Interview Preparation CMR/MMR First Class/Second Class Part-4 Dump failure and stability

61. Why a dump needs to be properly maintained?

In opencast mining part of overburden removed above the economic mineral has to be dumped outside.

A judicious dumping of the same ensures saving in ground and chances of any slide back or dangerous incidents / accidents in future.

While very low height and flat dumps could be ideal from the points of stability, these would not only occupy lot of ground space but also prove very expensive.

Hence, a balance has to be struck, maximum slope with minimum possible ground space to be occupied while ensuring that dumps do not slide and causes any untoward incident/ accidents. 

62. What are the parameters affecting the overburden dump stability?

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63. What steps can be taken to avoid dump slides?

Following steps may be taken to avoid dump slides:

1) For reliable dump stability two corridors may be formed, one at the level of rib top and one at the dragline sitting level.

2)Dumping by Shovel dumper may not done on or near the currently created dragline dump to avoid the dead loading of the partially consolidated dragline dump. In most cases dumping by shovel dumper may be done at least two cuts away.

3)Dumping of topsoil may be avoided at the bottom of the cut as it leads to instability. Dumping of soil and clayey material is done away from the working area, that is on the farther end of the dump so that formation of weak planes is avoided.

4)While extracting coal ribs holes may not be drilled right up lo the floor but 4 to 5 metres is left against the floor. This in turn avoids formation of weak zone between the coal rib and floor of the seam.

5) Sufficient time should be allowed before deployment of HEMM after blasting in the coal rib for safety.

6) Final regrading of internal dumps should be done in such a way that rain water is drained outside the mine through drains and culverts.

7)Retaining walls should be made in the mid entry roads to provide safety.

8)Constant vigil be kept on the condition of the dumps by experienced competent officials and if any signs of movement are observed (like development of Cracks etc.) precautions be taken to ensure safety of persons and machines.

9)Afforestation by planting trees helps a lot in improving stability of dumps by preventing erosion.

10)Height of the dragline bench should be judiciously selected by decapping about 15 m parting just below the Bottom seam by shovel dumper combination system.

64. What does TECHNICAL AND BIOLOGICAL RECLAMATION OF OB DUMPS consists?

• Stabilisation of OB dumps

• Construction of gabion retaining wall

• Construction of drain for drainage

• Provision of jute mesh for facilitating grass growth on slopes

• Provision of good soil mixed with manure and subsequent irrigation for growth of grass for anchorage on slopes

• Plantation

65. What are the ways to reduce the chances of surface ground control failure?

DGMS recommendations

There are several ways to reduce the chances of surface ground control failures as given below.

(a) Safe geotechnical designs

(b) Secondary supports or fall catchment arrangements and

(c) Monitoring devices for advance warning of impending failures.

Safe geotechnical design is with a single focused objective that of ensuring adequate stability against any failure while working, involving the complete study of local geologic formations/structures, rock mass properties, hydrologic conditions and the rainfall pattern about the openpit.

While a flatter slope always means better stability, it also involves permanent locking up of huge quantities of mineral reserves. On the contrary, steeper slope greatly increases the potential of failures. Therefore, a scientific balance has to be constantly established between the two situations catering to the expected life of the benches.

While it is very important to have good geotechnical designs of dumps and slopes, effective monitoring and examination of slopes for failure warning signs is the most important means of protecting exposed mine workers. Even, very carefully designed slopes may experience failure from unknown geologic structures, unexpected weathers or seismic shock etc. The problem of such failures are getting compounded manifold due to non-availability of land for dumps leading to increased height of the existing dumps, poor drainage systems, proximity to inhabited areas, non-separation of top-soil during excavation, etc. 

66. How would you monitor the sides and slopes in opencast workings?

Monitoring of Side and Slopes in Opencast Workings:

Conventional monitoring methods: It depend on regular surveying of the sides and slopes of the benches and dumps using the most modern survey instruments and associated software. Additional instruments can be used as part of a comprehensive system to register subsurface rock mass displacement, groundwater parameters, and blast vibration levels.

Challenges in this:

• Aside from visual inspection, these methods provide displacement information only for a single site, or at best, at discrete number of sites.
• If the monitored sites are too widely separated or if displacement occurs between sites, early indications of an impending slope failure might go unnoticed.
• In addition, these monitoring tools are difficult to install at many quarries and surface coal mines where steep highwalls and lack of benches limit access to areas above the working floor which is very common in dragline dumping in coal mining.
• It is costly and time consuming, can be dangerous on unstable slopes.
• Point-by-point monitoring of every potential failure block in a mine or dump slope is not practical.

