© 2006 RogueTurtle.com
A lot of work for a lot of shelter
Any person who has spent any time in the military knows all about sandbags. They are heavy, awkward, and a tremendous amount of work to use. But, they work and work very well. Most civilian uses for sandbags consist of holding back some errant body of water from entering your home. Flooding creeks, construction runoff, flash flooding, and that kind of thing. |
To keep costs down and the bags waterproof, most bags are made of a man-made fiber that is, unfortunately, susceptible to ultraviolet (sunlight) deterioration. Two years in the sun is the maximum you can expect from a sandbag without protecting it from direct sunlight. But, during those two years, you have a pretty good shelter. Some sandbags are made of burlap, but burlap will quickly rot if exposed to sun and moisture. Burlap bags are not a good choice for the survivalist.
In battle, the major advantage of sandbags is the protection they give from gunfire. The bags of dirt will absorb a lot of flying metal before they give up. But, that is not their only use. In settings other than the battlefield, you can use sandbags for many projects.
The first that comes to mind is a wind break for tents. Built up around a tent area, these walls will withstand a lot of wind, hail, snow, and falling branches. As a side line, if they are gradually covered with dirt or brush, they tend to hide your campsite from casual view giving it a "stealth" quality I so enjoy. The dirt covering the sandbags also protects them from ultraviolet deterioration.
Sandbags have been used for years to strengthen dams and river banks to stop flooding. In Florida, the counties most affected by predicted hurricane flooding will provide both sandbags and sand for homeowners to protect their homes. Labor, however, is NOT provided, even for the elderly. Used in conjunction with plastic sheeting, sandbags make good temporary dams against flooding.
Sandbag Wind Proofing for Tents
The chart below shows the number of sandbags needed for (up to) a four foot levee, 100 feet long. I have added an orange "wall" to show you that if it is at all possible, use an inside vertical supporting wall. The number of sandbags needed is cut in half by using the interior supporting wall. Later, I'll show you a method of cutting down this number even lower.
I show you this chart to give you an idea of how many sandbags you would need to purchase and (hopefully) pre-position in your previously purchased bug out location. It can also give you a vague idea of how long it will take (in hours) to build this structure. It takes two people to fill sandbags. One with a shovel, and one holding the bag. The bags don't necessarily have to be tied closed unless they are to be moved or stored for future use.
By placing your tent area behind the vertical retaining wall, you can create a sloping wind break that will protect you and your tent from high winds. The higher your tent, the higher the walls need to be.
The interior size and shape depend entirely on the size and shape of your tent. Add to the tent size any walkway spaces or fire pit space you may want to add. Keep in mind, for every foot of wall you add to your tent, you have to add in a whole lot more sandbags.
Filling Sandbags for Flooding|
If you haven't done any manual labor lately, "Get ready to rumble." With a willing partner and a large pile of dirt or sand, start filling each bag ONE HALF FULL ONLY.
If you are setting them in place right away: Fold over the top of the bag, carry it into its final position and place it as shown on the diagram on the right. The expected water flow (if any) should run along the long side of the bag. Fold the end of the bag under the body of the bag, and tamp it firmly in place with your feet.
If you are transporting or storing bags: Fill the bags, but tie off the top of the bags to prevent spilling. When placing the tied bags later on, once again fold the tied end of the bag under the body of the bag. This "tucking under" gives the next row of bags a flat surface to build on.
One bag down, 7,000 to go. Considering a typical thirty to thirty-six inch sandbag weighs about 30 pounds, you literally have tons of earth to move by hand. It's no wonder the Army troops hated sandbagging. It's a hot, dirty, backbreaking amount of labor. However, for Army troops under fire, it WILL save your life.
Placing Sandbags for Water Protection|
For a levee or dam built in a pyramid shape, the rule of thumb is to make the base of the sandbag wall three times the height of the wall. For a windbreak wall with a flat inside supporting wall, make the base one and one-half times the height of the wall.
In the following diagrams, you will see how to place sandbags for water protection, such as a levee or to protect your home.
Remember: Sandbags are NOT WATERPROOF. However, if you use sandbags to hold sealed plywood or plastic in place around door openings, you will get only a minimum of leakage. Notice I didn't say water proof – it will only slow it down and keep the leakage manageable.
Shown on the left, this is a typical Florida problem for blowing wind and storm surge problems. The events will be over in a few hours, so permanent masonry construction is not needed. However, a LOT of water can enter a home if you don't do something to slow it down. It is the plastic sheeting that gives this type sandbagging it's waterproof covering – not the sandbags. The bags are ONLY there to provide enough weight to prevent the plastic from being washed away. Will it still leak? Yes, but very slowly. If you do it right, you can keep up with the leakage with only a bucket and a mop.
