Chris Haberfield Member Name
Principal Geotechnical Engineer
Andrew Russell Member Name
Principal, Regional Leader - Victoria, Australia
Whether a retaining wall stands, moves or falls is rarely a matter of luck. Like tunnels, retaining walls are an inherently risky business. In both cases, a section of stable ground is being converted into something inherently unstable and with the potential to fail. Unlike tunnels, however, retaining walls are not a self-supporting geometry and are usually designed and constructed with significantly less engineering input.
When we create a void into which earth and water can move, we’re working with powerful elements of nature and very large forces. The consequences of not getting the design right can be costly, damaging and dangerous, including collapse or partial collapse, loss of serviceability, displacement of the wall or surrounds, water inflows, and widespread settlement due to groundwater drawdown. While collapse is relatively less common, excessive movement and poor performance are far too common and should be avoided.
Understanding the causes of failure is an investment in success. The causes of failure are many and varied, but they often cluster around inadequate ground investigation, inappropriate choice of wall, unsuitable design and poor construction or quality control issues. Once we understand the causes of failure, we can take action to avoid them, resulting in a far greater likelihood of stable, robust structures.
How well do you really know the ground conditions?
Many designs assume too much about the ground conditions and properties rather than carefully measuring them. How much can be said to be truly unforeseen rather than due to inadequate investigation? You need to have a good understanding of the geology and not be over-optimistic about ground conditions and parameters. Two different boreholes may yield different rock levels but we need a fuller picture, not only within the site, but also in areas beyond the site.
It’s essential to fully investigate the soil and rock strength, stiffness and earth pressures. It is usually not appropriate to apply soil mechanics techniques to rock, as they are very different materials. Even though rock is considered to be strong, its strength can be less than that of the weakest soil. Even using low-risk, conservative scenarios, the ground can produce surprises. Unforeseen movements have occurred because of ground conditions not being well understood outside the site, lower bedding strength than expected, or higher horizontal in situ stresses than expected.
And then there’s water, which impacts on everything. ‘No water, no worries.’ Wherever there’s water, there is potential for trouble. Water will always find a way in and the impacts can be far reaching and significant. Ignore groundwater at your peril.
Is the choice of wall appropriate and is the design adequate?
When you choose a type of retaining wall, you need to ask yourself not only what you’re planning to do to the ground, but also how the ground will respond. The choice of wall must take into account the height of the retained soil, the ground strength and stiffness, the effect of groundwater, the required level of watertightness, what level of movement can be allowed, the available space for construction and proximity to other services and other movement-sensitive structures, and what durability and design life is required.
You need to select a retaining structure that is safe and practical to build, suits the application (including the geology and groundwater and the desired design life) and protects adjacent assets and life. When you consider the cost, also consider the cost of failure. Safety in design and the requirements for temporary support are too often poorly considered in retaining wall design, often because of cost considerations. The result is poor choice of retaining wall and significant oversights with regard to safety.
Is there adequate construction documentation, supervision and quality control?
Another contributor to retaining wall failure is poor design and construction documentation. Don’t leave it up to the builder to make the decisions. Documentation must be sufficiently detailed to ensure that wall is built as designed in every aspect. Specifications must be detailed and targeted, with construction stages clearly described and illustrated in the drawings and key components of the wall identified, tested where appropriate and confirmed.
Competent oversight is needed to confirm that plans and specifications are followed. Unfortunately, this is all too rare. Detailed quality control documentation and checking are also infrequent or erratic.
For deep excavations, monitoring of movement is essential, and it needs to be proactive and regularly provided to the designer, as it gives a useful early indication of whether something might go wrong.
Does the design take a long-term view?
Of course, not all failures can be prevented through the original design. Two very common scenarios of retaining wall failure occur well beyond the wall’s design life. These scenarios include extra loads being applied on the wall by subsequent owners of the site or from development of a nearby site, and inadequate monitoring and maintenance over the long term. Additional surcharge is the responsibility of the future owners and designers creating the increased load. Owners are also responsible for monitoring and maintenance to verify that retaining walls on their land are still in good condition.
This does not, however, obviate the need for designers to think beyond the present. Retaining walls can last a very long time beyond their originally envisaged design life. No designer can foresee every future event, but some potential changes in conditions can be reasonably anticipated. Might there be impacts from fire, changes in groundwater levels, failure of a water main, or nearby excavation? Could the design withstand such events?
For safe, robust retaining walls that stand firm against the pressures of earth, water and time, we need to learn from failure or risk repeating it. With intelligent design, sound construction and rigorous quality control, retaining walls can last a lifetime or even far beyond.