Designing Your Retreat to Protect it Against Low Velocity Ballistic Attack

Whether intentional or accidental, a vehicle can very effectively smash through most unprotected building exteriors.
Whether intentional or accidental, a vehicle can very effectively smash through most unprotected building exteriors.

You probably already know that your retreat needs to be reasonably protected against high velocity ballistic attack, which is a fancy way of saying it should be more or less ‘bullet-proof’.  But have you thought about low velocity attacks?

What do we mean by low velocity attacks?  We mean instead of having small rapidly moving things (ie bullets) impacting on the side of your retreat and possibly penetrating through the walls, having much larger but slower moving things impacting on the side of your retreat and again possibly penetrating the walls.

The first example of such an object would be a traditional battering ram.  We’re going to hope that there’s not much chance of an enemy being able to deploy a battering ram against your retreat doors or windows, because that would presumably expose the people with the battering ram to sustained fire from you and your fellow defenders, to the point that by the time they’d positioned their ram and started to use it, they’d have experienced unacceptable casualties, causing them to break off the attack.

But there’s another type of low velocity ballistic weapon which you need to consider.  What say the bad guys simply drive a car or truck at full speed up to your retreat and crash it in through the walls?

To put things in perspective, a typical pistol bullet has in the order of 300 – 900 ft lbs of kinetic energy.  A .308 rifle round is in the order of 2000 – 2500 ft lbs.

A 5,000 lb car or truck, traveling at 30 mph, has 150,000 ft lbs of kinetic energy.  Get its speed up to 42 mph and it has 300,000 ft lbs of energy.  If you can get it to 60 mph, it will have 600,000 ft lbs of kinetic energy.

A passenger car, even at fairly low speeds, can crash right through the exterior of a typical stick built home, and end up in the living room (we know this from experience).  Add extra weight, reinforce the front with a ram to focus all that energy in one concentrated area, and get the speed up a bit, and any attacker has a formidable weapon.

It isn’t even a very high-tech weapon.  Any old vehicle, as long as it can be driven, will work perfectly well.

One more point.  With safety belts (and possibly air bags), even at quite high speeds, the person driving the vehicle into your house is unlikely to be harmed.  The reason for this is that the impact event is spread out over time and distance, so instead of a deadly sudden impact lasting only a few hundreds of a second, it is a survivable impact that extends over a second or longer.

There’s another issue, too.  At even a fairly slow 30 mph, the vehicle is covering 45 feet every second.  If you have, say, a 250 ft ‘clear zone’ around your retreat, that means it takes the vehicle just over five seconds from when it first appears to when it is crashing through your wall.  That’s not much time to respond to the threat – even if you were on active sentry duty, at least a second to recognize the threat and start a response, another second or more to get your rifle shouldered, sighted, and safety off, and that leaves less than three seconds to try to do something.

With the vehicle being a moving target and its angle to you probably rapidly changing (meaning you need to appreciably lead the target with your sighting – how much practice do you have at that?), your chance of getting any effective fire on it is minimal.  Even if you could get some rounds landing on the vehicle, and while that might injure or kill the people inside, you’re almost certainly not going to stop the vehicle continuing on and into your wall.

Sacrificing a vehicle to break into your retreat might seem like an extreme move that not many attacking groups would willingly do, but if that’s what you think, maybe you should think again.

Our prediction is that in a Level 2/3 situation, vehicles will be a dime a dozen.  What will be in precious short supply is fuel for the vehicles, but how much fuel does it take to start a vehicle, gun the engine, and drive it at full power a couple of hundred yards?  A pint or two of petrol should do the trick.

An adversary could have a team of horses tow the vehicle to close to your retreat and would only need to use precious petrol for the last short powered dash of the vehicle into your outer wall.

Such a method of attack – unless you had prepared for it – would be very hard to resist.  It behooves you to consider this risk and devise appropriate defensive measures.

The obvious solution might be to strengthen your walls still further, but that’s not necessarily the best, and certainly is far from the only possible response.  Generally, we’d recommend the better thing to do is to prevent potentially threatening vehicles from being able to approach your retreat at any speed other than a crawl.  Keep the battle at a safe distance from your retreat whenever possible.

There are two ways to do this.  The first involves ‘taking things away from the ground’ and the second involves ‘adding things to the ground’.  In other words, digging holes and ditches, or adding obstructions and obstacles.

Obstructions and Obstacles

An obstacle doesn’t need to be large to be effective.  It can be either permanently mounted (ie dug deep into the ground to hold it in place) or can be relatively mobile.  There are many different types of obstacles, but for simplicity, we’ll consider four primary types.

