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
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 ConditionsSelecting 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 systemsDrainage 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 HighInstability 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.