THE INFLUENCE OF SOIL CONDITIONS ON THE BLAST INTENSITY FOR SAND, CLAYEY SAND AND GRAVEL WITH SILT

Soil conditions, as defined by soil type, particle dispersion, moisture content, degree of saturation and other parameters have a crucial influence on the blast intensity of a buried explosive charge. None of the various existing standards presents an unambiguous, uniform definition of soil conditions. Soil moisture content, in particular, was found to have a significant effect on the total impulse inflicted on a structure by a buried explosive charge (Fiserova 2006, Hlady 2004, RTO-TR-HFM-089 2004). A considerable volume of the experiments and research conducted in institutions concerning the investigation of the effects of these parameters has been done on small explosive devices of 25-100 grams (Hlady 2004, Bergeron, Walker and Coffey 1998, Foedinger 2005). Such small scale experiments do not accurately reflect the effects of the relevant parameters on larger, full-scale explosive devices. In light of this situation, Plasan's research is being carried out on explosive devices with a minimum mass of 2 kg in soil representing standard experimental conditions. The experimental study was conducted on three different soils - sand, clayey sand, and graded gravel with silt. The experiments included the use of a Mine Impulse Pendulum situated above a pre-prepared soil pit. In order to characterize the boundaries of the study, additional tests were conducted by setting the charge in a steel pot (AEP-55, Vol 2 2006) and by substituting the soil with water. Both standard laboratory tests and in situ measurements were performed in order to classify the soils and determine their physical and mechanical properties. A complete cover age of the soil-moisture content spectrum was not achieved; however, sufficient data allows outlining the tendency of each soil, thereby, creating an overlay of total impulse with respect to soil saturation ratio. The experiments proved that the degree of soil saturation has a critical effect on the blast intensity. They demonstrated that soils of low density with a high degree of saturation produce higher blast intensity than denser soils with a lower degree of saturation. For example: the blast intensity of a charge in clayey sand with a saturation ratio of 60% and bulk density of 1700kg/m3 is 50% higher than the blast intensity produced by soil which answers NATO's standard (graded gravel with silt), with bulk density of 2000kg/m3 and saturation ratio of 40%. In the paper we shall describe the experiments and the quantitative results obtained.