slope and dump stabolity - sankar

1

SLOPE STABILITY AND

DUMP STABILITY U.Siva Sankar

Sr. Under ManagerProject Planning

Singareni Collieries Company Ltd

E-Mail :[email protected] or [email protected]

Visit at:www.slideshare.net/sankarsulimella

� Introduction

� Types of slope failure

� Factors Affecting Slope Stability

� Slope stability Analysis Methods

� Stabilizing methods

� Monitoring and instrumentation

2

Slope Stability Introduction

Introduction:

� Slopes either occur naturally or are engineered by humans� An understanding of geology, hydrology, and soil pr operties

is central to applying slope stability principles properly.� Analyses must be based upon a model that accurately

represents site sub surface conditions, ground be havior, and applied loads.

� Time of Analysis � Safe and economic design of excavations, embankment s,

earth dams, landfills, and spoil heaps .

Slope stability problem is greatest problem faced b y the open pit mining industry. The scale of slope stability probl em is divided in to two types:

Gross stability problem:It refer to large volumes of materials which come d own the slopes due to large rotational type of shear failure and i t involves deeply weathered rock and soil.

Local stability problem:This problem which refers to much smaller volume of material andthese type of failure effect one or two benches at a time due to shear plane jointing, slope erosion due to surface draina ge.


3

Aim of slope stability:

� To understand the development and form of natural a nd man made slopes and the processes responsible for diff erent features.

� To assess the stability of slopes under short-term (often during construction) and long-term conditions.

� To assess the possibility of slope failure involvin g natural or existing engineered slopes.

� To analyze slope stability and to understand failur e mechanisms and the influence of environmental factors.

� To enable the redesign of failed slopes and the pla nning and design of preventive and remedial measures, where n ecessary.

� To study the effect of seismic loadings on slopes a nd embankments.


� Safe, properly designed, scientifically engineered slope.

� Profitability of open cast mines.

� Design engineer/ scientist

•Excessive steepening:

� Slope failure � Loss of production, � extra stripping costs to remove failed

material, � DGMS may close the mine

Aim of slope stability:

4

TYPES OF ROCK SLOPE FAILURES

Failure in Earth and Rock mass

�Plane Failure�Wedge Failure�Circular Failure�Toppling Failure�Rock fall

Failure in Earth, rock fill and spoil dumps and Emb ankments

�Circular�Non-circular semi-infinite slope�Multiple block plane wedge�Log spiral (bearing capacity of foundations)�Flow slides and Mud flow�Cracking �Gulling �Erosion �Slide or Slump

Figure. Simplified illustrations of most common slo pe failure modes.

5

Fig. Failure mechanisms for the sliding failure mode (After Brown,1994): a) single block with single plane; b) single block with stepped planes; c) multiple blocks with multiple planes; d) single wed ge with two intersecting planes; e) single wedge with multiple intersecting planes; f) multiple wedges with multiple intersecting planes; and g) si ngle block with circular slip path

Plane FailureSimple plane failure is the easiest form of rock sl ope failure to analyze. It occurs when a discontinuity striking approximately paralle l to the slope face and dipping at a lower angle intersects the slope face, enabling the material above the discontinuity to slide.

6

Plane Failure

Geometrical Conditions for sliding on single Plane failure:

� The plane on which sliding occurs must strike parallel or nearly parallel (±20 0) to the slope face

� The failure plane must “daylight” in the slope. This means its dip must be smaller than the dip of the slope face

� The dip of the failure plane must be greater than angle of internal friction

� Release surfaces which provide negligible resistance to sliding must be present in the rockmass to define the lateral boundaries of the slide. Alternatively, failure can occur on a failur e plane passing through the convex “nose” of a slope.

Wedge failure

Wedge failure can occur in rock masses with two or more sets of discontinuities whose lines of intersection are app roximately perpendicular to the strike of the slope and dip to ward the plane of the slope.

7

Toppling Failure

Toppling failures occur when columns of rock, forme d by steeply dipping discontinuities in the rock structure and i t involves overturning or rotation of rock layers

� Circular failures are generally occur in weak rock or soil slopes. � Failures of this type do not necessarily occur alon g a purely circular

arc, some form of curved failure surface is normall y apparent. � Circular shear failures are influenced by the size and mechanical

properties of the particles in the soil or rock mas s.

