Landslides, Debris flows, Creep, Slumps

This is just an introduction and an overview. There are many things that a property owner or a general contractor can do to reduce the chance of earth movement before it happens, or to mitigate it after the fact. 

If you're looking for some ideas to deal with water erosion (ocean waves, rivers & streams washing away or undercutting banks, levees, etc.) then you might want to also look at our flood control page.

Beyond these pages, we encourage you to expand your search. There are great websites (especially government sites) and books that offer a much more comprehensive treatment of a complex topic than we do. But there's no substitute for getting a qualified & licensed engineer, architect, or geologist on site, especially if safety or protection of property is at stake.

Slope failure (or "mass wasting") can be described as downslope movement of soil or rock debris. It can be slow and gradual (creep), or sudden (slumps, debris flows, and landslides). 

Gravity is always at work on a slope. A "stable slope" can be broadly described as being about 30 degrees or less (angle of repose) and possessing a solid footing at its bottom. A slope of more than 30 degrees can be stable, and one less than that can be unstable. Factors like a lack of vegetation, soil composition & compaction, moisture, surface friction, presence of rocks or boulders, etc. can all quickly contribute to slow movement ("creep") or sudden collapse. 

There are a variety of other things that can make a slope unstable or cause it to move. Water results in saturated soil and rapid erosion. This is the most common cause of failure. Heavy rains, a backed-up culvert, a broken underground water main can all contribute to a slope failing within hours, through lubrication and liquifaction of the soil and by increasing the weight of the soil dramatically.

Fires can be disastrous to slope stability. They burn off the vegetation and kill the roots that hold the soil together. Enhanced flooding may result because the heat from the blaze can scorch the bare soil to a hard glaze ("burn scar") that rainfall just sheets off - resulting in fierce flow volume & velocity.

  Other causes of slope failure can include heaving (the cycles of wetting &  

  drying, especially with clay-rich soils), or freezing and thawing; earthquakes;

  vibration from heavy nearby construction or traffic; and overloading the top

  of the slope, such as building a house or a high-traffic road too close to the

  edge.

  Among man-made causes, backcutting the foot of the slope (or failing to 

  ensure its adequate drainage, allowing it to become oversaturated & 

  unstable) is perhaps the most common. It's what we do when we build roads or houses at the bottom of a slope, and it's like pulling an orange from the bottom of a pile in the supermarket's produce section. Equally dangerous is overloading the top of the slope (such as building houses, condos, or roads) on the edge; their weight puts downward pressure on the slope, and they contribute significantly to rainwater runoff which can accelerate erosion.

Recognizing instability:

Signs that a slope might be unstable or creeping include cracks in asphalt or cement paving, leaning trees, fenceposts, or utility poles (either towards or away from the hillside); offset fence lines; and changes in vegetation (suggesting the new presence of water). 

If there's a structure (on the slope, at its top, or at its bottom), signs of movement can include cracks or soil moving away from the foundations, sticking doors and windows, cracking concrete floors, sunken or down-dropped spots in roads & driveways, broken water pipes, and noticeable movement or separation of decks & patios in relationship to the building.

Steps to take:

Again, this page is just an overview. In some cases, it can be a quick fix. In other cases, you can spend a lot of time, effort, and money, including securing the services of a professional engineer or geologist. And sometimes you might be flogging a dead horse - the conditions may just be impossible (see above photo). Some things to look into doing can include:

• eliminating or redirecting sources of water that might be overly saturating the soil;
• covering the slope with geotextile material;
• planting vegetation; 
• terracing or benching the slope;
• installing lateral french drains or culverts;
• minimizing or eliminating elements that cause surface erosion;
• using straw wattles or bales to break the velocity of runoff & to catch sediment;
• stabilizing the slope with shotcrete or gunite, and covering with wire mesh.

Equally important are redirecting flow sources to cut down erosion & soil saturation, and stabilizing & reinforcing the foot of the slope with retaining walls. These are where sandbags can be helpful.

Overhangs: On steep slopes here in the Southwest, the differentiated strata that's so common can often result in hard rock that's underlain by easily eroded softer rock. This can result in a slope face in which hard rocks are hanging over a concave face that's been eroded away. A vertical prop wall can be built with sandbags, filling in the concavity, and reaching up to support the overhanging rocks. (Think of a filling in a tooth cavity - same concept.) This can be secured in place with fencing, and will help to prevent further erosion to the softer underlying rock.

