Defense – Obstacles Part III: Placement And Man-Improved Obstacles

In the last entry we covered what natural obstacles were and why they work.  In this entry we’re going to begin to address man-improved obstacles and their placement.  Remember a man-improved obstacle is one that implies a natural obstacle has been improved or an obstacle created through human effort.   In military doctrine these are normally referred to manmade however I want to draw a distinction between man-improved and manmade (an obstacle human placed that consumes resources like wire, posts, etc.).  Normally man-improved obstacles take less effort than manmade ones and what I refer to as man-improved obstacles normally don’t require significant external resources.

During this entry I’ll be using the phrase “cut a/the trail”  This implies actually severing a trail and not creating one like we would normally use the term.  Why would you cut a trail?  If you have multiple routes in your AO or on your land you may want to eliminate some of them to vehicle traffic (remember a guy on foot is going to be damn hard to stop) which could decrease the numbers of avenues of approach the threat could take while mounted (in a vehicle).  And with cutting a trail the old adage applies – “Any obstacle that’s going to make it hard for the threat to get in is going to make it hard for you to get out“.  Use these examples only by weighing that along with all of the pros and cons that putting them in brings.

One of the basic tenants of obstacles I presented in the introduction was the fact that it’s much easier to halt or delay a vehicle than a person on foot with an obstacle.  We’re going to focus on delaying vehicles in this entry because it’s a given that a determined threat on foot is going to find a way in.  So for our purposes we’ll look at trails on flat land, uphill/downhill, and side slopes.   IMHO these types of vehicular obstacles are best placed a good distance away from your homestead.  Why?  It they do stop a vehicle completely and the threat dismounts to continue on it’s going to cost him time and energy humping it.  Once he gets to your homestead he still has to assault.  And in the middle of summer with a 100 degrees outside he’s probably going to already be beat.  Since we’re looking at vehicles let’s take another look at the performance stats I presented in the natural vehicle entry.

The HMMWV is a .gov staple and fairly representative of a good four wheel drive truck so we’ll be using it to determine our obstacle characteristics.  By designing an obstacle to counter the performance of a given threat we disrupt the threats ability to easily move along a route and cost him time by forcing him to either bypass or fight through our obstacle.  This is where placement can be critical. We don’t want the threat to have an easy bypass (route around the obstacle) so we need to place it where they can’t simply just drive around it.  Looking at the example diagram below The red arrow is the threat direction of travel and the lettered boxes are our obstacles:

The obstacle marked “A” has enough room around it that if the terrain is passable the threat’s just going to bypass it by driving around it.  Obstacle “B” is placed in an area where the treeline is much denser and closer to the trail.  Another key point in our illustration is the width of the obstacles.  See how obstacle “B” covers the entire width of the area between the treelines whereas “A” is just across the road?  This is critical in denying the threat the capability to bypass an obstacle.

As I’ve stated previously the most effective obstacle is one that can be covered by fire and observation.  If you plan on using obstacles in conjunction with manned positions like Observation Posts (OPs) or Checkpoints (CPs) then you’ll want to afford yourself some standoff distance to prevent a threat from roaring up on your location.  That distance can translate into reaction time.  By placing an obstacle where you can both observe and fire on it (frustrating the threat’s attempt to fight through it) and have some distance between you and the threat if he dismounts and begins to fire and maneuver you’ve increased your survivability significantly. As illustrated by the following diagram The “RDF”” is our Rural Defense Force or simply tribe members:

Obstacle “A” is in what we would consider a protected curve.  It’s going to allow the threat some  measure of cover from long distance fire while at the same time requiring you to be a lot closer to the threat to engage.  Not a good deal.  If the terrain dictates that you use this kind of placement then it’s a good idea to back it up with some other measures (i.e. homemade spike strips before it, wire obstacles, etc.).  Obstacle “B” allows us to maintain that valuable standoff distance.  An added plus is that if you place an obstacle just around a curve and the threat is hauling ass they may not have enough reaction time and end up slamming into it.  If you’ve done your homework and invested some sweat in that obstacle you probably just took a vehicle and its occupants out of the fight at least temporarily.

The next location for obstacle placement we’ll look at is along a trail that is on the side of a hill or ridgeline.  These are usually cut with a steep bank on either side which we can use to our advantage.  In the illustration below the thin lines that run along the road represent contour lines (the natural curve of the terrain feature).  We’re concerned with three types of terrain on this trail.  The first type which Obstacle “A” is placed on is called a draw.  A draw is an area between two ridges or spurs.  If you are standing in a draw, the ground slopes upward in three directions and downward in the other direction. for our purpose just remember that a draw points to a hilltop.  The next obstacle (“B”) is placed parallel and along the hillside.  The final obstacle (“C”) is placed on a spur.  A spur is a short continuous sloping line of higher ground normally jutting out from the side of a ridge or hilltop.  A spur points “away” from the high terrain.  Looking at our illustration:

Obstacle “A” in the draw denies any significant room to maneuver and deal with an obstacle – especially if the terrain is steep.  Likewise with obstacle “C”.  Obstacle “B” gives the threat more room to move vehicles back and forth.  Later in this entry we’ll look at how to capitalize on both a spur and draw to cut a trail effectively.