Fig: Circular Failure types

Circular Failure

8

Types of circular failure

Circular failure is classified in three types depending on the area that is affected by the failure surface. They are:-

� Slope failure : In this type of failure, the arc of the rupture surface meets the slope above the toe of the slope. This happens when the slope angle is very high and the soil close to the toe posses the high strength.

� Toe failure : In this type of failure, the arc of the rupture surface meets the slope at the toe.

� Base failure : In this type of failure, the arc of the failure passes below the toe and in to base of the slope. This happens when theslope angle is low and the soil below the base is softer and more plastic than the soil above the base.

Rock Fall

In rock falls, a mass of any size is detached from a steep slope or cliff,

along a surface on which little or no shear displac ement takes place, and

descends mostly through the air by free fall, leapi ng, bouncing, or rolling

9

Cracking

� It is due to differential settlement of

the mine waste and suction level,

exceeding the tensile strength, is

reached.

� Due to further drying, or in

subsequent dry periods, cracks can

grow until finally, the complete

thickness of the sealing layer is

penetrated

Gulling

� The gulling was observed in many dumps and it is qu ite

dominant erosion mechanism.

� Gullies involve incision to depths often well in ex cess of a metre,

and remove large quantities of soil

10

Formation of gullies due heavy rain water flow

Gully formationGully formation

Slide or Slump

� Shallow failures involving slumping of saturated or partially saturated

dump materials. Concentrated surface flows dischar ging over the

dump crest.

� Slides, either in rock or soil, will have rotationa l or translational

movement.

� The sliding of material along a curved surface call ed a rotational slide

or slump.

� A common cause of slumping is erosion at the base o f a slope

11

Extensive soil erosionExtensive soil erosion

Long term impacts of river

H igh e st f lo o d le v e l in m o n so o n

B er m al on g t h e u n b r ok en ar ea

H igh e st f lo o d le v e l in m o n so o n

B er m al on g t h e u n b r ok en ar ea

12

Weathering

13

A First Incident Begins.

A 170 Ton capacity rear dump truck flees the effect of some oncoming miscalculation

The Coal face has begun to fall

Here it is cargo that is moving transport equipment !

14

There is no escape from this slide of the coal

benches

FACTORS AFFECTING SLOPE STABILITY

� Geological discontinuities of Rock Mass

� Geotechnical Properties of slope

� Groundwater and Rainfall (Force Due To Seepage of Water )

� Geometry of slope (Gravitational Force )

� State of stress

� Erosion of the Surface of the Slopes due To Flowing Water

� Seismic effect (Forces Due To Earthquakes )

� Dynamic Forces due to Blasting and HEMM Movement

� Slope modification, Under cutting

� Temperature and Spontaneous Heating

� Presence of UG galleries

Slope Stability Factor affecting slope stability

15


Geological discontinuities of Rock Mass

� Joints

� Bedding Joints

� Joint spacing

� Joint direction and dipping

� Faults

Fig: Idealized diagram showing transition from intact rock to jointed rock mass with increasing sample size

Geological Structure:The main geological structure which affect the stability of the slopes in

the open pit mines are:� amount and direction of dip� intra-formational shear zones� joints and discontinuities

� Reduce shear strength� Change permeability� Act as sub surface drain � Plains of failure

� faults� weathering and alternation along the faults� act as ground water conduits� provides a probable plane of failure

Factors Affecting Slope Stability

16

Spacing, Persistence, Aperture

Geotechnical Properties of slope

� Shear strength of rock mass

� Cohesion (C)

� Angle of Internal friction (Ø)

� Density

� Permeability

� Moisture Content

� Particle size distribution

� Angle of Repose

“Angle of repose” is the angle of steepest slope at which material will remain stable when loosely piled;


17

• Cohesion : It is the characteristic property of a rock or soil that measures how well it resists being deformed or brok en by forces such as gravity. In soils/rocks true cohesion is ca used by electrostatic forces in stiff over consolidated cla ys, cementing by Fe2O3, CaCO3, NaCl, etc and root cohesion.

However the apparent cohesion is caused by negative capillary pressure and pore pressure response during undraine d loading. Slopes having rocks/soils with less cohesion tend t o be less stable

• Angle of Internal Friction: Angle of internal friction is the angle (Ø), measured between the normal force (N) and result ant force (R), that is attained when failure just occurs in respon se to a shearing stress (S).

