# A Review of Blast Loading in the Urban Environment
## 2 Current Knowledge
### Degree of Confinement  
[PERSON] et al. [47] also explored the effects of urban confinement on blast loading. They defined the near-field as a \"_range where scaling rules do not apply_\" and state that this regime extends further when urban confinement is present. This is termed the \"extended near-field regime\" and typically extends to larger-scaled distances for wider streets. Regarding pressure, in the case of a straight street, the \"extended near-field regime\" exists within the scaled distance where primary reflections have not yet merged with the incident blast wave. Maximum overpressures on structures within this space did not change in magnitude  
Figure 3: Plots showing pressure and impulse data measured for various street widths (10 cm and 25 cm), supplemented with free-field data from an RP80 and theoretical curves for a free-field scenario, adapted from [PERSON] et al. [42]. compared to a singular-walled scenario; only impulse enhancement was generally seen to occur. This agrees with the blast wave behaviours discussed in [PERSON] et al. [42]. Beyond the scope of the near-field regime within straight-street tests, significantly greater overpressures were present compared to a free-field scenario. This occurs due to the street-reflected and incident blast waves combining to form an ultimately stronger blast wave, a phenomenon otherwise termed \"blast wave coalescence\". The magnitude of this increase was greater for a narrower street than a wider one, assuming the street is suitably long for coalescence to occur. It was also seen to occur at a shorter distance for a narrower street compared to a wider street; all scenarios assumed the same explosive mass was used. [PERSON] et al. [47] also examined the afterburning effects of the detonation products. \"_Afterburning occurs when fuel-rich explosive detonation products react with oxygen in the surrounding atmosphere. This energy release can further contribute to the air blast, resulting in a more severe explosion hazard, particularly in confined scenarios\"_[48]. Indeed, increases in overpressure and impulse were prevalent for all street widths tested. These effects were attributed to increased fireball temperature and size, resulting in the acceleration of blast reflections. Metallised TNT was seen to enhance these effects further. A new parameter was proposed, termed the \"degree of confinement\". It is described as a quickly-attainable metric that can determine the potential extent of urban environment effects on average scaled impulse for a given urban blast. Equation (3) details the basic formula, which includes a closed-area \"wall ratio\" and a \"corner ratio\". These ratios are defined in Equations (4) and (5). As seen in Equation (3), these ratios are weighted equally, although it is stated that further refinement of the general concept is required. \[DC=(WR+CR)/2 \tag{3}\]  
* \(WR\) is the wall ratio. * \(CR\) is the corner ratio. \[WR=\frac{\sum\limits_{i}^{r}}{\sum\limits_{i}^{all}}{H(Z_{\text{cutoff}}-Z_{i} )A_{i}Z_{i}^{-1}} \tag{4}\]
* \(H\) is a Heaviside function (returns 1 if positive or 0 if negative). * \(Z_{\text{cutoff}}\) is the limiting scaled distance (m/kg\({}^{1/3}\)). * \(Z_{i}\) is the scaled distance to the \(i^{th}\)-reflecting surface (m/kg\({}^{1/3}\)). * \(Z_{i}\) is the area of the \(i^{th}\)-reflecting surface (m\({}^{2}\)). \[CR=\frac{\sum\limits_{i}^{c}}{\sum\limits_{i}^{c}}{c_{i}Z_{i}^{-1}H(Z_{\text{ cutoff}}-Z_{i})} \tag{5}\]
* \(c_{i}\) is the corner factor and has pre-defined values depending on the corner type. * Ground diffraction corner, \(c=2\);
* Top of building diffraction corner, \(c=1\);
* Ground re-entrant corner, \(c=4\);
* Top of building re-entrant corner, \(c=4\). The utility of the degree of confinement concept was tested by analysing a set of scaled urban blasts. It was seen that the degree of confinement parameter generally produced values as expected: the most confined scenario returned the highest degree of confinement.