Heap

Piles are mixtures of solid particles ( granular matter ) that are loosely mixed or firmly pressed or baked together.

The term "heap" comes from the mining industry , where the rock lumps that were stacked in piles and broken out of the mine were meant that did not contain any usable parts ( dead rock ).

In modern mining, debris defines any accumulation of material, regardless of whether it was heaped up (for example a spoil dump or backfill by pre-screening) or whether it was created, for example, by blasting with which work is being carried out. The pile accordingly also consists of previously unseen material (example after the rock has been detached from the rock by blasting), which is sorted and conveyed and only sampled afterwards. The screening of the material in advance is usually carried out randomly through core drilling and analysis of the samples in the laboratory.

In Austria, the term "Hauwerk" is used for material obtained through mining. The origin of the word “Hauwerk” is explained by the fact that the material obtained is “the work (the work) of the hunter (the miner)”. “Mountains” (deaf rock) are also “Hauwerk”, so the restriction described above does not apply in the Austrian miner's language. The term "Hau f factory" in itself is considered in Austria as "unbergmännisch".

Material science basics

The principal force between the particles in a pile is friction and adhesion , and in the case of cohesive pile, cohesion . In the case of non-round particles, self-locking also acts ; in the case of rough micro-interlocking , electrostatic forces may also act .

Under the influence of gravity , an equilibrium of the shear stresses on the contact surfaces forms in the pile , which gives the pile its stable shape.

The particles can have different grain sizes and compositions. They can consist of basic substances , e.g. B. from various active pharmaceutical ingredients in tablets , or in turn from firmly interconnected mixtures , z. B. with building materials such as gravel.

Parameters

The methods of particle measurement technology and dispersity analysis can be used to investigate heaps .

• The geometry of the particles
• The roughness of the surfaces ( rheology )
• The proportion of particles per grain size is indicated by the grading curve. A steady grading curve is obtained with heaps with many different grain sizes, without certain grain sizes being missing. In by sedimentation sorted according to grain size material is used, the grain size network .
• The measure for the proportion of the void in the pile is called porosity . The voids between the contact points of the particles are called gussets .
• The true density is the density based on the volume without cavities, including the gross density on cavities.
• As an empirical parameter in powdery material, the specific surface area is taken as the ratio of material volume to internal surface area per unit weight
• The angle of repose is a measure of the internal friction in the pile : the larger the angle of repose, the stronger the material holds. It can be measured on the material cone . With cohesive and caked material, angles of repose> 90 ° are even possible.

Effects of the parameters

Mixing different piles (such as sand and coarse-grained gravel ) creates piles with new characteristics and correspondingly different uses.

Steady grading curves are desirable in certain applications because the mixture then contains few voids. In concrete production , for example, higher strength is achieved if the grading curve of the pile used is constant.

Other applications avoid the constant grading curve in the pile used in order to achieve a certain porosity and pore size. Internal porosity are approximately water-permeable concrete paving desired, which will allow the rapid infiltration of rainwater ( eco-paving , Sickerpflaster , drain pavement ). A grain size of around 5 mm is predominantly used here, with shares of fine sand with a diameter of 0–1 mm. The grading curve therefore has a characteristic gap between 1 and 5 mm. Such a product is then also referred to as pore -like. It is used where rainwater is to be fed into the groundwater, instead of first running off the surface and then running off through the sewer system .

Modeling

• The simplest heaps can be modeled mathematically using spherical packings .
• From a topological point of view, the pile forms a body that is compact overall, but whose pores are both (at least idealized) everywhere and continuous; it is a sponge , not foam.
• Self -similar structures are created in particular with continuous or partially continuous grading curves and sorted grain sizes . Then the important parameters correlate with the fractal dimension , which is less than 3 in volume and less than 2 in each cross section.

The mechanical behavior of non-round particles becomes much more complex. An extreme example is the haystack , the stalks of which already have interesting properties. A transition from the pile to the stack and its mechanical properties ( stacking mechanics ) takes place here.

When heaps of heaps start to move, they behave like fluids and can be described by fluid dynamic formulas.

Examples

• In physics one speaks of granular matter , in physical chemistry of the disperse phase of a material system
• Loose and dry debris is referred to as bulk material in logistics
• From a geological and mining point of view , embankments are called heaps , from a petrological point of view, loose rock
• Sediment is mostly loose rock that formed on the earth's surface through erosion or certain other processes and is deposited in heaps.
• In hydrology, the heaps of piles on the bed and bank, as well as in the current itself, are summarized under the term bed load

literature

• Alex Wulf: Krümelkunde - Common presentation of the Krümelwesen and related areas. Script, July 1, 2016, (online , PDF; 15 MB).