1) Ground control involves techniques to regulate and prevent collapse of mine openings by studying rock behavior under changing stress conditions and providing support systems. 2) Insitu stresses exist naturally in rock and are influenced by gravity, tectonics, and geology, while excavation induces additional stresses; both must be considered for stable mine design and support. 3) Proper planning through factors like layout, geometry, reinforcement, and sequencing can manage stresses to prevent rock failure, while support installation can further control fractured or blocky ground conditions.

1. U Siva Sankar
Email: uss_7@yahoo.com
Mining Ground Control
Ground Control : A collective term given to the techniques that are used
to regulate and prevent the collapse and failure of mine openings.
Ground control is the science that studies the behaviour of rockmass in
transition from one state of equilibrium to another.
It provides the basis for the design of the support systems to prevent or
control the collapse or failure of the roof, floor, and ribs both safely and
economically.
Ground pressure - The pressure to which a rock formation is subjected
by the weight of the superimposed rock and rock material or by
diastrophic forces created by movements in the rocks forming the earth's
crust. Such pressures may be great enough to cause rocks having a low
compressional strength to deform and be squeezed into and close a
borehole or other underground opening not adequately strengthened by
an artificial support, such as casing or timber.
1

2. Rock Stresses
Insitu (Virgin) Stresses Induced Stresses
Exist in the rock prior to any Occurs after artificial disturbance e.g.
disturbance. Mining, Excavation, pumping, Injection,
Energy extraction, applied load, swelling etc.
Tectonic Stresses
Residual Stresses Gravitational Terresterial Stresses
•Diagenesis Stresses •Seasonal tpr. variation
•Metasomatism (Flat ground surface •Moon pull(tidal Stress)
•Metamorphism & topography effect) •Coriolis forces
•Magma cooling •Diurmal stresses
•Changes in pore
pressure
Active Tectonic Stresses
Remnant Tectonic Stresses
Same as residual stresses but tectonic
activity is involved such as jointing,
faulting, folding and boundinage
Broad Scale Local
•Shear Traction •Bending
•Slab pull •Isostatic compensation
•Ridge push •Down Bending of lithosphere
•Trench suction •Volcanism and heat flow
•Membrane stress
Proposed by Bielenstein and Barron (1971)
Insitu and Induced stresses and their representation on
Mohr’s Circle
2

3. THE MINING ENVIRONMENT
IN-SITU STRESSES
3

4. 1. Magnitude and orientation of Insitu stresses vary considerably within geological
systems.
2. The pre-existing stress state changes dramatically due to
excavation/construction therefore load must be redistributed.
3. Stress is not familiar – it is a tensor quantity and tensors are not encountered in
everyday life.
4. It is a means to analyze mechanical behaviors of rock.
5. It serves as boundary conditions in rock engineering problems as a stress state
is applied for analysis and design.
6. It helps in understanding groundwater fluid flow.
7. At large scale shed some light on the mechanism causing tectonic plates to
move or fault to rupture with the added uncertainty in that there is no constraint
on the total force, as is the case with gravity loads.
Insitu stresses
virgin stresses or undisturbed in situ stresses are the natural stresses
that exist in the ground prior to any excavation. Their magnitude and
orientation are determined by
– the weight of the overlying strata, and
– the geological history of the rock mass
In situ vertical stress
For a geologically undisturbed rockmass, gravity provides the vertical
component of the rock stresses. In a homogeneous rockmass, when the
rock density γ is constant, the vertical stress is the pressure exerted by
the mass of column of rock acting over level.
The vertical stress due to the overlying rock is then:
σz =γ h
4

5. Insitu horizontal stress
The source of horizontal stress is mainly due to the tectonic
activities which have resulted in the formation of major
geological structures such as faults and folds.
Since there are three principal stress directions, there will
be two horizontal principal stresses.
In an undisturbed rockmass, the two horizontal principal
stresses may be equal, but generally the effects of material
anisotropy and the geologic history of the rockmass ensure
that they are not. The value of K
K = σh σv
Horizontal stress
Lithostatic stress occurs when the stress components at a
point are equal in all directions and their magnitude is due
to the weight of overburden.
σx =σy =σz
The other assumption is that rock behaves elastically but is
constrained from deforming horizontally.
This applies to sedimentary rocks in geologically undisturbed
regions where the strata behave linearly elastically and are built
up in horizontal layers such that the horizontal dimensions are
unchanged. For this case, the lateral stresses σx and σy are
equal and are given by:
µ
σ x = σ y = σ z. Terzaghi and Richart
(1 − µ )
(1952)
Later this relation found to be not true as Horizontal stress is
always more than vertical stress
5