Its tangent (S/N) is the coefficient of sliding fri ction. It is a measure of the ability of a unit of rock or soil to withstand a shear stress. This is affected by particle roundness and particle size. Lower roundness or larger median particle size resu lts in largerfriction angle. It is also affected by quartz conte nt.


Lithology• The rock materials forming a pit slope determines t he rock mass

strength modified by discontinuities, faulting, fol ding, old workings and weathering.

• Low rock mass strength is characterized by circular raveling and rock fall instability like the formation of slope in mas sive sandstone restrict stability.

• Pit slopes having alluvium or weathered rocks at th e surface have low shearing strength and the strength gets further red uced if waterseepage takes place through them. These types of sl opes must be flatter.

Ground Water• It causes the following:• alters the cohesion and frictional parameters and• reduce the normal effective stress• Ground water causes increased up thrust and driving water forces and

has adverse effect on the stability of the slopes. Physical and chemical effect of pure water pressure in joints filling mat erial can thus alter the cohesion and friction of the discontinuity surface.

• Physical and the chemical effect of the water press ure in the pores of the rock cause a decrease in the compressive streng th particularly where confining stress has been reduced.


18

Groundwater and Rainfall Water in Crack

� Presence of water – Flow of water - Not a big problem.

� Water flow checked – water storage- hydro. pressure

Groundwater and Rainfall : Water in pores

19

Slope Geometry:

� The basic geometrical slope design parameters are h eight, overall slope angle and area of failure surface.

� With increase in height the slope stability decreas es. � The overall angle increases the possible extent of the development of the

any failure to the rear of the crests increases and it should be considered so that the ground deformation at the mine peripher al area can be avoided.

� Generally overall slope angle of 45 ° is considered to be safe by Directorate General of Mines Safety (DGMS).

� Steeper and higher the height of slope less is the stability.

Fig: Typical Pit slope Geometry

Figure: Typical slope failure and relationships be tween critical slope heights and slope angles

20

Figure: Typical slope failure and relationships be tween critical slope heights and slope angles

Mining Method and Equipment

Generally there are four methods of advance in open cast mines. They are:� strike cut- advancing down the dip� strike cut- advancing up the dip� dip cut- along the strike� open pit working• The use of dip cuts with advance on the strike redu ces the length and

time that a face is exposed during excavation. Dip cuts with advance oblique to strike may often used to reduce the stra ta

• Dip cut generally offer the most stable method of w orking but suffer from restricted production potential.

• Open pit method are used in steeply dipping seams, due to the increased slope height are more prone to large slab /buckling modes of failure.

• Mining equipment which piles on the benches of the open pit mine gives rise to the increase in surcharge which in tu rn increases the force which tends to pull the slope face downward a nd thus instability occurs. Cases of circular failure in spoil dumps ar e more pronounced.


21

State of stress


In some locations, high in-situ stresses may be pre sent within the

rock mass. High horizontal stresses acting roughly perpendicular to

a cut slope may cause blocks to move outward due to the stress

relief provided by the cut. High horizontal stresse s may also cause

spalling of the surface of a cut slope.

Erosion


Two aspects of erosion need to be considered. The f irst is largescale erosion, such as river erosion at the base of a cliff. Thesecond is relatively localized erosion caused by gr oundwater or surface runoff.

22

Seismic effect

� Seismic waves passing through rock adds stress whic h could cause

fracturing.

� Friction is reduced in unconsolidated masses as the y are jarred apart.

� Liquefaction may be induced.

� One of the major hazards of earthquakes is the thre at of landslides.

� This is particularly so because the most unstable p arts of the earth are

at the plate boundaries and it is also here that yo ung fold mountain

belts are formed and there are high relief and stee p slopes

� Most open pit operators are familiar back break for m blast, but most

people only consider the visible breakage behind th e row of holes of the

blast.

� Blasting has a significant influence upon stability of slopes.

� Uncontrolled blasting-

� over breaks, overhangs and extension of tension cracks.

� Opening & loss of cohesion between weak planes.

� shattering of slope mass and �allowing easier infiltration of surface water�unfavourable ground-water pressures.

