ABSTRACT

The advance-and-relieve method benefits from lateral

destressing associated with mining in laminated rocks and a high 

horizontal stress regime. This stress control method is based on 

measurements showing that occurrence of rock failure in the roof 

and floor of an entry (or caving of roof strata in a panel gob) results 

in redistribution of stresses in adjacent entries. By locating other 

entries within the shadow zone of the first entry (or gob),

improvements in stability can be achieved.

Numerical modeling proved useful in studying the basic

mechanics of lateral relief while investigating the sensitivity of 

results to different geologic and mining parameters using controlled 

experiments. It was shown that failure of rocks near an entry 

results in redistribution of horizontal stress and shifting of the stress 

to higher horizons. Measurements from two mines are consistent in 

showing significant horizontal stress reductions in comparison with 

the far-field stress regime within the destressed zone. Although the 

far-field stress regime is very anisotropic, these measurements 

show near-equal secondary principal horizontal stresses, or perhaps 

a switch in orientation, as a result of destressing. Stress relief is 

achieved through lateral movement and relaxation of rocks along 

weak bedding planes toward adjacent caved zones (or softened 

zone).

Because of cave geometry in the advancing panel, horizontal 

stress concentrations occur near the cave line both in front of the 

face and to the sides. The horizontal stress concentration reaches 

1.7 times the far-field stress ahead of the face. This stress increase 

is significant and may cause structural damage in this zone

benefiting from additional support. In the next advancing panel 

located within the shadow zone of the gob, horizontal stresses are 

significantly reduced in the roof (by 50%). Thus, the stability of 

future advancing panel can be improved through prudent layout 

designs and sequencing. The width of the relief zone is

significantly influenced by the height of the softened (or cave zone) 

and rock mass properties. 

INTRODUCTION

As the most favorable coal reserves are depleted, modern 

mines are being developed under more extreme conditions, such as 

deep cover, multiple seams, and/or high horizontal stresses. The 

trend toward mining reserves under more difficult conditions has 

accelerated dramatically in the past year with the increased demand 

for coal. Room-and-pillar mines, in particular, are facing severe 

ground control problems and are actively searching for solutions.

National Institute for Occupational Safety and Health (NIOSH) 

researchers have been studying the mechanics of stress

redistribution associated with retreat mining for more than a

decade. A particular approach of interest is a new stress control 

method called “advance-and-relieve” mining. This method

involves the extraction of a pillar row and part of the barrier on one 

side of a panel as the panel is developed. A cave is then created 

along the side of the panel as the pillars are extracted that advances 

with, but, just behind the panel face development. The width of the 

cave zone does not encompass the width of the panel but is limited 

to the extracted pillar and barrier. Both zones of stress relief and 

concentration are developed in the panel adjacent to the cave. 

Maximizing stress relief while simultaneously minimizing

unwanted stress concentrations is the key to the technique. During 

the past 3 years, researchers from NIOSH have completed two field 

studies aimed at developing the advance-and-relieve stress-control 

method (1). These field studies have provided much needed 

information, but are expensive and time consuming and results are 

difficult to generalize to other studies. Numerical modeling offers 

an alternative approach to studying the basic mechanics of lateral 

destressing for better utilization of the advance-and-relieve method.

The advance-and-relieve method benefits from lateral

destressing associated with mining in laminated rocks and under a 

high horizontal stress regime. This concept is based on the premise 

that rock failure in the roof and floor of one entry (or caving of roof 

strata in a panel gob) results in a redistribution of stresses in 

adjacent entries. By locating other entries within the horizontal 

shadow zone of the first “sacrificial” entry (or gob), improvements 

in stability are achieved. Republic Steel utilized this concept and 

realized improvements in the stability of adjacent entries 30 to 50 ft 

from the first entry. Practical implementation of this concept 

involved using a Dosco miner to mine an arched entry in the 

middle entry ahead of other entries and installing external support 

while allowing the rocks above the steel sets to fail (estimated cave 

height 25 to 30 ft). This sacrificial entry then shielded side entries 

against lateral stresses. Researchers using numerical modeling to 

study the results found that failure or movements along bedding 

planes was possibly required to develop the extent of stress relief (2, 

3). 

ROCK MECHANICS STUDY OF LATERAL DESTRESSING FOR THE 

ADVANCE-AND-RELIEVE MINING METHOD

Hamid Maleki, Principal

Maleki Technologies, Inc.