If your expected flooding is more than just wind-blown rain and a rising (but slow moving) pond, you have to place the bags as shown in these diagrams.
This is the case where flash flooding or an over-flowing creek needs to be diverted away from your property or campsite.
Fold the open end of the partially filled bag to form a triangle. Tuck it under the bag. If tied bags are used, flatten or flare the tied end.
Place the partially filled bags lengthwise and parallel to the direction of flow, with the open end facing against the water flow. Tuck all the flaps under, keeping the unfilled portion under the weight of the sack to prevent it from opening.
Place succeeding bags on top, offsetting by one-half (1/2) filled length of the previous bag, and stamp into place to eliminate voids, and form a tight seal.
Stagger the joint connections when multiple layers are necessary. For unsupported layers over three (3) courses high, use the pyramid placement method.
Did you watch the TV news when the citizens of New Orleans were trying to reinforce the levees? Not one bag was placed correctly . . . Most were merely "dumped" or thrown haphazardly into place. The levees never had a chance.
Pyramid Placement Method
The pyramid placement is used to increase the height of sandbag protection.
Place the sandbags to form a pyramid by alternating header courses (bags placed crosswise) and stretcher courses (bags placed lengthwise).
Stamp each bag in place, overlap sacks, maintain staggered joint placement, and tuck in any loose ends.
Empty sandbags can be bulk-purchased and stored away for future use. How many you need or want depends entirely on what you will use them for. At best, it will be an estimate on your part. Since the cost per bag is actually very low, I strongly recommend you give it your best guess on what you will need, and then double it. It is better to have too many bags than not enough to finish the job.
The example shown here from the Jacobs Trading Company is typical for the costs of most sandbags.
Jacobs Trading Company: "911 Sandbags"
Pricing as of 8-30-06
Call for quantities more than 2000 bags.
Materials Required for 100' Lineal Sandbag Wall
(Pyramid, Flood Control Type)
For the structure to be stable, it should be 3 times as wide at the base as the levee is tall. For example, a 4 foot levee wall should be 12' wide at the base.
SANDBAG WALLS – REVETMENTS|
Sandbag walls have been used for military use ever since the bag was invented. Many walls were hastily constructed and when the walls fell apart the position was abandoned to return to nature. However, while they were in place, they provided shelter from rain, snow, bullets and bombs (unless it was a direct hit). With the addition of heavy beams to make a roof, many bunker walls were turned into cozy bunkers. Perhaps "cozy" is not the correct word. "Livable" would be better.
Since keeping out torrential flooding was not usually part of the battle scenario, the base of the walls didn't have to be tapered in the fashion of the pyramid walls. GI's used the sandbags like floppy concrete blocks, stacking one on top of the other until the desired height was reached. No engineers I know of ever signed off on the construction of sandbag bunkers made in the heat of battle. No building codes exist for sandbag shelters. They are "temporary" shelters, and if abandoned, quickly return to their natural form – mud.
The US Air Force built revetments (1.) out of corrugated steel roofing and sandbags to provide protection for parked aircraft inside the war zone in Vietnam. These were thick walls held in place by using the corrugated steel as wall covering (vertical) to protect the dirt inside the walls. These thick walls would prevent any accident (such as your plane blowing up) from damaging the other aircraft parked next door. They were three sided shelters since the aircraft had to enter and leave on a daily basis.
The Marines and Army usually used the sandbags "as is". The bags were stacked one time, and only replaced if damaged in battle. However unpopular the construction of the sandbag revetments were, they were quickly appreciated when the bullets started flying. For civilian use, you (hopefully) won't be being attacked by hostile forces aiming to drive the infidels out of their area. The sandbags primary use would be for wind protection, and with some supporting lumber, rain and snow protection.
For civilian use, a "revetment" is some form of interior wall to hide behind, with some form of dirt built up on the outside of the wall to provide protection from whatever. Today's sandbags weren't available in the middle ages. Had they been, they would have been used extensively because it is an easy way to move dirt exactly where you need it.
On the first page of this article, I showed a chart labeled "Typical Pyramid Sandbag Placement". I drew in a small orange wall to show you that you can use half as many bags by merely adding an inside wall – and still maintain a wall that has enough slope to deflect strong winds. This slope forces the wind to push "up", taking some of the direct wind load off of the vertical inside wall. The gentle 45 degree slope reduces the flat areas and corners, which helps reduce wind loading, and the earth sheltering helps to spread out loads.