Let’s deal with a basic flat trail.  Remember from our illustration above it should be placed in a location that doesn’t give the threat an easy bypass.  Used in conjunction with a natural obstacle on either side (like dense treelines) it can be pretty effective at halting vehicles or at least slowing their advance significantly.

In this illustration we have cut a pit 12 feet long with a back wall as near as vertical a minimum of 3 feet deep.  We took the spoil and using some old tires to prevent quick erosion piled the dirt up on the backside of the obstacle.  It’s a good idea to keep the pile at least 6 inches from the edge of the pit to prevent it from backfilling.  That creates a vertical wall well in excess of what the HMMWV can scale.  At this point unless the threat possesses some decent bridging equipment it’s time to dismount, get out the shovels, or turn around.  Now why the 12 feet?  So a pair of car ramps (like you see on U-Haulers which are roughly 6 feet long) can’t be used to cross with little effort.  So how do ya dig this monstrosity?  It’s best left to a dozer or backhoe back dragging the dirt however you can do it with a tractor which is much more time consuming.  Leave one spot to cross back and finish it off with either shovels or a backhoe.   Given the performance parameters of the HMMWV it ain’t getting across this without some significant effort.

Next up we’ll look at your basic hill.  This technique is good for cutting trailheads that lead either up or downhill and is fairly easy to implement.

Notice our original grade was less than 60 percent.  That’s well within the performance parameters of a HWMMV and it would walk right up that hill.  However by using a blade to cut a good chunk out of the side of the hill we have created a vertical wall that the HWWMV is incapable of climbing.  Using the spoil to create a hump further defeats the performance characteristics by denying a straight off shot up or downhill.    Ramps could work with this design however it would take some serious shoveling to get them stable enough to drive across.  Downhill this kind of obstacle is especially nasty at night as a vehicle that drops off of that embankment is probably going to nose down and flip.

The most effective way to cut this kind of obstacle is with a blade on a dozer.  After that using either the blade/bucket on a tractor or a backhoe would be next.  Whatever you do don’t forget to think out how you’re going to get your equipment back on the other side of the obstacle.  I had to use this method to cut the trailhead to the pipeline that runs across my property to keep ATVs from using it.  And before you go getting your panties in a wad and being all judgmental I did it  because it was more of a matter of liability in our sue happy society than being a buzzkill.

The final type of man-improved obstacle we’ll look at in this entry is one that cuts a trail that runs along a hillside.  Referring back to the terrain definitions above the most effective place to put one of these is either going to be on a draw or a spur.  That way if a vehicle tries to negotiate it there’s more likely a chance that the vehicle is going to flip downhill because of the angle of the terrain and it’s center of gravity.  The most effective placement of this type of obstacle is going to be on a spur as the center of gravity and turning of the vehicle are away oriented away from the terrain (kind of like the way a car leans out going around a curve).  Taking advantage of that little piece of physics makes our obstacle more effective.

Once again the dashed line represents the original grade which was less than 40 percent and well within a HMMWVs capability to traverse.  By doing some digging and piling we create a grade that is almost impossible for a vehicle to stay upright while negotiating as indicated by the improved grade which is over 60 percent.   On smooth ground a HMMWV could possibly cross this but in rougher terrain it’s going to be a total crapshoot.  To make matters even worse make sure you leave no part of the obstacle that is close to horizontal more than 3 feet wide.  This denies the ability to “straddle” the steep portions.  The length of this obstacle should ideally be at least 12 feet long.  Why 12 feet?  that requires the majority of the vehicle to enter the obstacle if it tries to cross it.  Once that center of gravity on that vehicle is committed either going forward or back becomes a much hairier prospect for the threat.  Depending on the width of the trail and the amount of digging this kind of obstacle is probably best cut with a backhoe.  Given the slopes involved moving either a tractor or dozer along that slope could be pretty dangerous. Blasting is another option if you have the resources.  It’s also a good idea to take some erosion control measures at the top of this kind of obstacle to prevent it from backfilling and washing level.  That’s where those old tires come in real handy.

Hopefully these examples have got your gears turning.  Make sure you look at your terrain and be creative when thinking about where to place them.  A few things of note when you start cutting trails.  Be very conscious of what you leave behind and what was there when you began.  Things like logs or downed trees can be drug into position to help breach an obstacle.  We don’t want to leave anything the threat can use to fight through.  That includes piles of dirt.  Better to use them to increase or complicate the obstacle rather than sitting on the side of the trail.   Also remember that man-improved obstacles are more effective in conjunction with natural obstacles.  Don’t give the threat room to bypass or maneuver and you’ll frustrate the piss out of his plan.

In the next entry we’ll continue to look at man-improved obstacles and look at a couple of additional methods of cutting trails and address some issues with cutting roads (both improved and unimproved) and bridges.

About Treaded

Semi-retired career and contract troop. I own and maintain my own small ranch out here in beautiful rural America.
This entry was posted in Defensive Measures, Hardening the Homestead, Security Planning, Terrain and AO Development. Bookmark the permalink.

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