Spokane, Washington USA

Dennis R. Dolinar, Mining Engineer

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

Jeff Dubbert, Manager of Technical Services

Blue Mountain Energy, Inc.

Rangely, Colorado USA
The U.S. Bureau of Mines (USBM) researchers used numerical 

modeling to investigate the effectiveness of lateral destressing and 

pillar softening due to the transfer of loads to the sides and 

improving cutter roof problems (2). Slotting the pillar at the roof 

and at mid-height proved ineffective, while lateral softening in 

conjunction with a reduction in pillar width resulted in significant 

stress reductions. Pillar width was reduced from 30 to 12 ft before 

significant reductions in horizontal stresses were calculated in 

adjacent entries. For an elastic rock mass, the shadow zone 

surrounding an entry is very narrow and thus requires very narrow 

pillars to achieve lateral destressing (4). Because narrow pillars are 

less stiff than coal on the sides (side block), they will always 

provide some lateral destressing by transferring loads to the sides 

through the action of a “pressure arch.” This results in lower 

horizontal and vertical stress concentrations on the inside entries. 

In this paper, the authors review the results of field measurements 

in two mines to study stress distribution in laminated rocks. 

Numerical modeling is then used to evaluate the failure mechanism 

and to determine the effect of panel width, geology, and other 

factors on the stress distribution and the extent of stress relief. It is 

also demonstrated that destressing can have a significant impact on 

the interpretation of stress measurements taken to obtain the

far-field stress while the identifying characteristics of

measurements in destressed zones is given. 

REVIEW OF FIELD DATA

Three recent stress measurement sites provide evidence of

lateral destressing. The first set of data was collected at the 

Deserado Mine (Mine 1) through cooperative work among Spokane 

Research Laboratory (SRL) NIOSH, Maleki Technologies, Inc.

(MTI), and Blue Mountain Energy, Inc. The last two sets were 

obtained by the Pittsburgh Research Laboratory (PRL) NIOSH at 

the Tanoma Mine. One of PRL measurements was intended to 

measure the far-field stress regime while the last set was designed 

to quantify stress changes associated with the advance-and-relieve 

method. Measurements at both mines are consistent in showing 

significant horizontal stress reductions in comparison with the farfield stress regime within the destressed zone. Although the farfield stress regime is very anisotropic, these measurements show 

near-equal secondary principal horizontal stresses, or perhaps a 

switch in orientation, as a result of destressing. Both measurements 

were obtained using the overcoring method with the USBM

deformation gauge. The measurements were assumed to be at 

sufficient distance (10 to 19-ft) from the excavation so that far-field 

horizontal stress could be determined. However, because of

significant lateral destressing, the measurements were actually 

obtained within the destressed zone. In both field studies,

measured horizontal stresses were significantly lower than the farfield stress regime. 

Measurements at Mine 1

The Deserado mine is located near Rangely, Colorado, 

extracting B and D seams from the base of the Lower Williams 

Fork Formation, Cretaceous in age. During the extraction of the D 

Seam, far-field horizontal stress was measured at four locations 

over a distance of several miles (figure 1) (5, 6). These 

measurements were taken with the USBM borehole deformation 

gage and analyzed using site-specific measurements of rock

deformation modulus (5). The measurements indicated a maximum 

principal stress of approximately 2,000 psi. Minimum principal 

stress was approximately 1,400 psi at a depth of 600 ft. Subsequent 

measurements at greater depths indicated a slight increase in 

horizontal stress caused by gravity effects. The orientation of 

maximum principal horizontal stress was consistent (N 73 °E to 

N 88 °W, approximately east-west). The higher stress of 6,100 psi 

at site 1 was measured in a very stiff limestone that was locally 

present. Because of its high stiffness, the limestone absorbed very 

high stresses (4).

Three stress measurements were recently taken in the mine roof 

in the B Seam some 40 ft below the D Seam. These measurements 

were obtained from a vertical drillhole at 10.0, 11.5, and 12.3 ft 

above the roof in crosscut 7 along the B Mains. The B Mains are 

oriented approximately north-south (N 25 °E) at a distance of a 

mile from other workings in the D Seam. Because of the

uniformity of geology and lack of faulting in the area, the far-field 

stress regime in the B seam could be assumed to be similar to that 

measured in the D Seam (figure 1).

