Introduction
The importance of safe, properly designed and scientifically engineered slope is well known. The benefit of an openpit operation largely depends on the use of the steepest slopes possible, which should not fail during the life of the mine. So, the design engineer is faced with the two opposite requirements, stability and steepness, in designing the deep openpit slopes. Steepening the slopes, thereby reducing the amount of material to be excavated, can save a vast sum of money. At the same time excessive steepening may result into slope failure leading to loss of production, extra stripping costs to remove failed material, reforming of benches, rerouting of haul roads and production delays. Therefore, it is necessary that a balance between economics and safety should be achieved. Slope stability and slope monitoring studies are not yet included as an integral part of the total pit design in India. The subject gets importance only when slope failure takes place putting in danger the entire mining operations or when a failure is impending. Geotechnical studies conducted in the early exploratory or detailed prospective stage will be helpful for economically successful mining. The stability of the slope primarily depends on the strength properties of the slope materials and groundwater condition within the slope. The orientation of the discontinuity planes with respect to slope face determines the types of failure possible within that slope. Generally planar, wedge, toppling and buckling types of failure occur in rock slopes, while in soil slopes and highly weathered rock slopes circular failure is possible. The failure mostly occurs in the cut fissured variegated clays. The failures are progressive in nature. It occurs due to opening up of fissure followed by strength reduction. The presence of white clay bands as a constituent of variegated clays is also mobilizing failure in these weak clays. The slope instability is generally related to different types of clays associated with lignite seams. The problems are mainly due to sudden changes in the orientation of weak planes. The changes are so local that it is difficult to detect them in advance i.e. during the initial design stages of mine development. These sudden changes in the orientation of weak planes when exposed in the slope may cause slope failures which may not be predicted in advance. The lignite deposit is part of major lignite occurrences in the Gujarat State. Broadly the geological succession in the mine is as follows:
Black cotton soil at the top, followed by red clay- white clay- yellow clay (collectively named as variegated clay), gray to gray- greenish clay, lignite and basalt. Layers of sand and conglomerate are occurring in between the clay beds. The average annual rainfall is around 1200 mm with a minimum and maximum of 500 and 2300 mm respectively over the last 10 years. Ground water is present in the topsoil aquifer, at the contact of basalt and clays, contact of clays and lignite and in the cracks of the basalt with secondary porosity. The depth of groundwater is from three to eight metres with a fluctuation of ground water level between pre-monsoon and monsoon from about three to four metres. The clays overlying the lignite seam are impermeable hence the water seeps into the mine from topsoil aquifer and at the contact of lignite and clays, which leads to bottom heave problem under heavy water pressure. It was observed that the main source of water seepage is the contact of basalt and the underlying clays. The source of the contact water will be confined aquifer within the cracks of basalt. Sometimes even the scientifically engineered slope in sedimentary sequences related to lignite deposits may also experience some instability problems. These problems are most often related to geologic structural features such as unexpected changes in bedding dip or the presence of unfavourably oriented structural discontinuities. These features are often of limited extent and it is extremely difficult, if not impossible, to recognise during the initial design stages of mine development. These relatively small, unforeseen situations can have significant implications for the stability of the pit walls as they are developed. The different fissured weak sedimentary litho units are characterised by highly varying depositional features, i.e. the dip amount and dip direction of these litho units are varying to great extent within the pit. The failures occur frequently in cut slopes of fissured clays, primarily due to opening up of fissure followed by strength reduction due to their long exposure time to natural weathering agencies. The repeated cycles of wetting (due to rains) and drying reduce the strength of fissured and other consolidated clays to great extent. Initially tension cracks are developed, at the crest of the slope after getting the white clay band exposed and finally the failure occurs along the white clay. Thin band of white clay was observed in the vertical slopes of failed zone. The occurrence, behavior and orientation of this very weak formation are very erratic and changing sharply after small distance. The dip amount is about 17° near the basalt zone. As we move away from the basalt sub crop the dip is shallow (about 5°). This white clay is very weak and it becomes almost cohesion less when it comes in contact of water or in other words its surface becomes very slippery. The overlying slope mass fails along this slippery plane during the monsoon. In dry season, the white clay loses its inherent moisture after getting exposed. It shrinks and parts along the contact plane of overlying slope mass. Although there are many bands of white clays in the mine with highly varying orientation but a few bands are only adversely oriented which cause failures. It is very difficult to detect/ predict any specific weak band in advance because usually these are weak, thin and local geological formations with changing dip at short distance and get exposed at the later stages of mining.
