岩石爆破危害.docx

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1、Blasting damage in rockThe development of rock mechanics as a practical engineering tool in both underground and surface mining has followed a rather erratic path. Only the most naively optimistic amongst us would claim that the end of the road has been reached and that the subject has matured into

2、a fully developed applied science. On the other hand, there have been some real advances which only the most cynical would discount.One of the results of the erratic evolutionary path has been the emergence of different rates of advance of different branches of the subject of rock mechanics. Leading

3、 the field are subjects such as the mechanics of slope instability, the monitoring of movement in surface and underground excavations and the analysis of induced stresses around underground excavations. Trailing the field are subjects such as the rational design of tunnel support, the movement of gr

4、oundwater through jointed rock masses and the measurement of in situ stresses. Bringing up the rear are those areas of application where rock mechanics has to interact with other disciplines and one of these areas involves the influence of blasting upon the stability of rock excavations.Historical p

5、erspectiveBy far the most common technique of rock excavation is that of drilling and blasting.From the earliest days of blasting with blac powder, there have been steady developments in Our understanding of the mechanics of rock breakage by explosives.It is not the development in blasting technolog

6、y that is of interest in this discussion. 1t is the application of this technology to the creation of excavations in rock and the influence of the excavation techniques upon the stability of the remaining rock.As is frequently the case in engineering, subjects that develop as separate disciplines te

7、nd to develop in isolation. Hence, a handful of highly skilled and dedicated researchers, frequently working in association with explosives manufacturers, have developed techniques for producing optimum fragmentation and minimising damage in blasts. At the other end of the spectrum are miners who ha

8、ve learned their blasting skills by traditional apprenticeship methods, and who are either not familiar with the specialist blasting control techniques or are not convinced that the results obtained from the use of these techniques justify the effort and expense. At fault in this system are owners a

9、nd managers who are more concerned with cost than with safety and design or planning engineers who see both sides but are not prepared to get involved because they view blasting as a black art with the added threat of severe legal penalties for errors.The need to change the present system is not wid

10、ely recognised because the impact of blasting damage upon the stability of structures in rock is not widely recognised or understood It is the authors aim, in the remainder of this chapter, to explore this subject and to identify the causes of blast damage and to suggest possible improvements in the

11、 system.A discussion on the influence of excavation processes upon the stability of rock structures would not be complete without a discussion on machine excavation. The ultimate in excavation techniques, which (cave the rock as undisturbed as possible, is the full-face tunnelling machine. Partial f

12、ace machines or readheaders, when used correctly, will also inflict very little damage on the rock. The characteristics of tunnelling machines will not be discussed here but comparisons will be drawn between the amount of damage caused by these machines and by blasting.Blasting damageIt appears to m

13、e, a casual reader of theoretical papers on blasting, that the precise nature of the mechanism of rock fragmentation as a result of detonation of an explosive charge is not fully understood. However, from a practical point of view, it seems reasonable to accept that both the dynamic stresses induced

14、 by the detonation and the expanding gases produced by the explosion play important roles in the fragmentation process.Duvall and Fogelson (1962), Langefors and 1Chilstmm (1973) and others, have published blast damage criteria for buildings and other surface structures. Almost all of these criteria

15、relare blast damage to peak particle velocity resulting from the dynamic stresses induced by the explosion. While it is generally recognised that gas pressure assists in the rock fragmentation process, there has been little attempt to quantify this damage.Work on the strength of jointed rock masses

16、suggests that this strength is influenced by the degree of interlocking between individual rock blocks separated by discontinuities such as bedding planes and joints. For all practical purposes, the tensile strength of these discontinuities can be taken as uro, and a small amount of opening or shear

17、 displacement will result in a dramatic drop in the interlocking of the individual blocks. It is easy to visualise how the high pressure gases expanding outwards from an explosion will jet into these discontinuities and cause a breakdown of this important block interlocking. Obviously, the amount of

18、 damage or strength reduction will vary with distance from the explosive charge, and also with the in situ stresses which have to be overcome by the high pressure gases before loosening of the rock can take place. Consequently, the extent of the gas pressure induced damage can be expected to decreas

19、e with depth below surface, and surface structures such as slopes will be very susceptible to gas pressure induced blast damage.An additional cause of blast damage is that of fracturing induced by release of load (Hagan, 1982). This mechanism is best explained by the analogy of dropping a heavy stee

20、l plate onto a pile of rubber mats. These rubber mats are compressed until the momentum of the falling steel plate has been exhausted. The highly compressed rubber mats then accclcrate the plate in the opposite direction and, in ejecting it vertically upwards, separate from each other. Such separati

21、on between adjacent layers explains the tension fractures frequently observed in open pit and strip mine operations where poor blasting practices encourage pit wall instability. McIntyre and Hagan (1976) report vertical cracks parallel m and up to 55 m behind newly created open pit mine faces where

22、large muld-row blasts have been used.Whether or not one agrees with the postulated mechanism of release of load fracturing, the fact that cracks can be induced at very considerable distance from the point of detonation of an explosive must be a cause for serious concern. Obviously, these fractures,

23、whatever their cause, will have a major disruptive effect upon the integrity of the rock mass and this, in turn, will cause a reduction in overall stability.Hoek (1975) has argued that blasting will not induce deep seated instability in large open pit mine slopes. This is because the failure surface

24、 can be several hundred metres below the surface in a very large slope, and also because this failure surface will generally not be aligned in the same direction as blast induced fractures. Hence, unless a slope is already very close to the point of failure, and the blast is simply the last straw, t

25、hat breaks the camels back, blasting will not generally induce major deep- seated instability.On the other hand, near surface damage to the rock mass can seriously reduce the stability of the individual benches which make up the slope and which carry the haul roads. Consequently, in a badly blasted