Scanning laser rangefinders: It may be used where ground displacements are detected by comparing successive scans

Challenges in this:

• Processing requirements and scan rates have so far made repeat pass intervals too great for effective and timely slope monitoring.
• In addition, the range and accuracy of these systems are impaired by differences in the reflectivity of the rock, the angle of the rock face, weather, and other factors.

Radar technology: It used widely in a variety of fields for several decades, has found its utility of recent in monitoring ground movements in mining applications.

This technology sports distinct advantages over conventional methods in its ability to cover large areas on the surface for true two-dimensional monitoring day and night in almost any weather, and atmospheric dust and/or haze have little effect.

Slope Stability Radar (SSR) developed on the above radar technology, is now being widely used in several countries to provide real time monitoring and advance warning signals before any slope or dump failure in opencast mines.

The SSR system can detect and alert movements of a wall with sub-millimeter precision, with continuity and broad area coverage. This monitoring occurs without the need for mounted reflectors or equipment on the wall or slope and the radar waves adequately penetrate through rain, dust and smoke, 24 hours a day. The SSR system produces data for interpretation 4 usually within minutes. 

67. What are the factors affecting dump stability:

A variety of factors act in combination to control the stability of a mine dump. The main factors are discussed below.

Dump Configuration

The configuration and the size of a mine dump have a direct impact on its stability and potential size of failures.

The primary geometric variables are:

• Height of the dump: It is defined as the vertical distance from the dump crest to the ground surface at the dump toe. Dump height typically ranges from 20m to more than 400m. It is an important characteristic for stability, mode and speed of failure, and potential runout distance. In general, dump height is predetermined by the mine site physiography and mining rate.

• Volume: Small dumps are considered to contain less than about 1 million m3, while large dumps have more than 50 million m3 . Medium sized dumps have volumes in the range of 1 to 50 million m3
• Slope Angle: The overall dump angle is measured from crest of the uppermost platform to the toe. The normal range of dump slopes is between 26° to 37°, the lower value commonly adopted for reclamation whereas the upper value corresponds to the free dumped cohesionless rockfill.

Foundation Conditions: Poor foundation conditions are cited as the most frequent cause of dump slope instability. The foundation of a dump may have a variety of rock formation from saturated soil to competent bedrock.

The presence of water within the voids of a dump seriously affects the dumps stability.

Presence of weak saturated soil or similar zones in the foundation affects stability of a dump by allowing deformation of the toe material.

Surface runoff is controlled to a large degree by the physiography of the site, however manmade diversion systems can be constructed to minimize this amount. The potential presence of groundwater springs needs to be investigated prior to dumping.

Dump configurations such as valley fills or cross valley fills require an assessment of the drainage characteristics of the foundation and fill material, to prevent built up of water within the dump.

Dump Material Properties: Properties of dump materials include gradation, shear strength and durability, etc. The most favourable dump materials are composed of hard and durable coarse rock with little or no fines and are commonly associated with metal mines. The least favourable materials are formed of soft and degradable rocks with significant fines, such as mudstones or shales.

Dumping method: Mass dumping leads to rotation failures where the fine grained material behaves as a weak homogeneous mass. In the end-dumped dump state, it is very sensitive to pore pressure increases caused by rapid loading, precipitation, and foundation seepage pressures. Face dumping causes formation of a plane of weakness along the face. Under such condition, continued use of the dump, even for dumping of good quality material is susceptible to failure along this plane.

Dumping Rate: The impact of loading rate on physical stability of dump is very important. The placement of loose blasted rock or soils in a manner which does not allow for sufficient time for the material to develop strength at reasonable density will experience significant settlement. Rapid loading and increase in toe loading associated with slope and crest deformations overload the toe block. High rate of dumping may result in generation of excess pore pressures. In such cases, dumping rate may have to be controlled and pore pressure monitored during construction to ensure that excess pore pressures are effectively dissipated and foundation stability is maintained.

Seismicity and Dynamic Stability: The most significant impact on stability due to earthquakes appears to be potential liquefaction of susceptible foundation materials.

Topography: Topography has a direct influence on dump stability in different ways. Mine dump instability on steep terrain may be due to the difficulty in achieving adequate dump density and therefore the maximum shear strength of the material. Other problems include slope regions with increasing slope angles in the vicinity of the toe or side flanks. Flat or near horizontal slope is more stable whereas, steep slopes approaching the natural angle of repose for the overburden dump material are likely to exhibit ravelling, or other planar instability.

Dump drainage condition: Proper drainage needs to be maintained.

Theory related to Dump stabilty and failure is very important for this year interview due to recent mishap of Lalmatiya Mines.

Stay tuned for next part