Avoid Common Mistakes When Building Retaining Walls

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Concrete Rubble block Retaining Wall    Note the cap

Concrete Rubble block Retaining Wall      Photo by r.a.kukkee

 

Retaining Wall Failure: Is it Predictable?

 Perhaps a loose-piled, simple  flower-bed retaining wall in your back yard failed during  a recent  storm because  the soil under it  washed away.  With little harm done, a few hours of work might be required to restore the simple structure to it’s former glory.  Just down the street, they weren’t so lucky; a high, carelessly-built retaining wall collapsed, and the slumping hillside blocked a main city artery, blocking egress for ambulance and fire crews responding to emergencies.What happened?   Perhaps the retaining wall failed simply because the builder underestimated the requirements for a retaining wall of that size, or was not aware how to avoid common mistakes when building retaining walls.   It may be a happy circumstance that the majority of retaining wall failures fall in the smaller category, or at least somewhere between these extremes, --but they do occur. Retaining walls become stressed, crack,  deform, sag, lean,  break and fall over. Why?  Is failure predictable?  It may well be if common mistakes were made in building.  

‘Some stone and block retaining walls are over a hundred years old. What characteristic have  made them withstand the test of time?'  

  The immediate physical causes of failure are water damage, ground pressure, stress, inherent structural weakness, structural  instability and materials failures .  There are many complex reasons for damage or complete structural failure of retaining walls, but  a combination of contributing factors and mistakes may include:
  •  ·         Inadequate assessment of site conditions 
  •        Underestimating the forces of nature
  • ·         Inadequate design
  • ·         Choosing a  marginal structural construction system  
  • ·         Confusing the aesthetic requirements of a wall with structural necessity

The most common physical causes of  retaining wall failure are

  • Inadequate footings:   Cost of footings and other unseen infrastructure  is always an issue, when building, but inadequate footings may also result in unseen, progressive damage and heaving from freeze-thaw cycles--and ultimately, failure. 
  •  Incorrect construction, inadequate construction and  poor quality of workmanship 
  • Incorrect choice of materials: examples may unsuitable stone, dimensioned timbers that are too small, wrong choice of wood species, and untreated timbers that rot.
  • Unstable ground conditions and slumping, friable, or plastic soils. Can be caused  and exacerbated by local traffic and vibration.
  •  Inadequate drainage: wet ground soil conditions, unrelieved subsurface water pressure
  •  Surface  water damage from constant, or unexpected, uncontrolled surface water runoff
Bottom line, if a retaining wall is not designed properly and correctly built to meet all reasonable conditions expected, it will be subject to damage. Failure of the wall is inevitable.  Avoid these common errors when constructing any retaining wall:

Failure to assess the site and application correctly  

An inadequate structure can result from  failure to assess the site properly and accurately  identify conditions that will be encountered. Keep in mind that   a “100-year storm is seldom  predictable.   In your geographical  location, what type of wall construction appears to be most durable?  Provide a reasonable assessment of requirements by looking at older, long-established walls. What materials are they built of, and how?  A common mistake is to base a wall design only uponappearance, orcost. Consider obtaining professional advice if the wall must be over 6' in height.  

 Inappropriate Design for the Application:

 In some locations, arbitrarily choosing a building material for your retaining wall based upon appearance may be an expensive mistake where the ground is unstable, wet, and constantly slumping.  A steep hillside of unconsolidated soil is equally unstable.  The construction method chosen for building a retaining wall should be determined by conditions observed on the specific site, and the correct choice is critical for wall stability, performance and safety.  For example, on locations where soil conditions are known to be friable and  unstable,  consider building a layered, compacted geogrid reinforced wall.  Be observant. In your location,  which types  of wall construction are the oldest and have remained functional, standing upright boldly and remain as straight as the day they were built?