26、slope, the overall slope may be reasonably stable, but the face may resemble a rubble pile.In a tunnel or other large underground excavation, the problem is rather different. The stability of the underground structure is very much dependent upon the integrity of the rock immediately surrounding the

27、excavation. In particular, the tendency for roof falls is directly related to the interlocking of the immediate roof strata. Since blast damage can easily extend several metres into the rock which has been poorly blasted, the halo of loosened rock can give rise to serious instability problems in the

28、 rock surrounding the underground openings.Damage controlThe ultimate in damage control is machine excavation. Anyone who has visited an underground metal mine and looked up a bored raise will have been impressed by the lack of disturbance to the rock and the stability of the excavation. Even when t

29、he stresses in the rock surrounding the raise are high enough to induce fracturing in the walls, the damage is usually limited to less than half a metre in depth, and the overall stability of the raise is seldom jeopardised.Full-face and madheader type tunnelling machines are becoming more and more

30、common,particularly for civil engineering tunnelling.These machines have been developed to the point where advance rates and overall costs are generally comparable or better than best drill and blast excavatiol methods.Tlle Iack of disturbance to the rock and the decrease in the amount of support re

31、quired are major advantages in the use of tunnelling machines.For surface excavations, there are a few cases in which machine excavation can be used to great advantage. In the Bougainville open pit copper mine in Papua New Guinea, trials were carried out on dozcr cutting of the final pit wall faces.

32、 The final blastholes were placed about 19 m from the ultimate bench crest position. The remaining rock was then ripped using a D-10 dozer, and the final 55 degree face was trimmed with the dozer blade. The rock is a very heavily jointed andesite, and the results of the dozcr cutting were remarkable

33、 when compared with the bench faces created by the normal open pit blastingtechniques.The machine excavation techniques described above are not widely applicable in underground mining situations, and consideration must therefore be given to what can be done about controlling damage in normal drill a

34、nd blast operations.A common misconception is that the only step required to control blasting damage is to introduce pre-splitting or smooth blasting techniques. These blasting methods, which involve the simultaneous detonation of a row of closely spaced, lightly charged holes, are designed to creat

35、e a clean separation surface between the rock to be blasted and the rock which is to remain. When correctly performed, these blasts can produce very clean faces with a minimum of overbreak and disturbance. However, controlling blasting damage starts long before the introduction of pre-splitting or s

36、mooth blasting.As pointed out earlier, a poorly designed blast can induce cracks several metres behind the last row of blastholcs. Clearly, if such damage has already been inflicted on the rock, it is far too late to attempt to remedy the situation by using smooth blasting to trim the last few metre

37、s of excavation. On the other hand, if the entire blast has been correctly designed and executed, smooth blasting can be very beneficial in trimming the final excavation face.Figure 1 illustrates a comparison between the results achieved by a normal blast and a face created by presplit blasting in j

38、ointed gneiss. It is evident that, in spite of the fairly large geological stmctures visible in the face, a good clean face has been achieved by the pre-split. It is also not difficult to imagine that the pre-split face is more stable than the section which has been blasted without special attention

39、 to the final wall condition.The correct design of a blast starts with the very first hole to be detonated. In the case of a tunnel blast, the first requirement is to create a void into which rock broken by the blast can expand. This is generally achieved by a wedge or burn cut which is designed to

40、create a clean void and to eject the rock originally contained in this void clear of the tunnel face. Figure 1: Comparison between the results achieved by pre-split blasting (on theleft) and normal bulk blasting for a surface excavation in gneissIn todays drill and blast tunnelling in which multi-bo

41、om drilling machines are used, the most convenient method for creating the initial void is the burn cut. This involves drilling a pattern of carefully spaced parallel holes which are then charged with powerful explosive and detonated sequentially using millisecond delays. A detailed discussion on th

42、e design of bum cuts is given by Hagan (1980).Once a void has been created for the full length of the intended blast depth or pull,the next step is to break the rock progressively into this void. This is generally achieved by sequentially detonating carefully spaced parallel holes, using one-half se

43、cond delays. The purpose of using such long delays is to ensure that the rock broken by each successive blasthole has sufficient time to detach from the surrounding rock and to be ejected into the tunnel, leaving the necessary void into which the next blast will break.A final step is to use a smooth

44、 blast in which lightly charged perimeter holes are detonated simultaneously in order to peel off the remaining half to one metre of rock,leaving a clean excavation surface.The details of such a tunnel blast are given in Figure 2. The development of the burn cut is illustrated in Figure 3 and the se

45、quence of detonation and fracture of the remainder of the blast is shown in Figure 4. The results achieved are illustrated in a photograph reproduced in Figure 5. In this particular project, a significant reduction in the amount of support installed in the tunnel was achieved as a result of the impl

46、ementation of the blasting design shown in Figure 2.HolesnoDiammExplosiveTotal wl kgDtorsetonaBurn1445Gelamex 80,18 sticks/hole57MillisecLifters945Gelamex 80,16 sticks/hole33Half-secPerimeter2645Gurit,7 sticks/hole and Gelamex 80,13 sticks/hole26Half-secOther4445Gelamex 80,13 sticks/hole130Half-secR

47、elief375No chargeToal96246 Figure 2: Blasthole pattern and charge details used by Balfour Beatty一Nuttall on the Victoria hydroelectric project in Roman numerals refer to the detonation sequence of while Arabic numerals refer to the half-second delays in the remainder of the blast. Layout of holes Milliacond delay V1 Millisecond delay 0 Milliacond delay 2 Millisecond dely 3 Millisecond dely 4 Millisecond delay1 Half-second delay5 Half-second delay2 Half-second dealy6 Half-second dealy3 Half-second dealy7 Half-second dealy4 Half-second dealy8 Half-second dealy5