A Crying Boy. 1955 Photo. N G Nair Kerala travel 2010.
TOO MUCH BURDEN
If the distance to reasonable free faces is too great (excessive burdens), much of the explosion energy will be dissipated in excessive crushing of the rock immediately around the blast hole and much more energy will be released in the form of vibration. Because of the lack of moment the extension of cracks by explosion gases cannot occur, and therefore, fragmentation and muck pile looseness suffer.
TOO LITTLE BURDEN
In sufficient burden causes high pressure explosion gases to vent prematurely, resulting in air blast and fly rock
Optimum free face blasting requires that the distances to free faces (which may include the top of the bench) are sufficient to contain the explosion gases but are short enough for shock induces cracking and spalling to occur. This forces explosion gases to do useful work fragmenting rock. The nature of the rock mass dictates correct blast placement. Even within the same area of the mine, changing rock charteries can affect the ideal blast geometry.
In buffer blasting, broken rock from the previous blast lies against the solid rock face. The effective burden is therefore larger than the drilled burden. This increased burden (as soon by the blast hole charge), reduces the moment of blasted rock and some rock breakage processes cannot occur. As a result, fragmentation may not e significantly affected, but rock looseness and diggability are reduced. A higher powder factor is often needed in order to maintain productivity.
Effects of Rock Properties. The most important rock mass properties which affect blasting include:rock strength, and intact rock properties;
rock mass structure (joints and/or bedding planes) and
rock mass variability.
Rock strength. In-situ rock samples can be collected, examined and tested to quality the physical characterizes which have an important effect on blasting results.
Compressive/Tensile Strength. Dynamic compressive strength of rocks are very much greater than the dynamic tensile strengths, to tensile breakage accounts for the greater part of the new fracture surfaces created in a blast.
Dynamic Compressive Strength
The dynamic compressive strength of a rock may be up to 10 times its static compressive strength. Where the dynamic compressive strength of rock is low, a greater proportion of explosion energy is immediately around the blast hole. In rock with a high dynamic compressive strength, blast hole crushing is minimal (several millimeters) and a greater proportion of the explosive’s strain energy is utilized in generating crack networks.
Dynamic Tensile Strength
Value of tensile strength differs widely with the rock type.
Elasticity and Toughness
Some rocks are able to sustain considerable pressure and deformation before they crack and break. The elasticity of rock is comely described in terms of the Young’s Modulus (E). This is the ratio of stress or pressure applied relative to the amount of strain of deformation. Rocks with a low Young’s modulus deform more before they fall, absorbing more energy.
Correlation between burden and blast-induced vibrations in open-pit mines. For this purpose, two different mines were studied. In these mines, the vibrations caused by explosions at burdens having widths ranging from 3 to 14 m were measured from various distances. From the results, it was found for these cases that burden width has a significant impact on vibrations. Consequently, it was proven that vibrations decrease as burden increases.