� Due to effect of blasting and vibration, shear stre sses are momentarily increased and as result dynamic acceler ation of material and thus increases the stability problem in the slo pe face. It causes the ground motion and fracturing of rocks.

Dynamic Forces

23

Slope Modification –Modification of a slope either by humans or by natural causes can result in changing the slope angle so that it is no longer at the angle of repose. A mass-wasting event can then restore the slope to its angle of repose.

Undercutting - streams eroding their banks or surf action along a coast can undercut a slope making it unstable.

24

What do you do with a burning Coal face?

Coal Face on fire

25

Dynamite was used to loosen the Coal for collection by a powerful electric Shovels.

But heat from the explosion & an exposed Coal seam can sometimes be a bad combination.

Fire erupts from the Coal face!

Fig. Plot of slope displacement versus time for prediction of failure.

A. Plot of fastest moving point in the slope.B. Plot of slowest moving point in the slope.C. Prediction of slope failure date based on existing data (extrapolation).D. Predicted and actual date of failure.

26

Manual or Conventional Opencast MinesIn alluvial soil, morum, gravel, clay, debris or other similar ground –

� the sides shall be sloped at an angle of safety not exceeding 45 degrees from the horizontal or such other angle as permitted by Regional Inspector of mines

� the sides shall be kept benched and the height of any bench shall not exceed 1.5 m and the breadth thereof shall not be less than the height:

� In coal, the sides shall either be kept sloped at an angle of safety not exceeding 45 degree from the horizontal, or the sides shall be kept benched and the height of any bench shall not exceed 3m and the width thereof shall not be less than the height.

� In an excavation in any hard and compact ground or in prospecting trenches or pits, the sides shall be adequately benched, sloped or secured so as to prevent danger from fall of sides. However the height of the bench shall not exceed 6 m.

� No person shall undercut any face or side or cause or permit such undercutting as to cause any overhanging.

DGMS Guidelines for Benches or slopes design

Mechanized opencast working .-

� Before starting a mechanized opencast working, design of the pit, including method of working and ultimate pit slope shall be planned and designed as determined by a scientific study.

� The height of the benches in overburden consisting of alluvium or other soft soil shall not exceed 5 m and the width thereof shall not be less than three times the height of the bench

� The height of the benches in overburden of other rock formation shall not be more than the designed reach of the excavation machine in use for digging, excavation or removal.

� The width of any bench shall not be less than –

(a) the width of the widest machine plying on the bench plus 2m, or(b) if dumpers ply on the bench, three times the width of the dumper, or(c) the height of the bench, whichever is more.

DGMS Guidelines for Benches or slopes design

27

(1) While removing overburden, the top soil shall be stacked at a separate place, so that, the same is used to cover the reclaimed area.

(2) The slope of a spoil bank shall be determined by the natural angle of repose of the material being deposited, but shall in no case exceed 37.5 degrees from the horizontal. The spoil bank shall not be retained by artificial means at an angle in excess of natural angle of repose or 37.5 degrees whichever is less.

(3) Loose overburden and other such material from opencast workings or other rejects from washeries or from other source shall be dumped in such a manner that there is no possibility of dumped material sliding.

(4) Any spoil bank exceeding 30m in height shall be benched so that no bench exceeds 30m in height and the overall slope shall not exceed 1 vertical to 1.5 horizontal.

(5) The toe of a spoil-bank shall not be extended to any point within 45m of a mine opening, railway or other public works, public road or building or other permanent structure not belonging to the owner.

DGMS Guidelines for Formation of Spoil Banks and Du mps

28

Methods for Slope Stability Analysis

� Limit equilibrium -

�Analytical (software), �Chart methods

� Kinematic analysis, To determine the types of above mentioned failure.

� Sensitivity analysis

�Classification method –SMR

�Probabilistic method, and

�Numerical modelling method.

Limit equilibrium method ,

� It is the most widely accepted and commonly performed design tool in

slope engineering

� Sliding occurs when a limit equilibrium condition is reached, i.e., when

the resisting forces balance the driving forces.

� These methods are the most widely accepted and commonly used design methods and they permit a quantification of slope performance

with the variations in all the parameters involved in the slope design.

� The basic idea behind the limit equilibrium approach is to find a state of

stress along the critical surface so that the free body, within the slip

surface and the free ground surface, is in static equilibrium. � This state of stress is known as the mobilized stress, which may not be

necessarily the actual state along this surface.

