam mines 

can be used. It should be noted that laboratory values tend to overstate the 

actual in-situ properties. Consequently, it is appropriate to apply a reduction 

factor, based upon specimen size, to the laboratory values. As an example: 

strength reduction factors of 1/5 for 2-inch cubes and 1/4 for 3-inch cubes have 

been used to estimate in situ coal strength from test data. When no site-related 

data is available, general coal and mine roof rock properties can be utilized, or 

the default values offered in the software, can be employed. Regardless of the 

source of data, it cannot be overemphasized that they represent only a first 

estimate of mine roof and rock properties that must be validated. 

3. Model Observed Areas - The third step of the process involves modeling each of 

the areas observed underground. The properties estimated above are tested 

 

1 Karabin, G.J., and M. A. Evanto. Experience with the Boundary Element method of Numerical Modeling 

as a Tool to Resolve Complex Ground Control Problems. Proceedings of the 13th International Conference 

on Ground Control in Mining, WV Univ., August 1994, pp. 201-213. 
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under various geometric and overburden conditions to determine their 

suitability to accurately model observed areas. Successfully modeling many 

areas under a variety of different conditions increases confidence in the 

properties used. 

4. Verify Model Accuracy - This is the most critical step in the entire simulation 

process. Each of the areas modeled must be closely examined to ensure that the 

results correlate with observed conditions. If reasonable correlations cannot be 

made, the model must be recalibrated (material properties adjusted) and the 

process repeated. It should be noted that relating the output of numerical 

models (stress, convergence, etc.) to observed conditions (pillar sloughing and 

roof or floor deterioration) is often difficult given the complexities of the 

underground environment. The use of a deterioration index rating system, 

discussed later in this PIB, can simplify the task of verifying model accuracy. 

5. Establish Threshold Limits - Once the accuracy of the model is verified, 

threshold limits delineating acceptable and unacceptable mining conditions must 

be established in order to evaluate the effectiveness of proposed design 

alternatives. Stress or convergence levels corresponding to deteriorating ground 

conditions can be identified. Other factors such as the extent of pillar yielding or 

predicted pillar, roof and floor conditions can also be utilized. 

6. Model New Configurations - Having established an effective model and a 

means of evaluating the results of analyses, new mining techniques can be 

simulated. Generally, several alternatives are modeled under the conditions 

expected at the mine location where the design will be implemented. 

7. Evaluate New Configurations - The various alternatives can be evaluated 

relative to the threshold limits established. For instance, if specific stress and 

convergence values were found to correspond to deteriorating ground 

conditions, an alternative that produces levels lower than those values would be 

desired. However, if none of the configurations evaluated meet the threshold 

requirement for stable conditions, then new alternatives must be developed and 

analyzed. 

8. Implement Best Alternative - Once the best alternative is identified (either 

meeting the threshold criteria or providing the most favorable conditions), it can 

be cautiously implemented. The level of confidence in achieving a successful 

design is directly proportional to the breadth of the evaluation and the degree of 

correlation noted in the model verification process. In any event, conditions 

should be closely monitored as the design is implemented, and any deviations 

from the expected behavior would warrant reassessing the mining plan and 

recalibration of the model. 
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Deterioration Indices

As mentioned previously, the most critical phase of the simulation process is verifying 

the accuracy of a model through correlation with actual underground conditions. To 

aid in the evaluation of in-mine ground conditions and verification of model accuracy, a 

set of deterioration indices should be established to quantify pillar, roof and floor 

behavior. For example, observed in-mine locations could be assigned a numerical 

rating on a scale of 0 - 5 (0 being the best condition and 5 the most severe) in each of the 

three categories: pillar, roof and floor. The deterioration index levels should be 

reasonably well defined to minimize subjectivity of observations and promote 

consistency in ratings from site to site and from observer to observer. 

Guidelines for the Boundary Element Method 

While the above simulation process and deterioration indices can be applied to 

numerical modeling in general, the following topics specifically address the boundary 

element method (BEM) of numerical modeling for coal mining applications. 

Mining Geometry

An essential step when using the boundary element method is creating a model grid 

that duplicates the in-mine geometry. The seam must be broken into elements of a size 

that allows the entry, crosscut and pillar dimensions to be accurately reproduced. Seam 

elements must be small enough to model details of the mine geometry and produce 

discernable differences in performance, yet large enough to allow broad areas of the 

mine to be included in the simulation. 

As a general rule, setting the element size at one half the entry width has provided 

acceptable results in most coal mining applications. A 10-ft. element width (for a 20-ft.-

wide entry/crosscut configuration) should enable a large area to be modeled and yet 

provides the stress and convergence detail needed to effectively evaluate conditions. 

Both larger (1-entry width) and smaller (1/4-entry width) element sizes can be used for 

specific applications, but are limited in application to scenarios where detail (small 

elements) or influence area (large elements) are considered critical for the analysis. 

A number of other geometric guidelines have been identified that can aid in creating an 

effective boundary element model: 

• To the extent possible, locate model boundaries over solid coal or known stable 

areas to reduce the likelihood of erroneous loading conditions (transferred stress 

from adjacent yielded areas not propagating into the zone of interest). 

