## 4 Conclusions  
This research investigates the behavior of sand-tire shred mixtures under various surface explosion hazards and scenarios, which is a unique application. The aim was to evaluate the ability of this mixture to dissipate the energy of surface explosions and reduce peak blast pressure as the leading destructive impact energy for underground services and structures. To this end, mixtures of fine-grained sand and shred tire were subjected to surface explosions with varying charges. Sand-tire shred mixture layers with thicknesses of 2, 4, and 6 m and rubber contents of 10%, 20%, and 30% were investigated under 100, 500, 1000, and 5000 kg surface explosion scenarios. The results proved the success of a 6-m-thick sand-tire shred mixture in reducing peak blast pressure in all simulations when compared with pure sand. The most significant reduction was 90%, which was achieved at a depth of 2 m under a 100 kg charge, followed by an 84% reduction at a depth of 3 m under a 1000 kg charge, an 83% reduction at a depth of 3 m under a 500 kg charge, and an average reduction of above 20% at depths between 8 to 12 m under a 5000 kg charge. Further investigation showed that the thickness of the mixtures layers has a significant effect on their behavior under explosion hazards, especially at lower depths. The peak blast pressure under a 100 kg charge was reduced by up to 81% when the thickness of the mixture layer increased from 2 m to either 4 or 6 m. This reduction was between 79% to 81% under 500 and 1000 kg charges at a depth of 3 m. However, the thickness of the mixture layers had a less significant impact on reducing peak pressure at greater depth than the mixture and sand layer interface. This finding would suggest that the optimum thickness of the mixture layer should be determined by considering the distance of the underground structure from the surface explosion charge and calculating the required amount of energy dissipation to avoid explosion hazards. An investigation on the mixture layer's tire shred (rubber) content indicated that it does not significantly affect peak blast pressure for any explosion scenarios with the 2-m-thick mixture layer. However, increasing the rubber content from 10% to 20% and 30% for the 6-m-thick mixture layer resulted in a more significant reduction in peak blast pressure under 200 and 1000 kg charges. The formation of a deep crater under a 5000 kg charge resulted in the least noticeable effect on peak pressure by increasing the rubber content. Overall results indicated that increasing the rubber content significantly affects peak pressure reduction in lower depths and under lower power explosions. Figure 5: Effect of rubber content on peak blast pressure for 2- and 6-m-thick mixture layers under surface explosion with charge weights of 500, 1000, and 5000 kg: (**a**) 2-m-thick mixture layer and 500 kg charge. (**b**) 6-m-thick mixture layer and 500 kg charge; (**c**) 2-m-thick mixture layer and 1000 kg charge; (**d**) 6-m-thick mixture layer and 1000 kg charge; (**e**) 2-m-thick mixture layer and 5000 kg charge; (**f**) 6-m-thick mixture layer and 5000 kg charge. This research has proven the adequate performance of the sand-tire shred mixtures in reducing peak blast pressure, which is the leading cause of damage to underground structures under surface explosion. The thickness and rubber content of the mixture layer need to be carefully designed based on each explosion scenario by considering the distance of the explosion charge from the structure, the weight of the explosion charge, and the safe peak blast pressure to protect the structure from surface explosion hazards. It is recommended that further research in this area integrate soil-structure interaction in the analysis based on specific characteristics of different underground structures. Conceptualization, M.H. and M.K. ; methodology, M.K. and P.G.A. ; software, P.G.A. ; validation, H.D. ; formal analysis, H.D. and R.K.; investigation, R.K.; writing--original draft preparation, H.D. and M.H.