Earlier this week I was asked by a long time reader if I could discuss ways to protect critical assets from an attack by a vehicle borne improvised explosive device. The facility at which he worked was trying to figure out what they could do to address this particular attack scenario. As always I am more than happy to express my thoughts on security related topics. Standard caveat; I am not an engineer; I am a chemist by schooling. I do have some experience with explosives from the Army, but I am not an explosives expert and I have never detonated anything as large as a VBIED (if someone wants to let me push the plunger on one in a controlled test, I will certainly be there). Finally, before you actually install some protective device make sure that you are dealing with an engineering firm that has some experience in the field. Prevent Detonation The most obvious protective technique is the prevention of the detonation of the explosive device. While most of the responsibility for this lies with the intelligence and law enforcement people, the facility does have some basic techniques that they can employ to aid in the effort. The most obvious is the employment of an active counter-surveillance program. Any effective terrorist attack is going to be preceded by a variety of surveillance efforts. The earlier efforts may be harder to detect, but as attack planning advances the terrorists will have to acquire more detailed information about facility security procedures. Facility employees and security personnel should always be on the watch for suspicious personnel hanging around the facility. The facility counter-surveillance plan should include an educational component to make personnel aware of the potential threat and their responsibility to be aware of what goes on around the facility. There needs to be a clear reporting procedure and the reports need to be promptly forwarded to local law enforcement for follow-up investigation. Standoff The closer you can get an explosion to your target the more effect it will have. Conversely the further you can keep the explosion away from critical areas of the facility the less effect it will have. For a standard, non-focused explosion the force of the explosion should falloff as a square of the distance from the explosion. This means that even a small increase in the distance from the explosion can have a significant effect in force reduction. Of course, the size of the explosion has an effect on the distance as well. I mean if you have a dry-box trailer packed with 40,000 lbs of commercial grade explosives the standoff distance will have to be much larger than if you have 500 lbs of homemade explosives in a panel van. So, to determine how far you have to keep the VBIED away from the potential target, you have to know how large a VBIED the terrorists will use against that target. Obviously the terrorists are not going to tell you how large a VBIED they are going to use. So you have to guess; hopefully an educated guess, but a guess none the less. The guess should be based upon the potential risk; I would expect that a Tier 1 facility should expect a bigger VBIED than would probably be used against it than against a Tier 4 facility. Don’t expect me to tell you what size to use; I just don’t have enough information to make even a lucky guess. Hopefully your security consultant will be able to provide a rationale for what ever size you pick. Once you have established your planning VBIED size you should be able to calculate (well the experts should be able to calculate, I haven’t seen the formulas) how far you have to keep the VBIED away from the target to have a reasonable chance of surviving without catastrophic damage. If you want to keep your consultant on his toes, ask about max pressure and impulse effects. But, remember, you are going to need an expert. Anyone that tells you that there is a reasonable distance at which no damage will occur is either exaggerating or doesn’t understand explosions (a mile away from a 500 lb VBIED I would feel comfortable predicting little or no damage). With a VBIED attack if you can avoid catastrophic damage (ie: a quick, total drainage of a release-toxic COI from a large tank) you have achieved a reasonably successful defense. Your release mitigation techniques should be able to handle the results of non-catastrophic damage. Blast protection The last type of protection is the one that probably requires the most expertise, physical blast protection. This can either be some sort of hardening of the target so that the blast will not affect it, or putting some sort of barrier between the potential blast and the target. Both require some special engineering skills and experience to properly design. Once again, the size of the VBIED is a key design variable. If you dramatically underestimate the size of the device your blast barrier will become flying fragments that will contribute to the destruction of your target. One design element that needs to be considered is ‘line-of-sight’. Over longer ranges flying stuff from an explosion follow what is called a ‘ballistic arc’; the pieces fly up and out then fall to earth along an arc. The closer the initial trajectory is to 45° the further the projectile will fly. At distances where blast effects predominate, however, the blast and projectiles are flying in essentially straight lines. Thus, any barrier must block the line of sight from the blast to the target. But, don’t forget to harden the top of the target to protect against falling debris. Combination Plate The most effective VBIED protection scheme will utilize all three protective elements. Preventing detonation is, in my book, the most important element of the program. The lack of an explosion is the best protection. Remember, that the larger the VBIED, the more likely that there will be extensive pre-operational surveillance. Putting together a large VBIED takes a lot of resources so the terrorists will do what ever is necessary to optimize their chance of a successful deployment; this means more surveillance. The last two, standoff and blast protection both require the services of someone who understands blast effects and blast protection engineering for optimal results. If you cut corners here you had better hope that your prevention program is very effective. A poorly designed protective program may actually increase the risk of a successful VBIED attack.
Special NOTE: Since this is a rather specialized field of study and application, I would certainly like to hear from practioners in the field. Discussion of the blast effect protection techniques would be particularly instructive.