The same litho unit will geotechnically behave differently in different parts of pit, depending up on its orientation. Sometimes in this situation, the prediction of slope instability along these clays becomes difficult.
Corrective Measures for Failed Slope
The geometry of the slope has a significant influence upon the stability. The alignment of the slope face with respect to weak planes in the slope mass will improve the stability of the slope. Relatively small changes in the position or alignment of the slope face can result in considerable improvements in stability. The slope gets good lateral restraint by the curvature of the face (Hoek and Londe, 1974). This philosophy will help in safe mining during the coming years. The failed benches should not be reformed by simply filling the subsided zone from top. It will add dead weight on the previously failed slope mass which can reactivate the failure surface. As a consequence, another failure will occur. The only solution for long term stability in the failed zone is to push back benches so that the new benches are formed in in-situ mass. The toe of the failed slope should not be disturbed until systematic mining is done from top. Otherwise, it will lead to another failure. In case, it is not possible to back push the benches then start making small benches (preferably of 3m height) in the failed slope with the help of dozer from bottom to top after applying suitable lateral restraint in the form of consolidated bund. It is done to consolidate the failed mass. If the numbers of reconstructed benches are more then the consolidation will be more and it will make safe slopes. An extra wide safety berm/ bund (minimum width not less than the height of failure) should be made near the toe of the failed mass. The toe of failed mass can be located by the location where floor heaving was observed after first failure. Then start filling and making benches from this safety berm to the upper surface of the mine. The extra weight of filled material on slided part will get consolidated. If the filling is done on the top of slide mass, without initially making benches of less height in the slided mass (necessary for consolidation of the failed mass), then the slided mass will again show the instability sign. It has been observed that the presence of white clay and/ or the contact of basalt with other clay beds are more vulnerable to instability in the lignite pits, which is generally within 40m depth from the surface. The high bench slope in the basaltic contact zone has shown unstable signs especially due to water saturation. So during operating stage, the top 40 m slope should be flattened by providing extra wide benches. The fourth bench from top should be extra wide. This type of staggering of the pit is effectively best option to take care of failures related with lignite mining. This way, the entire pit can be divided in to different segments depth wise. If there is any failure at any particular depth then the failed mass would be arrested on the extra wide bench provided for staggering at different depth intervals and mining activity at rest of the staggered levels would continue without any problem. This strategy worked very well in many lignite mines in Rajasthan. The bench height should be 3 m in the top 40 m basaltic contact zone. Below this zone, the maximum bench height may be 6 m. The bench width should be so adjusted that the overall slope angle does not exceed from the above mentioned slope angle. The bench slope angle should be 70°. Benching should also be done in lignite where ever the thickness of the lignite is more than 6m till the final stage of the excavation and start of the reclamation.
It may be noted that progressive small scale failures may subsequently cause a big failure. If two or three benches are made steeper at any level in any part of the pit then it may initiate failure. Although the overall slope angle may be quite low but the steeper slope angle of these benches may
increase the stress at the toe of relatively steeper part of the slope which may cause failure. Two or three such small failures may cause a big failure. So, benching should be done properly from top to bottom. The mine management should not make steeper slopes/extra high or less wide benches in clays and lignite even after the urgent demand for lignite. Initially it will cause formation of tension crack in the slope which may cause collapse. Even in the peak demand period, the recommended benching parameters must be followed in the slopes from top lignite seam to bottom most lignite seam. It is necessary for better economics of the mine in long run. The sharp triangular outward edges should be avoided at the corners or at any places of the pit during mining activity. The concentration of unfavourable stresses will be higher along these edges. These sharp edges are potential unstable parts of the pit. It should be circular. The stability of slopes in fissured clay is time dependent because of changes in their material properties. The slope will stand with its peak strength only for limited period of time, since the clays undergo substantial softening and get weaker with time. It can be expected that if the cycle time between turnover cuts increases, the chances of failure can likewise be expected to increase. The mining should be so synchronised at the pits from top to bottom that the slope is required to stand for a minimum period of time, preferably one monsoon season. The standing period of the slope will decrease and by the time the clays loose their strength due to exposure, a fresh cut in the slope will be made. This practice will help in increasing the stability of standing slopes. It is sure that no slope can remain stable for long time in these weak clays. The clays loose their strength fast after exposure. The mine management should think about the practicability of this suggestion. While rehandling the failed soil mass, it may be possible that the mining operation is started in in-situ condition in the upper portion of the failed mass, but at the lower levels, the mining operation may touch the toe portion of the failed mass. In this situation, the bench parameters must be changed from in-situ condition to failed mass condition. Other wise the rehandled slope would once again fail. This situation could be avoided by starting the mining activity from sufficiently backwards from the existing crest of failed mass for rehandling of the failed mass. Then only the benches would be formed in in-situ condition from top to bottom. In case it is difficult to implement two different designs (in in-situ and failed mass) at different depths then the design suggested for the failed mass should be implemented uniformly from top to bottom, even in in-situ mass also. The pit should be provided with garland drain/ bund / barrier on the upper surface of pit to divert the run-off of rainwater away from the pit. It should be kept effective during the monsoon. The discontinuance of the pre- monsoon preparation at any location will jeopardise the whole effort of maintaining the designed slopes. The open tension cracks should be filled with permeable material. This filled material should be consolidated by dozer. At the top, any impermeable material may be spread to avoid entry of water to lower level. The contact of sand and other clay beds are more vulnerable to instability due to water saturation. Proper drainage of the water seeping from sand should be arranged. The water should be directed to the pit sump in a controlled channel to avoid saturation of soil. If heavy seepage is observed then advance pit dewatering by submersible pumps should be done to depressurize the slopemass of the benches.