• Orient the model such that the primary areas of interest are positioned away 

from the model boundaries to minimize end effects. 
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• Known or potential yielding pillars should not contain linear-elastic elements 

which could erroneously affect the stress transfer to adjacent areas. 

• Known or potential yielding pillars should contain an odd number of elements 

across the minimum dimension to ensure accurate pillar strength and peak core 

stress calculations. 

• Care should be taken when entries or crosscuts are not oriented at 90° angles to 

ensure that the effective widths and percent extraction match the actual mine 

geometry. 

Rock Properties

The rock mass properties needed for BEM modeling are minimal since the assumption 

of a linearly elastic material is inherent and in most BEM models the rock mass is 

composed of a single unit. Initially, it would appear that treating a complex rock 

structure in such a simplistic manner would not be appropriate. However, considering 

that stresses on pillars within the seam are generated through massive main roof 

loading (generally remaining in elastic compression), it is not unreasonable to expect an 

effective representation of pillar loading. 

One widely used BEM program, LaModel, represents the rock mass as a stack of layers 

piled atop one another. The layered formulation in LaModel utilizes an additional 

input parameter, layer thickness, that can be adjusted to allow more flexible and 

realistic strata behavior. In LaModel, it is important to recognize the effect of layer 

thickness. Using thin laminations will result in roof which tends to sag readily into the 

mine openings and load the edges of pillars. As a result, the rock mass is less apt to 

span across openings or failing pillars and does not transfer load over a long distance. 

Coal Properties

Establishing representative coal properties for a BEM analysis is the most critical step in 

model formulation. Yielding seam capability is needed to accurately simulate the 

complex underground environment where localized coal failure results in the 

redistribution and concentration of stress into adjacent areas. The suitability of 

assigned coal properties can be assessed by comparing the simulation output to 

observed pillar conditions. Test models should include underground areas (varying 

depths and pillar sizes) where definite observed pillar behavior can be documented and 

reflects the differences in depth and pillar size. For instance, if a model with 8-ft.-wide 

elements predicts corner yielding, significant sloughing and crushing for a length of 8 

ft. from the pillar corner should be obvious. A similar condition would be expected 

along the sides of pillars if perimeter yielding were projected. In general, more 

observed pillar deterioration than projected by the model suggests that the coal strength 

has been overestimated and less sloughing than predicted indicates it has been 

underestimated. There are occasions, however, where the element size itself can 
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contribute to erroneous interpretations. For example, a model using 10-ft. elements 

may indicate elevated stress at the pillar corners, but no yielding. However, 

underground observations reveal 4-ft. crushed zones at the pillar corners, suggesting 

that the model coal strength has been overestimated. Remodeling the area using 4-ft. 

elements (with corresponding recalculation of element properties) may result in the 

prediction of corner yielding that would match the in-mine conditions. 

When constructing calibration models to verify coal strength, it is essential that: 

• the element size selected is appropriate to illustrate phenomena (yielding) 

observed underground; 

• element properties are recalculated when element sizes are changed, as smaller 

elements have lower strength values than larger ones because of their proximity 

to the free face. 

Gob Properties

When numerical models contain large mined areas such as longwall or pillar line gobs, 

some mechanism must be employed to simulate caving and stress relief associated with 

those areas. Without it, the full weight of the overburden would be transferred to 

adjacent areas and result in a significant overestimation of abutment loads. The stress 

redistribution process is complex and is comprised of caving, bulking and subsequent 

compaction of the gob material. As with other material properties, the suitability of the 

gob material properties that essentially treat the gob as a backfill must be verified. The 

use of a gob material that is too stiff will result in excessive gob loading and reduced 

abutment loads. Conversely, a gob material that is too soft will generate excessive 

abutment loads and low-gob stress. The modulus of elasticity of the rock mass and 

other geometric parameters (panel width, lamination thickness, etc.) can have a 

significant impact on gob backfill loading and must be considered. Examining gob 

backfill stress can indicate the amount of stress redistribution simulated by the model 

and can be compared to known or anticipated cave heights associated with those areas. 

Summary

Successful numerical simulation requires a substantial effort including the observation 

of in-mine conditions in many areas and the often repetitive process of calibrating 

model parameters. The use of techniques such as a deterioration index can facilitate the 

linking of observed and simulated mine conditions. It cannot be over-emphasized, 

however, that in order to be of value, a numerical model must be validated and provide 

a realistic representation of the underground environment for which it is applied. 
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Authority

The Federal Mine Safety and Health Act of 1977, as amended, 30 U.S.C. § 801 et seq.; 

30 C.F.R. §75.203. 

Internet Availability

This PIB may be viewed on the Internet by accessing the MSHA home page at 

http://www.msha.gov "Compliance Info" and "Program Information Bulletins." 

Issuing Office and Contact Person(s)

Coal Mine Safety and Health 

Stephen Gigliotti, (202) 693-9479 

E-mail: Gigliotti.Stephen@dol.gov

Technical Support 

Joseph A. Cybulski, (412) 386-6920 

E-mail: Cybulski.Joseph@dol.gov

Distribution

MSHA Program Policy Manual Holders 

Underground Coal Mine Operators 

Coal Special Interest Groups