The Effect of Discontinuities Characteristics - PowerPoint Presentation

Slide1

The Effect of Discontinuities Characteristics

on Coal Mine Stability

Kazem Oraee (Prof) Arash Goodarzi (Eng) Nikzad Oraee-Mirzamani (Phd)-Parham Khajehpour (Eng)

34th International Conference on Ground Control in MiningSlide2

Roof rock failure

has generally been the most dangerous

hazard for underground coal mining in the world.although artificial support mechanisms have been used extensivelySlide3

Roof stability

is primarily determined in

many underground mines in developing countries by a number of methods that often resort to subjective criteria.Slide4

Failure of rock

around the

opening-high rock stress conditions-the presence of structural discontinuitiesSlide5

the engineering

behavior of

rock massesThe properties of discontinuities affectwedges or blocks-falling from the roof-sliding

out of the wallsSlide6

Failure of rock

around shallow coal mine openings

oftenresults from loosening of blocks of rockon the weakened planes under the influence of gravity.The strength of roof rocks varies widely depending both on the rock type and on

planes of

weakness

present within

itSlide7

Roof rock failure

in small mines

may take the various formsfrom tiny cracks in immediate roofto a large scale movement of blocks of rock

The

weight

of

the failing

zone

could

therefore vary from a few

kilograms

to as much as

hundreds of

tones

, that far

exceeds

the

support capacity

of typical

small mine

systems.Slide8

Determining the

active structural discontinuities:

is related to mapping and modeling discontinuities.the fall of wedges-blocks from the roof-sliding out of the wallsSlide9

The discontinuities of

coal and its surrounding rock

-post-peat accumulation:-with peat accumulation:-faults-joints-cleats-bedding planes-

slickensides

Occurring:Slide10

A numerical modeling method

is required which can model the presence of discontinuities

as well as the failure mechanism of the coal material.UNWEDGE is a computer software program that calculates the possible wedges that can form around the excavation by the intersection of the three discontinuity planes of the excavationSlide11

iii. The shear strength of the discontinuities

τ: shear strength of the discontinuity

τ=c+σntanφc: cohesionσn: normal stressφ: friction angle of the discontinuity planeMohr-Coulomb MethodSlide12

Sketch of

Eshkeli coal mine

(×: accident place)A huge collapse occurred on 14 December 2010 at 11:30(stope 103)4 miners were killedthe average thickness of the coal bed: 3 mthe average dip of the coal bed: 35○Slide13

Hojedk

Coal Mine

Eshkeli Coal MineBab-Nizou Coal MineThe major proportion of developing countries’ coal is excavated through artisanal methods in small-scale mines,Slide14

The

region’s topography is

rough and many hills and valleys are in the regionSlide15

using small-scale coal mining in developing countries

Stull

Stope Miningthin and steep coal beds with weak hanging and foot wallsSlide16

ii. The

stope

axis orientation is defined by the trend / plunge of the stope axis. The collapsed stope (stope 103)Collapsed area in the stope8.95 mSlide17

The cracks were displayed on surface of failure zoneSlide18

The

displacement

in the yielding steel setsSlide19

Ground control

is accomplished by timbering (

wooden square-set) between the foot and hanging wall or backfilling with waste for a large stope.The geometry of stope

26.82 m

Length

4.5 m

Width

3 m

Height

35

○

Dip

69.5

○

Dip directionSlide20

Stull

Stope MiningSlide21

The orientations of three distinct discontinuities may be defined by their respective geophysics profiles.Slide22

Several geophysical methods are available for locating discontinuities.

The discontinuities are located by

the interpretation of the natural geological boundaries at depth even if they are not outcropping at the surface.Slide23

Two main separate faults

that affected the

failure zone are shown as 1, 2 discontinuitiesMagnetic MethodThe magnetic method involves the measurement of the earth’s magnetic fields’ intensity by a magnetometer.Slide24

Resistivity Method

Electrical conductivity

is governed by the earth’s crust material properties (density, permeability, porosity, and etc.). Coal has much higher relative conductivity than discontinuities, where a fracture or a failure zone increases the volume of porosity and consequently the electrical resistivity

Failure zone started from

the level of 2355 m

and extended to

the level of 2200 m

, where there was

the coal bed

.

The coal bed

acted as

a weakness planeSlide25

Bed separation

and

failure can occur along the bedding planes, especially when the thickness of the coal seam is not uniform.Slide26

Assuming C=0 and φ=26.5○ for all discontinuities.

Dip and dip direction angles of discontinuities

Plane Type Dip Dip direction 1Fault62○32○2Fault

35

○

126

○

3

Bedding Plane

35

○

122

○

Discontinuity orientation

describes

the attitude of the discontinuity

in space.

A discontinuity’s orientation

is

the most important property

in opening stability.Slide27

Failure is largely structurally controlled under low stress levels in shallow underground mines.

The discontinuities

were a primary cause of the Eshkeli shallow underground mine collapse and therefore the wedges analysis is appropriate in this case.Slide28

Stereographic

projection of the

discontinuities orientations and stope axisSlide29

wedge #2 is stable completely.

wedge #1

wedge #1 has been scaled by the discontinuity 3 (bedding plane)-volume ≈267 m3-shear force ≈ 230 tonesThe occupational accident in stope 103 is a result of falling wedge#1.Slide30

Thank you very much for your attention.