Last November, 15 people were evacuated from their homes in the Cleveland (south of Brisbane) street of Middle street after excavators ‘dug too deeply’ at the site and an excavation collapsed.
One neighbouring carport had to be demolished because of unstable ground.
That echoed a similar event nearly three years ago in the Melbourne east suburb of Mount Waverly. In that case, about a dozen residents had to be evacuated after the collapse of an excavation left their homes teetering over a 15-metre deep pit.
These cases raise questions about whether engineers throughout Australia afford adequate attention to the safety and design of large-scale retaining walls which are used for either embankments or excavations. According to one geotechnical engineer, the answer is no.
In a recent presentation delivered to members of Engineers Australia in Melbourne, Golder Associated principal geotechnical engineer Dr Chris Haberfield said many engineers were not observing adequate caution in respect of the retaining walls which they design.
Retaining walls, Haberfield said, represent one of the most hazardous types of structures which engineers design. Essentially, he says, a retaining wall represents a means of steepening batters often from a situation which is stable to one which is not stable and creates a void into which earth and water can fall. As a result, they carry risks not dissimilar to those of tunnels.
Yet whilst tunnels are often given extensive engineering, he says the same cannot be said about retaining walls. With retaining walls, he says many engineers apply generic soil properties and formulas without adequate consideration about whether they are suitable for the site in question given ground conditions and properties of the wall. Important considerations such as impacts to the ground and groundwater, temporary support and the long-term performance of the wall are often neglected.
As a result, he says, engineers often select the wrong type of wall. This can lead to issues in respect of occupational health and safety, construction, stability and ground movement.
“Unlike tunnels, we don’t often give retaining walls the attention they need,” Haberfield said. “With tunnels, we do a lot of work. It’s an open excavation in the ground. There is a lot of engineering.
“(With) retaining walls, for some reason, we don’t give it the same attention.”
What will the wall do to the ground?
When designing retaining walls, Haberfield says they must be given due respect as a structure which has to support loads which need to be calculated. Treating it like a fascia or simply spaying shotcrete up a wall in hope that this will remain in place is not acceptable. Nor is relying on suction in the ground to hold the wall up.
Most important, he says, engineers need to think about what the wall will do to the ground and how it will impact surrounding ground, structures, roads, services, buildings and other assets.
First, there is the issue of loading. By excavating into a slope for a basement, Haberfield says you unload the ground. By contrast, filling for an embankment results in loading of the ground. This is important as the ground exhibits different properties when it unloads as compared with when it loads. In addition, both loading and unloading cause ground movements. Whilst these movements will hopefully be stable, it is also possible that they may not, and may result in failure.
Excavations can also impact and potentially lower the groundwater table and regime. Especially where you have an aquifer beneath the retention system, this can have implications for kilometres.
On a related note, there are questions about how the ground will respond to the wall. This could include wall failure, excessive settlement both of the wall itself and the area behind the wall, excessive horizontal movement and alterations to the groundwater flow regime, as well as possible piping and liquefaction or ground consolidation.
When thinking about these issues, it is important to consider the height of the retained soil; the strength, compressibility and stiffness of the ground; the available space to construct the wall; construction impacts and temporary works to construct the wall; the proximity of structures which are sensitive to movement and groundwater.
With these considerations in mind, it is important to think about the right type of wall structure. This must be suitable for the ground conditions (geology and groundwater), practical and safe to build, and must protect assets and life.
Embankments or excavations?
Retaining walls can support either embankments or excavations.
Provided you are on reasonably solid ground, Haberfield says the engineering associated with embankments is relatively straightforward. The supporting structure (retaining wall) is erected in a controlled manner as the embankment is raised. The material being supported is also engineered fill and has properties which are select, are reasonably consistent, and in many cases will remain consistent over time. As the wall is erected above the ground, groundwater is usually not an issue.
By contrast, excavations are more hazardous and are often misunderstood and abused. Too many engineers, Haberfield says, rely upon suction to hold these up.
With excavations, Haberfield says the wall becomes a supporting structure which replaces the ground. In some cases, temporary support is needed until the permanent structure is in place. The actual process of construction for retaining walls is hazardous.
Excavations also involve natural and unengineered material. Often, the engineer is not certain what the material behind the excavation actually is (boreholes are often not drilled behind the site) and needs to use geology to make educated guesses. The properties of these materials are inherently variable and change with time. Groundwater is often a significant issue – issues such as watertightness, drainage and discharge often need to be considered.
