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Retaining Wall Challenges

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:

  1. Tipping:  The entire wall begins to lean and eventually tips over
  2. Buckling:  The wall buckles and bends, often the breaking point is about 1/4 to 1/3 of the way up the wall.
  3. 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.”
The owner has identified that there is a drainage problem with the wall.  The practice of having the drainage pipe part way up behind the wall comes from other engineered retaining wall systems such as “L” shaped poured concrete walls.  In a gravity wall, with proper draining material, it belongs below the bottom of the foundation, and this is especially true with a dry stone wall.  In most dry stone walls, water flows freely through them, so a drain pipe part way up the back is utterly useless.  The cut stones in this wall fit tightly enough that water penetration will be slow, making it more imperative that there is good drainage.
It is important to view retaining wall construction as SYSTEM. All the aspects of it must work together.  A drain pipe water can’t get into either because it is in the wrong place or surrounded by the wrong material will not be effective.  Likewise the best drained wall in the world will still fail if it was not stacked right or did not have enough mass to retain the weight behind it.  Site conditions are very important in determining the correct system and the variables in it.  Lots of contractors (and designers) rely on specific pieces of knowledge relating to a specific material or method without thinking about the FULL SYSTEM needed to achieve the desired result.  This is likely the reason there is a wide range in answers.  People are answering a specific questions making assumptions about the system that are widely varying.
Lets get to the owners specific questions:
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.
Answer:  The selected stone used can work fine.  While the sawn surfaces have lower friction than some stones, they arguably have more than others (such as round river rock.  The problem is not the stone selected but rather how it was used and stacked, and the drainage behind and below it.   An 8″ thick wall does not have the needed mass to retain soil at that height, even if the drainage was excellent, 8″ is inadequate in a case such as this.  A 39″ tall retaining wall should generally have a base with of 26″ to 30″ or more.  An engineer will tell you that is an excessive in terms of mass, and that is true.  But the wall also has to be stiff and essentially rigid.  That thickness is needed to build a dry stone wall strong enough to be rigid.  The single stack nature of the way this wall was built does little to prevent buckling or sliding between the stones, as seen in the wall pushing out at about 1/3 the way up.  The wall needs to be built as a double stack wall with proper stacking techniques for a dry stone wall.  Note: The back stones do not need to be the same as those that make up the front face of the wall, a much less expensive stone can be used.  In this case, the 8″ wide stones are a bit narrow and are traced (long edge parallel to the wall face) instead of properly length in.  However the evenness and flatness makes this less of an issue than it would be with other types of stone.   It would be strongest to turn the stones so that the ends of each stone forms the viable face of the wall.  However in this case using the stone traced, provided all other aspects of the are build to compensate for this and the total wall is suitably thick, is likely acceptable.  In this case adding mortar to a wall built like this would be counter productive as it further prevent drainage and would not solve the underlying problems.  The wall is not failing because it is dry wall.  But because the retaining system was not adequately designed.
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.
Existing wall showing the movement. Tipping, and buckling, and sliding (several courses up) are all present in this wall.
Photo showing overall wall and site
Photo showing extent stones are pushing out
Another photo showing the movement of the stones
A dry stone wall built with one type of stone for the face and a much less expensive stone for the back. Crushed stone and filter fabric visible behind
Not the thickness of this dry stone wall which will be about 6 ft tall.
Standard TypicalStone Wall Construction Detail
Typical Dry Stone Retaining Wall Construction Details
Answer:  This gets to the heart of drainage issue.  River run round stone can mean different things to different people and refer to different products.  The objective is to back fill the wall with a free draining material that holds no water and has plenty of air space so that if water does freeze in it, it will expand into the air spaces, rather push the wall over.   River round stone or pea stone, provided there is no sand or fines in it,  accomplishes that, but it also has a very low angle of repose.  It is a bit like back filling your wall with marbles.  It has no structural stability on its own.  A much better choice if available locally is clear crushed stone.  This is angular stone that has had all the fines screened out so it is free draining.  1 1/2″ dia. is ideal for back filling a wall this size.  3/4″ would be okay and is easier to shovel if placing by hand, but it less structurally stable.
The volume of crushed stone needed is site and soil specific.  In general due to the wall tapering in thickness as it goes up, and the angle of the native soil behind the wall it will be less at the base, widest part way up and then reduce to very little at the top of the wall (see typical section).  For this wall in the clay soil conditions described, 1 ft wide at the base would likely be fine (behind a properly thick wall).  At mid way up the wall it should be about 2 ft. wide as minimum.  More is often better.  If you leave a width of 6″ or more of crushed stone exposed at the surface behind the top of the wall it will collect surface runoff and help prevent your top course of stone from pushing off the wall.
Just as drainage behind the wall is important, drainage below the wall is equally important.  For a wall such as this in clay soil, the bottom of the 1st course of wall stone should be about 3 to 4 inches below the finish grade of the patio.  Beneath that, 12 to 18″ of clear crushed stone that has been thoroughly compacted should be used (compaction needs to be done with a jumping jack, not a vibratory plate compactor).  The depth of the crushed stone varies because the bottom of the foundation trench should pitch 1/4″ per ft along its length. Perforated drain pipe should be installed in the bottom few inches of the crushed stone.  If the holes on the pipe are installed facing down it should be on top of a few inches of crushed stone.  Alternatively it can be installed with holes to the side or up and placed directly on the bottom of the trench (this is a topic that can be gone into at length…).  I typically recommend using SDR35 pipe (teal green color) as it is much stronger and wont shatter when cold.  Avoid flexible coil pipe that crushes easily and is almost impossible to lay evenly without hollows which will trap sediment and clog the pipe eventually.
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.
Answer:  Again this is site and soil specific.   I am assuming the ‘fiber’ you are referring to is a non woven filter fabric.  This looks like felt that is abut 1/16″ thick or less.  In general filter fabric is most applicable sandy or silty soils that erode easily when water flows through them.  The concept of filter fabric is that the voids between the fibers catch the soil particles while allowing the water to flow through.  In clay soil, when compacted, it generally does not erode quickly.  The tiny particle size in clay soils will also usually clog a filter fabric making it more like a sheet of plastic.  This prevents drainage and causes problems.  Depending on the specific project situation common solutions are to either not use filter fabric, or to place a layer of sand between the fabric and existing clay soil.  A resonable solution from the information given on this project is to use filter fabric only on the upper portion of soil back fill where it is above the crushed stone stone to use a layer of 6″ of sand on top of it before back filling with the native soil.  Alternatively not using any fabric or sand would probably be ok as well, provided the clay is well compacted (loose or aerated clay soil is very erodible).

