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Nuffield Research Report


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Nuffield Research Report

  1. 1. Nuffield Research Placement Ben Pace’s Report From the University of Sheffield, Department of Civil Engineering, Computational Engineering Research Group September 2014 Introduction Dr Clarke and his team at the Sheffield University Engineering Department were researching the effect that soil type had on the impulse, and the variation of impulse, given from a shallow buried explosive. To be able to help infer meaning from the large quantity of experimental data, I was required to learn the basics of Soil Mechanics from an Undergraduate textbook, learn how to program in Matlab, and read through three research papers that my supervisor, Dr Clarke, and his colleagues had published. After being given the raw data, I organised it and wrote a program to produce graphs illustrating the relationships uncovered. These discoveries shall later be used in formal papers and offered to leading Academic Journals for publication.
  2. 2. Abstract This report details an experimental setup to discover the relationship between soil type and the spread of explosive impulse from a shallow buried charge. No relationship was discovered between bulk density and impulse spread, or between moisture content and impulse spread. A relationship was discovered between the spread of particle size within a soil, and the spread of impulse, whereby a soil which was more uniform in spread of particle size had much lower spread of impulse.
  3. 3. Theory, Experiment, Findings This, the largest section of the report, will describe the experiments done, detail some of the relationships discovered, and discusses their implications for this research area. 1. Geotechnical Theory This section will start with the basics of Soil Mechanics. ‘Soil’ is the term for the looser materials contained within the Earth’s crust, and typically consists of the following three ‘phases’: 1. Solid e.g. mineral particles 2. Liquid e.g. water 3. Gas e.g. air, water vapour Further information:  The space in a soil mass occupied by liquid and/or gas is called ‘void.  A soil is o Dry, if the void is filled with gas o Saturated, if the void is filled with liquid o Partially Saturated, if the void contains both liquid and gas  The weight of the gas phase is assumed to equal zero.  A ‘soil phase diagram’ shows the phases separately, with their weights and volumes labelled. Figure 1 From Clarke et al. 2013 Figure 2 From Clarke et al. 2013
  4. 4. Some examples of phase relationships used later in this report:  Water Content o The ratio of water, to the weight of soil. o Expressed as a percentage. o Water Content = MW / MS  Bulk Density o The weight of soil per unit o Bulk Density = M / V 2. Experimental Setup1 Figure 3 shows a large steel frame supporting a hollow, cylindrical mass of 1,500 kg (here-on referred to as the reaction mass). This mass can move freely upwards by a distance of 0.8m, at which point it is stopped by a steel plate. Explosive charges beneath the reaction mass are set off, and by observing how quickly the mass rises, and the maximum height reached (in the cases where it doesn’t collide with the steel plate at the top), a measurement of explosive impulse is made. Another plate was attached to the bottom of the cylindrical mass, and the deformation of this plate as a result of the explosion was another measurement of impulse. The explosive charges were contained within a wide steel bin sat beneath the cylinder, and were buried in soil (Fig. 4). Two high-speed cameras recorded the explosion, and from the recordings the velocity and peak height of the cylinder is recorded. This data, combined with a recording of the initial velocity and mass of the reaction cylinder, allows impulse to be derived. Impulse = Change in Mass*Velocity Figure 3 – Test Rig (without soil bin) From Clarke et al. 2013 Figure 4 – Steel Bin (containing explosive charge, submerged in soil) From Clarke et al. 2013
  5. 5. Figure 5 shows the displacement of the reaction mass plotted against time, as a black line. The red line shows the displacement of a fixed point on the frame, recorded by the second camera. It can be seen that the cameras were displaced by the explosion. The cameras were fixed to a common frame, meaning that they were displaced identically, and so subtracting the red line from the black line gives the true displacement of the reaction mass. Figure 6 has a green line representing the result of subtracting the red line from the black. A fourth-order polynomial has been fitted to the data, represented by the blue-dot line. Figure 5 Figure 6