Table 1 presents the calculated maximum horizontal stress at 

the measurement sites in the B Seam. These measurements are 

significantly lower than previous measurements of far-field stresses 

in the D Seam. Considering the accuracy of the overcoring method, 

the measured stresses are nearly equal. The measured maximum 

stress is oriented approximately north-south, and minimum stress is 

oriented east-west. These data give the appearance that the

principal stresses have switched orientations. The north-south 

orientation of maximum stress is not consistent with the observed 

failure pattern along the rooms in these mains, and thus it is 

believed that recent measurements are not a measure of far-field 

stress. Had additional measurements been taken deeper into the 

Figure 1. Horizontal stresses measured in Mine A D Seam 

(After Maleki and others 1997).
roof, far-field stresses may have been measured that were

consistent with regional measurements in the D Seam. 

Table 1. Calculated secondary principal horizontal stress and 

orientation of maximum stress, B Seam.

Overcore distance 

above roof, ft

Maximum 

stress, psi

Minimum 

stress, psi Bearing

10.0 540 420 N 0 °E

11.3 470 400 N 9 °E

12.3 560 440 N 17 °W

Average 11.3 to 12.3 515 420 N 4 °W

Recent measurements in the B Seam are influenced by the 

height of failure, or “softened” zones, about the entries. Since most 

of the stress-induced failures occurred along the rooms, the authors 

believe that the maximum far-field horizontal stress is oriented 

near-perpendicular to the entries (approximately east-west). 

Failure of laminated rocks along the entries shifts horizontal

stresses to higher horizons. This failure process appears to relieve 

horizontal stress within the crosscuts at some distance behind the 

face, particularly in rocks that contain layers having low shear 

strength values. This results in a reduction in horizontal stresses 

particularly the maximum horizontal stress within the immediate 

roof rocks in the crosscuts. Because of roof damage in the 

northeast orientated entries, the stresses measured in the crosscuts 

have been significantly altered from the expected regional stress 

field as measured in the D-seam. Essentially, the regional

maximum horizontal stress (east-west) has been reduced by stress 

relief with this reduction also affecting the measured minimum 

stress. In the lower roof, with less damage in the crosscuts, stress 

can be transferred more efficiently in the roof in the north-south 

direction than in the east-west direction where damage occurred to 

the entries. Therefore, a higher stress can be sustained in the northsouth direction than in the east-west direction. This assertion is in 

agreement with numerical modeling results and detailed stress 

mapping within the entire mine. Modeling results from this mine 

are not included in this paper. Results are, however, similar to that 

presented later in this paper for Mine 2, East Mains.

Stress Measurements at Mine 2, East Mains

One successful stress measurement was obtained at a distance 

of 17 ft above the roof in the East Mains just outby the entrance to 

panel E15 at the Tanoma Mine (figure 2, stress site, SS). This mine 

is located 10 miles northeast of Indiana, PA, and extracted coal 

reserves from the 4-ft-thick Lower Kittanning Seam. The East 

Mains are oriented N 60 °W. Roof lithology and mechanical

properties are summarized in table 2. 

Table 2. Roof lithology and average mechanical properties at the 

measurement location.

Distance above 

roof, ft Lithology

Young’s 

modulus, 106 

psi

Uniaxial 

compressive 

strength, psi

0-14 Shale 2.5 10,800

14-23 Sandy shale 3.3 14,850

23-26 Sandstone 4.5 29,300

23-30 Shale NA NA

To estimate the far-field stress regime, the authors reviewed 

available measurements in the Kittanning (1) and Freeport coal 

seams. Considering geologic conditions, the authors expect the 

following pre -mining horizontal stress range for the Tanoma Mine:

maximum stress ( P) = 2, 400 to 2,800 psi,

minimum stress (Q) = 1,750 to 2,100 psi, and

maximum stress direction = N 70 °E.

The measured secondary principal horizontal stress regime is 

summarized below. Considering the accuracy of the overcoring 

method, secondary principal stresses are near-equal, and the

maximum measured stress is significantly below the assumed farfield stress. The stresses are 

maximum measured stress = 1,975 psi,

minimum measured stress = 1,710 psi, and

maximum stress direction = N 78 °W. 