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:

  1. If you put a garden hose stream of water on it should flow through almost instantly, not form a puddle.
  2. If you hold it up to the sky you should see a substantial amount of light through it.
  3. 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?
Answer:  This is referring to the ‘batter’ of the wall.  What you describe works out to a 1:12 batter ratio.  1″ in for 12″ up.  The typical range for batter on a retaining wall like this would be 1:6 to 1:8, so a 1:12 batter is a bit steep, though with the flat stone used a 1:12 batter is not out of the question, it does slightly weaken the wall so you would want to compensate in other ways (i.e. more through stones, thicker wall, better length in on the stacking etc).  An related issue is that the stones should all be pitched back into the wall.  at a 12:1 ratio so that 1 ft into the wall the stone is 1″ lower than at the face.  Thus in order for a stone to push out it has to actually move up, thus using the weigh of the wall above in addition to the friction between stones to resist the outward pressure.
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.
Answer “Dead men” is a term typically used in wooden retaining walls were long pieces are placed perpendicular to the wall so they extend into the soil behind the wall and often have a cross piece at the back end to structurally tie the wooden timbers into the soil behind.  In dry stone walls, this can most closely be compared to through stones, which are very important to the structure of the wall.
In conclusion, when designing or constructing a retaining wall it is important to remember what the forces are on the wall and plan how you will resist them.  It is important to plan a drainage system that will remove the water and have minimal potential to become clogged.  In most cases we get caught up with what material to use and not how it is used or thinking back to what the goal is and if it is being achieved.   For example, drainage systems using no pipe at all can be designed and built very effectively, but simply removing the pipe from a system that was designed for it, or adding pipe to a poorly thought out system wont achieve the desired results.  Most importantly how a wall is stacked cannot be over emphasized.  A poorly stacked wall will still fail no mater what is done for back fill and drainage.  For more information be sure to check out the standard specifications for dry stone wall retaining systems published by the stone trust and available free here.
One of the best ways to learn about building a retaining wall is to attend one of our retaining wall workshops.  Click here for more info!