As with all permanent placements of polypropylene bags, the bags need to be covered from the sun or plastered (mud's fine, so long as it doesn't wash away; geotextile fabric or shotcrete is better) so UV rays don't cause the bags to disintegrate. The 1600 hours that our bags are treated for is good for only a couple of months, at best. If moisture isn't a problem, burlap bags might be your best bet. A more expensive (but higher insurance) alternative would be to use premium bags, such as our 4-year heavy-duty black polypropylene bags or polyethylene Durabags.

After collapse:

If it's safe to do so, the priority should be to prevent any further sliding or damage as much as possible. If there's a building, road, or stream immediately downslope, you can stack sandbags and try to create a wall or barrier that will contain or divert mud, rocks, and any further collapse of soil. If it's raining, doing this right away will help prevent the fallen material from liquifying & spreading, especially desirable if - again - there's a house or a road.

If there's any significant water flow on the upper part of the slope that might aggravate further collapse, it might be possible to divert or redirect it with a sandbag check dam (see below). Odds are, however, that it's best at this point to evacuate & come back in a few days with heavy machinery.

As mentioned above, support can be provided to overhanging rocks that are at risk of coming down with a vertical prop wall of sandbags, and eroded concavities can be filled in with bags to help stabilize the slope & prevent further erosion .

Diverting and redirecting flows:

If slopes have any purpose, it's to transport water downhill to drainage systems (creeks, arroyos, rivers). It can be concentrated (like in a gully) or it can be a sheet of water covering the entire slope. Such surface flows can be diverted by channeling the flow sideways to what can be a "safe" drainage. Safe drainage can include pipes, gullies, and swales (that allow backed-up water to seep through absorbant soil) which are away from the slope that's being protected.

Diversion methods can include building channels with rocks or sandbags, elevated berms, check dams (see photo) and/or channels with rocks or sandbags. Berms and check dams generally don't need to be any more than two feet high, and don't necessarily require any digging (though a bonding trench will help to prevent the bags from washing away in large flows). Placing barbed wire between the layers of bags, and/or covering them with wire mesh can make them solid & resistant to collapse. Covering them with earth, concrete, stucco, or a basic earth plaster (clay-rich dirt, sand, and straw) will do the same, and protect the bags from the sun's UV rays & abrasive deterioration. If using bags, burlap bags may be a better choice.

On the other hand, it's worth noting that slowing down runoff can lead to ponding, which gives water a chance to seep into the soil and undermining your barrier. Sandbags that aren't sealed won't provide a 100% waterproof barrier; water will seep through them, which helps prevent ponding. It's a trade off, and depends on if you're looking at a temporary or a permanent fix. Sealing your bag barrier - say, with plastic sheeting - will require some forethought about how you're going to deal with any possible standing water issues.

In addition to dealing with water flow, another source of concern is mud and debris flows. These can be disastrous - not only because of their destructive weight, but because they can precipitate a domino effect when they rush down a slope that's already saturated. The best you can hope for is prevention. If your check dams are successful at catching them and retaining them, you're going to want to clean them out after the storm.

Stabilizing the base:

People like to build roads, houses, and patios at the base of hills and slopes, and tidy them up & maximize the real estate by removing the wedge. Doing this, however, weakens the slope - because the slope exerts downsloping pressure (even if it's not actively "on the move", or "creeping"), this is where the weight of the slope is concentrated. 

  A well-built retaining wall (toewall) at the base of the slope should follow the 

  line (angle) of the slope as much as possible. Where is the direction of force

  coming from? The retaining wall should meet this, and smoothly transfer the

  downward weight of the slope. It should be wider at its base, narrowing as it

  rises, and lean solidly against the slope. Even better would be excavating

  the base of the slope and building the retaining wall into it.

  Sandbags are well-suited for this; they're portable, easy to work with, and

  can be solidly tamped into place. Again, strips of 4-point barbed wire can be

  placed between the courses, locking the bags together. Since UV rays tend to break the polypropylene fabric down over time, it's a good idea to plaster or stucco the bags after the wall is built.

