A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
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Loads on underground structures 81<br />
If the soil surrounds the structure <strong>and</strong> rests at its natural angle <strong>of</strong> repose, its<br />
presence will considerably reduce the value <strong>of</strong> the coefficient <strong>of</strong> drag on the<br />
front face <strong>and</strong> at the same time buttress the structure against lateral movement.<br />
Underground structural loading is also influenced by the flexibility <strong>of</strong> the<br />
structure in relation to the properties <strong>of</strong> the surrounding soil <strong>and</strong> the soilstructure<br />
interaction effects that occur as the structure begins to deflect under<br />
shock pressures. The structure <strong>and</strong> its surrounding soil form a composite body,<br />
the loading on which can be influenced by soil-arching <strong>and</strong> the disturbance to<br />
the natural properties <strong>of</strong> the soil.<br />
Much <strong>of</strong> the early work in this field was driven by the need to assess nuclear<br />
weapon effects, <strong>and</strong> there are good summaries <strong>of</strong> the problem in the reports<br />
by Newmark <strong>and</strong> others, <strong>and</strong> by Glasstone <strong>and</strong> Dolan, discussed earlier in<br />
references [4.2] <strong>and</strong> [4.5]. Using their work as a basis, we will discuss first the<br />
structural loading that comes from blast-induced ground shock. We saw earlier<br />
that there are two types <strong>of</strong> air-blast-induced ground motion. In that where the<br />
velocity v <strong>of</strong> the shock front exceeds the dilational seismic velocity (u) <strong>of</strong> the<br />
soil, there is a sloping shock front below ground. If v=u, the ground shock<br />
front becomes almost vertical near the surface, <strong>and</strong> if v=u, the shock front in<br />
the ground outruns that in the air. The relationship <strong>of</strong> pressure with duration<br />
will attenuate as greater depths <strong>of</strong> soil are considered, <strong>and</strong> Figure 4.5, from<br />
the work <strong>of</strong> Newmark [4.8] illustrates this. As the depth increases the rise<br />
time is longer, there is a lower peak pressure <strong>and</strong> a longer decay. The total<br />
impulse (i.e. area under the pressure-time curve) remains about the same, <strong>and</strong><br />
it was established experimentally that the rise time at any depth was about<br />
half the time <strong>of</strong> transit <strong>of</strong> the shock front to the point considered. The changes<br />
in shape <strong>of</strong> the pressure pulse were a direct result <strong>of</strong> the stress-strain<br />
characteristics <strong>of</strong> the soil in unidirectional compression. A simplified empirical<br />
equation was proposed by Newmark <strong>and</strong> Haltiwanger [4.9], which gave<br />
Figure 4.5 Attenuation <strong>of</strong> pressure wave with depth (from Newmark, ref. 4.8).