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.

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.

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

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

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

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

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

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)

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

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.