In sequence from easy and simple to largest and most complex, the first type is very simple – known as caltrops, these are nothing more than metal spikes/stars that you can sprinkle over the ground.  At least one of the spikes always sticks up and so will puncture a vehicle’s tire(s).

The best caltrops are made out of hollow tubing, so as to increase the certainty and speed of the tire puncturing.

The nice thing about caltrops is they are low tech, simple to make, and quick and easy to deploy and similarly not difficult to remove again.  You might be thinking ‘if I can easily remove them, so too could an attacker’, and that is half-true.  But while the attacker is out there, sweeping aside your caltrops, he is very exposed and vulnerable to your defensive fire, and he has lost the ability to surprise you with a vehicle coming from nowhere and cannoning into your retreat unexpectedly.

You could even have a caltrop deploying device – a slingshot or something – that would enable you to throw more caltrops onto any cleared pathway.  You’d be tossing them out from behind cover, while the bad guys would have to be exposed in the open to clear them.

A second type of low tech very simple obstacle is almost laughable in its simplicity, but people who have seen this in action assure us of its effectiveness.  Simply strew old car/truck tires where you wish to impede on-coming traffic.

We are told that this will massively slow down an oncoming vehicle, as it suddenly finds itself having to go up and down over tires that are probably each 6″ – 12″ high.  Like the caltrops, the tires can be spread out and cleared on an ‘as needed’ basis, and while an opposing force could also clear the tires, that would again involve exposing themselves to your defensive fire.

A really determined enemy could devise a ‘snow plow’ or ‘cow catcher’ front to put on their vehicle that would deflect tires and caltrops away, although it would probably have to do so at lower speed.

The bottom line for both these approaches (and indeed for all of them) is that a truly comprehensive defense involves using multiple layers and strategies, and you have to realistically decide where to draw the line and how much is enough.

The third type is what are sometimes called Czech hedgehogs.  They are structures that might be freestanding, or possibly weakly anchored.  They might be individual units or possibly may be linked together, and might be made out of metal or concrete.  They work best when they are only slightly taller than the clearance underneath the class of vehicles they are designed to stop – that is a reasonably known quantity when defending against tanks, but in your case, you might be confronting anything from a low slung sports car to a ridiculously raised 4×4.

So these are not so good as a general purpose ‘one size fits all’ device.

The fourth type are large heavy concrete blocks – possibly in pyramid shape, possibly just pillars or some other shape.  These objects are more commonly securely mounted in the ground.  They have been called dragon’s teeth (and many other names too).

Unless they are ridiculously solid, they are vulnerable to being knocked over by a sufficiently strong oncoming vehicle.  But in the course of being knocked over, they will absorb a great deal of the vehicle’s energy.  They are particularly good, in this ‘sacrificial mode’ when reasonably close to your retreat exterior, so in the event of them failing, the vehicle won’t have enough distance to build up speed again.

Noting that attacking vehicles might have been raised up on trick suspension, we’d suggest you need to make any sort of obstruction fairly tall, and we don’t really much like that concept too much because you may then be providing cover for enemy forces to shelter behind when attacking you.

Obviously, obstacles can also be large and prominent.  You could have a three-foot high and three-foot thick wall (perhaps a type of Hesco bastion, for example).  But then you run the risk of providing a defensive cover for your attackers to shelter behind, hence our focus on smaller sized obstacles that don’t have this type of downside associated with them.

There’s another type of simple but effective device for slowing down vehicles, but it is not without its limitations in our sort of environment :  A gravel pit, such as you sometimes see going down long steep hills for runaway trucks.  If you’ve ever seen one of those, you may have noticed how short it seems – those gravel pits do a great job of slowing down and stopping a speeding truck on a steep slope and would do the same for any vehicles approach you at speed too.

Arrester beds typically have a depth of between two and three feet of small-sized gravel in them.  Some locations use sand, which provides a much stronger decelerating force, but in cold winters, the sand will freeze into solid blocks and cease to function as a dragging medium, instead it becomes a smooth surface for the vehicle to drive over, unimpeded.  A loose gravel pit will probably still work as intended in freezing weather.

The shape of the gravel is important – ideally it should be fairly well-rounded, enabling each piece to move and slide over each other piece, rather than causing pieces to interlock together and present a firmer more solid platform for the vehicle to drive on the top of.

A truck at 45 mph typically takes 200 – 250 ft to stop in an arrester bed.  That becomes a massive waste of space if being used to defend a retreat on all sides, unfortunately, making arrester beds less practical for retreat defense.