  6. 6. 3. Findings Within the literature on blast experiments, there is often a large variance (e.g. ±15%) in the recorded data. For example, in Netherton and Stewart (2013), which measured the pressure and impulse resulting from a bare explosive detonation, they experienced percentage spread in the range of 50- 130%, and this was in a situation without soil (which would have otherwise provided further capability for variance). Whilst this error is normally explained as inherent in the nature of explosions, Dr Clarke and colleagues suggest that this error can be minimised, especially through proper control of soil conditions – Clarke, Warren & Tyas (2011) ran blast experiments with errors of ±3%, providing strong evidence that the large variance is not necessary. Appendix A compares Netherton and Stewart’s data with a more similar experimental data set from Rigby et al. (2014). The following graphs plot for a variety of different soils. It should be noted that the plot for ‘Minepot’ records the results of a pure metal container, and is meant to serve as a standard for the charge’s explosive power. Figure 7 suggests that moisture content has little effect on the spread of impulse. If the very high LBFa result is considered an outlier2 , all of the other results lie within ±6.5%. Figure 8 shows the same data, with Bulk Density along the x-axis. Furthermore, in our data set, no other variable recorded showed significant correlation with percentage spread of impulse. Figure 7 Soil Types Soil Types Figure 8
  7. 7. Another integral part of soil mechanics is a discussion of grain size. The individual particles in a soil can, depending on the type of soil and how it was formed, range from less than 2 µm (0.002mm) to over 300mm. It was hypothesised that if a soil had a wider range of grain sizes, this would allow for more possibilities in the arrangement of those particles and also more variability in data acquired using that soil. Figure 9 shows the range of grain sizes the soils in the given data set. The green line representing LB has a high gradient, showing that the majority of the grains were all of a very similar size. The red line representing LBF, however, stretches across a wide range of grain sizes, showing a lot more variance. The term ‘well-graded’ refers to a soil which has a range of different-size particles, such as LBF, and the term ‘uniform’ refers to a soil whose particle-size lies within a small bound, such as LB. Figure 10 shows the same data as on the previous page, but has sorted the soils into the two categories ‘well- graded’ and ‘uniform’. It can be seen that almost all of the uniformly graded soils have variance of less than ±2% whilst the well-graded soils reach ±6%. This is strong confirmatory evidence for the hypothesis. Figure 9 Figure 10
  8. 8. Conclusion It was shown that bulk density and moisture content of soil does not have a significant effect on percentage variance in the explosive impulse of a buried charge. It was further shown that minimising particle size variance within a soil significantly decreases percentage error in impulse. Further research in this area could include looking to see whether plate deformation is at all correlated with impulse variation. This would show that impulse rather than peak pressure is the driving factor in plate deformations. Appendix Appendix A Figure A1 shows a series of experiments in Netherton and Stewart (2014). Each vertical grouping represents an identical setup. The data has over- predictions of up to 50%. Figure A2 shows a highly similar experiment, with a significantly narrower spread of recorded pressure. All the data lies within ±8%. Figure A1 From Netherton and Stewart (2014) Figure A2 Peak Pressure Variation from From Rigby et al. (2014)
  9. 9. Notes 1 - A more detailed account of the setup can be found in (Clarke et al. 2014). 2 – The soil type LBFa in fact has a very precarious balance of moisture and bulk density, making it highly sensitive to external forces, and thus an incredibly unreliable soil. References Clarke, S D, Warren, J A & Tyas, A., 2011, The influence of soil density and moisture content on the im-pulse from shallow buried explosive charges. Proceedings of the International Symposium on Interaction of the Effects of Munitions with Structures, September 19-23, Seattle, US. Clarke, S D, Warren, J A, Fay, S D, Rigby, S E & Tyas, A., 2012, The role of geotechnical parameters on the impulse generated by buried charges. 22nd International symposium on the Military Aspects of Blast and Shock, November 5-9, Bourges, France. Clarke, S D, Warren, J A, Fay, S D, Rigby, S E & Tyas, A., 2014,Repeatability of Buried Charge Testing. 23rd International symposium on the Military Aspects of Blast and Shock, September 7-12, Oxford, UK. S. E. Rigby, A. Tyas, S. D. Fay, S. D. Clarke & J. A. Warren, Validation of Semi-Empirical Blast Pressure Predictions For Far Field Explosions – Is There Inherent Variability In Blast Wave Parameters? To be presented at: 6th International Conference on Protection of Structures against Hazards 16-17 October 2014, Tianjin, China M.D. Netherton and M.G. Stewart. The Variability of Blast-loads from Military Munitions and Exceedance Probability of Design Load Effects. In 15th International Symposium on the Interaction of the Effects of Munitions with Structures (ISIEMS), Potsdam, Germany, 2013. Bibliography Core Principles of Soil Mechanics – by Sanjay Kumar Shukla Matlab 7 – by Rudra Pratap
  10. 10. Acknowledgements The Nuffield Foundation’s generous scholarship has had a great impact on my view of academia, and also my future, and for this I am very grateful. My thanks go to Sam Clarke for his excellent direction and for the time he has kindly given. My thanks also go to Sam Rigby and Darren Lincoln for their invaluable guidance and advice in matters of programming, engineering, and lunch. Chris Smith’s conversation about the physics of sailing boats was also very stimulating.