Stress Measurements at Mine 2, E15-E17 Panels

The last set of measurements was obtained in the L2 entry of 

E15 panel (figure 2). At this site, six multi-component deformation 

gauges were installed in the roof at a distance of 2 to 20 ft above 

the seam. The intent of these measurements was to quantify lateral 

destressing that occurred in the L2 entry as a result of caving in the 

E17 panel as the panel was advanced. In this case the cave was 

produced by the advance and relieve mining method. The

instruments were at a distance of 120-ft from the cave line. E15 

Figure 2. Location of test site (TS) in the L2 entry, stress site 

(SS).
and E17 panels are oriented N 30 °W, or near- perpendicular to the 

maximum far-field stress (P).

Figure 3 presents the history of horizontal stress changes for 

four advancing positions of the cave in panel E17. In spite of 

variability in the data, there is a consistent trend for horizontal 

stress changes in the roof. 

• The horizontal stress acting perpendicular to the panel (P or 

maximum far-field stress) is reduced by 700 to 1,200 psi.

• The horizontal stress acting parallel to the panel (Q, or

minimum far-field stress) is slightly increased at the face 

position followed by a slight decrease behind the face position. 

This stress change is generally within the accuracy of the 

measurement technique (about 200 to 400 psi).

Considering the limitations in this field program, the authors 

expect these measurements to be accurate within a 25% margin in 

the magnitude of stress changes. Nevertheless, the trends indicate 

significant reduction in maximum horizontal stress (P), in particular 

at large distances from the gob that cannot be readily accounted for 

using elastic solutions. 

BACK-ANALYSES OF STRESSES AT THE TANOMA MINE

Numerical modeling is used as a tool for back analyses of 

stresses and for examining the mechanics of lateral stress relief in 

laminated, inelastic rocks. In this section, the authors provide 

analyses of stress distribution along the measurement locations 

using both elastic and inelastic constitutive models and a

comparison of results with available field data. The intent is to 

enhance the understanding of the mechanics of lateral destressing. 

For this purpose, the authors used both two - and three-dimensional 

finite-difference codes (8) with multi-material elastic and inelastic 

constitutive laws. 

Modeling Methodology

The authors started the modeling exercise by using elastic 

material models and then incorporating inelastic models. Elastic 

models are advantageous because they require the least amount of 

input data while incorporating basic lithology and material

properties. In selecting the material models for inelastic runs, the 

authors examined the laminated nature of the rocks that have been 

always associated with significant lateral destressing. The authors 

chose several constitutive models that can simulate the effects of 

bedding planes with low shear-strength properties, as well as 

crushing the rock mass layers to a residual strength. These models 

were: 

• Ubiquitous joint model.

• Mohr-Coulomb model.

• Strain-softening rock mass model with ability to crush and 

soften under increased strain.

• Strain-softening model for the rock mass with interfaces

located at the boundary of laminated rock units. The

horizontal interfaces with low shear-strength properties “soft 

interfaces” are suitable for simulating lateral movements and 

the associated stress relief.

Two approaches were investigated to simulate the compressive 

failure of the rock mass about the entries. First, a strain -softening 

model was used for laminated rocks. With this model, one can 

simulate crushing and unloading of material by reducing cohesion 

and/or the angle of internal friction from peak values to residual 

values after failure . Alternatively, failed elements in the roof were 

removed because failed material can be considered to have

insignificant load-carrying capacity (9). By removing the failed 

rocks for an elastic rock mass bounded with soft interfaces, the 

authors simulated horizontal stress relief that was in agreement 

with field measurements. 

Initially, the authors used two -dimensional models to develop 

modeling procedures and to select material models and suitable 

ranges for the material properties (table 3). A three-dimensional, 

finite-difference code (FLAC3D) was used for complementing the 

two -dimensional stress analyses so that the three-dimensional 

nature of the stress field and mine excavations could be modeled 

realistically. FLAC3D (Fast Lagrangian Analyses of Continua in 

Three Dimensions) simulates the behavior of rock and soil

structures that behave elastically or undergo plastic flow when 

yield limits are reached. 

Table 3. Range of material properties used in models.

Property Roof and 

floor

Coal 

seam

Bedding planes

(soft to stiff)

Young’s modulus, mpsi 1 to 4.5 0.30

Poisson’s ratio 0.25 0.25

Cohesion, psi NA NA 40 to 70

Angle of internal friction 

(degrees) NA NA 15 to 45

Normal stiffness (psi/in) NA NA 40,000

Shear stiffness (psi/in) NA NA 40 to 400

NA = Not applicable.

