ANGLE OF REPOSE

ANGLE OF REPOSEGurkaran Singh2011CH10083

INTRODUCTION Topic Assigned Angle Of ReposeDefinition - Steepest angle between the surface of the pile and the horizontal surface, without slipping of powder particlesDenoted by R

FACTORS AFFECTING ANGLE OF REPOSE The individual material, a reflection of the different coefficients of friction between different substancesSize of the particles Fine grained OR Coarse grainedMoisture Gravity!METHODS TO MEASURE ANGLE OF REPOSE Tilting Box Method

Fixed Funnel Method

Revolving Cylinder Method

NARROWING DOWN THE PROBLEM Novel measurement techniques being used & their correctness OR

Application of Angle Of Repose in predicting Avalanches from the properties of the granular materials

AVALANCHESMass of snow which becomes detached and slides down a slope, often acquiring great bulk by fresh addition as it descends.Three common types-SLAB AVALCNHES - Formed from the snow that has been deposited, or redeposited by wind. Characteristic appearance of a block of snow POWDER SNOW AVALANCHES - These consist of a powder cloud, which overlies a dense avalanche. These are formed from turbulent suspension currents.WET SNOW AVALANCHES - Low velocity suspension of snow and water, with the flow confined to the track surface. REASONS FOR AVALANCHESThe load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger the failure in the snowpack and avalanches FACTORS AFFECTING AVALANCHES Internal Friction of movement or Kinetic FrictionIf is the angle of inclination of the natural slope, =tan is the coefficient of kinetic friction. Dry snow without cohesion remains in equilibrium at a slope related to the shape of the crystals. For spherical particles, this angle is 22to 23. In cuplike crystals, has a natural slope of 32 to 37 . CohesionIf the snow crystals are attached to each other, the snow acquires a certain cohesion, which depends on the number and surface of the contacts. Temperature has an important influence on cohesion. For equal contact surfaces, the cohesion is stronger at lower temperatures. A degree of humidity holds the flakes together.Resistance To Shear The resistance to shear is the sum of static friction and of cohesion, which opposes the downward movement of snow.

POSSIBILITIES OF RUPTURE OF THE AVALANCHE The snow cover on a slope is drawn downwards by its weight, in a component parallel to the slope, known as the shear stress or the tendency to shear. For an avalanche to be released, five breaks must occur: one in tension at the top, two lateral shear breaks, one failure in compression at the bottom of the slope, and one shear failure on the supporting stratum. Failures usually occur one after the other so that a single break, known as the primary rupture, can often bring on the other breaks, called secondary, and so detach a mass of snow. If the resistances are strong enough to withstand the primary rupture, no avalanche results

PROPERTIES AFFECTING THE ANGLE OF REPOSE OF SNOW PARTICLES 1)SNOWPACK STRUCTURE AND ITS EFFECTS

Dendritic or stellar crystals have the highest angle of repose(upto 80), decreasing to 35 for rounded forms. It is known that slush can avalanche off of the slopes of 15 or less also.

Source- The Avalanche Handbook By David McClung, Peter A. Schaerer

PROPERTIES AFFECTING THE ANGLE OF REPOSE OF SNOW PARTICLES 2)EFFECT OF GRAVITY A study was conducted to determine if angles of repose of avalanching sediments depend on gravity and thus differ on planet Mars or the Moon compared to Earth

Source-Reduced gravity causes larger-volume and lower-angle granular avalanches with less stratification by Sebastiaan J. de Vet, Maarten G. Kleinhans, H. Markies, A.C. in 't Veld, F.N. Postema, J.J.W.A. van Loon

PROPERTIES AFFECTING THE ANGLE OF REPOSE OF SNOW PARTICLES It was found that in reduced gravity:number of inter-particle contacts decreases, distances increasecomponent Finter in (Ffriction+Finter) reducesreduction of total friction, lowering the threshold angle during avalanchingwhich implies that the addition of a surface force by e.g. static electricity would increase the total friction, leading to an increase in the repose angle.

Source-Reduced gravity causes larger-volume and lower-angle granular avalanches with less stratification by Sebastiaan J. de Vet, Maarten G. Kleinhans, H. Markies, A.C. in 't Veld, F.N. Postema, J.J.W.A. van Loon

PROPERTIES AFFECTING THE ANGLE OF REPOSE OF SNOW PARTICLES 3)EFFECT OF GRANULAR SIZE OF SNOW ON THE ANGLE OF REPOSE IN A CHUTE

The angle of repose in the chute has exponential decay with , which is the reciprocal of granular diameter d, =1/d.

Source- ZHOU Ying and ZHANG GuoQin. Effect of granular size on the angle of repose in the chute. Sci China Ser G-Phys Mech Astron, Apr. 2009, vol. 52, no. 4, 563-565

PROPERTIES AFFECTING THE ANGLE OF REPOSE OF SNOW PARTICLES 4) MOISTURE IN SNOW As liquid is added to the snow spheres, it is observed that particle clusters (clumps) develop, whose size increases with liquid content. The presence of such clusters leads to the dependence ofr on the values of tliq (thickness of liquid layer)

Development of clusters - a transition from a regime where the bulk properties are associated with the dynamics of individual grains to a regime where long-range correlations control the material behavior

Source- Albert R, Albert I, Hornbaker D, et al. Maximum angle of stability in wet and dry spherical granular media. Phys Rev E, 1997, 56(6):R6271-R6274 The data also reflects the development of clustering as an increase in the width of the distribution of our measured values of r with increasing tliq. This width corresponds to the roughness of the crater surface. The development of such clusters appears from these data to be rather sudden, suggesting a transition from a regime where the bulk properties are associated with the dynamics of individual grains to a regime where long-range correlations control the material behavior

14CONCLUSION An attempt to :a) Clearly understand the concept of Avalanches and the forces behind itb) Explain the mechanism of Avalanches c) Describe the factors affecting Avalanches with suitable importanceREFERENCEShttp://en.wikipedia.org/wiki/Avalanche The Avalanche Handbook By David McClung, Peter A. Schaerer Reduced gravity causes larger-volume and lower-angle granular avalanches with less stratification by Sebastiaan J. de Vet, Maarten G. Kleinhans, H. Markies, A.C. in 't Veld, F.N. Postema, J.J.W.A. van LoonNowak S, Samadani A, Kudrolli A. Maximum angle of stability a wet granular pile. Nat Phys, 2005, 1(1): 50-52Lee J, Herrmann H J. Angle of repose and angle of marginal stability:Molecular dynamics of granular particles. J Phys A-Math Gen, 1993,26(2): 373-383Liu C H, Jaeger H M, Nagel S R. Finite-size effects in a sandpile. Phys Rev A, 1991, 43(12): 7091-7092Albert R, Albert I, Hornbaker D, et al. Maximum angle of stability in wet and dry spherical granular media. Phys Rev E, 1997, 56(6):R6271-R6274ZHOU Ying and ZHANG GuoQin. Effect of granular size on the angle of repose in the chute. Sci China Ser G-Phys Mech Astron, Apr. 2009, vol. 52, no. 4, 563-565

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