In addition to gravel and sand arrester beds, there is a special type of collapsible concrete material used at some airports as a run-off zone at the end of runways.  This material stops planes safely and quickly and in less distance.  This ‘engineered material’ has also been used to protect the approaches to secure locations, being a discreet way of providing protection against vehicular attacks.  It is more expensive, but takes up less space.

Unlike gravel, which is a relatively low tech product that can be probably fairly easily replaced or just raked back into place, once the collapsing concrete blocks have stopped a vehicle they need to be replaced.  This would be difficult in a Level 2/3 situation, and you’re still looking at a requirement for at least 100 ft of arresting zone.

Ditches and Moats

A defensive ditch can be a great vehicle stopper.  It can also slow the advance of regular infantry forces, although note that any such ditch needs to be designed so it doesn’t inadvertently provide a secure position for enemy troops to regroup in and then press their attack against you.  In other words, the closer the ditch is to you, the more readily you’ll be able to shoot down into the ditch, and the less cover it will provide an attacking force.

If you have abundant supplies of water, adding water to a ditch makes it into a moat, which can slow down the progress of enemy forces and vehicles even more effectively.

A moat can be expensive to create, especially if you line it with concrete, although it would be possible to line a moat with puddled mud as an alternative.  Depending on where your retreat was located and the wind and weather conditions, you could expect appreciable evaporation in summer, and if it froze over the winter it would not be very effective while frozen solid.  A possibly easy solution to that would be to partially or completely drain the moat in fall and refill it in spring, assuming water is plentiful.

Moats and trenches can be bridged over, and if they are not very wide, maybe by nothing more sophisticated than 2″x12″ planking, although such an arrangement would not allow for a vehicle to proceed over at high-speed.

Traditionally, castle moats were at least 12 ft wide, and ranged in-depth from as little as 3 ft to as much as 30 ft.  Ideally they would be too deep to allow an attacker to wade across.  A 30 ft depth is of course not so essential, but one benefit of a deeper moat was (and still could be) that it protected the structure inside the moat from tunnelers.  Unlike medieval sieges, we feel it unlikely that any attacking force against your retreat would go to that degree of effort, whereas the depth of moat might interfere with your own escape tunnels out of your retreat.

Clearly the width is an important parameter – the wider, the better, at least up to a certain point.  But it is also necessary to consider cost.  For example, if we say your retreat exterior measures 40′ x 100′, and if we say there is a three-foot space between the building’s exterior walls and the start of the moat, and let’s make the moat 15 ft wide and 8 ft deep, that’s a lot of excavation (44,000 cu ft of dirt).  You could use some of that to build up the raised berm on which your retreat stands, and you’re sure to find a landscaping or strategic use for the rest, but you’ll still have to pay a considerable sum for the earthworks.

If you decide to line your moat with concrete, and if you have a six-inch thick floor and walls (some sources suggest thicker walls), then using the same moat dimensions as in the previous paragraph, you need 5624 cu ft of concrete, or 210 yards.  At a total cost for the concrete and pouring of perhaps $200/yard, that’s close on $45,000 for the concrete alone.

As for the water required, let’s say you decide to fill the 8′ deep moat with 6′ of water.  That would require 245,000 gallons of water.

One more interesting factor about a moat.  On a warm summer’s day with a gentle breeze, you would lose at least 1,000 gallons of water a day to evaporation.  Note that the evaporative loss rate is unaffected by the depth of water – it is proportional to the surface area of water, not its depth (and also to the temperature, humidity, and wind speed), so simply reducing the depth of moat water would do nothing to reduce the rate of evaporation.

Of course, a moat is effective whether wet or dry.  If dry, it is a ditch and still prevents vehicles transiting over it, and slows people down – they have to climb down 8 ft (in the example we have been using) and then climb up 8 ft at the other side, and then have only a very slim 3ft ‘work area’ pressed up hard against your retreat’s exterior walls.  The more water in the moat, the slower their progress across it will be, and the more difficult it will be to take equipment with them.

If you find yourself choosing between a deeper moat and a wider moat, then assuming you have a minimum width of 12′ or slightly greater, you would be best advised to make your moat deeper rather than wider.  There is little or no added value in increasing the width – the difficulty of a moat is mainly in the ‘transitions’ from level ground to whatever depth/differential the moat is, and then from whatever level the moat is to the ground on the other side.

Ideally, we’d recommend the water depth be 6′ or slightly greater so that people can’t conveniently walk across, and then any extra depth should be used to create a vertical air space/wall between the water level and the ground level.  Getting in and out of a moat is even more difficult if you have to ascend or descend more than a convenient number of feet, and with water rather than solid ground at the lower level.