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Date

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0

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0 400 800 1,200

-1,400

-1,400

90 -1,000

-1,000

180 -600

-600

270 -200

-200

360 200

200

A

B

Direction

Cave position, ft

KEY

 Q

P

Figure 3. Measured stress changes from advance and relieve 

mining at the Tanoma mine. (A) 2-ft depth and (B) 10-ft 

depth. Delta P and Q are the maximum and minimum stress 

changes. The azimuth direction is to delta Q.
Stress Analyses of East Mains, Mine 2

East mains are a set of 18-ft wide entries oriented near parallel 

to the maximum horizontal stress direction of N 70 °E (Figure 2). 

The entries are separated by crosscuts on 100-ft centers. Because 

of unfavorable orientation of crosscuts with respect to the

maximum horizontal stress, there was higher structural damage in 

the crosscuts than the rooms. The only successful stress

measurement was obtained in a room at a distance of 60 ft from the 

center of the crosscut.

Figure 4 presents mesh geometry, material layers, and the 

coordinate system. The mesh includes one of the East Mains’ 

entries, three crosscuts, and two pillars. Because of symmetry 

conditions applied on the model boundaries, the functional modeled 

area is many times larger than shown in figure 4. The model is 

loaded with far-field minimum horizontal stress (Q) in the Xdirection and maximum stress (P) along the Y-direction. Geologic 

conditions are analyzed using four materia l models. Instead of

removing the failed roof rocks within the crosscuts, the authors 

utilized the strain -softening model. Peak cohesion varied from a 

maximum of 150 to 50 psi for the sandy shale and shale layers, 

respectively. The residual cohesion for softened rock was reduced 

to a minimum of 25 psi in some analyses. The peak and residual 

angle of internal friction were 38 to 20 degrees.

Selective results are summarized in table 4 and figure 5. There 

is a good agreement between the calculated and me asured stresses 

particularly when using the lower bound for the far-field stress 

condition. Note that the calculated stresses are near-equal, and 

maximum stress is now oriented in the X-direction.

Based on the results pertaining to modeling procedures, the 

authors believe that an elastic model with soft interfaces and 

inclusion of cave geometry is the best approach for simulating 

lateral destressing. Because the plastic strain is mesh-dependent in 

FLAC formulation, the predicted results for the strain -softening 

model may not be adequate for the relatively coarse mesh

resolution. The authors have, therefore, used elastic models for roof 

rocks while simulating the failure/softening of roof rocks by

removing the material for the remaining part of this study. 

Table 4. Calculated horizontal stress at point A instrument location. Measured range varies from 1,700 to 2,000 psi.

Model Syy, psi Sxx, psi Comment

Elastic 2,750 2,200 Very limited lateral relief

Mohr-Coulomb plasticity for roof rocks with

cohesion = 50 psi 2,200 2,300 Near equal stresses

Strain softening and soft interfaces 2,350 2,350 Using peak cohesion of 150 to 90 psi for 

sandy shales and shales

Strain softening and soft interfaces 2,050 2,350 Using peak cohesion of 50 psi for both 

sandy shales and shales

Strain softening and soft interfaces and lower bound 

for far-field stress* 2,000 2,070 Using peak cohesion of 50 psi for both sandy shales and shales

*Using the lower limit for far-field stress SYY =2,400 and SXX= 1,750 psi

FLAC3D 2.10

Maleki Technologies, Inc.

Mining & Geotechnical Engineers

Step 5500 Model Perspective

15:55:06 Tue Dec 10 2002

Center:

 X: 1.700e+001

 Y: 9.900e+001

 Z: 2.000e+001

Rotation:

 X: 10.000

 Y: 35.000

 Z: 45.000

Dist: 5.356e+002 Mag.: 1

Ang.: 22.500

Job Title: Tanoma Mains, Interface with Elastic Model, sxx 1750 syy 2400

Block Property shear

1.730000e+007

5.760000e+007

1.440000e+008

1.900000e+008

2.590000e+008

Axes

 Linestyle

X

Y

Z

Figure 4. Mesh geometry and model properties, Tanoma 

Mine, East Mains, 3D analysis.

