Free face

It will be seen that the reflection of the shock wave from a free face is of considerable importance in the use of explosives. In practice, the most economic use of an explosive can only be achieved if it is flred at a suitable distance from such a free face. This distance is known as the burden of the shot. If, as in tunnelling, no free face exists naturally, the first shots fired are heavily loaded and arranged so as to produce such a free face for the later charges. 

The boreholes may be drilled in many patterns, differing essentially in the arrangement by which the first section of rock is removed and a free face developed. A method which is applicable under most circumstances is known as the wedge cut and this is illustrated in Fig. 14.3 for a mediumsized tunnel. In Fig. 14.3 the boreholes are shown in elevation and section the figures in brackets refer to the period of delay of the detonator used in each individual hole. 

Around the wedge are drilled a number of holes known as easers, arranged in approximately concentric rings. The detonators used in these holes have delays which gradually increase in number as the hole is farther from the wedge. This is to ensure that the inner holes nearest the free face always fire first and can produce a fresh free face against which the farther shots can work. Finally, an outer row of holes, known as trimmers, are drilled close to the intended outline of the tunnel. The trimmers have the longest delays, particularly at the top of the tunnel. 

The sequence which occurs on firing is that the rock in the wedge is first blown out and then the hole thus produced is expanded outwards as each successive ring of charges detonates. Apart from the first six holes, which in any case are angled suitably, the explosives in the other boreholes all work towards a free face. 

Burden. In blasting, the shortest lateral distance between a borehole and a free face. 

Free face. In blasting, a face of rock or coal approximately parallel to the line of boreholes (see p. 138). 

Solid coal. Implies coal which is being worked without the provision of a free face by undercutting or similar means. 

Spalling. In explosives technology implies the breaking off of a scab of material from a free face as a result of the reflection of shock waves (see p. 135). 

Undercutting. In coal mining, the production of a free face by cutting out mechanically the lowest 10 to 12 cm of a seam (see p. 142). 

Wedge cut. A method of tunnelling, etc., in which a free face is first produced by blowing out a wedge of rock. 

The pentatungstate can be seen as a fragment of the hexatungstate and is obtained by removal of one octahedron (162). It consists of five W06 octahedra, two of which have free faces. According to calcula-... 

The vacuum applies the sheet onto the laminate, which creates a compression of the composite and improves the aesthetics of the free face. 

A semi-infinite explosive slab of thickness h, -bounded on both free faces by vacuum (or air), is initiated instantaneously at one free face. We wish to compute the impulse delivered at the other free face. From the polytropic equation of state assumption it follows that ... 

The writer has measured detonation velocities and impulses delivered for four different detonating gas mixtures. His results (quoted in Ref 5) are used for the comparison of experimental and theoretical ID s shown in Table 2. Each of the slabs of detonating gas, of necessity, had a thin plastic sheet on its free face. Both slab thickness and plastic sheet thickness were varied... 

The strain measurement test set-up consisted of linear array of shat and gage holes, as shown in Fig Ex 15. The holes were drilled in solid rock about 25 ft deep and at a distance from any free face, so that charges detonated at the bottom of the shot... 

Flyrock may originate from bench tops and not the free face and... 

To sum up, the Gurney approach provides reliable max flyrock ranges for flyrock from free faces. Although a similar treatment can be used to estimate max flyrock ranges from bench tops (see Ref 25), these estimates are less reliable because of uncertainties in computing c/m for these systems... 

Fig 2 Typical values of peak particle velocity as a function of scaled distance for blasting to a free face. Charge weight is per-delay-period of eight msec or more. Blasts made under tight confinement may give values five times more than typical under normal confinement (from Ref 4)... 

Theoretically, to fragment the block of oil shale by detonating wellbore charges of pelletized TNT, the area around center well 5 should be enlarged or sprung. This would be accomplished by repeated wellbore shots from bottom to top of the test zone. The broken and enlarged area surrounding the wellbore would serve as a free face to enhance effects from later simultaneous wellbore shots across the pattern. 

The distance between boreholes measured parallel to the free face toward which rock is expected to move. 

The perpendicular distance from blast hole to the free face measured at the floor elevation of the quarry pit. 

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