A properly built retaining wall should last decades, and quite possibly over a century. However, there are several reasons retaining walls could fail prematurely. In this article we will discuss some of the common causes of retaining wall failure.
One of the leading causes of retaining wall failures is improper drainage. There are many reasons for this, but one main reason is that saturated soils behind walls cause excessive weight. This additional surcharge may cause the wall to lean outward and eventually collapse. Additionally, soils with high moisture content will expand during freeze thaw cycles which can slowly and incrementally force the wall outward, like a wedge. While not drainage related, tree or vegetation roots and vines can cause a similar issue to that of freeze thaw cycles by slowly prying and deteriorating the integrity of the wall. Properly installed drainage systems may consist of drainage aggregate placed directly behind the wall building material. Filter fabric should be used to separate native and backfill soils from drainage stone. Drainage pipes and weep holes may also be used to allow collected water to flow around the wall or directly out the front face. Drainage systems over time may become compromised if they become clogged or fail in another way.
There are many different types of building materials that can be used in building retaining walls. Some common construction materials in the landscape industry include railroad timbers, landscape timbers, paver concrete blocks and natural stones or boulders. Other materials often used by masonry contractors include poured concrete, bricks, stones and concrete blocks, or concrete masonry units (CMU) block. Typically concrete and stone products will last the longest although there are some inferior products to watch out for. Wooden building materials typically have a shorter shelf life. For this reason we do not recommend railroad ties or landscape timbers for retaining wall construction. Wooden timbers are typically better utilized in decorative garden and planter boxes.
It is important with every retaining wall type to ensure the foundation is strong. Proper compaction is essential to a long lasting retaining wall. Retaining walls should always be built on native ground and not built on imported fill dirt. If the elevation for the foundation needs to be raised, proper compatible materials should be used and compacted to 95% Standard Proctor. In addition to the foundation being properly compacted it is also important to backfill and compact the back side of the wall. Non-compacted fill material can cause settling and movement of soil after the wall is built.
As mentioned before, a properly built retaining wall should last decades. Equally so, an improperly built retaining wall can fail in years, months or even days. Therefore proper construction and design techniques are crucial for longevity. Most materials used for retaining walls have the same key design elements in building a proper retaining wall. But implementation for each may be slightly different. For example, wooden retaining walls will typically use dead man timbers to anchor within the soil backfill layer. Some timber and concrete walls will also use tieback anchors or rods for a similar purpose. On the other hand, engineered paver block retaining walls will typically use geotextile fabric to perform the same function.
Poured concrete and cmu block stem walls will use rebar and an ‘L’ shaped footings that acts as a cantilever. Failing to understand, and properly implement correct design elements in each particular wall type can lead to leaning, cracking or buckling in your retaining wall. Eventually total collapse is possible.
One last item to mention with regards to the design is the intended surcharge on a retaining wall. Typical retaining walls are designed to carry the weight of the soil directly above the wall. However if there is a shed, building or other structure; including a road or parking area for vehicles, this added weight should be included in the design and ultimately implemented in the final construction. If a surcharge is added to an existing wall not intended for it, this could fail prematurely, even if it was properly designed and constructed.
When we are initially contacted to consult on retaining walls that are in a state of failure or have completely failed these are some of the more common reasons for retaining wall failure we have seen. There may be some other potential causes for retaining wall failure but each wall type, function and location will vary. Each site is unique and causes different obstacles to overcome. It is typically apparent what the cause for failure is in each circumstance depending on how the wall has begun to move.
Click this link for more information on Building a Retaining Wall with Concrete Paver Blocks.
Jasper Ridge landscaping provides “Professional Landscape Services” to the Joplin and surrounding areas; including Carl Junction and Webb City. Our service area is contained within Jasper and Newton Counties in Missouri (MO). For more information about Jasper Ridge LLC please contact us using the contact form.