On the demolitons site, everything changes once the explosive charges are layed out. These charges are very sensitive and could go off prematurely if jarred. To prevent this, experts will often place sandbags over the detonators in order to absorb a shock in case something were to fall on them. Because the risk of having a charge not go off could lead to an incomplete demolition or worse yet, send rubble flying in a dangerous direction, not long before the blasting date, the blasters check over everything to ensure that all of the explosives are wired coreectly. Once this is done, in addition to briefing local emergency personnel in the event of a disaster and rechecking the setup of the rigged structure for the last time, the demolitions team is now ready to blast.
The day of the blast is probably the most nerve-racking day for the demolitions crew. Although everything has been layed out and checked ad nauseam, there are still many variables which could lead to a potential disaster. Here are a few of these such things which experts concentrate on to try and avoid once the charges are set off:
As previously stated, flying debris can be a real problem and has, on occasion, severely injured and even killed people. There are a few different ways in which demolitions experts try to combat this problem. First of all, as I also mentioned, the charges are checked just prior to the blast to ensure that they all go off when they are supposed to. Besides this, the charges are often wrapped in a mesh-like material to contain the blast. Precautions are also taken to make sure that an excessive amount of explosive is not being used. Still another method employed is by ensuring that everyone is a safe distance from the blast and that all important property is protected by either being moved or covered over.
I think at this time it is fitting to mention the large cloud of dust which is often generated as the structure collapses. While this cloud maybe bothersome, aside from people with sensitive respiratory systems, these clouds are harmless. In fact, experts believe that less dust is generated from an explosion than with a wrecking ball, the main difference being that during a blast demolition, all of the dust material is created within few minute timespan rather than over the weeks it takes for a wrecking ball to bring a structure down. Because the dust is often very fine and light, air currents may spread this dust for miles before it settles.
We now know that explosions work by rapidly increasing the amount of pressure of a gas (i.e. the air) around the supports of the structure, but it is important to remember that these shock waves continue out past the site and can be felt a great distance away, which could lead to the damage of nearby property and personal injury. Anyone standing near a demolition site when it goes off will experience these concussions as loud booms. The rumbling of a structure crashing towards the ground will also create potentially harmful vibrations as well. In air, these concussions travel at the speed of sound (around 343 m/s) but will also vibrate rapidly through any other medium at an often faster rate (partiucularly buldings and cars) which is what leads to damage. To help mimimize this, blasters can do a couple of things.
First of all, one of the most common ways of going about this is by setting off the charges at different times, by doing this, in stead of one giant shock wave being generated, several smaller ones are created in stead. Another action which they take is to secure all nearby property (if any) especially by boarding up any adjacent windows and removing anything which can be moved that could be disturbed from the blast.
While many might not think about it, one of the most important things to take into account in the case of a building being blown up is how the air inside of the structure shall escape. Approximately 85% of a building is nothing other than air and if it isn;t allowed to escape properly, this could potentially turn the structue into one giant mechaical explosive or lead to a damaging rapid current of air being released onto another property. In order to guard against this, the blasters will check to make sure that the air has places to escape in directions which are least harmful.
Remaining Building Sections
Another worst fear of the demolitons expert is to find a significant part of a structure still standing once the dust cloud clears. If that were to happen, the experts would have to risk serious injury by going in to finish the job on an area whose stability has been compromised by a blast. One of the biggest ways to prevent this is through the "test blasting" that I had mentioned before which determine the proper amount of explosives to be used.