# A Review of Blast Loading in the Urban Environment
## 2 Current Knowledge
### Blasts Around Obstacles  
[PERSON] et al. [33] analysed the interaction between a shock wave and a single obstacle through small-scale experimental work. The shock wave is generated via the detonation of a gaseous charge contained within a soap bubble (discussed further in Section 4.3.1). Two types of obstacles were tested, a cylinder and a parallelepiped. Each configuration was tested twice, once with the pressure sensors present and the other without. This was to avoid visual disturbances from the presence of the pressure sensors. Two cameras and an arrangement of mirrors permitted a front view and a plan view of the obstacles to be filmed. The visual and overpressure data were then superimposed in post-processing. It is stated that superimposing _\"was possible only thanks to the good repeatability of the tests\"_. The imaging setup used in the aforementioned work is discussed further in Section 4.1.3.  
The findings highlight the potential benefits of multiple cameras, pitched perpendicularly to each other, to expose three-dimensional effects that cannot be detected using a single camera or overpressure data alone. Regarding shock wave interaction with a finite object or an array of simple obstacles, it appears necessary to focus on imaging to ensure three-dimensional effects are detected [34].  
[PERSON] et al. [35] also analysed the interaction between a blast wave and a wall of finite length. The thickness and height of the wall remained unchanged throughout at 18 mm and 82 mm, respectively. This ensured that the behavioural differences observed could be largely due to the length of the wall. Pressure sensors were placed on the same and opposite sides of the wall (relative to the charge centre) to allow for a free-field comparison.  
Multiple wave behaviours were imaged clearly, allowing for a qualitative explanation of why the overpressure data varied from free-field propagation. The so-called \"bypass\" wave resulted from the incident wave that diffracted over the top side of the blast wall, and the trajectory of this wave remained similar for all configurations as the wall height was kept constant. The lateral waves consisted of two shocks that travelled along the length of the wall and diffracted around the lateral faces. They appeared to be slightly offset in the imaging; however, this resulted from a parallax from the imaging technique used (see Section 4.1.3), and they are, in fact, symmetrical. The trajectory of the lateral waves varied with each test as the wall length was altered.  
For larger-length walls, the reduction in peak overpressure occurs in the immediate central area downstream of the wall, extending out to a distance similar to the height of the wall. This distance is seen to extend up to twice the height of the wall in other studies, [36]. Here, the wall sufficiently delayed the lateral waves so they do not interact with the bypass wave or its reflection near the wall. Further downstream for the medium-length walls, the maximum overpressure increased up to 40% compared to free-field propagation. This was attributed to the coalescence of the lateral and bypass waves. This is an important finding, as protecting a \"close proximity\" area to the wall may worsen loading at another area further downstream of the blast. It is hypothesised that only an infinite wall (or one of a conservatively substantial length) can guarantee overpressure mitigation. This also indicates that the assumption of free-field propagation in the presence of even a single and simple obstacle can be inaccurate.