Nuffield Research Placement
Ben Pace’s Report
From the University of Sheffield, Department of Civil
Engineering, Computational Engineering Research
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
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
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.
From Clarke et al. 2013
From Clarke et al. 2013
Some examples of phase relationships used later in this report:
o The ratio of water, to the weight of soil.
o Expressed as a percentage.
o Water Content = MW / MS
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
Figure 6 has a green line
representing the result
of subtracting the red
line from the black. A
has been fitted to the
data, represented by the
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
content has little
effect on the
spread of impulse.
If the very high
LBFa result is
, all of the
other results lie
Figure 8 shows the
same data, with
Bulk Density along
Furthermore, in our
data set, no other
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
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.
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
From Netherton and Stewart (2014)
Peak Pressure Variation from From Rigby et
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.
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.
Core Principles of Soil Mechanics – by Sanjay Kumar Shukla
Matlab 7 – by Rudra Pratap
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.