6. Vertical and Horizontal stresses
Vertical Stress (after Brown Townend and Zoback, (2000)
and Hoek, 1978)
Ratio of Horizontal to Vertical Stress
Sheory,1994
 1
K = 0.25 + 7 Ek  0.001 + 
 z
where Ek (GPa) is the average deformation modulus of the upper part of the
earth’s crust measured in a horizontal direction.
6

7. VERTICAL STRESS CONCENTRATED IN RIBS
HORIZONTAL STRESS CONCENTRATED IN ROOF & FLOOR
Characteristics of Coal Measure Roof Strata
Cover - The overburden of any deposit.
Overburden – Layers of soil and rock covering a coal seam.
Overburden is removed prior to surface mining and replaced
after the coal is taken from the seam.
Lithology - The character of a rock described in terms of its
structure, color, mineral composition, grain size, and
arrangement of its component parts; all those visible features
that in the aggregate impart individuality of the rock. Lithology
is the basis of correlation in coal mines and commonly is
reliable over a distance of a few miles.
Bed - A stratum of coal or other sedimentary deposit.
Roof
The stratum of rock or other material above a coal seam; the
overhead surface of a coal working place. Same as "back" or
"top."
7

8. Characteristics of Coal Measure Roof Strata
Immediate Roof
The roof strata that is immediately above the coal seam. This
is the strata requires support for the mine openings to remain
competent.
Primary roof - The main roof above the immediate top. Its
thickness may vary from a few to several thousand feet.
Secondary roof - The roof strata immediately above the
coalbed, requiring support during the excavating of coal.
Competent rock - Rock which, because of its physical and
geological characteristics, is capable of sustaining openings
without any structural support except pillars and walls left
during mining (stalls, light props, and roof bolts are not
considered structural support).
Characteristics of Coal Measure Roof Strata
Fissure - An extensive crack, break, or fracture in the rocks.
Fracture - A general term to include any kind of discontinuity in a
body of rock if produced by mechanical failure, whether by shear
stress or tensile stress. Fractures include faults, shears, joints, and
planes of fracture cleavage.
Joint
A discontinuity in the rock strata where there is no sign of relative
movement.
A divisional plane or surface that divides a rock and along which
there has been no visible movement parallel to the plane or surface.
Cleat
The vertical and Parallel cleavage planes or partings crossing the
bedding. The main set of joints along which the coal breaks more
easily than in any other direction.
Face cleat - The principal cleavage plane or joint at right angles to
the stratification of the coal seam.
8

9. Characteristics of Coal Measure Roof Strata
Butt cleat - A short, poorly defined vertical cleavage plane in a coal
seam, usually at right angles to the long face cleat.
Slickenside - A smooth, striated, polished surface produced on
rock by friction.
Slip - A fault. A smooth joint or crack where the strata have moved
oneach other.
Fault - A slip-surface between two portions of the earth's surface
that have moved relative to each other. A fault is a failure surface
and is evidence of severe earth stresses.
Fault zone - A fault, instead of being a single clean fracture, may be
a zone hundreds or thousands of feet wide. The fault zone consists
of numerous interlacing small faults or a confused zone of gouge,
breccia, or mylonite.
Fig.: Influence of Joints
Fig: Joints exposed in the sandstone roof
Fig: orientation of Cleats and coal seams
Fig: Face and Butt Cleats in the Coal Pillar
9

10. Normal Fault
Slickensides along the slip plane
Strike Slip Fault
Reverse Fault
Characteristics of Coal Measure Roof Strata
Sandy Strongly Sandstone Sandy Sand stone
shale Jointed over shale
over shale over
Shale Sandstone
Classifications of typical coal measures roof strata
(modified after Peng & Chiang, 1984)
Study of characteristics of coal measure strata is important to
Determine the stability of Openings
Determine Caving Characteristics & proper design of support
system
Design of Mine layout
10

11. PRESSURE ARCH CONCEPT
Arching - Fracture processes around a mine opening,
leading to stabilization by an arching effect.
Stress Distribution Above a Small
Pressure arch formation around mine
Mine Opening
opening (After Dinsdale, 1937)
Abutment Pressures
When an opening is created in a coal seam, the stress that was present before the
opening was created is re-distributed to the adjacent coal pillars that are left. The areas
within the remaining coal where the vertical stress is greater than the average are called
abutments and hence the stresses in those areas are called abutment pressures.
Minor Pressure Arch
Major Pressure Arch
Minor pressure arches can form independently from pillar to pillar when
the strength of the pillars in situ exceeds that of the abutment pressure,
If the pillars yield or fail because of excessive pressure, their load is
transferred to neighboring barriers or abutment pillars and a major
pressure arch
11