The confined aquifer may be present in the cracks of basalt. This water may seep along the contact of basalt and clay layers. The surface/ rain water may enter in to the slope through the fissured clay. If the entry of this water is not checked then it may cause unstable condition. Advance trenching in virgin land or by drilling bore wells for advance pumping in the proximity of the basalt contact will help to depressurize/ dewater the ground/ area/ slope. Conclusions and Recommendations The failure will not be uncommon in the weak formation of Lignite Mines. The pit slope stability could be achieved by adopting the following measures.
Staggering of the pit by providing extra wide benches at different depth levels to take care of failures at particular levels without compromising the working at other levels between staggered zones.
Effective drainage to take care of groundwater and rainwater.
Proper benching from top to bottom of the pit.
Different designs for failed and in-situ slope mass.
By minimizing the exposure period of clays by synchronizing the cuts.
The importance of safe, properly designed and scientifically engineered slope is well known. The benefit of an openpit operation largely depends on the use of the steepest slopes possible, which should not fail during the life of the mine. So, the design engineer is faced with the two opposite requirements, stability and steepness, in designing the deep openpit slopes. Steepening the slopes, thereby reducing the amount of material to be excavated, can save a vast sum of money. At the same time excessive steepening may result into slope failure leading to loss of production, extra stripping costs to remove failed material, reforming of benches, rerouting of haul roads and production delays. Therefore, it is necessary that a balance between economics and safety should be achieved. Slope stability and slope monitoring studies are not yet included as an integral part of the total pit design in India. The subject gets importance only when slope failure takes place putting in danger the entire mining operations or when a failure is impending. Geotechnical studies conducted in the early exploratory or detailed prospective stage will be helpful for economically successful mining. The stability of the slope primarily depends on the strength properties of the slope materials and groundwater condition within the slope. The orientation of the discontinuity planes with respect to slope face determines the types of failure possible within that slope. Generally planar, wedge, toppling and buckling types of failure occur in rock slopes, while in soil slopes and highly weathered rock slopes circular failure is possible. The failure mostly occurs in the cut fissured variegated clays. The failures are progressive in nature. It occurs due to opening up of fissure followed by strength reduction. The presence of white clay bands as a constituent of variegated clays is also mobilizing failure in these weak clays. The slope instability is generally related to different types of clays associated with lignite seams. The problems are mainly due to sudden changes in the orientation of weak planes. The changes are so local that it is difficult to detect them in advance i.e. during the initial design stages of mine development. These sudden changes in the orientation of weak planes when exposed in the slope may cause slope failures which may not be predicted in advance. The lignite deposit is part of major lignite occurrences in the Gujarat State. Broadly the geological succession in the mine is as follows:
Black cotton soil at the top, followed by red clay- white clay- yellow clay (collectively named as variegated clay), gray to gray- greenish clay, lignite and basalt. Layers of sand and conglomerate are occurring in between the clay beds. The average annual rainfall is around 1200 mm with a minimum and maximum of 500 and 2300 mm respectively over the last 10 years. Ground water is present in the topsoil aquifer, at the contact of basalt and clays, contact of clays and lignite and in the cracks of the basalt with secondary porosity. The depth of groundwater is from three to eight metres with a fluctuation of ground water level between pre-monsoon and monsoon from about three to four metres. The clays overlying the lignite seam are impermeable hence the water seeps into the mine from topsoil aquifer and at the contact of lignite and clays, which leads to bottom heave problem under heavy water pressure. It was observed that the main source of water seepage is the contact of basalt and the underlying clays. The source of the contact water will be confined aquifer within the cracks of basalt. Sometimes even the scientifically engineered slope in sedimentary sequences related to lignite deposits may also experience some instability problems. These problems are most often related to geologic structural features such as unexpected changes in bedding dip or the presence of unfavourably oriented structural discontinuities. These features are often of limited extent and it is extremely difficult, if not impossible, to recognise during the initial design stages of mine development. These relatively small, unforeseen situations can have significant implications for the stability of the pit walls as they are developed. The different fissured weak sedimentary litho units are characterised by highly varying