Pillar collapse

The pillars are stable up to 400 °C. At higher temperatures, the pillars collapse in height, as indicated by the shifting of the (001) line to higher angles in the powder X-ray diffraction patterns. 

The mechanics of pillar collapses are very different from either coal bumps or the more common slow pillar failure. Collapses occur when the pillars are so thin that they cannot carry any weight once they fail. Pillar collapses can be prevented by increasing the stability factor of the pOar design, or contained by conducting high extraction in localized compartments that are protected by barrier pillars. 

As a rule of thumb, 50 to 55 percent of coal can be extracted using continuous mining. To improve this extraction ratio, a pillar-recovery process usually is applied when mining reaches the end of the panel and the direction of the mining is reversed. The continuous miner mines into the pillars, recovering as much coal as possible, as the roof is allowed to systematically collapse. Usually this can increase the extraction ratio by up to 5 percent. 

Most of the pillared structures are thermally stable up to about 500°C, and keep the specific surface area as large as 300-500 m /g. The bismuth [11] and the chromium oxides pillared clays collapse on heating to 300°C, the pillars being removed out of the interlayer spaces, although the chromium oxide with a larger basal spacing of 21 A is more thermally stable in a nitrogen atmosphere [10]. 

It has to be noted that the introduction of Li into the structure of the clay before pillaring and a calcination temperature lower than 3(X)°C increase the surface area of the solids. A calcination temperature higher than 5(X)°C gives amorphous solids. The Li clay structure collapses. In addition, these solids treated at 700°C present the same surface area as the Na montmorillonite. 

A small increase of the (d 001) basal spacing is observed for the Li containing Zr pillared clays. However, the thermal stability of these solids drastically decrease. At high temperature, the collapse of the strucutre is also supported by the decrease of the surface area which is, at 700°C, almost identical to those measured for the montmorillonite. Different hypothesis may be proposed to explain the increase of the interlayer distance at low temperature (i) a better polymerization of the intercalated complex (ii) a modification of the distribution of the pillars (iii) a lower interaction between the pillar and the silica layer. The first hypothesis may easily be eliminated since the small variation of the height of the pillars (less than 1 A) cannot be explained by structural changes of the... 

Collapse of previously meta-stable old bord-and-pillar workings... 

After the coal is mined, the miners retreat while removing coal comprising the pillars. This usually causes the collapse of rock strata above the seam, thus leaving an underground area, known as gob, which is not safe to enter. 

Related to guanidinium sulfonates are analogous cation phosphonate structures which can adopt either pillared or zeotype structures. Recently novel tubular morphologies have also been discovered for both classes of compound which may have promise for improving porosity in these types of material. At present, like the guanidinium sulfonates, few phosphonates, are really porous and removal of the guest template generally leads to the collapse of the structures.15... 

Deep (> 1.5 pm) PDMS channels were prone to collapse, either spontaneously (because of gravity) or during suction [1025]. In one report, support pillars were put within channels to avoid bowing of the deep PDMS channels (100 pm wide and 3 pm deep) during bonding to glass [985]. Furthermore, lateral collapse of PDMS channels was observed when they were not spaced sufficiently apart, as shown in Figure 2.29 [247]. 

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