Slope Stability

Slope

• Mathematicians have developed a useful measure of the steepness of a line, called the slope of the line.

• Slope compares the vertical change (the rise) to the horizontal change (the run) when moving from one fixed point to another along the line

Slope is expressed by

1. An angle from horizontal i.e 24°45°2. As a gradient i.e 1 in 4as a vertical fall3. centimetre per meter

Gradient

• The method to calculate the Gradient is:• Divide the change in height by the change in

horizontal distance• Gradient = Change in Y/Change in X

Types of open pit slopes• Slopes in quarries can result from a range of activities associated with the

working of the site. There are three principal types of slope to be considered:1. Excavated slopes (in which in situ ground is exposed); 2. Tipped slopes (where loose material is formed into mounds either above

ground in placed in existing voids); and 3. Natural slopes (which may be affected by excavations or placement of loose

materials). On the basis of duration pit slopes can be4. Short term slopes – stockpiles and soil mounds, intermediate quarry faces; 5. Medium term slopes – soil mounds, quarry faces, screening bunds, etc.;

and 6. Long term slopes – quarry faces, spoil mounds, lagoon embankments,

screening bunds.

Types of open pit slopes• Unimportant slope• Average slope• Critical slope• Unimportant slope;- slope angles unimportant

economically and flat slopes can be used• Average slope;- slopes angles are important but not

critical in determining economics of mining• Critical slope;- slope angle critical in terms of both

economics of mining and safety of operations

Types of open pit slopes• In open pit mining, mineral deposits are mined from the

ground surface and downward. Consequently, pit slopes are formed as the ore is being extracted.

• The various types of pit slopes are 1. Bench slope2. Overall pit slope3. Current overall pit slope4. Haul road slope,5. Ramp slope6. Inter ramp slope7. dump slope

Types of Slope Failure

• Bench slope or bank angle: it the angle measured in degree between the horizontal and an imaginary line connecting bench toe to the bench crest.

It is an inclined plane of the bench which limits it on the side worked out space.

• overall pit slope angle: it the angle measured in degree between the horizontal and an imaginary line connecting bottom bench toe to the top bench crest.

• Current overall pit slope;- it the angle measured in degree between the horizontal and an imaginary line connecting existing bottom bench toe to the existing top bench crest at a given time.

• Ramp slope;-The gradient on a ramp is the grade line profile along the road centre line, in the vertical plane.

• Dump slope;- it the angle measured in degree between the horizontal and an imaginary line connecting toe of dump to the existing top crest at a given time.

• The interramp slope angle is measured from toe to toe or crest to crest exclusive of any ramps. By excluding ramps or other offsets the interramp angle is kept static no matter how many benches are measured. The interramp angle is similar to the face angle because it is static.

PIT SLOPE GEOMETRIES • There are numbers of slope which inters into pit design. Care is needed so that there is no confusion

as to how they are calculated and what they mean . The main are bench slope, overall pit slope , , ramp slope, interramp slope etc.

• There are three major component of a pit slope1. Bench configuration2. Interramp slope3. Overall slope• The bench configuration is defined by bench face angle, the bench height and bench width.• The interramp angle is the slope angle produced by a number of benches. Where there are haul

roads ,working levels or other wide benches• The overall pit slope is the angle of the line from the toe to the crest of the pit.and will be flatter

than interramp slope. • It is important to consider all three components in slope design• In order to make a quantities estimate of the stability of aslope analytical models amenable to

mathematical solutions must be used• The requrements of these modeles are the failure geometry and assumption regarding materila

properties and stress distributions.• While active mining is under way ,some working benches would be included in the overall slope.• At the end of mining it is desired to leave the final pit slope as steep as possible, some of safety

benches will be reduced in width while others may be eliminated entirely.`

Types of Slope Failure

PIT SLOPE GEOMETRIES • The primary components of a pit design are as follows:1. Bench Geometry – • The height of the benches is typically determined by the size of the shovel chosen for the

mining operation. • The bench face angle is usually selected in such a way as to reduce, to an acceptable level, the

amount of material that will likely fall from the face or crest.• The bench width is sized to prevent small wedges and blocks from the bench faces falling

down the slope and potentially impacting men and equipment. • The bench geometry that results from the bench face angle and bench width will ultimately

dictate the inter-ramp slope angle. Double or triple benches can be used in certain circumstances to steepen inter-ramp slopes.