� This state of stress is then compared with the available strength, i.e.

the stress necessary to cause failure along the slip surface.

Stability Analysis of Mine Slopes

29

� To represent the slope performance other than the equilibrium

condition, it is necessary to have an index and the widely used index used to be factor of safety.

� Factor of safety is calculated as the ratio of shear strength to the

available shear stress required for equilibrium, integrated through the

whole slide.

� It is constant throughout the potentially sliding mass. Due to scatter of test results and the uncertainty of these input parameters, a factor of

safety greater than one is necessary to ensure an acceptably low

chance of failure.

Guidelines for the Equilibrium of a Slope

Fig. Effect of ground water on rock slope (source: Abramson, 1995)

Plane Sliding – Stability Analysis

30

Slope Stability Stability Analysis of Slope

With no tension crack and no water pressure

W cosθ

W

W sinθ

R

Block A

sShearStres

gthShearStrenFactor of safety =

s

c

τφσ tan+

Factor of safety =

A

wA

wc

θ

φθ

sin

tancos+

θφθ

sin

tancos

w

wcA +Factor of safety = =

A

w )sin(θσ =Normal Stress;

A

w )cos(θτ =Shear Stress ,

Planar failure Analysis


Tension crack present in upper slope surface

Depth of tension crack; θαα tan)cot(tan HbbHZ c +−+=

Weight of unstable block; ( ))cot2

1 2 bZbHXXHW ++= α

)cottan1( αθ−=X

Area of failure surface; θα sec)cot( bHA +=

2

2

1wwZV γ=

AZU wwγ2

1=

Driving water force;

Uplift water force;

31


Tension crack present in slope face

)tan)(tancot( θαα −−= bHZ

−

−= )1tan(cotcot12

12

2 αθθγH

ZHw

θα sec)cot( bHA c −=

2

2

1ww ZV γ=

AZU wwγ2

1=

βθθφβθθ

sincossin

tan)cossincos(

TVW

TVUwcA

−++−−+

Depth of tension crack;

Weight of unstable block;

Area of failure surface;

Driving water force;

Uplift water force;

Factor of safety =

Slope Stability Numerical

Circular Failure Analysis

W

32

Slope Stability Numerical


FOS =

s

c

τφσtan+



Wn

φστ tan+= cFOS =

s

c

τφσtan+

θφθ

θ

φθ

sin

tancossin

tancos

w

wLc

L

wL

wc +∆=

∆

∆+

[ ]

[ ]∑

∑=

=

=

=+∆

pn

nnn

pn

nnnn

W

WLc

1

1

sin

tancos

α

φα

FOS =

FOS =

33

Software based on Limit equilibrium Method

SLIDE (rocscience group)

GALENA

GEO-SLOPE

GEO5

GGU

SOILVISION

Overview of GALENA

34

Software for water pressure simulation

HYDRUAS

GEOSLOPE/ SEEP (GEOSTUDIO)

SOILVISION /Water

GMS

FEFLOW

Software based on Numerical modeling

PHASES2

PLAXIS

FLAC-SLOPE / UDEC / PPF

ANSYS

FEFLOW

GEOSLOPE/SIGMA

SOIL-VISION

35

The average orientations of the discontinuity sets determined from the geotechnical mapping were analysed to assess kinematically possible failure modes involving structural discontinuities

Kinematic Analysis

Slope Unfavourable Slope favourable

Kinematic Analysis to know Type of Failure

36

Sensitivity analysis � The sensitivity analysis was done with an aim

to know the influence of water on the factor of safety.

� This study is highly beneficial to choose the best method of remedial measure for any critical slope.

� The influence of groundwater on factor of safety is remarkable.

� The stability analyses of highwall slope have been conducted in undrained geo-mining condition also

� It is evident that the highwall slopes are stable in drained condi-tion with cut-off safety factor of 1.3 is unstable, if the slopes are subjected to undrained condition with safety factor less than 1.3.

� In order to avoid undrained condition, attention must be paid to avoid entry of rain/ surface water in the slope by providing suitable drainage in and around the quarry, failing which the slope can become unstable. It should be taken up well before the onset of monsoon.

