© 2013 by Raymond Alexander Kukkee
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.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:‘Some stone and block retaining walls are over a hundred years old. What characteristic have made them withstand the test of time?'
- · 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
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? + 
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