The following definitions may help you understand the concept of revetments and other associated military terms. If you're confused, use my civilian definition, above.
- Revetment - a retaining wall constructed to support the interior slope of a parapet (2). Made of logs, wood planks, fence rails, fascines (3), gabions (4), hurdles(5), sods, or stones, the revetment provided additional protection from enemy fire, and, most importantly, kept the interior slope nearly vertical. Stone revetments commonly survive. Historically, a few log revetments have been preserved due to high resin pine or cypress and porous sandy soils. After an entrenchment was abandoned, many log or rail revetments were scavenged for other uses, causing the interior slope to slump more quickly. An interior slope will appear more vertical if the parapet eroded with the revetment still in place.
- Parapet (It. parapetto, shield the chest) - a linear mound of earth built to defend against incoming fire. The thickness of a parapet was determined by the armament that it was expected to withstand-for musketry, 5-7 feet; field artillery, 8-16 feet; for siege or naval guns, up to 35 feet. The parapet consisted of an interior slope, usually revetted with logs, planks, rails, stones, sandbags, or fascines, so as to be nearly vertical, the superior slope or crest, which inclined slightly downward toward the enemy, and the exterior slope or outer face, which took the brunt of enemy fire. The exterior slope typically inclined 45 degrees, the natural angle of repose for most soils.
- Fascine - tightly bound bundle of saplings used to reinforce a parapet or in revetment.
- Gabion - a large basket of interwoven vines and saplings used to strengthen or shape an earthwork. The gabion was set into position, then filled with earth on site.
- Hurdle - revetment formed by interlacing vines or saplings through a series of posts set upright against the interior slope of the parapet.
What the above definitions should tell you is the ancient warriors used whatever was on hand to build the inside walls of their fortifications. The same is true for the survivalist. While it is true a vertical wall provides wind protection, it is also true a flat wall is under a lot of pressure by high winds. Without a backing wall, or without a sloping wall (which deflects the wind rather than stops it), a long, tall stack of sandbags may collapse due to the pressure of the wind.
This aerial view of a circular shelter for livestock shows that blizzard winds will create the desired shape by filling in the walled area with blown snow. Notice the majority of the interior of this sheltered area is mostly snow-free. Had you pitched your tent inside this shelter, it would have been jostled around, but kept safe from the hazardous snow build up that can collapse any tent. The protection of this walled in area reduces the wind by approximately 60%.
Since this design is for livestock, the entrance to this sheltered area is left open and is fairly large. For people, this can be blocked off with short walls to make it almost perfectly secure no matter which way the wind blows.
Solid Fences (Includes solid sandbag walls)
As wind slams into a solid fence, it is either forced up and over the obstacle or around it. Air pressure increases on the upwind (windward) side, and a slight vacuum is created on the downwind (leeward) side. As the wind is forced over the top of this solid barrier, its velocity increases. After crossing the top of the barrier, the vacuum created on the downwind side causes air turbulence which dissipates the wind's energy. The wind no longer retains enough energy and velocity to carry a snow load; therefore, the snow drops out.
In the above drawing of a flat wall or fence, the wind will drop the snow in two places: (1) on the upwind side of the fence at a 45-degree angle for a distance of about one-fence-height, and (2) on the downwind side for a distance equal to about five times the fence height (Figure 1). The distance of snow drop on the downwind side is not directly proportional to fence height, and the effect is somewhat less than five times the height for very high fences. The area of wind and snow protection for a solid fence, then, extends from about five to fifteen times the fence height downwind. For example, a 6 foot fence will have a protected area behind the fence of from 30 to 90 feet. Unless you are living in a circus tent, this is ample room for your tent.
Windbreak fences require sturdy construction, since they take the wind's full force. Pressure created by the wind increases rather significantly as fence height increases. For instance, as wind speed increases from 10 to 30 miles per hour (mph), the force on a solid fence increases nine times.
Wind velocity and fence height determine the size of the protected area. For instance, when the wind is blowing at 10 mph, a 6-foot-high porous fence will reduce that velocity to a minimum 10 feet downwind from the fence. When the wind is 20 mph, the minimum velocity point will be 65 feet from the fence; and at 30 mph, the area protected is about 90 feet downwind.
As a general rule, wind protection is usually adequate for a distance equal to about 20 times the fence height, whereas the major snow drop is in an area downwind equal to about 10 times the fence height (Figure 3).