Slope Mass Rating (SMR)

37

Adjustment rating of F1, F2, F3 and F4 for joints

Classification of Rock Slope according to SMT

38

Slope Stability Stabilization Techniques

STABILIZATION OF SLOPE

Drainage System

Stabilization through Support

Rock Mass Improvement and Stabilization Methods

Drainage System

Surface drainage

Subsurface Drainage

Fig: Slope Drainage and depressurization methods


Surface Drainage Systems: Surface drains and landscape design are used to direct water

away from the head and toe of cut slopes and potential landslides, and to reduce

infiltration and erosion in and along a potentially unstable mass

Sub-Surface: The main functions of subdrains are to remove subsurface water directly

from an unstable slope, to redirect adjacent groundwater sources away from the subject

property and to reduce hydrostatic pressures beneath and adjacent to engineered

structures �Objective

� Decrease water pressure

�Effective garland drain , directed away from excavated pit.

�Proper and effective drainage

� 5 to 10 deg. increase in slope angle

95% slide triggered by poor water management.

39



• Ground InclusionsGround anchorSoil Nails

Rock Bolt

Ground inclusion: It is a metal bar that is driven or

drilled into competent bedrock (rock which is not

highly fractured or broken up) to a provide stable

foundation for structures such as retaining walls and

piles, or to hold together highly fractured or jointed

rock.



Piles

• Piles are long, relatively slender columns positioned vertically in the ground or

at an angle (battered) used to transfer load to a more stable substratum.

• Piles are often used to support or stabilize structures built in geologically

unstable areas.

• Piles used as foundation for structures constructed on compressible soil or

weak soil.

• Grouped piles used as a retaining wall: Anchors are generally used to increase

the effectiveness of pile walls

40



• Retaining Walls

Engineered structures constructed to resist lateral forces imposed by soil

movement and water pressure

Retaining walls are commonly used in combination with fill slopes to reduce

the extent of a slope to allow a road to be widened and to create additional

space around buildings



Geosynthetics

Grouting

Chemical Stabilization

Biological Stabilization

41



Geosynthetics are porous, flexible, man-made fabrics which act to reinforce and

increase the stability of structures such as earth fills, and thereby allow steeper cut

slopes and less grading in hillside terrain. Geosynthetics of various tensile

strengths are used for a variety of stability problems, with a common use being

reinforcement of unpaved roads constructed on weak soils.

Grout is a cement or silicate based slurry, fluid enough to be poured or injected

into soil and thereby fill, seal, or compact the surrounding soil. Grouting is the

pressure injection of this slurry through drilled holes into fissured, jointed,

permeable rocks and compressible soils to reduce their permeability and increase

their strength.



Chemical stabilization is a soil improvement method that increases the load

bearing capability by mixing the soil with powders, slurry, or chemicals. Stability

is developed in a number of ways; for example, the admixtures can fill soil voids,

bond together individual grains, change the permeability of the soil

Biological Stabilization

42

�Controlled placement of spoil

� Impermeable material increases water pressure.

�weak top layer – swelling minerals,

�base of the dump – permeable material.

Dump Slope Stability

� Improving drainage at the base of the dumps,

•Blasting/ ripping of the floor,• Garland drain/ bund near toe of dump,

• all along the periphery of dump edges,

•5 m away from the toe of the dump – toe cutting.

�Proper spoil levelling

� To check rainwater ponding at top,

� Dumping in depressed zone,

� Liquefaction of dump toe,

� Planting of self-sustaining grass and plants

� to check the soil erosion,

� to avoid the formation of deep gullies, form terraces, 1 m wide at the height of each about 6m.

Dump Slope Stability

� Rejection dump – near crest of slope – dead wt. on sl ope

�No unplanned dump – Near the crest.

43

Stability analysis of active mine slope without overlying dump

Factor of safety 1.25

Stability analysis of active mine slope with overlying dump

Factor of safety 1.1

44

Slope Monitoring

�Objective & why desired

�If detected in the early stage and later stage.

�Techniques

� Instrumentation� Photogramammetric� GPS� Satellite imageries� Survey based techniques

�Most widely used, �Precision, Repeatability, �Direct displacement.