Inadequate Footings

 To build any tall, heavy structure upon an inadequate footing or weight-bearing structure  is asking for trouble. Ensure your site is prepared correctly and the footings adequately designed to carry the weight and stresses expected on the wall.Unless you are constructing a retaining wall standing on solid bedrock, a footing or base for a retaining wall must be built to bear the load placed upon it.   For a rigid, heavy , mortared stone, cement  block or concrete wall, a footing should be no less than 3 times the thickness of the wall, and in softer, unstable  conditions, the wider and thicker the  better    Excavate all soft organic topsoil,  boggy or loose soils,  and fill the excavated area with gravel compacted for increased stability prior to forming or  pouring any concrete footing. Poured concrete footings have little tensile strength and must also  be steel reinforced to avoid cracking and vertical  failure, especially for heavy walls in locations subjected to deep frost penetration.  Failure of footings under concrete block walls, even if slight, will result in the wall cracking  Footings for timber retaining walls, and ballasted, dry stone or mortarless stone construction can be allowed to flex  to a greater degree --without the same extent of  damage  rigid walls will suffer under the same conditions.    With average dry soil conditions, footings for these “flexible” structured  walls, if prepared  with care,   may be crushed rock or even a thick layer of compacted gravels in place of a poured concrete footing. Minor vertical flexing will not damage timber,  or dry-stone walls, however, care must still be taken in anchoring them laterally to avoid leaning from lateral ground  pressure.    

Selecting the Correct Materials for Conditions

Selecting permeable  clay brick for wall construction where soils are constantly wet or  freeze-thaw cycles predominate will virtually guarantees destruction and early failure .   Mudstone, soft sandstones, and some other natural  rocks are not durable, and many will crumble within a few years  under wet conditions.  Harder, less porous rock will last for centuries.                         It is self-evident that building a high stone wall with poor mortar and  rounded or poor quality stones on a poor foundation is a pending disaster for the simple fact of gravity. Choose rock, stone, and block materials carefully and make that selection based upon the characteristics of the wall you wish to build. Familiarize yourself with the  durability of the materials you are considering.  Choose materials proven reliable by use in similar conditions.  Logically if a  4' high wall on an adjacent property with a similar slope has remained  intact and straight after 20 years in service, your project wall should also be long-lived if built equally well using the same construction techniques and  materials.   Look at  older, established  neighbourhoods.  Some stone and block retaining walls are over a hundred years old. What characteristic have  made them withstand the test of time?  What kind of stone was used?  Similarly, choice of timbers is important. Undersized timbers will not be strong enough, and choosing of untreated wood that decays quickly is false economy.  

Insufficient Drainage, or improperly placed drainage systems

Drainage is a primary and serious concern for retaining walls, and the requirement  for adequate drainage can not be overstated.   Many retaining walls are built with little thought  to drainage and the extensive  potential damage that can be done by  water content in the soil.   Substantial damage can be done by running water running over,  along, through,  or under a retaining wall.  To prevent erosion of the wall structure, prevent water damage by placing placing  weeping tile  at the bottom of the footing of the  retaining wall.  Backfill the wall with free-draining gravel on the up-hill side to ensure water is drains  down to the weeping tile placed along the footing for superior wall performance.   Do not allow drainage water to run down the face of your retaining wall. To do so, especially with a mortared stone or block wall is to invite water stains, but worse,damage from water penetration and ice.                                                                                    Where excessive surface water causes surface erosion further up slope, plan a system to remove the water systematically.  Collect the water at it’s source, if possible.  Consider building a French drain to prevent excessive surface water from washing down the hill,  over or through the retaining wall.  

 Walls Built Too High

 Instability results in any structure built to excessive height  if not  adequately  supported.    For every foot of  increased height, exponentially increased  lateral, or sideways  pressure is exerted by the soil.  Care in construction becomes increasingly critical with additional height.   Higher timber retaining walls must be built significantly  stronger with more deadmen timbers and anchors included to prevent the wall eventually leaning out under ground  pressure.     
  • The thickness of a stone or concrete  retaining wall must be substantially increased at the base as as the height is increased. In addition,  depending upon the design and materials,
  •  the wall must be constructed  leaning in toward the hill to compensate for  additional height.  When building  higher walls, lean the wall into the hill at a  minimum of 1" or more for each additional foot of height for stability.  
  • Consider  providing  right-angle buttress walls for extra support on long spans of high walls.   
  • Worth noting,  as a design safety issue,  it is often  better to build two lower, more stable  walls instead instead of a single high wall,  and use the space  between them creatively.   
              