12. Formation of major pressure
arches due to Longwall Mining
(After Stemple, 1956)
For very wide openings such as those created by longwall mining, major
pressure arch formation is likely to create points of excessive pressure in seams
above and below
Arching stresses can either hinder or benefit mining in overlying or underlying
seams.
The extradosal ground forms the zone of high compressive stress that can
cause ground control problems in the roof, floor and pillars.
The intradosal ground or tension zone is actually a distressed region in relation
to the surrounding strata and conceivably the stress encountered in this zone may
actually be less than that created by the cover load.
MECHANISM OF STRATA FAILURE
• Failure through intact material due to
overstressing
• Failure along bedding surface due to
overstressing
• Localized failure of discrete joint bounded blocks
• Localized failure of thinly bedded roof sections
• In coal measure strata
– Bedded, low to moderate strength rock types
• Subjected to varying stress levels
– Expected behavior of strata
• Function of roadway shape, lithology & stresses acting on
the roadway
12

13. Idealized Ground Response Curve and Support line.
Idealized Ground Response Curve (GRC) and support line
Prior to excavation, the excavation boundaries are subject to pressure
equal to the field stresses (point A).
After the excavation is created the boundaries converge and the
pressure required to prevent further convergence reduces as arching
and the self-supporting capacity of the ground develops (point B).
A point is reached (point C) where loosening and failure of the rock
occurs and the required support resistance begins to increase as self-
supporting capacity is lost and support of the dead weight of the failed
ground is required (point D).
The effect of the support system can also be plotted on the chart.
Equilibrium is achieved when the support curve intersects the ground
reaction curve (point B).
Ideally, support should be designed and installed to operate as close
as possible to point C, which allows the available strength of the rock
mass to be utilized while minimizing the load carried by the support
system.
The second support has a higher ultimate capacity (point E) than the
first support (point F), but both reach the ground reaction curve at the
same spot. This shows that higher capacity does not necessarily
ensure better ground control.
13

14. Ground Reaction Curve approximation for outby loading conditions in a
longwall tailgate (Barczak, et.al;)
Strength = P/A
where, P= Load to break rock
A= Area
σ
Stiffness = Load per unit area(σ) /
ε
Strain(ε)
Strain = ∆L/ L
This is expressed as the modulus of
elasticity or Young’s modulus (E), so,
E = σ/ε
ε
As ε is dimensionless, E has the same
unit as σ. As the number becomes very
large, it is usually expressed in Giga-
Pascals (GPa)
1 GPa = 1000 MPa
14

15. STRESS AROUND A ROADWAY
HORIZONTAL STRESS LOADS THE ROOF AND FLOOR
AFTER EXCAVATION, HORIZONTAL STRESSES CONCENTRATE IN
THE STIFF (BRITTLE) BEDS IN THE ROOF AND FLOOR
PROPAGATION OF
FAILURE ABOUT
ROADWAYS
15

16. AS THE LOWER ROOF BEDS SOFTEN, STRESSES ARE
REDISTRIBUTED INTO HIGHER STIFF BEDS
UNBOLTED ROOF
THIS FAILURE ZONE WILL CONTINUE TO MIGRATE
FURTHER INTO THE ROOF
IF NO REINFORCEMENT IS INSTALLED
16

17. UNBOLTED ROOF
EVENTUALLY A LARGE FAILURE ZONE
WILL FORM ABOVE EXCAVATION
UNBOLTED ROOF
IF UNSUPPORTED THIS WILL LEAD
TO A FALL OF GROUND
FORMING A NATURAL ARCH
17

18. KEY FEATURES OF ROADWAY
BEHAVIOUR
• State of stress acting on a roadway is influenced by
– Geological structure
– Variation in lithology
– Topography
– Seam structure (warps/rolls, etc.)
– Tectonic setting
• It may be kept in mind that roadways are often developed in
a modified stress field as a result of adjacent workings,
overlying/ underlying workings, in abutment areas due to
pillar/ longwall extraction, etc.
– While analyzing a situation, these influences must be given
due importance
18