Slope Stability Slope Monitoring

SLOPE MONITORING INSTRUMENTS

Extensometers

Time domain reflectometry (TDR)

Inclinometers

Piezometers

Crack Meters

Borehole extensometers consists of tensioned rods anchored at different points in a borehole Changes in the distance between the anchor and the rod head provides the displacement information for the rock

Extensometers

Fig: slope with Extensometer

45


Time domain reflectometry

* lower installation costs

* no limits on hole depth

* immediate determination of movement

* remote data acquisition capability

In TDR, a cable tester sends a voltage pulse waveform down a cable grouted in

a borehole, If the pulse encounters a change in the characteristic impedance of

the cable, it is reflected. This can be caused by a crimp, a kink, the presence of

water, or a break in the cable. The cable tester compares the returned pulse with

the emitted pulse, and determines the reflection coefficient of the cable at that

point. The change in impedance with time corresponds qualitatively to the rate

of ground movement.


Inclinometers

Monitoring slopes and landslides to detect zones of movementMonitoring dams, dam abutments, and upstream slopes. Monitoring the effects of tunneling operations

46


• Vibrating wire

• Pneumatic

• Standpipe piezometers

Piezometers


Crack Meters

Crack meters can be very useful tools in the early detection of deforming

mass movements. These devices measure the displacement between two

points on the surface that are exhibiting signs of separation.

47

Prism Monitoring based on survey techniques

Prisms are installed on the highwalls at a regular spacing, 50m horizontally and 45m vertically, and on critical areas throughout the open pits. Surveyors collect and store data, while the rock engineers then analyse the data, looking for significant movement, and report any potential areas of slope failure to the mining personnel.

Laser MonitoringMounted laser scanners will scan the entire pit walls by dividing them into zones. A camera is attached to the side of the laser and takes photographs at the start of scanning. The data transmitted by laser scanner was downloaded to a computer and analysed using software.

Radar Monitoring

The GroundProbe slope stability Radar (SSR) uses differential interferometry to measure sub-millimetre movements on a rough rock face

Digital photogrammetry

SiroVision is a digital photogrammetry software program that enables safe and comprehensive mapping of dangerous and inaccessible highwalls, which are being captured in photographs with the use of high resolution digital camera.

Seismic Monitoring

Seismic monitoring aims to predict slope deformation by measuring micro seismic events caused by brittle movements within a rock slope. Analysis of micro seismic events using multiple tri axial geophones enables the location of source and therefore the discontinuity on which movement is occurring.

48


Monitoring by Observational Techniques : Total Station

Total station instruments consist of a device to measure horizontal

and vertical angles, and some form of Electromagnetic Distance

Measurement (EDM) capability to measure distances. These

instruments allow the surveyor to measure 3D coordinates of points

remotely


LASER - Remote controlled Monitoring

49

Slope Stability Radar Technology

� The Ground Probe SSR is a technique for monitoring open pit mine walls based on differential interferometry using radar waves.

� The system scans a region of the wall and compares the phase measurement in each region with the previous scan to determine the amount of movement of the slope.

� An advantage of radar over other slope monitoring techniques is that it provides full area coverage of a rock slope without the need for reflectors mounted on the rock face.

� The system offers sub-millimetre precision of wall movements without being adversely affected by rain, fog, dust, smoke, and haze.

� The system is housed in a self contained trailer that can be easily and quickly moved around the site.

� It can be placed in the excavation, or on top of a wall or on a bench to maximize slope coverage whilst not interfering with operations.

� The scan area is set using a digital camera image and can scan 320 degrees horizontally and 120 degrees vertically.

� The system provides immediate monitoring of slope movement without calibration and prior history. Scan times are typically every 1-10 minutes.

� Data is uploaded to the office via a dedicated radio link. � Custom software enables the user to set movement thresholds to warn

of unstable conditions. � Data from the SSR is usually presented in two formats. � Firstly, a colour “rainbow” plot of the slope representing total movement

quickly enables the user to determine the extent of the failure and the area where the greatest movement is occurring.

� Secondly, time/displacement graphs can be selected at any locations to evaluate displacement rates.

� Additional software can also be installed to allow the data to be viewed live at locations remote to the SSR site such as corporate offices and at the offices of geotechnical consultants.

Slope Stability Radar Technology

50

Fig: Slope Stability radar

Typical problems, critical parameters, methods of a nalysis and acceptability criteria for slopes.

51

Thank You

slope and dump stabolity - sankar

Documents