Parametric analysis of groundshock loading on steel plates and reinforced concrete slabs

Hardened underground structures are purposely constructed beneath the ground surface for additional protection. The protection and sustainability of such a structure is provided by the soil overburden, supported by the structure's roof slab, also called a primary slab. The soil overburden usually contains one or more layers of protective reinforced concrete slabs, defined as secondary slabs. Hardened sub-surface structures can experience a degree of damage from sub-surface explosions, caused by the detonation of airborne weapons at variable depths within the soil overburden layer. The research objective was to evaluate the degree to which material and geometric parameters influenced the magnitude of groundshock loading and hence the severity of damage to sub-surface positioned reinforced concrete slabs. The work was of an experimental nature, performed within a purposely built test cell. The experimental set-up involved positioning cylindrical charges within the soil overburden at variable standoffs from an initially positioned steel plate, then from a primary slab, which was subjected to a series of cumulative loadings. The structural response of both the steel plate and a reinforced concrete slab were evaluated using an energy balance procedure. A parametric study was performed, determining the influence each geometric parameter had to the magnitude of groundshock loading. The variability of data associated with the material parameters was analysed and compared to the published literature. Numerical simulations were performed at the end of the experimental stage (using the non-linear finite element programme AUTODYN21)) to investigate phenomena that could not be investigated experimentally. A visual damage assessment in the form of a crack pattern analysis and chronological order of occurring mechanisms upon reinforced concrete slabs was performed. The severity of damage to the primary slabs was then associated with the cumulative load impulse history. The research yielded the following conclusions: I. An idealised half sine-wave load distribution approximated the pressure-time history profiles recorded by pressure gauges. 2. The steel plate and one concrete slab (for which conclusive data was obtained) responded impulsively to groundshock loading. 3. The magnitude of groundshock loading was most sensitive to a change in the charge standoff. 4. The reduction in the soil overburden above the top face of a secondary slab did not influence the groundshock loading induced into a primary slab. 5. The effect of the propagating groundshock wave overrode any initial form of soil compaction between the charge and target. This was due to the significantly high stresses induced into the soil, which were well above the initial insitu stresses. 6. Internal weakening of a primary slab, which was unidentifiable from the external damage, caused significant loss in structural strength and stiffness.