Debris throw

This chapter deals with this problem. Because for the QD approach only boundary values have been of interest, in many areas there is little reliable data for the development of suitable prediction models over a whole area. Information is particularly sparse about debris throw, especially from underground installations in rock with insufficient rock overburden leading to a crater in case of an explosion. 

In Section 26.2, this accidental explosion is described in detail, followed by a description of the comprehensive evaluation of the rock-debris throw originating from the crater produced. Section 26.3 shows how these data, together with other sources, were used to enhance the current knowledge in this field and to develop a new prediction model for crater debris [3] to be used in the Manual of NATO Safety Principles and the Swiss regulation TLM 75 [4]. 

As a basis for evaluating the physical effects of the explosion, the damage pattern was recorded by topographical maps, terrain sections, and aerial as weU as terrestrial photos. Detailed documentation was elaborated for 53 pieces of large single debris and 40 debris collection fields [8]. The data from these debris fields were used for evaluating the debris throw from the crater, which is described in detail here. 

For the evaluation of the debris throw from the crater, only debris fields nos. 81 to 95 and 109 to 116, (a total of 23) were used. Together with the fact that the maximum crater debris throw distance was on the order of 600 to 700 m, the graph in Fig. 26.14 was developed. This shows the debris mass density in relation to the distance from the center of the crater. Although the data scattering was not as small as one would have liked it, this debris mass density versus distance curve represented the physical facts reasonably well. Based on this curve, the debris mass density contour lines in Fig. 26.15 could be drawn. 

This curve does not yet show an angular dependency of the crater debris throw. Although some of the debris fields sideways to the axis showed somewhat smaller debris mass densities, it was decided at that time, because the number of data points was comparatively small, to draw a single curve through the data points as in Fig. 26.14. A more detailed investigation of the data points during the development of a new crater debris throw model showed, however, that there is a distinct dependency of the density of crater debris on the angle of the slope of the overburden in the area where the crater is formed. This effect is discussed in Section 26.3. 

On the technical level, it could be shown, based on a realistic case, that the widely used NATO safety criteria for debris throw might be too conservative, as already suspected by many experts. Furthermore, it is shown that the approach presented above gives more plausible results, which, together with a risk-based approach and respective safety criteria, would allow a more economic use of such installations. 

Modeling of debris throw from craters produced by explosions in underground storage installations is one of the most complex problems to be dealt with when analyzing the effects... 

FIGURE 26.21 Parameters influencing debris throw from craters. 

Discussions and data showed that in addition to these factors, the rock quality (which influences the size of the crater) and the venting characteristics of the storage chamber (which influence the total energy available for the throw-out of the crater material) play some role. For several reasons, however, it was decided to develop a model valid only for strong (good) rock and unvented chambers as a first step. Since venting of the chamber reduces the pressure driving the debris throw, this is a conservative assumption. 

One explanation for this might be that the observed maximum debris launch velocities documented in many reports are not really the maximum velocities but only the maximum velocities of the debris throw front, and that part of the debris behind the front have a higher velocity due to a longer acceleration by the escaping gases of the explosion. A second weak point of the trajectory calculation approach is that it does not take into account the total amount of material that is thrown out of a crater. A simple example calculation using this two-part procedure shows this important fact. 

It can easily be understood that despite the debris throw distance being the same for both explosions, the debris density is not the same, as the total amount of debris displaced to the surroundings is much greater for the 100 t explosion. 

Tests. Until now, only a few tests have been performed concerning debris throw from crater-forming mechanisms, some of them being only small-scale tests. Small-scale tests for debris throw are subject to problems of scaling laws (e.g., gravity effects) and the difficulty of simulating real rock material with its joints, cracks, and fissures. 

Raufoss trials (1968) A series of tests (scale approximately 1 3 to 1 4), the largest with 5400 kg of explosives producing a crater, was performed in Norway in 1968. Only a summary report with very sparse information concerning crater debris throw was available for this study. The few data reported are documented in Ref. 30. The crater area lay in a wooded area where it would have been difficult to collect all the debris, especially as the ground was covered with snow at that time. Furthermore, a substantial amount of debris probably would have been stopped by the trees. 

UusikylH (1965) This accident, also involving up to several hundred tons of munitions, is not documented very well, at least not regarding the debris throw to the surroundings. For the available information, see Refs. 42 and 43. 

Other accidents For other accidents in underground storage installations, such as Prtim, Germany (1949) [44], Mitholz, Switzerland (1947) [45], and Waikalua, Hawaii (1946), either no crater or no typical crater was produced or the available information about crater debris throw was not sufficient to be used for developing a debris throw model. 

Data Used for Calibration. In general, the most emphasis was laid on the China Lake test data because this test was specially designed for studying the effects of debris throw and the geometry of the installation was typical for an underground ammunition storage. 

The General Model. Based on the main factors influencing debris throw from craters, the following empirical formula was proposed for calculating the distance for a given debris density D ... 

For hard and moderately strong rock, it is assumed, based on the above-mentioned references, that for a scaled cover depth above 1.2 no cratering occurs that produces relevant debris throw into the surroundings. This does not imply, however, that there will be no explosion effects at all at the surface. Loose rock may be displaced, and spalling and a... 

The Loading Density Parameter. The influence of the chamber loading density y on the debris throw distance was taken into account by means of a nondimensional decoupling factor called loading density parameter/ (Fig. 26.25). 

FIGURE 26.27 Debris throw increase and decrease factors. (Top line = 10 m/s bottom line = 200 m/s)... 

Comparison with Accident and Test Data. To show the accuracy and applicability of the new approach, data from accidents and tests were compared with the new and existing NATO debris throw model eonceming hazardous distances in Table 26.3. Table 26.2 shows the main data of the installations used for the comparison. The new model seems to be quite accurate and fits the real data much better than the current one in AASTP-1. 

Thus, the crater debris throw model presented here may be used within the following boundaries only ... 

The debris throw model presented here shall not be used to calculate maximum missile or debris ranges. 

The newly developed crater debris throw model presented here was reviewed by the NATO AC/258 experts as well as by U.S. Waterways Experiment Station (WES), Vicksburg, Mississippi, and Swiss experts in the explosives safety field. It will be included in the revised NATO AASTP-1 safety manual as well as in the Swiss regulation for the storage of ammunition, TLM 75 [4, 11]. 

Evaluation of the Debris Throw from the 1992 Explosion in the Steingletscher Installation in Switzerland... 

Debris Throw from Undergroimd Ammunition Magazine in Granite Model Test... 

Debris Throw frmn Cratei-s AC/258 Undetgroimd Storage AHWP CH Status Report as at Marcli 6, 1998... 

AC/258 Underground Storage AHWP Debris Throw from Craters... 

---