Slopestability

SLOPE STABILITY
AND
DUMP STABILITY
U.Siva Sankar
Sr. Under Manager
Project Planning
Singareni Collieries Company Ltd

E-Mail :ulimella@gmail.com or
uss_7@yahoo.com
Visit at:
www.slideshare.net/sankarsulimella

Introduction Slope stability Analysis Methods
Types of slope failure Stabilizing methods
Factors Affecting Slope Stability Monitoring and instrumentation

1

Slope Stability Introduction

Introduction:

Slopes either occur naturally or are engineered by humans
An understanding of geology, hydrology, and soil properties
is central to applying slope stability principles properly.
Analyses must be based upon a model that accurately
represents site sub surface conditions, ground behavior,
and applied loads.
Time of Analysis
Safe and economic design of excavations, embankments,
earth dams, landfills, and spoil heaps .


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

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

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

2


Aim of slope stability:

To understand the development and form of natural and man
made slopes and the processes responsible for different
features.
To assess the stability of slopes under short-term (often during
construction) and long-term conditions.
To assess the possibility of slope failure involving natural or
existing engineered slopes.
To analyze slope stability and to understand failure mechanisms
and the influence of environmental factors.
To enable the redesign of failed slopes and the planning and
design of preventive and remedial measures, where necessary.
To study the effect of seismic loadings on slopes and
embankments.

Aim of slope stability:

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

3

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 Embankments

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 slope failure modes.

4

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 wedge with two intersecting
planes; e) single wedge with multiple intersecting planes; f) multiple
wedges with multiple intersecting planes; and g) single block with circular
slip path

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

5

Plane Failure

Geometrical Conditions for sliding on single Plane
failure:
The plane on which sliding occurs must strike
parallel or nearly parallel (±200) 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 failure
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 approximately
perpendicular to the strike of the slope and dip toward the plane of the
slope.

6

Toppling Failure
Toppling failures occur when columns of rock, formed by steeply
dipping discontinuities in the rock structure and it involves overturning
or rotation of rock layers

Circular Failure

Circular failures are generally occur in weak rock or soil slopes.
Failures of this type do not necessarily occur along a purely circular
arc, some form of curved failure surface is normally apparent.
Circular shear failures are influenced by the size and mechanical
properties of the particles in the soil or rock mass.

Fig: Circular Failure types

7

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 the
slope 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 displacement takes place, and
descends mostly through the air by free fall, leaping, bouncing, or rolling

8

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 quite
dominant erosion mechanism.
Gullies involve incision to depths often well in excess of a metre,
and remove large quantities of soil

9

Gully formation

Formation of gullies due heavy rain water flow

Slide or Slump

Shallow failures involving slumping of saturated or partially saturated
dump materials. Concentrated surface flows discharging over the
dump crest.
Slides, either in rock or soil, will have rotational or translational
movement.
The sliding of material along a curved surface called a rotational slide
or slump.
A common cause of slumping is erosion at the base of a slope

10

Extensive soil erosion

Long term impacts of river

Ber
m a lo
H ig ng th e
hes unb
t f lo rok
o d le en a
ve l in rea
mon
so on

11

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!

13

There is no
escape from this
slide of the coal
benches

Slope Stability Factor affecting slope stability

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

14


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

Factors Affecting Slope Stability
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

15

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;

16

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

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

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

Its tangent (S/N) is the coefficient of sliding friction. 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 results in larger
friction angle. It is also affected by quartz content.

Lithology
• The rock materials forming a pit slope determines the rock mass
strength modified by discontinuities, faulting, folding, old workings and
weathering.
• Low rock mass strength is characterized by circular raveling and rock
fall instability like the formation of slope in massive sandstone restrict
stability.
• Pit slopes having alluvium or weathered rocks at the surface have low
shearing strength and the strength gets further reduced if water
seepage takes place through them. These types of slopes 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 material can thus alter the
cohesion and friction of the discontinuity surface.
• Physical and the chemical effect of the water pressure in the pores of
the rock cause a decrease in the compressive strength particularly
where confining stress has been reduced.

17

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

18

Slope Geometry:

The basic geometrical slope design parameters are height, overall slope
angle and area of failure surface.
With increase in height the slope stability decreases.
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 peripheral 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 between
critical slope heights and slope angles

19

Figure: Typical slope failure and relationships between
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 reduces the length and
time that a face is exposed during excavation. Dip cuts with advance
oblique to strike may often used to reduce the strata
• Dip cut generally offer the most stable method of working 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 turn increases the
force which tends to pull the slope face downward and thus instability
occurs. Cases of circular failure in spoil dumps are more pronounced.

20


State of stress

In some locations, high in-situ stresses may be present 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 stresses may also cause
spalling of the surface of a cut slope.