2.Inter-ramp Slope – The maximum inter-ramp slope angle is typically dictated by the bench geometry. However, it is also necessary to evaluate the potential for multiple bench scale instabilities due to large-scale structural features such as faults, shear zones, bedding planes, foliation etc. In some cases, these persistent features may completely control the achievable inter-ramp angles and the slope may have to be flattened to account for their presence

3. Overall Slope – The overall slope angle that is achieved in a pit is typically flatter than the maximum inter-ramp angle due to the inclusion of haulage ramps. Other factors that may reduce the overall slope angles are things such as rock mass strength, groundwater pressures, blasting vibration, stress conditions and mine equipment requirements

Mechanism of pit slope failure• Mechanism of slope failure; when driving force exceed the

resisting force• Factor of safety; the ratio of resisting force to the driving force, if

FS≤1 the slope will fail, if FS >the slope is theoretically stable

shear stress is a stress state where the stress is parallel to the surface of the material, as opposed to normal stress when the stress is vertical to the surface. Shear stress is relevant to the motion of fluids upon surfaces, which result in the generation of shear stress.

A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force.

Shear strength is a material's ability to resist forces that can cause the internal structure of the material to slide against itself. Adhesives tend to have high shear strength.In engineering, shear strength is the strength of a material or component against the type of yield or structural failure where the material or component fails in shear.Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain.

Strength ;-the ability to resist being moved or broken by a forceStress ;-pressure or tension exerted on a material objec

• Before mining the horizontal stress flows horizontal and vertical stresses due to weight downward and are in equilibrium state. when an excavation is made the flows horizontal and vertical stresses disturbed and equilibrium state break.

• With the excavation of the pit , the pre existing horizontal stresses are forced to flow beneath the pit bottom an and around the pit

• The vertical stresses are also reduced through the removal of the rock overlying the final slopes. This means that the rock lying between the pit outline and theses flow lines largely distressed

• As a result of stress removal cracks/joints can open with a subsequent reduction in cohesive and friction forces restraining the rock in place

• Further more ground water can more easily flow through these zones reducing the effective normal force on potential

Failure plane• As the pit is deepened the extend of this distressed zone increases and The consequence of failure becomes more severe .the presence of adverse structure like fault, dykes, weak zonesEtc further reduce resisting force As soon as driving force exceed the resisting force slope failure takes place

• 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: 1. 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.

2. 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.

TYPES OF ROCK SLOPE FAILURES • Failure in Earth and Rock mass1. Plane Failure 2. Wedge Failure 3. Circular Failure 4. Toppling Failure Rock fall FailFailure in Earth, rock fill and spoil dumps and Embankments5. Circular 6. Non-circular 7. semi-infinite slope8. Multiple block plane wedge 9. Log spiral (bearing capacity of foundations)10. Flow slides and Mud flow11. Cracking 12. Gulling 13. Erosion Slide or Slump Figure.

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.

• A rock slope undergoes this mode of failure when combinations of discontinuities in the rock mass form blocks or wedges within the rock which are free to move. The pattern of the discontinuities may be comprised of a single discontinuity or a pair of discontinuities that intersect each other, or a combination of multiple discontinuities that are linked together to form a failure mode.

• A planar failure of rock slope occurs when a mass of rock in a slope slides down along a relatively planar failure surface. The failure surfaces are usually structural discontinuities such as bedding planes, faults, joints or the interface between bedrock and an overlying layer of weathered rock.

• Plane failure can occur on the bench scale ,interramp scale and Pit wall scale• The favorable conditions of plane failure are as follows: 1.The dip direction of the planar discontinuity must be within ( ±20o) of the dip direction of the slope face 2. The dip of the planar discontinuity must be less than the dip of the slope face (Daylight) 3.The dip of the planar discontinuity must be greater than the angle of friction of the surface

• In open pit mining, mineral deposits are mined from the ground surface and downward. Consequently, pit slopes are formed as the ore is being extracted.

• It is seldom, not to say never, possible to maintain stable vertical slopes or pit walls of substantial height even in very hard and strong rock.

• The pit slopes must thus be inclined at some angle to prevent failure of the rock mass

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.

