Principal, Regional Leader - Victoria, Australia
Principal Geotechnical Engineer
As cities become more and more congested, pressure increases on accommodation and parking. For an owner or developer to extract the most from the limited land footprint – and reap the greatest profit – the trend is toward taller buildings with deeper basements. However, while bigger might be better for the profit margin, taller and deeper bring into play a set of challenges to ensure that the building will be stable, safe, and not at risk of subsidence or groundwater problems.
The number one priority at the outset of such a project must be in-depth investigation of ground conditions. And that means involving a geotechnical engineer as early as possible, as the geotechnical aspects can make or break the project.
Getting to grips with the ground
One of the all-too-frequent problems in the industry is the reluctance of some developers to pay adequately for a thorough ground investigation. Money spent on this early stage is an extremely important investment in the success of the project. The consequences of insufficient analysis can be devastating down the track, but it’s not just a matter of avoiding the overt risks to the project. There’s also potential to save on construction costs by investing in uncovering a clever geotechnical solution.
With suitable investment in a comprehensive ground investigation, the geotechnical engineer will have the budget needed to undertake the analysis required to come up with a good solution. That solution is more likely to be successful, and may well be more cost-effective than the prudent solution that would be required in less-understood conditions. For example, while it might seem extravagant to spend $100,000 more on the geotechnical investigation, that extra investment could possibly save millions by optimising the foundations alone.
Although the construction costs of basements and foundations of tall buildings is only a small proportion of the total project cost, any errors made in the ground/subsoil assessment and planning can result in significant building damage, construction delays and additional costs. So, there’s a great deal of value to be gained and risk to be avoided by ensuring that ground conditions are professionally and thoroughly assessed.
Digging into difficult soils
In established cities such as Sydney, Melbourne or Hong Kong, many new tall buildings are built on difficult ground, so the work can be expensive and challenging depending on the ground conditions. In many cases, it is possible to support the weight of the tower where ground conditions are reasonable, but challenges arise on deep soft soils, where it can be difficult to design foundations to satisfactorily carry wind and earthquake forces, or when we start digging holes for basements where water ingress becomes problematic.
It’s essential that the design of retaining structures isn’t overlooked. Ultimately, with a retaining wall, a section of stable ground is being converted into something inherently unstable and with the potential to fail. Designing a structure to hold back that unstable face long into the future must take account of the properties of the soil and the risks associated with soil and water – which, though hidden, are foreseeable.
Relieving the pressure on retaining structures
For successful drainage and management of the hydrostatic pressures on retaining walls, you need to understand the soil and not underestimate what water can do. In clayey soils, for example, drainage behind the wall is often required even though water seepage is minimal to make sure hydrostatic pressures can’t build up very easily. However, in reactive clays providing drainage can be a problem as it can provide a water source allowing the clay to wet up and expand and cause major problems. On the other hand, in sand or fractured rock where there is significant flow, the retention system needs to be tanked to prevent water inflow as the flows are too large to be manageable.
Success will come from thinking hard about the specific circumstances of this soil, this water and this project, rather than applying a standard theoretical approach.
Once a retaining structure has been built, some key aspects need to be checked to ensure the integrity of the structure for the long term. The first of these is keeping the drainage clear. Once filter material starts clogging up, hydrostatic pressure will increase.
Another factor is movement. Many retaining wall issues are due to too much movement, because displacement wasn’t adequately estimated. If the wall starts leaning, it’s likely to have been under-designed or there are factors present that were not designed for. Whether that wall is unstable or not depends on many factors. A small amount of lean may not be a significant issue for the ongoing performance of the wall, unless for example the lean is due to a loss of durability of the materials used to construct the wall. If those materials are being compromised, action is required.
Designing for the foreseeable future
Every retaining wall is different. Good design depends on understanding what already exists and on what you can reasonably foresee. For example, if building next to a railway tunnel easement, you need to design for the potential for future railway tunnels to go past.
There are plenty of services around retaining walls, so if there’s a 100-year-old water main sitting behind your wall, you need to design for the possibility that the water main could break, and lead to a rise in the groundwater table.
For your tall building to get off the ground or your deep basement to stay safe and dry, you’ve got to get back to basics and understand the ground. The upfront investment in understanding your site’s ground conditions will pay for itself in broader design options, quicker construction, and avoided risks. With a solid understanding of ground conditions, translated effectively into an appropriate design solution, all you now need is a sensible system in place to ensure the actual design is built and built safely.