Erosion

Two aspects of erosion need to be considered. The first is large
scale erosion, such as river erosion at the base of a cliff. The
second is relatively localized erosion caused by groundwater or
surface runoff.

21

Seismic effect
Seismic waves passing through rock adds stress which could cause
fracturing.
Friction is reduced in unconsolidated masses as they are jarred apart.
Liquefaction may be induced.
One of the major hazards of earthquakes is the threat of landslides.
This is particularly so because the most unstable parts of the earth are
at the plate boundaries and it is also here that young fold mountain
belts are formed and there are high relief and steep slopes
Most open pit operators are familiar back break form blast, but most
people only consider the visible breakage behind the row of holes of the
blast.

Dynamic Forces

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 stresses are
momentarily increased and as result dynamic acceleration of material
and thus increases the stability problem in the slope face. It causes
the ground motion and fracturing of rocks.

22

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.

23

What do you do with a burning Coal face?

Coal Face on fire

24

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.

25

DGMS Guidelines for Benches or slopes design
Manual or Conventional Opencast Mines
In 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.

26

DGMS Guidelines for Formation of Spoil Banks and Dumps

(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.

27

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.

Stability Analysis of Mine Slopes
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.

28

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

Plane Sliding – Stability Analysis

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

29

Slope Stability Stability Analysis of Slope

Planar failure Analysis

With no tension crack and no water pressure

Block A

R
ShearStrength
Factor of safety = ShearStress
W sinθ
W cosθ
W
c + σ tan φ
Factor of safety =
τs

w sin(θ )
Normal Stress; σ =
A

w cos(θ )
Shear Stress , τ=
A

w cos θ
c+ tan φ
A cA + w cos θ tan φ
Factor of safety =
w sin θ
= w sin θ
A


Tension crack present in upper slope surface

Depth of tension crack; Z = H + b tan α c − (b + H cot α ) tan θ

Weight of unstable block; W =
2
(H cot αX + bHX + bZ ) )
1 2

X = (1 − tan θ cot α )

Area of failure surface; A = ( H cot α + b) sec θ

1
Driving water force; V= γ wZ w
2

2

1
Uplift water force; U= γ wZw A
2

30


Tension crack present in slope face

Depth of tension crack; Z = ( H cot α − b)(tan α − tan θ )

1 
2  Z
2

Weight of unstable block; w = γH 1 −  cot θ (cot θ tan α − 1)
2 
 H 


Area of failure surface; A = ( H cot α c − b) sec θ

1
Driving water force; V= γ wZw
2

2

1
Uplift water force; U= γ wZw A
2

cA + ( w cos θ − U − V sin θ + T cos β ) tan φ
Factor of safety = W sin θ + V cos θ − T sin β

Slope Stability Numerical

Circular Failure Analysis

W

31

Slope Stability Numerical


FOS = c+σ tan
φ
τs



FOS = c+σtan
φ
τ = c + σ tan φ
τs
w cos θ
c+ tan φ Wn
∆L c∆L + w cos θ tan φ
FOS = =
w sin θ w sin θ
∆L
n= p

∑ [c∆L
n =1
n + Wn cos α n tan φ ]
FOS = n= p

∑ [W
n =1
n sin α n ]

32

Software based on Limit equilibrium Method
SLIDE (rocscience group)
GALENA
GEO-SLOPE
GEO5
GGU
SOILVISION

Overview of GALENA

33

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

34

Kinematic Analysis

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

Slope Unfavourable Slope favourable

Kinematic Analysis to know Type of Failure

35

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)

36

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

Classification of Rock Slope according to SMT

37

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.

38


Stabilization through Support

• Ground Inclusions
Ground anchor
Soil 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

39



• 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


Rock Mass Improvement and Stabilization Methods

Geosynthetics
Grouting
Chemical Stabilization
Biological Stabilization

40



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

41

Dump Slope Stability

Controlled placement of spoil

Impermeable material increases water pressure.

weak top layer – swelling minerals,
base of the dump – permeable material.

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.

Dump Slope Stability
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.
Rejection dump – near crest of slope – dead wt. on slope

No unplanned dump – Near the crest.

42

Factor of safety 1.25

Stability analysis of active mine slope without overlying dump

Factor of safety 1.1

Stability analysis of active mine slope with overlying dump

43

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

Fig: slope with Extensometer
Extensometers

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

44


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 movement
Monitoring dams, dam abutments, and upstream slopes.
Monitoring the effects of tunneling operations

45


Piezometers

• Vibrating wire
• Pneumatic
• Standpipe 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.

46

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 Monitoring
Mounted 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.

47


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

48

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.

Slope Stability Radar Technology

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.

49

Fig: Slope Stability
radar

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

50

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