Fencing, or the use of sandbags without a backing, provide a lot of protection from wind and snow. But, as the above article from Purdue University shows, they will not keep out ALL the wind and snow. If you can get a wind-speed rating for your tent, you can decide what type of wall or protection you want to consider. A flat wall stops 60% of the wind. That means a 100 MPH wind will only be 40 MPH inside the protected area. If your tent is only rated for 30 MPH, you can expect problems with tent integrity. If your tent is rated for 50 MPH, you shouldn't have too many problems.
The trick is to build the best windbreak you can. By using sandbags to put a slope on the protective walls, you not only strengthen the protective wall but create more turbulence over the top of the shelter, further dissipating the strength of the wind.
As you all know, on cold days the stronger the wind blows the colder it gets. I published a wind-chill guide earlier this year and it should show you how much warmer it will be when you build any wall or protection system that can lower the wind by 60%.
Designs That Work Depend on Where You Are
If you are camped in an area where the wind is predictably from one direction only, then you only need a wind break between you and the wind. If the wind directions are totally unpredictable, then you should consider a full circle of wind protection. It all depends on the site.
Factors you should consider when using sandbags to construct your windbreak:
- Sandbag walls offer the maximum protection from wind-blown objects that can maim and kill you, as well as tear gaping holes in your tent.
- Flat sandbag walls offer wind protection but are subject to high pressure failure on long spans that are not supported by a backing wall.
- Using flat walled sandbags with an interior supporting wall (or fence) may double the protection from extremely high winds. Using sloping walled sandbags increases the number of bags to be placed, but offer the maximum protection.
- Tornado winds are of such high velocity and short duration that a full-circle of sloping walls around your tent may just allow a tornado to "skip over" your site. At the very least, the inside corners of the sandbags (where the sandbags meet the ground) are a natural protected area to hide in. Flying objects will only come from the contents of the tent, and the tent itself.
- The site itself may offer some natural protection in the form of trees or hills. By siting your tent in one of these protected areas, and then adding additional protective walls, you will maximize the wind protection and cut down drastically on the number of sandbags you need to fill and haul.
- If at all possible, use a sloping wall of 45 degrees. This has been proven to be the optimum protective angle to divert damaging winds.
If you didn't notice, the use of flat walls create a minor problem behind the wall. As the winds increase, a certain percentage of the wind will circulate behind the wall in the form of turbulent down-drafts. These down drafts can be quite strong depending on how fast the wind is blowing. If you look at the aerial photo of the circular feed lot in the snow, you can see some snow is also piled behind the wall. It has been placed there by these down drafts.
FYI: In aviation, this turbulence area creates a "Lenticular Cloud" (lens shaped). This is a dead giveaway that the down drafts on the side opposite the wind direction are EXTREMELY STRONG. Many an aircraft has crashed because it could not pull out of the winds created by the downdraft. It occurs when the wind strikes the mountain at a ninety degree angle. Also, on the windward side of the hill, tremendous UP drafts can and do occur.
360° Wind Protection Wall for Tents
Add Drains at base of wall on downhill side to let out trapped rain water.
Wall height depends on height of the tent. Wall should be at least as high as the tent, or one layer higher for maximum wind protection.
Walking area should be gravel to prevent puddles and mud holes.
Entry areas should be 3 feet wide to allow large items to enter tent area.
Berm up the outer walls with sandbags, dirt, or brush to slope away from walls and give wind an "uplift" to blow over the walls rather than press directly on them.
The higher the wall, the wider the base must be to keep the wall stable.
Gray area is the recommended wall area around tent and does not necessarily reflect a flat-surfaced interior wall. Unsupported, this wall should be Pyramid shaped with sloping walls. It is NOT a bunker design, but is for wind protection only.
Walking space around the tent is needed for routine tent maintenance.
Drawback: On hot days, it will be "windless" inside the sheltered area. To avoid the loss of all air circulation, openings in the wall – aligned with tent windows – can be built into the wall. These can be covered during high wind conditions with lumber, sandbags, etc.
For use beyond 2 years, some form of cover must protect the sandbags from ultraviolet light: Concrete, stucco, etc.
This protected tent area would be best built at your permanent "bug out" location if you decide to NOT build a more permanent protected building. More than one area can be added by constructing adjoining areas and sharing walls. Care must be taken so rain runoff from one site does NOT enter another site, compounding the flooding potential. Using existing hills as protection is a good idea PROVIDED YOU ARE NOT IN A FLASH FLOOD AREA.
Stealth: Easily seen from the air, the area over the tent could be covered with camouflaged nets or brush supported on a wooden framework. Sides are easily camouflaged by adding dirt (which also protects the sandbags from sunlight) and planting local vegetation with shallow root systems. Hiding your vehicles is another problem not easily solved.