Allowing for Legitimate Vehicle Passage

You need your own vehicles to be able to travel through your obstructions, and you’ll also have legitimate friendly traffic coming and going too.

If you have a ditch or moat, obviously the solution for that is to provide a drawbridge – either a device like in old-fashioned castles that raises up and lowers down, or alternatively one which extends out from your retreat to the other side, or one which rotates/swivels.  Of course, whichever design you use needs to be very secure so that an opposing force can’t take over its mechanism and lower/extend/rotate it to span the moat against your will.

If you are relying on a series of above ground obstacles, you need a lane through them that vehicles can travel, and by including tight turns, you ensure that the vehicles have to move slowly.

The best type of tight turn is to require a three-point (or sometimes called ‘Y’) turn.  Because it requires the vehicle to actually stop, reverse, then proceed on forwards again, this is a very effective delay.

Of course, having had to very slowly work through the obstacles would cause the enemy to be in the open and exposed to your defensive fire for an extended period, plus means that when they finally do arrive close to your retreat wall, they are moving slowly rather than quickly.

Note that when you are planning for how you will allow legitimate traffic to come and go, you need to consider the dimensions and turning abilities of the largest probable type of vehicle that may legitimately visit.  This could be as large as a semi truck/trailer unit with a 56′ long container behind the tractor unit; alternatively, you might decide to have a staging point somewhere remote from your retreat where larger vehicles can be unloaded to smaller vehicles.

That is a much less efficient arrangement, so if it is possible to allow for at least moderately large semi trucks to get through your ‘maze’ that would be the best option.

A Last Line of Physical Defense

Naturally you want to stop vehicle attacks some safe distance from your retreat.  Even a slow speed vehicle, when approaching your retreat, can provide cover for enemy attackers sheltering behind or alongside the vehicle as it slowly moves forward, so some type of obstacle arrangement is essential.

Unavoidably, the further away from your retreat you place such defenses, the more extensive they will have to be.  If you have a retreat with perhaps 40′ x 100′ exterior walls, the walls themselves have a length of 280 ft.  If your vehicle barriers are a mere 100 ft away from your retreat (a distance that a sprinting attacker could cover in four seconds or less) then they will be a massive 1080 linear feet in size.

So possibly you’ll have a partial line of obstructions at a distance to slow down incoming vehicles, then a more severe obstacle design closer in to stop them.  This is the concept of layered defenses – it also helps you to understand the intentions of people approaching your retreat, requiring them to show aggression earlier on.

In addition, if the budget allows for it, maybe it makes sense to have a last line of defense that is actually immediately adjacent to – or even part of – your retreat wall.

One form of this would be an immediately adjoining moat.  Another form would be to build your entire retreat above grade on a berm.  Maybe the first three feet or so of the retreat is nothing other than a solid earth foundation.  In addition to its three-foot (or more) height, f you had this solid earth foundation extend out even only another foot or so (due to it being liable to be compressed if hit by a vehicle) that would be a very strong stand-off layer.

Having your retreat an extra three feet higher is a good thing for all reasons, giving you a better commanding view of your surroundings.  And presumably at least part of that extra three feet of height would be used to get you a head start on excavating basements, too – your below grade basements would in effect have their first three feet above ground level, but still be treated as basement.

In the event that you were blessed with a water table that was very close to the surface, raising up the ground level of your retreat structure is obviously very beneficial.

Whether you raise up your retreat or not, you definitely should make sure there’s not a downhill slope running towards your retreat.  That just makes it so much easier for an incoming vehicle to gather more speed and more energy.  It isn’t only bad feng shui to have your house lower than the surrounding area, it is bad strategy too.


There will be three major categories of physical threat against your retreat.  Bullets, heavy weights and vehicles, and people.  A good series of obstructive defenses will help keep vehicles away from your retreat walls, and will slow down the speed at which people can reach your walls too.

It is easy to underestimate how far away a person can be and still be an immediate danger.  A person 100 yards away can cover that distance, if there are no obstructions, in about 10 seconds, whereas if you’re just going about your ordinary lives in your retreat, there’s no way you can mount an effective defense with that little warning.

Rather than creating impractically huge clear zones around your retreat, you need to slow down the speed at which people as well as vehicles can get to your retreat, to buy you time to prepare an effective defense.

Above ground obstructions have some value in slowing down vehicles, but are less effective against individuals on foot.  Fencing works well against people (but probably not vehicles).  Ditches and moats will stop most vehicles and slow down people.

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