Active vs. passive systems
Designed primarily for stability and to resist stress caused by the soil or ground pushing up against the wall, passive wall systems essentially sit in place and react to weight or some other system. They develop their resistance and retaining forms through displacements. Examples of passive wall systems include gravity walls, cantilever walls, reinforced earth walls, soil nail walls and bolted rock walls.
These structures are ideal for embankments or for temporary unsupported excavations. They should not be used in close proximity to assets, services or buildings which are sensitive to movement, as they may damage these structures.
When designing passive systems, it is important to be careful about swelling soils, which can wet up and ‘push’ the retaining wall.
Some important considerations should be noted about the type of wall in construction.
With a gravity wall, you have a block, active earth pressure and base friction generated by the weight of the wall. When designing for these, you have to resist sliding, overturning, bearing capacity and the structural capacity of the wall. To do this, Haberfield says a rule of thumb dictates that you would need a width equal to at least 50 or 60 per cent of the height. This can be reduced, but only if you design accordingly to accommodate a lower width. When designing these types of walls, Haberfield says a global stability check is necessary but is unfortunately often ignored.
With soil nails and rock bolted walls, these essentially stitch the front of excavation together to form a gravity block. Accordingly, they are similar to gravity wall systems and will suffer similar displacements as gravity systems.
Cantilever walls can be embedded or non-embedded. Embedded cantilever walls (usually supported with pylons) are ideal for support of excavations and are put in prior to an excavation to protect workers and provide some protection to any nearby surrounding structures. Non-embedded cantilever walls can be used for support of embankments or for temporary unsupported excavations where displacements are not of concern.
Active wall systems use props or anchors to prevent the system from moving. This is done by inserting the retaining wall into the ground and adding the anchors or props as you go. Stability and movement are both important. These must be designed to limit stable ground movement. To do this, displacement analysis using finite element analysis is necessary. Haberfield says the AS4678 standard (earth retaining structures code) is not applicable to these walls. Nor is limit state design. Rather, it is important to do a best estimate of the ground properties. Out of that, you can get working stresses and working reactions in the piles and then factor those.
Since you are pulling earth back with the anchors, earth pressures in respect of these walls are much higher. Earth pressure distribution is also more complex.
Haberfield says you still need to consider many of the same factors as with passive systems, such as sliding or overturning. Bearing capacity is critical as you have anchor forces which are inclined and are pulling the wall down. Indeed, Haberfield says walls have failed because they have been pulled down, have not had sufficient bearing capacity and that has caused the wall to move out.
It is also important to think about the connection between the anchors and the walls. If that connection moves, then the wall will move out. Australia, Haberfield says, is poor at dealing with anchors, adding that these are often poorly installed.
Finally, there is water, which impacts on all parts of the wall system. This includes the stability, wall pressures, soil strength (suction), wall movement, anchor performance, seepage and much more. If you have anchors in sand, for instance, and you have groundwater level rise (for instance through a broken water main), you halve the strength of the anchor and the wall moves out. Thus if you are putting a retaining wall next to a 100-year-old water main, you need to think about the possibility that that water main could fail.
A common solution is to provide for drainage to get rid of the water. In doing this, Haberfield says you need to think about how long the drainage will last, how you will provide for it, whether you need to provide a system to clean it out, and the type of material you have in the ground.
With clay, not much water comes through and drainage becomes more of a pressure relief situation. With sand, you have flowing water which needs to be tanked because of the volume of water coming through. When you have intermediate situations such as in weathered fractured rock and you have water coming along seams, you might have a lot of water inflow and may need a method by which to maintain the drainage structure.
Another consideration is reactive clays. Many engineers do retaining walls in reactive clays and put a gravel backfill behind it to act as drainage. Since getting water out of reactive clays is difficult, Haberfield says this does not have much effect and merely introduces water into the ground from the surface which in turn causes problems with the wall. If you are going to do this, he says, you need to seal the top to prevent water inflow. Houses, he says, have been blown apart because of water getting in.
In summary, Haberfield says it is important when designing retaining walls to understand the setting you are in (geological, hydrological and urban), check to see what existing assets are there and protect these, select the correct type of retaining wall and apply prudent engineering to ensure these are correctly designed and constructed.
“There are a lot of cowboys out there who are willing to take shortcuts,” Haberfield says.
“We don’t need to be part of those. I would encourage you not to be one of those. I know we get pressures on us to steepen up batters and so on. But we need to think about safety. It only takes one batter out of 100 to fail and hurt somebody and you will never design a steep batter again and will be in deep trouble.
“Excavations and retentions are a risky business. They need to be given due attention.”