depositional features, i.e. the dip amount and dip direction of these litho units are varying to great extent within the pit. The failures occur frequently in cut slopes of fissured clays, primarily due to opening up of fissure followed by strength reduction due to their long exposure time to natural weathering agencies. The repeated cycles of wetting (due to rains) and drying reduce the strength of fissured and other consolidated clays to great extent. Initially tension cracks are developed, at the crest of the slope after getting the white clay band exposed and finally the failure occurs along the white clay. Thin band of white clay was observed in the vertical slopes of failed zone. The occurrence, behavior and orientation of this very weak formation are very erratic and changing sharply after small distance. The dip amount is about 17° near the basalt zone. As we move away from the basalt sub crop the dip is shallow (about 5°). This white clay is very weak and it becomes almost cohesion less when it comes in contact of water or in other words its surface becomes very slippery. The overlying slope mass fails along this slippery plane during the monsoon. In dry season, the white clay loses its inherent moisture after getting exposed. It shrinks and parts along the contact plane of overlying slope mass. Although there are many bands of white clays in the mine with highly varying orientation but a few bands are only adversely oriented which cause failures. It is very difficult to detect/ predict any specific weak band in advance because usually these are weak, thin and local geological formations with changing dip at short distance and get exposed at the later stages of mining.
The same litho unit will geotechnically behave differently in different parts of pit, depending up on its orientation. Sometimes in this situation, the prediction of slope instability along these clays becomes difficult.
Corrective Measures for Failed Slope
The geometry of the slope has a significant influence upon the stability. The alignment of the slope face with respect to weak planes in the slope mass will improve the stability of the slope. Relatively small changes in the position or alignment of the slope face can result in considerable improvements in stability. The slope gets good lateral restraint by the curvature of the face (Hoek and Londe, 1974). This philosophy will help in safe mining during the coming years. The failed benches should not be reformed by simply filling the subsided zone from top. It will add dead weight on the previously failed slope mass which can reactivate the failure surface. As a consequence, another failure will occur. The only solution for long term stability in the failed zone is to push back benches so that the new benches are formed in in-situ mass. The toe of the failed slope should not be disturbed until systematic mining is done from top. Otherwise, it will lead to another failure. In case, it is not possible to back push the benches then start making small benches (preferably of 3m height) in the failed slope with the help of dozer from bottom to top after applying suitable lateral restraint in the form of consolidated bund. It is done to consolidate the failed mass. If the numbers of reconstructed benches are more then the consolidation will be more and it will make safe slopes. An extra wide safety berm/ bund (minimum width not less than the height of failure) should be made near the toe of the failed mass. The toe of failed mass can be located by the location where floor heaving was observed after first failure. Then start filling and making benches from this safety berm to the upper surface of the mine. The extra weight of filled material on slided part will get consolidated. If the filling is done on the top of slide mass, without initially making benches of less height in the slided mass (necessary for consolidation of the failed mass), then the slided mass will again show the instability sign. It has been observed that the presence of white clay and/ or the contact of basalt with other clay beds are more vulnerable to instability in the lignite pits, which is generally within 40m depth from the surface. The high bench slope in the basaltic contact zone has shown unstable signs especially due to water saturation. So during operating stage, the top 40 m slope should be flattened by providing extra wide benches. The fourth bench from top should be extra wide. This type of staggering of the pit is effectively best option to take care of failures related with lignite mining. This way, the entire pit can be divided in to different segments depth wise. If there is any failure at any particular depth then the failed mass would be arrested on the extra wide bench provided for staggering at different depth intervals and mining activity at rest of the staggered levels would continue without any problem. This strategy worked very well in many lignite mines in Rajasthan. The bench height should be 3 m in the top 40 m basaltic contact zone. Below this zone, the maximum bench height may be 6 m. The bench width should be so adjusted that the overall slope angle does not exceed from the above mentioned slope angle. The bench slope angle should be 70°. Benching should also be done in lignite where ever the thickness of the lignite is more than 6m till the final stage of the excavation and start of the reclamation.
