Using the university’s unique Explosives Arena, a team led by senior geotechnical engineering lecturer Dr Sam Clarke carried out around 250 test explosions using different soil samples. While much of the previous work in this area has focused on desert conditions – often considered to be a worst-case scenario in terms of pressure loading and fragmentation – this new study looked primarily at clay soil.
“The AEP-55 standard [used in most investigations] mandates the use of sandy gravel in soil based tests. That’s a particularly severe case for dismounted troops and local perforations of thin-walled armour, but what we’ve come to understand is that it might not be the worst total impulse scenario,” said Dr Clarke. “As you increase moisture content in the soil, the pressure distribution becomes more like a discrete wave, maintaining a higher pressure as it propagates outwards; drier soils tend to result in a more uniform temporal loading that decays more rapidly.”
The tests used a series of 17 Hopkinson pressure bars arranged on a flat steel plate 150mm above the surface of a clay test pit. Experimental data from the blasts was backed up by a numerical model developed and applied as part of a study supported by the Engineering and Physical Sciences Research Council (EPSRC). This will be fed into a wider investigation into the effects of IEDs and landmines on armoured vehicles carried out by the Defence Science and Technology Laboratory (DSTL).
“Through better understanding of how these devices behave with the soil we hope to be able to improve protection against them without increasing weight or cost,” said Dr Clarke. “It becomes increasingly relevant if you consider new materials. Hard targets such as steel may be less sensitive to the distribution of the loading, because you know they’re not going to perforate, so it becomes a total impulse problem. But if you’re dealing with a lightweight composite structure where localised failure might be an issue then you really do need to know the discretisation of the pressure over the structure as a whole.”