• Wedge failure of rock slope results when rock mass slides along two intersecting discontinuities, both of which dip out of the cut slope at an oblique angle to the cut face, thus forming a wedge-shaped block

• Wedge failure can occur in rock mass with two or more sets of discontinuities whose lines of intersection are approximately perpendicular to the strike of the slope and dip towards the plane of the slope. This mode of failure requires that the dip angle of at least one joint intersect is greater than the friction angle of the joint surfaces and that the line of joint intersection intersects the plane of the slope.

• The necessary structural conditions for this failure are summarized as follows: 1. The trend of the line of intersection must approximate the dip direction of the slope face. 2. The plunge of the line of intersection must be less than the dip of the slope face. The line of

intersection under this condition is said to daylight on the slope. 3. The plunge of the line of intersection must be greater than the angle of friction of the surface

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

• This failure can occurs in soil slopes, the circular method occurs when the joint sets are not very well defined. When the material of the spoil dump slopes are weak such as soil, heavily jointed or broken rock mass

• The conditions under which circular failure occurs are follows:1. When the individual particles of soil or rock mass, comprising the slopes aresmall as compared to the slope.2. When the particles are not locked as a result of their shape and tend to behave as soil.• Types of circular failureCircular failure is classified in three types depending on the area that is affected by the failuresurface. They are:-(a) 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.(b) Toe failure: In this type of failure, the arc of the rupture surface meets the slope at the toe.(c) 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.

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

• Toppling failures occur when columns of rock, formed by steeply dipping discontinuities in the rock rotates about an essentially fixed point at or near the base of the slope followed by slippage between the layers

• . The centre of gravity of the column or slab must fall outside the dimension of its base in toppling failure. Jointed rock mass closely spaced and steeply dipping discontinuity sets that dip away from the slope surface are necessary prerequisites for toppling failure. The removal of overburden and the confining rock, as is the case in mining excavations, can result in a partial relief of the constraining stresses within the rock structure, resulting in a toppling failure

FACTORS AFFECTING SLOPE FAILURE • Slope failure are often caused by processes that increase shear stress or decrease the shear strength of soil or rock mass. Residual soil and weathered bed rock

can be weekend by pre-existing discontinuities such as fault, bedding surface, foliation, cleavages, sheared zone, elict joints ,dikes and sills.• Slope failure occurs when the downward movements of material due to gravity and shear stresses exceeds the shear strength. • Therefore, factors that tend to increase the shear stresses or decrease the shear strength increase the chances of failure of a slope. • factors that tend to increase the shear stresses1.REMOVAL OF SUPPORTA. Erosions1. By streams and rivers, 2. By glacial3. By action of wave of water bodies and ocean4. By successive wetting and drying(e.g winds ,freezing) B. Natural slope movements(e.g falls, slides, settlements)C. Human activity1. Cuts and excavation2. Removal of retaining wall or sheet piles3. Drawdown's of bodies of water (e.g lakes ,lagoons)2. OVERLOADINGA. By natural causes1. Weight of precipitation(e.g rains and snow)2. Accumulation of material because of past slidesB. By human activity1. Construction of fill2. Building and other overload on the crest3. Water leakage in culverts, water pipes and sewers3. TRANSITORY EFFECT ( EARTH QUACKS)4. REMOVAL OF UNDER LYING MATERIAL THAT PROVIDES SUPPORT4. By rivers and sea5. By weathering6. By underground erosion due to seepage, solvent action7. By human activity e.g mining or excavation8. By loss of strength of underlying material5. INCRESE IN LATERAL PRESSURE6. By water in crack and fissures7. By freezing of water in cracks8. By expansion of clay

FACTORS AFFECTING SLOPE FAILURE• factors that tend to decrease the shear strength increase1. Factors inherent in the nature of the material• Composition• Structure• Secondary or inherited structures• Stratification2. Changes caused by weathering and physicochemical activities• Wetting and drying processes• Hydration• Removal of cementing material3. Effect of pore pressure4. Change in structure• Stress release• Structural degradation

Prediction of slope failure • Forecasting potential slope failure in open pit mines is integral to maintaining safety and mine productivity

1.Bulging ground appears at the base of a slope or a retaining wall *2. Water breaks through the ground surface or appears in a location near or at the base of a

slope *3. Fences, retaining walls, utility poles, or trees tilt or move *4. Cracks appear on the slope *5. Water pipes break *6. Cracks appear on the ground or in the foundation *7. Structures on slopes moving away from their original position8. Doors or windows start to stick or jam *9. Sunken or down-dropped road beds10. Slowly developing, widening cracks on the ground or paved areas such as streets or

driveways *11. Land movement and small slides could indicate unstable condition of the slope that may

lead to bigger failures *12. Outside walls, walks, or stairs begin pulling away from building.