It may be noted that progressive small scale failures may subsequently cause a big failure. If two or three benches are made steeper at any level in any part of the pit then it may initiate failure. Although the overall slope angle may be quite low but the steeper slope angle of these benches may
increase the stress at the toe of relatively steeper part of the slope which may cause failure. Two or three such small failures may cause a big failure. So, benching should be done properly from top to bottom. The mine management should not make steeper slopes/extra high or less wide benches in clays and lignite even after the urgent demand for lignite. Initially it will cause formation of tension crack in the slope which may cause collapse. Even in the peak demand period, the recommended benching parameters must be followed in the slopes from top lignite seam to bottom most lignite seam. It is necessary for better economics of the mine in long run. The sharp triangular outward edges should be avoided at the corners or at any places of the pit during mining activity. The concentration of unfavourable stresses will be higher along these edges. These sharp edges are potential unstable parts of the pit. It should be circular. The stability of slopes in fissured clay is time dependent because of changes in their material properties. The slope will stand with its peak strength only for limited period of time, since the clays undergo substantial softening and get weaker with time. It can be expected that if the cycle time between turnover cuts increases, the chances of failure can likewise be expected to increase. The mining should be so synchronised at the pits from top to bottom that the slope is required to stand for a minimum period of time, preferably one monsoon season. The standing period of the slope will decrease and by the time the clays loose their strength due to exposure, a fresh cut in the slope will be made. This practice will help in increasing the stability of standing slopes. It is sure that no slope can remain stable for long time in these weak clays. The clays loose their strength fast after exposure. The mine management should think about the practicability of this suggestion. While rehandling the failed soil mass, it may be possible that the mining operation is started in in-situ condition in the upper portion of the failed mass, but at the lower levels, the mining operation may touch the toe portion of the failed mass. In this situation, the bench parameters must be changed from in-situ condition to failed mass condition. Other wise the rehandled slope would once again fail. This situation could be avoided by starting the mining activity from sufficiently backwards from the existing crest of failed mass for rehandling of the failed mass. Then only the benches would be formed in in-situ condition from top to bottom. In case it is difficult to implement two different designs (in in-situ and failed mass) at different depths then the design suggested for the failed mass should be implemented uniformly from top to bottom, even in in-situ mass also. The pit should be provided with garland drain/ bund / barrier on the upper surface of pit to divert the run-off of rainwater away from the pit. It should be kept effective during the monsoon. The discontinuance of the pre- monsoon preparation at any location will jeopardise the whole effort of maintaining the designed slopes. The open tension cracks should be filled with permeable material. This filled material should be consolidated by dozer. At the top, any impermeable material may be spread to avoid entry of water to lower level. The contact of sand and other clay beds are more vulnerable to instability due to water saturation. Proper drainage of the water seeping from sand should be arranged. The water should be directed to the pit sump in a controlled channel to avoid saturation of soil. If heavy seepage is observed then advance pit dewatering by submersible pumps should be done to depressurize the slopemass of the benches.
The confined aquifer may be present in the cracks of basalt. This water may seep along the contact of basalt and clay layers. The surface/ rain water may enter in to the slope through the fissured clay. If the entry of this water is not checked then it may cause unstable condition. Advance trenching in virgin land or by drilling bore wells for advance pumping in the proximity of the basalt contact will help to depressurize/ dewater the ground/ area/ slope. Conclusions and Recommendations The failure will not be uncommon in the weak formation of Lignite Mines. The pit slope stability could be achieved by adopting the following measures.
Staggering of the pit by providing extra wide benches at different depth levels to take care of failures at particular levels without compromising the working at other levels between staggered zones.
Effective drainage to take care of groundwater and rainwater.
Proper benching from top to bottom of the pit.
Different designs for failed and in-situ slope mass.
By minimizing the exposure period of clays by synchronizing the cuts.
References
Hoek, E. and Bray, J.W. (1981). Rock Slope Engineering. Institution of Mining and Metallurgy, London.
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