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Protective measures must often be established during explosive ordnance disposal (EOD) operations to reduce the effects of unexploded ordnance (UXO). Adequate models for the design of protective constructions against the jet from shaped charge ammunition are lacking. Two conditions are unique for EOD operations on shaped charges: constructions built from sandbags and long standoff distances. For these conditions, verified test data are very limited.

To study these conditions, full-scale experiments have been conducted at the Swedish EOD and Demining Centre (SWEDEC) with the objective to generate data for the development of a useful tool for EOD operations. Three series of five shots using a 107 mm warhead have been fired against both sandbag and homogenous sand/gravel targets, at standoff distances from 10 to 100 calibers.

The result indicates that the hydrodynamic penetration theory, based on the Bernoulli equation, is not suitable for these target materials and these standoff distances. The actual penetration was more than twice what was expected from calculations based on this theory. Furthermore, the penetration was found to be significantly smaller when the sand/gravel was packed in sandbags – a result in contradiction to the same theory.

Standoff curves which also take into account if the target material is packed in sandbags or not have been developed. The general shape of these curves is different from what is characteristic for materials such as metals and concrete. Increasing standoff decreases penetration only marginally.

The jet is fully fragmented into smaller segments when it hits the target, but still has good penetration capability in these target materials. This phenomenon is related to the cut-off velocity, the lowest jet velocity that gives a contribution to the penetration, which is considerably lower than for more resistant target materials. Combining the influence of the cut-off velocity with the hydrodynamic penetration theory is used in the explanatory model in the paper.

The experiments also demonstrated that the target material surrounding the jet moved forward, resulting in a growing penetration channel after the jet was fully consumed. This is related to phase three penetration. The contribution to the total penetration is considerable and gives a plausible explanation as to why sandbags give better protection.

The results have been used to develop a new model for the design of protective measures against jet penetration. The military utility for EOD operations has been the main criteria during all development phases. The final result is a simple tool that can be used under field conditions. Approximately one year after the first experiments, the result has been implemented in regulations and training for EOD personnel in the Swedish Armed Forces.