Poor Construction Techniques:

 Failure to alternate structural joints is a very common mistake.   It may be tempting and easier to avoid cutting blocks, logs, or timbers and install  the joints  “straight up”instead of staggering the joints.  Although it is convenient to build in that fashion with less cutting,  doing so is a dangerous practice.  Joints of several courses lined up vertically from the top of a wall to the bottom, introduce  substantial weakness  into an otherwise stable, strong design..   To avoid that problem, carefully   stagger all  joints in each subsequent course during construction regardless of style or type of materials used in construction. The mason’s rule is applicable:   place  one over two, and two over one. Staggering the joints substantially increases the potential  strength of any assembly.  The sole exception to this rule is where timbers are long enough to extend the whole length of the construction, but even in that situation, if  the wall is high, deadmen timbers must be installed for strength, which may involve cutting the timbers and resulting in joints, or alternatively,  installing anchor cables.

Failure to Cap Hollow Walls

A concrete block wall must be capped to prevent water collecting in block cavities and freezing. When water freezes it expands as ice, and the pressure will eventually destroy the blocks.  Fill the top course with mortar and cap the wall to seal out moisture.  Walls constructed of softer, permeable  types of stone should also be capped for the same reason.

Incorrect Selection of Timbers:  

 Using untreated timber or the wrong species of wood as a cost-saving measure is false economy.    Wood in contact with damp earth rots very quickly because of natural organisms and the dissolution  of chemical components of the wood. Retaining walls  built of untreated timbers or logs are short-lived and will show signs of decay quickly. Depending upon conditions, some  may even fail completely within two or three years.  Using cedar, redwood, or other rot-resistant timbers will delay deterioration for a substantial length of time,   but chemically or creosoted timbers will survive many years under damp and less than ideal  soil conditions. 

Avoid Damaging Hidden Superstructure

“Call BEFORE you Dig” is not just a trendy  television commercial.   Tearing out a gas or water line, or damaging communication cables, electrical equipment and other superstructure can not only  be very expensive, it can be extremely dangerous,  even fatal.   Call the utilities companies serving your area before you excavate for any footings and save yourself the potential cost, danger  and even evacuation of the neighbourhood  that can be a result of damaging a  buried line or pipe.     Any utility company will happily advise you of underground utilities and services buried in the vicinity of  your project.

 Additional Notes

  Safety  Building a Retaining wall or any structure can be hazardous, working with machinery, tools, concrete, stone, and heavy timbers. Keep safety first at all times.  Always use appropriate  protective equipment and use safety protocol on  building and construction projects.

Protect Onlookers

As a property owner, you may be held responsible and liable  for any injuries  that occur   on your  construction project.  Provide a safe viewing area, particularly for curious  children.   Install a temporary safety barrier  if necessary. Safety requires appropriate safety equipment and suitable clothing.  Wear safety footwear.  Wear safety goggles if cutting stone, block or brick. Wear leather gloves and long-sleeved clothing  if handling creosoted or chemically-treated timbers!  Wounds from slivers of wood from any treated timbers can be difficult and slow-healing. Seek medical help sooner instead of later with wounds caused by creosoted or chemically-treated wood.  

Bottom Line

 If you have taken extra care in building your wall and  avoided these  common mistakes,   it will stand securely and  proudly in place for many years. Good luck with your retaining wall construction project.  ##   Is that Incoming I hear? +       Facebooktwittergoogle_plusredditpinterestlinkedinmailFacebooktwittergoogle_plusredditpinterestlinkedinmail

2 Responses to Avoid Common Mistakes When Building Retaining Walls

  1. Ed Jansen says:

    f you build a wooden retaining wall along a property line between two houses and the length of the retaining wall is going from the front and downhill to the backyard, is drainage with a weeping tile still necessary given that moisture going onto the higher ground behind the retaining wall can still flow downhill to the backyard?

    The reason I ask is that we want to build a fence with posts going down four feet into the ground next to the retaining wall on the property line, and not have to set back the fence because of weeping tile.

    • Ed, this is a good question and quite subjective depending on all flow conditions, soil conditions and height of the wall. If the backfill you use against the retaining wall is coarse, free-draining gravel, it may not be a problem, water can obviously run down along the retaining wall. However, under high flow conditions, problems with stability may develop. The lateral movement of water will still be from the high side of the retaining wall to the low side under higher flow conditions. For a solution, set the drainage tile back from the retaining wall, far enough back ie 12″ or so, to allow your posts to be installed next to the retaining wall on the property line, simply install adequate free-draining gravel over the height of the tile, ie. cover the drainage tile with sufficient free-draining gravel to protect the tile, and remember, ‘holes face down’ and the bottom of the tile should be at the height of the bottom of foundation for the best result. Best wishes with your project. ~R

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