Don't overlook the importance of dealing with drainage. If hillside saturation is an issue, one approach is "weep holes" in your wall - spaces between bags every three feet or so, downward-sloping if possible. While a polypropylene sandbag retaining wall won't necessarily wick up water, failing to providing an outlet for it as it backs up behind the bags can supersaturate & weaken the surrounding soil. Or, if the soil behind the bags is rich in clay, it can cause that soil to swell & expand, pushing your bags out, ruining your wall at the least and possibly resulting in total slope failure.

Working with sandbags:

Sandbags come in two basic materials. Burlap is subject to rot and is attractive to insects and rodents. Polypropylene is rip-proof and water resistant, but prone to deterioration in the sun (our poly bags are treated with 1600 hour UV inhibitors, which in New Mexico summers gives them a life span of only a couple of months). Either material will last longer if sealed with stucco or cement. A basic earth plaster can be made on-site out of clay-rich dirt, sand, and water (a little straw will give it strength). On a summer day, it will bake in the sun and may last for years with a little maintenance. If your project is temporary, you can simply cover the bags with tarps or shovel some dirt over them to keep the sun off.

It's not a bad idea to pick through or sift your dirt before filling your sandbags, removing sharp twigs & branches, as well as root balls and clusters (which can sprout & burst through the bag). Bags are generally filled 1/2 way, then tied off & laid horizontally. The opening is folded under as the bag is laid, and the bag is shaken to distribute the contents equally. A hand tamper is then used to compact them.

As they're being laid, the bags should be overlapped and butted together (see image). If they're being subjected to lateral force (such as a check dam), the basic rule of thumb is that the base should be 3 times the height, which calls for a pyramid construction.

Depending on your fill material, a standard sandbag (14" x 26") will weigh about 25-30 lbs after being half filled. This means that every 65 to 80 bags will give you a ton (2,000 lbs) of reinforcement. One average person can fill, tie off, place & tamp 20 bags per hour. Several people, setting up a production line, can crank out 100 or more bags per hour (see our bag filler). If it's a big job, we can bring our sandbag machine out to your site, which is capable of cranking out over 800 bags per hour.

Burritos:

If you have the means to deploy them (they can be large & heavy), you can construct what we call  "sandbag burritos". They're essentially quick-and-dirty cylindrical gabions holding filled sandbags that take less than 10 minutes to construct. 

Roll out some 6-foot chain link fence, then cut out a 3-foot wide section. Next, lay a number of filled sandbags on it (say, two vertical rows of 3 or 4 sandbags), then roll it up & fasten the edges and the ends with hog rings. Six 50-lb sandbags gives you a 300-lb, 6'L x 2' round burrito. Eight 50-lb sandbags will give you 400 lbs. Twelve 50# sandbags will give you 600 lbs. You get the idea. 

The chain link allows you to link the burritos together after they're placed. Utilizing higher-quality sandbags and/or covering the burritos with dirt (so UV exposure doesn't cause the bags to disintegrate) will help ensure long life. Even better might be to use our snake bags.

Deploying burritos isn't always easy because of their weight and because erosion sites tend to be steep, uneven and unstable. Being cylindrical, they do roll off a steep edge nicely; the trick is making them stop rolling where you want them. Hooking up premeasured chain to them with a solid belay before rolling them over the edge helps make them stop where you want them; then hammering in a few fence stakes below them will keep them put. If your conditions call for them, they're easy, inexpensive, and far more effective than just using individual sandbags. If you like, we can supply them to you pre-assembled to your specs; write or give us a call.

In many areas, you may need a permit to build retaining walls. If you plan to file a claim with your property insurance, you might have it rejected unless you had an engineer or a licensed professional involved before you built.

In times of disaster, bags can sometimes be acquired from your local fire station or emergency response authorities - but they may be limited in number (some have told us that they've encountered limits of 10 to 20 bags per household).

For farmers & ranchers, businesses, government & tribal agencies, and anyone else, we can provide bags in any quantity, from one to 100,000 (larger quantities need to be pre-ordered). We don't stop at just selling them to you - if necessary, we'll do whatever it takes to get your bags to where you need them, and free you up to worry about other things. Visit our shopping page or contact us for prices & options.

For more in-depth information about controlling erosion, you can download a 64-page USFS booklet here (pdf). Also, we highly recommend An Introduction to Erosion Control from the Quivira Coalition in Santa Fe, available for download here (also in pdf format).