SXX Results for FLAC3D Mains Analyses

-6000

-5000

-4000

-3000

-2000

-1000

0

0 5 10 15 20 25 30 35

Distance from x=0 (ft)

Horizontal stress, SXX (psi)

Elastic and soft interfaces

Low strain softening and soft interface,

P=2,800 psi

in situ stress

Strain softening & soft interface

(compression as negative)

Entry

Pillar centerline

Measurement

SYY Results for FLAC3D Mains Analyses

-3000

-2500

-2000

-1500

-1000

-500

0

0 5 10 15 20 25 30 35

Distance from x=0 (ft)

Horizontal stress, SYY (psi)

Elastic & soft interface

Low strain softening & soft interface,

P=2,800 psi

in situ stress

Strain softening & soft inetrface

(compression as negative)

Entry Pillar centerline

Measurement

Figure 5. Comparison of the model results versus measured 

horizontal stresses, Tanoma mine, east mains, 17 ft in roof. 

Top is Sxx-direction and bottom is Syy-direction.
Stress Analyses of Panel E15 and E17, Mine 2

As illustrated in figures 2 and 3, changes in horizontal stress 

were measured in the mine roof at a test site located in the L2 

entry, panel 15. This five-entry panel was isolated by a 100-ft 

barrier from the cave developed from the advance and relieve 

method in panel 17. The width of the caved zone in panel 17 was 

approximately 120 ft. A significant reduction (700 to 1,200 psi) 

in maximum stress (P) was measured in the mine roof and 

attributed to the formation of the cave in panel 17. Assuming 

that the sandy shale-sandstone roof rocks remained elastic during 

the development of panel 15, the change in horizontal stress is 

related to formation of the cave and lateral displacement of

laminated roof toward the cave zone. The following threedimensional analysis addresses some of the mechanisms involved 

using a cave height of 20 ft. The sensitivity of results to different 

parameters is examined later in this paper.

Figure 6 presents the mesh geometry. The mesh includes 

half of the gob zone in panel 17 and half of the entry

development in panel 15. Because of the planes of symmetry on 

the model boundaries, the actual modeled area is many times 

larger than shown in figure 6. The model is loaded with far-field 

maximum horizontal stress (2,400 psi) in the X-direction and 

minimum stress (1,750 psi) along the Y-direction. Geologic 

conditions are analyzed using four material model properties.

Below are five face positions (or modeling steps).

Comparison of model results versus measured stress changes at 5 

different face positions from advance and relieve mining at the 

Tanoma Mine corresponding to retreat distances as shown in 

figure 3.

1. Panel 15 development with no pillaring or cave in panel 17. 

2. The pillar/cave line is near the instrument location.

3. The pillar/cave line passes the instruments by 400 ft.

4. The pillar/cave line passes the instruments by 800 ft.

5. The pillar/cave line passes the instruments by 1,200 ft.

Figure 7 compares the measured and model stress changes at 

five face positions and two instrument locations. There is a good 

agreement between measured and calculated results, particularly 

in the main roof. The model does not show any significant 

change in the horizontal stress pattern after a retreat of 400 ft 

from the instrument location. 

Figures 8 and 9 present the horizontal stress distribution (Sxx) 

during the retreat at the measurement horizon for steps 2 and 3. 

The direction of retreat in panel 17 is from the bottom of the page 

to the top.

• In panel 17 (advancing panel), horizontal stress

concentrations occur near the cave front both in front of the 

cave and to the sides. The Sxx stress exceeds 4,000 psi ahead 

of the face (stress concentration factor of 1.7). This stress 

increase is significant and may cause structural damage in 

this zone. 

Figure 6. Mesh geometry for cross section of panels 15 and 

17, Tanoma Mine, 3D analysis.

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-1,400

90 -1,000

-1,000

180 -600

-600

270 -200

-200

360 200

200

A

B

Direction

Cave position, ft

KEY

 Q

P

Model stress xx

Model stress yy

Figure 7. Comparison of model results versus measured stress 

changes at 5 different face positions from advance and relieve 

mining at the Tanoma Mine. (A) 2-ft depth and (B) 10-ft 

depth. Delta P and Q are the maximum and minimum stress 

changes. The azimuth direction is to delta Q.

FLAC3D 2.10

Maleki Technologies, Inc.