Without the use of sandbags, the walls will eventually erode away from rain and wind. However, some archeological remains of civil war "earthworks" can still be seen at some national battlefields. Eroded but still standing. A permanent covering of chicken wire and concrete will help this structure keep its shape. It will also prevent burrowing animals from taking up residence between the small gaps in the sandbags. Snakes immediately come to mind. This covering can be added later on.
VIETNAM ERA SANDBAG USE
Boy, does the RogueTurtle remember these things. The first week in country I always knew the shortest route to the nearest shelter in the event of a rocket attack. By the end of my tour I spent one evening sitting on the rooftop rooting for the rockets.
My barracks was way too close to the Officer's Club bar.
This is a model of a bunker used by the Army in Vietnam.
Source: Acheson Creations.com
"Many of the sandbag bunkers in the Long Binh and Bien Hoa area were covered with chicken wire and then coated with concrete."
The inside floor can be dug out (and usually was) to provide more headroom and protection from the ground itself. The drawback to digging out the bottom is that the torrential rains tended to make it a mud pile with walls. Many floors were lined with empty wooden pallets to at least give you something dry to sit on.
Almost all tents now have a natural curvature to them making them more wind resistant than the older square tents. For every foot up you raise your protective wall, you decrease the amount of wind pushing on your tent and trying to uproot your tent pegs. Protective walls take a lot of manpower and time to build. If most of the really bad storms in your area come from the North, then build the North wall first. The nice part about sandbags is you can always add more whenever you feel like it. Or, whenever you are forced to do so.
The goal of the protective tent wall is not to make a fighting position, but to keep your tent intact for a long period of time. If you are only going to be in the area for a couple of days – or even weeks – don't bother unless you are expecting severe weather for the entire duration of your stay. However, if you plan to return or stay for a prolonged time period, these walls will be worth the effort.
Every visit to the camping area can result in higher and higher walls, with full protection obtained faster (and easier) than you would think. Two people can add at least one layer of sandbags in one day, without putting your back in traction in a non-existent hospital.
Adding lumber: If you are lucky enough to have trees, lumber or other construction material available, then it is possible to build an interior supporting wall to keep the bags of a flat wall from falling over. This allows you to have square interior walls and sloping outer walls. This is the best of both worlds. However, this inside wall has to be STRONG to support the weight of the sandbags that will inevitably lean on the interior walls. It is possible to combine both systems by building a shorter interior wall out of square lumber, and then (when the lumber runs out) switching over to the pyramid-shaped wall design. There are no limits. The real advantage to square interior wall supports is you only use half as many sandbags as the (full) pyramid-shaped walls. This saves you half the work.
REVETMENT DETAILS USING DIMENSIONAL LUMBER
This wall design assumes you have pre-stocked pressure treated lumber available to construct a semi-permanent wind proof wall system. In northern areas, all posts should be sunk below the frost line.
Revetments are two parallel rectangular-shaped strong walls filled with earth, sandbags, stone, or whatever fill is available in your area. In Vietnam, the engineers used many types of these barriers to prevent mortar or rocket damage to aircraft. On a smaller scale, using pressure treated lumber walls (filled with sandbags) would give you great protection for a small tent area. The sandbags are sandwiched between the wall sides as the walls are built. The tall vertical posts should be one solid (4" x 4" or larger) beam with the bottom portion of the beams sunken into the ground. Drainage for rain runoff must still be considered. Since most dimensional lumber comes in about eight foot lengths, you can dig post holes two to three feet deep (eight feet apart) and have a really strong wall using 2" x 6" lumber for the wall sections. As you fill up the wall with sandbags, add another 2" x 6". They can be nailed in place or left "as is" using the sandbag weight to hold them in place. All lumber should be pressure treated to prevent rotting lumber from collapsing your walls. I would try to limit my walls to eight foot sections (in length). This type of revetment will last for years and years if the top of the walls are waterproofed. The spacing for the tall upright 4" x 4" walls should be wide enough 2 or 3 sandbags side by side. When filled, the sandbag walls are very heavy. You will need scaffolding to lay bags over shoulder height. This style of revetment would be expensive but it would last for a minimum of ten years or longer.
What's New: Stealth Walls for the Bunker
Weatherproof tarps may reduce bunker sandbag replacement
Stars & Stripes, January, 2001
EAGLE BASE, Bosnia and Herzegovina — The Army's old sandbag-shielded bunkers are in for some reinforcement.
Defense contractors are wrapping 75 bunkers with a new, heavy-duty waterproof tarp to prevent wear and tear.
The military hopes the black, woven fabric will add three more years to the life of the typical bunker, saving base camps across Bosnia time and money