We were recently emailed about a retaining wall that was starting to fail just 8 years after construction. This wall is a good example of the problems that retaining walls can have whether dry stone or constructed with other materials.
The stone this wall was built of is somewhat atypical for dry stone walls, in that it is sawn top and bottom. The pieces are 3″ thick and 8″ wide. Most are 16″ long. The wall is 39″ high and is in an area with ground frost and clay soil based on visual assessment.
There are a number of issues, and solutions that contributed to the premature failure of this wall. Before we get into those it is important understand how and why retaining walls fail.
A dry stone wall falls under the classification of a ‘gravity wall’ this is to say that it is a mass of material with sufficient weight to not be moved by weight of the material it is holding back. In order to function properly a gravity wall must be built so that it distributes the pressure of the retained soil throughout the structure without bending.
The weight behind a wall can very dramatically with soil and site. Soils have different angles of repose, which is the angle they will naturally slope to without a wall. A wall at the bottom of a slope has far more pressure on it than a wall retaining a level grade above. There can also be ‘live’ loads such as cars driving above the wall. A huge factor is waters. Some soils become saturated easily while others are free draining. Saturated soils can literally become fluid (think of mudslides) putting tremendous force on retaining walls.
Finally in climates that experience freezing weather, will experience ground frost. Because water expands when it is wet, wet soil expands when it freezes. And the expansion can be dramatic in some cases (think multiple inches per foot of frozen soil. With a retaining wall the pressure caused by freezing soil behind the wall push outward on the wall as it seeks the easiest way to expand.
The pressure exerted by freezing water varies with temperature, but pressures can in the 1000’s of pounds per square inch! It is impractical to resist the pressure of freezing water in most retaining wall situations. Thus adequate drainage behind the wall is needed to prevent wet freezing soil from putting pressure on the wall.
How retaining walls fail:
Gravity type retaining walls commonly fail in three ways:
- Tipping: The entire wall begins to lean and eventually tips over
- Buckling: The wall buckles and bends, often the breaking point is about 1/4 to 1/3 of the way up the wall.
- Sliding: The entire wall slides forward.
Tipping and buckling are the two most common ways dry stone walls fail, and it is often a combination of the two as seen in this wall. Sliding most often happens on walls that are built at the top of a slope, where there is little keeping the foundation stones in place.
So, lets look at the wall in question:
The owner writes:
“I have a 39” high dry stacked retaining wall that is 50’ long and there are perpendicular walls at either end. It is built out of cut limestone.It is 8 years old and is failing (pushing in at the lower 1/3 of the wall) partially because the contractor who built it did not put their drainage pipe at the bottom of the wall and left it up about 12”.We are redoing it with another contractor… but we do not know the correct way to build it for sure and every one we ask says something different.”
1. Is it possible to build a dry stacked wall 39” high with the material we are using and not have it fail after 8 years. The soil is clay but without testing it I know that is a difficult question to answer.
2. How much river run round stone should we back fill with. I have heard anywhere from a 1 foot trench behind the wall filled up to the top to a 3 foot wide trench. 3 feet seems overkill to me and it would seem that the water could make its way to the drainage pipe in a 1 foot deep trench.
3. Should we use a soil retaining fiber between the soil and the stone or will that just clog and stop proper drainage. The existing wall has the fiber.
Part of the answer is to have the right fabric, it is not a yes or no question because filter fabrics vary widely, it has to be viewed as part of a system. Many fabrics are too tightly ‘woven’. In general you want one with a flow rate over 100 gal/per min/sq ft. Many fabrics also tear very easily and wont work well because of that. Ideally the fabric void space should be paired with the partial size of the soil, but that gets complex for a project of this scope.A practical field test for filter fabric:
- If you put a garden hose stream of water on it should flow through almost instantly, not form a puddle.
- If you hold it up to the sky you should see a substantial amount of light through it.
- You should not be able to jab your thumb through it, and it should be difficult or impossible to cut with a shovel.
4. Each of our 13 3” layers of stone was set back 1/4” for a total of 3.25” that the wall leans back into the hill side. Is this enough?
5. Since this is a dry stacked wall my guess is that dead men will not achieve anything because they may stay put but the stone around them could still move.