Mining & Geotechnical Engineers

Step 9500 Model Perspective

13:50:34 Thu May 15 2003

Center:

 X: 2.403e+002

 Y: 3.866e+002

 Z: 5.421e+001

Rotation:

 X: 96.327

 Y: 0.780

 Z: 0.000

Dist: 5.599e+003 Mag.: 2.38

Ang.: 22.500

Plane Origin:

 X: 0.000e+000

 Y: 0.000e+000

 Z: 2.100e+001

Plane Normal:

 X: 0.000e+000

 Y: 0.000e+000

 Z: 1.000e+000

Job Title: Advance-and-Relieve, Step 2, Retreat face in E15 at instrument location

Contour of SXX

 Plane: on

 Magfac = 0.000e+000

 Gradient Calculation

-4.0000e+003 to -3.6000e+003

-3.6000e+003 to -3.2000e+003

-3.2000e+003 to -2.8000e+003

-2.8000e+003 to -2.4000e+003

-2.4000e+003 to -2.0000e+003

-2.0000e+003 to -1.6000e+003

-1.6000e+003 to -1.2000e+003

-1.2000e+003 to -8.0000e+002

-8.0000e+002 to -4.0000e+002

-4.0000e+002 to 0.0000e+000

 0.0000e+000 to 0.0000e+000

 Interval = 4.0e+002

X

Y

Z

Figure 8. Horizontal stress contours (Sxx) at 17 ft into roof, 

modeling step 2. (Lighter shades indicate stress concentration 

and darker shades indicate stress relief.
• In panel 15 development ahead of the retreat face in panel 17, 

horizontal stresses are concentrated in the roof, exceeding the 

premining stress of 2,400 psi.

• In panel 15 developments behind the retreat face in panel 17, 

horizontal stresses are significantly reduced in the roof.

Clearly, the entries are now within the shadow zone of the 

gob. The horizontal extent of lateral destressing is shown to 

be significant.

Having visualized the three-dimensional stress relief

mechanism, one can tailor special panel designs utilizing the 

advance-and-relieve method. Because horizontal stresses

concentrate ahead of the face in panel 17 before lateral

destressing is achieved (behind the face), the advance-and-relieve 

method may not be very effective in reducing ground control 

problems at the faces in panel 17. However, the stress shadow 

zone forming behind the retreat face provides protection for

entries located to the side of panel 17. Thus, significant stress 

reductions can be achieved through prudent mine layout designs 

and sequencing as follows:

1. Mine the first panel (advancing panel) and create a cavity in 

the roof by extracting the coal to the sides. Because of stress 

concentrations in this panel, the authors recommend use of 

additional support.

2. Mine the second panel (advancing panel) after retreat of the 

first panel. Locating the second panel within the stress 

shadow of gob of the first panel will benefit from lateral 

destressing. 

3. The degree of lateral destressing in the second panel depends 

on both geologic conditions and mine layout designs. 

Minimum width stable pillar designs should be used in the 

second panel (including both barrier and in-panel pillar 

designs). This is important because lateral relief is reduced 

as a function of distance from the gob.

SENSITIVITY ANALYSES

To utilize the potential benefits from lateral destressing, the 

authors used numerical models to quantify the maximum extent 

of horizontal stress relief as a function of geologic and mining 

conditions. Four specific geologic and engineering factors were 

evaluated:

1. Soft and stiff bedding plane properties.

2. Cave height.

3. Cave or panel length.

4. Cave width

The following factors were kept unchanged: Thickness of 

the interbedded zone in the mine roof and the far-field stress. 

Both two - and three-dimensional modeling was used in this 

sensitivity analyses while comparing the results with the Tanoma 

Mine measurements for reference. Using FLAC2D analyses, the 

authors evaluated both the influence of cave height and interface 

properties. Figure 10 shows minimal lateral stress relief for roof 

rock having either stiff or poorly developed interfaces. 

Obviously, the cave height influences the results significantly. 

For all cases, the maximum relief occurred near the L2 entry, 

gradually diminishing farther away from the gob. Thus to take 

the most advantage of lateral destressing, it is important to 

optimize pillar designs and consider both vertical and lateral 

loading conditions. Within the practical ranges used for the cave 

width (60 to 120 ft), the width of the gob zone does not influence 

results significantly.

The sensitivity of results to other parameters, including panel 

length, was evaluated using three-dimensional analyses. Figure 

11 presents the results for two panel lengths and three cave 

heights. Panel length does not influence the results significantly 

within the analyzed range. Cave height and bedding plane 

properties are the most significant factors using the results of 

both two -and three-dimensional analyses.

CONCLUSIONS

The advance-and-relieve method benefits from lateral

destressing associated with mining in laminated rocks and a high 

horizontal stress regime. This stress control method is based on 

measurements showing that the occurrence of rock failure in the 

roof and floor of one entry (or caving of roof strata in a panel 

gob) results in redistribution of stresses in adjacent entries. By 

locating other entries within the horizontal stress shadow zone of 

the first “sacrificial” entry (or gob), improvements in stability are 

achieved.