19. Effect of Horizontal Stress on Stability of Galleries in Mines
Ground Control Practices and Constraints
To ensure the stability of UG (Bord & pillar , Longwall, Highwall ) or OC
structures, designer must consider principles of rock mechanics to
determine
Overall Mine layout – the relative location & intersection of entries and
pillars, sections, or panels
Shape size and number of entries
Shape size and number of pillars
Optimum support systems for structural stability or controlled failure
Overall Mine layout, overall pit slope & dump slope , slope of individual
benches and spoil dumps
Dimension and number of benches, spoil dumps
Shape of overall pit, and spoil dumps
Constraints: Sometimes rock mechanics principles are need to be
completely ignored in normal mining operations such as Coal
Extraction, Coal haulage, and Ventilation
19

20. Ground Control Techniques or Practices
Ground is controlled in the first instance by proper mine planning. This
means controlling the extraction geometry and sequence in such a way
that stress levels and failure zones in the surrounding rock are kept
below some threshold or potential for failure.
It is not always possible to keep stresses low, and in these cases
support can be installed to control fractured ground. Support is also
used to keep blocky ground from unraveling and resulting in
unexpected groundfalls.
The following techniques can be used to manage stress and
accomplish control.
• Avoidance (change heading location and alignment)
• Excavation shape (can change stresses from tensile to compressive)
• Reinforcement (can provide the rock with additional strength)
• Reduction (i.e. leave protective pillars)
• Resistance (provide ground support)
• Displacement (alter the sequence to “chase it away”)
• Isolation (“keep it away”)
• De-stressing (actively change the stress by blasting)
Ground Control Techniques or Practices
Some ground control techniques serve more than one of the above
functions. For example, a rock bolt may provide for alteration of, and
resistance to, ground stress.
Avoidance
Stress is avoided in the first place by aligning entries, headings, and
boreholes to miss treacherous fault zones, dykes, sills, old workings, and
zones of subsidence by a wide margin. When a problem fault must be
traversed, the heading is aligned to meet it at near a right angle, rather
than obliquely.
Stress concentration is avoided by rounding the corners in a rectangular
heading.
Excavation shape
Tensile and bending stresses are altered to compressive stresses when
the back of a heading is arched. The same is true of a shaft or raise that is
changed from a rectangular to a circular cross-section.
Reinforcement
The ability of the rock mass to resist shear, tensile and bending stress is
reinforced when a cable bolt is tensioned because the friction in joints and
fractures is increased.
20

21. Ground Control Techniques or Practices
Reduction
The ground stress around one heading arising from its proximity to another
opening is reduced by a protective pillar (safe distance) between them.
The magnitude of the ring stress is reduced (and displaced) if a circular
shaft or raise is advanced by drilling and blasting instead of raiseboring,
because the fractured zone “pushes” the peak stress some distance into
the solid rock.
Controlled (“smooth wall”) blasting techniques are used to minimize
overbreak and crack propagation; however, their introduction to highly
stressed ground may have another, negative effect (ring stress
concentration). To reduce stress in deep shaft sinking, it is typical that
smooth wall blasting is abandoned near the horizon where discs were first
observed in the pilot hole drill core.
Resistance
Stresses are resisted with ground support. The support may consist of sets
(wood or steel), rock bolts, cable bolts, shotcrete, screen, strapping, or
concrete. Ground support is commonly evaluated for comparison purposes
by the average pressure that it is calculated to exert against the rock face.
Displacement
Ground Control Techniques or Practices
Isolation
In deep mining, perimeter headings may first be driven around a
stoping block to avoid wrongful stress transfer and minimize stress
buildup in stope ends.
e.g.1: At the current South Deep project in South Africa, the shaft pillar
at the reef horizon was deliberately mined out before shaft sinking
could reach it.
e.g.2:It was proposed (W. F. Bawden) that a ring heading around an
existing shaft will isolate it from stresses induced by future mining in the
near vicinity.
De-stressing
De-stressing displaces stress away from the walls of an entry or
heading and into country rock. When properly executed, de-stressing
creates a failure envelope that shunts stress away from the excavation.
21

22. Various approaches for development of strata control
techniques
Caving Mechanisms – Strata Mechanics – B&P
Typical layout of a Conventional depillaring panel with manner of pillar
extraction.
22

23. Caving Mechanisms – Strata Mechanics- B&P
Conceptual Models of Loading & Caving of overlying Roof
Strata in Bord & Pillar Caving Panel
AMZ includes all of the
pillars on the extraction
front (or "pillar line"), and
extends outby the pillar
line a distance of 2.76
times the square root of
the depth of cover
expressed in m.
Mining depth is the principal factor affecting
abutment loads.
Cave quality and massive strata in the
overburden are also recognized to affect
abutment loading. 46
23