Signs of immediate danger• THE signs of slope failure can materialise days, weeks, or months

before failure actually occurs. However, there are some unmistakeable signs that manifest moments before a landslide occurs. These should never be ignored and evacuative action needs to be taken. Simply put, a landslide is coming and you need to get out of its way immediately.

• A sudden decrease in creek water levels although rain is still falling or has just stopped.

• A faint rumbling sound that increases in volume which indicates the landslide is coming closer.

• Unusual sounds, such as trees cracking or boulders knocking together, which could indicate moving debris.

Short-term prediction of mass movement• Springs, seeps, or saturated ground in areas that have not typically been wet before.• New cracks or unusual bulges in the ground, street pavements or sidewalks.• Soil moving away from foundations.• Ancillary structures such as decks and patios tilting and/or moving relative to the main

house.• Tilting or cracking of concrete floors and foundations.• Broken water lines and other underground utilities.• Leaning telephone poles, trees, retaining walls or fences• Offset fence lines.• Sunken or down-dropped road beds.• Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil

content).• Sudden decrease in creek water levels though rain is still falling or just recently

stopped.• Sticking doors and windows, and visible open spaces indicating jambs and frames out

of plumb.• A faint rumbling sound that increases in volume is noticeable as the landslide nears.• Unusual sounds, such as trees cracking or boulders knocking together, might indicate

moving debris.

Slope Movement Monitoring • Slope movement monitoring, even in its simplest form, should be carried out in all mining

situations.• Regular visual inspection for signs of tension cracking, rock fall activity, slope raveling, bulging

in the slope face or heaving at the toe of a slope can provide advanced warning of potential instability.

1. Visual slope monitoring by routine walkover inspections by the Geotechnical Engineer. The Engineer compares the last visit observations with the latest one and records any deleterious slope stability changes that may have occurred. The recording of any changes that occur on the open pit slope faces by production personnel during the shift is another way of visual slope monitoring. To this end, slope hazard awareness lessons are conducted twice a year for all pit workers from supervisor to operator.

2. Slope monitoring using the Geodetic Monitoring System (Geomos) survey technique. 3. A slope monitoring report is issued containing the results of the visual monitoring and the

Geomos monitoring systems. The report has an action list of critical slope stability issues requiring attention with time lines and responsible individuals indicated. The report is issued to production, management and technical officials.

Depending on the duration of mining and the heights of the proposed mine slopes, an array of reflective survey prisms located near the pit crest provides a baseline of slope displacements, from which potential changes in slope behaviour can be assessed. In potentially unstable areas where tension cracks or slope

Hazard of slope failure

• Physical impact can be of three types1. Direct;- are those consequences incurred by direct physical contact with land slide itself2. Indirect ;-are change brought about properties and behaviour of natural system as a result of landslide activity3. Acute ;- immediate , short lived 4. Chronic;- delayed, long period• Hazard to human life and property• Injury and loss of life• Property damage• Failure of communication system• Social and economic disruption• Loss of productive land• Ecological impact• Change in hydrology• Change in ground profile (topography)and land use pattern• Change in soil and rock structure• Loss of scenic beauty• Loss of production,• extra stripping costs to remove failed material, • DGMS may close the mine

Aim of slope stability:1. To understand the development and form of natural and man made slopes

and the processes responsible for different features.2. To assess the stability of slopes under short-term (often during construction)

and long-term conditions.3. To assess the possibility of slope failure involving natural or existing

engineered slopes.4. To analyze slope stability and to understand failure mechanisms and the

influence of environmental factors.5. To enable the redesign of failed slopes and the planning and design of

preventive and remedial measures, where necessary.6. To study the effect of seismic loadings on slopes and embankments.7. Safe, properly designed, scientifically engineered slope.8. Profitability of open cast mines.9. Design engineer/ scientist