Field measurements provided important evidence of lateral 

destressing at distances exceeding 120 ft from the gob. For both 

anisotropic horizontal stress fields in the study mines, a signature 

of lateral destressing is shown to be a reduction in the maximum 

stress, giving the appearance that the horizontal stresses are nearequal within the relief zone or perhaps the stresses have switched 

orientations. This reduced stress pattern should alert researchers 

in the field that they are still within the relief zone and that the 

measurements should be continued deeper into the roof when 

making stress measurements in regions known to have

anisotropic stress fields. 

FLAC3D 2.10

Maleki Technologies, Inc.

Mining & Geotechnical Engineers

Step 14000 Model Perspective

14:07:17 Thu May 15 2003

Center:

 X: 2.293e+002

 Y: 5.721e+002

 Z: 3.675e+001

Rotation:

 X: 94.396

 Y: 0.662

 Z: 0.000

Dist: 5.599e+003 Mag.: 1.71

Ang.: 22.500

Plane Origin:

 X: 0.000e+000

 Y: 0.000e+000

 Z: 2.100e+001

Plane Normal:

 X: 0.000e+000

 Y: 0.000e+000

 Z: 1.000e+000

Job Title: Advance-and-Relieve, Step 3, Retreat face in E15 passed instrument location

Contour of SXX

 Plane: on

 Magfac = 0.000e+000

 Gradient Calculation

-4.0000e+003 to -3.6000e+003

-3.6000e+003 to -3.2000e+003

-3.2000e+003 to -2.8000e+003

-2.8000e+003 to -2.4000e+003

-2.4000e+003 to -2.0000e+003

-2.0000e+003 to -1.6000e+003

-1.6000e+003 to -1.2000e+003

-1.2000e+003 to -8.0000e+002

-8.0000e+002 to -4.0000e+002

-4.0000e+002 to 0.0000e+000

 0.0000e+000 to 0.0000e+000

 Interval = 4.0e+002 X

Y

Z

Figure 9. Horizontal stress contours (Sxx) at 17 ft into roof, 

Modeling step 3. (Lighter shades indicate stress 

concentration and darker shades indicate stress relief.
Numerical modeling proved useful in studying the basic 

mechanics of lateral relief while investigating the sensitivity of 

results to different geologic and mining parameters using

controlled numerical experiments. It was shown that failure of 

rocks near an entry results in redistribution of horizontal stress 

and shifting of the stress to deeper horizons both in the roof and 

floor. Within the relief zone, the maximum far-field stress is 

significantly reduced, locally to near-isotropic conditions. 

Horizontal relief is most effective in laminated rocks with low 

shear-strength properties. The width of the relief zone is

significantly influenced by the height of the softened or cave 

zone and the rock mass properties. Because of the

interdependence of cave height, rock mass properties, and panel 

layout designs, it is difficult to develop general guidelines for 

effective implementation of the advance-and-relieve method

without additional field studies and numerical modeling. Practical 

implementation of this method als o depends on the spatial

distribution and thickness of interbedded rocks with low shearstrength properties, and excavation timing and sequence.

ACKNOWLEDGMENTS

The management of Blue Mountain Energy, Inc. and the 

Tanoma Mining Company are greatly acknowledged for their 

permission to use their facilities during field investigations and 

providing logistic support. In addition, the authors would like to 

acknowledge the contributions of the following NIOSH

personnel, Dr. Chris Mark, Bob McKibbin, Eric Zahl, Jami 

Girard-Dwyer, and Mike Jones with great gratitude.

SXX Results for FLAC3D Panels 15 & 17 Analyses, Stress Profile at Measurement Height (20-ft)

1000

Panel 15 and 17, 2D, Stress Profile at Measurement Height

-3000

-2500

-2000

-1500

-1000

-500

0

500

0 50 100 150 200 250 300

Distance from center of gob, ft

Horizontal stress, psi

Mine coal, panel 17

23-ft cave, soft interfaces

20-ft cave, soft interfaces

23-ft cave, elastic, no interfaces

23-ft cave, elastic, stiff interfaces

20-ft cave, soft interfaces, narrow

panel

Panel 17 gob

0-23-ft caved

Panel 15

L2

Measured 

stress range

Figure 10. Horizontal stress profile for three interface conditions and two panel widths, 2D analysis.
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