24. When a gob area is
created by full
extraction mining
(depillaring), abutment
loads are transferred
to the adjacent pillars
or solid coal;
The abutment stresses
are greatest near the
gob, and decay as the
distance from the gob
increases;
From experience and from numerical analysis it is
found that the front abutment load reaches to zero
at about a distance give by the following equation
D = 5.14 H
Layout of Longwall Workings
24

25. Forces on supports due to lateral strata movement.
General pattern of Vertical and (a) Weak roof -- horizontal force acting away from face.
Horizontal stress redistribution (b) Strong roof -- horizontal force acting towards face.
(Gale 2008) Adapted from Peng et al. [1987].
Vertical stress distributions at seam level around single longwall face
(Brady and Brown 1992)
25

26. Three zones in overburden due to longwall mining
(Chekan at al., 1993)
Distinct Zones in Overburden of an Longwall Opening
Ranges
in Strata
Zones
Thickne Characteristics
ss
Complete
Strata fall onto mine floor, broken into irregular, platy shapes of
caving 3-6HL
various sizes, crowded in random manner.
region
Partial
Strata have significant degree of bending, leading to intense
caving 6-12HL
Caving zone fracturing or displacement.
region
Upper
Strata may separate along planes and fracture or joints may
limit of
12-20HL open; individual beds remain intact and displacements are less
caving
likely to occur.
zone
Strata are broken into blocks by fractures and cracks due to bed
Fracturing 20-
separation; bending is not as abrupt and fractures are less
zone 50HL
pronounced
Sagging 50HL to Bending of strata is gradual and distributed over a large
zone Surface horizontal distance, without causing any major cracks.
Note: HL = The mining height lower seam.
26

27. Caving Mechanisms – Strata Mechanics
For an easily caveable roof
stratum, the goaf gets packed
quite frequently during face
advance. Bulking factor of caved
material is important and the
face is unlikely to experience
dynamic loading.
Bulking factor controlled caving of weak and
laminated overlying strata.
Working face experiences large
overhang if the roof strata are
strong and massive in nature.
Under this condition, stress
meters may play important role
to visualise the nature and
extent of dynamic loading
during enmasse movement of
the roof strata.
Parting plane controlled caving of strong
and massive overlying strata.
Cavability of a rock formation
Quantitatively, it is difficult to define
cavability.
But, a roof may be considered to be ideally
cavable when the roof rocks cave in and
fill the goaf as soon as the supports are
withdrawn.
27

28. Parameters Influencing Cavability
σv
J1
J2
Stress relief zone σh (impedes caving)
Excavation
J1 = Sub-horizontal joint sets: Essential for caving
Sub-
J2 = Sub-vertical joint sets: Augments caving
Sub-
Parameters Influencing Caving Span
Tensile strength : Measured in laboratory
Rock density : Almost constant
Horizontal stress: Measured in field
stress:
Identification of layers and their thickness :
Difficult
28

29. Natural features that influence cavability of
rock are
• Geometry of the discontinuities.
•Shear and tensile strength of the
discontinuities,
•Strength of rock materials and in-situ
stress field
Cavability can be enhanced by a set of induced
features
•Undercut span
•Boundary slots, and
•Mass weakening by creating fractures
High horizontal stresses inhibit the roof caving.
The caving height is low
Caving occurs after a long face advance
29

30. Hydraulic fractures in the roof can stimulate the
caving.
Without creating fractures in the massive roof, the
caving height would be low and the caving would
occur after longer face advance
A fracture of large area in massive roof must be
created so that its area increases progressively to
initiate caving of the roof strata.
Caving Mechanism in B&P Panels
30

31. Caving Mechanism in B&P Panels – Local, Main & Periodic
Falls
Critical conditions of
strata behaviour
invariably occurred in
indian geo-mining
conditions after
extraction of two rows of
pillars with 50 – 60 m
span, and at an area of
extraction of 4,000 -
6,000 m² including the
ribs in the goaf.
Longwall Caving Diagram
Cut after cut, shear after shear the AFC & subsequently
Chock shield supports will be advanced and the
immediate roof rock may cave in or not.
31

32. Main Fall
As the retreat further proceeds substantial area of main
roof rock forms a plate & caves in by imposing load on
supports, known as main weighting.
TENSILE FRACTURES
CRACKS STARTS TO FORM IN
MID SPAN
For 150m Longwall face length, Mainfall fall is taking place
After an area of exposure of 8000 to 12000 Sq.m for coal as immediate roof
and
After 7000 to 8000 sq.m for sandstone as immediate roof conditions
Periodic Fall
Periodic falls occur at 18 to 25m and 10 to 16m progress intervals for coal
and sandstone as immediate roof conditions respectively
32