Whether in New Zealand, Japan, Indonesia, California — or any place in the world that is prone to earthquakes — building on ‘shaky ground’ is a major challenge. It is neither possible to reliably predict earthquakes, nor to stop one; but smart geotechnical and structural engineering methods can be used to design safer and stronger foundations.
In New Zealand’s 2010-11 Canterbury earthquakes, soil liquefaction caused by strong ground shaking resulted in major damage to infrastructure and homes around the Canterbury region, and particularly in Christchurch City. In the aftermath of the severe earthquake effects, New Zealanders started to realize the potential risks from building on earthquake-prone ground. New Zealand building owners, local communities and regulators now see the potential benefits of using engineered ground improvement techniques to create more earthquake-resilient foundations.
Because most of New Zealand is subject to earthquake hazards, all construction work in the country needs to consider how to deal with unsuitable ground conditions. The goal is to minimise the effects of soil liquefaction on building and public safety. Reducing the potential impacts of soil liquefaction can be complex and time consuming, and generally increasing the cost of construction.
There are many possible geotechnical and structural measures that can be used to mitigate potential effects from earthquake shaking. These measures have different implications for life safety, and construction schedule and cost. So, when starting out on a new project, what is the right approach?
Gather a range of advice and maintain an open mind
When building in an earthquake-prone area, the geotechnical and structural design approach must allow for the earthquake loads that will be put on the soil, foundations, and the building structure. Load requirements must be assessed correctly so that the design concept will be safe and effective. The overdesign of structures needs to be avoided too because it is not only expensive but also unnecessary to build safely.
Owners, developers, engineers, and architects all benefit from the assessment of different options as early as possible in the design process so construction costs and other aspects such as construction time or environmental impact can be assessed accurately. Real value for the project is created when a geotechnical expert, a specialist contractor and/or a structural designer are involved from the very beginning of the project.
Whether it’s a building, bridge or railway line, the structure must be supported by a safe foundation system. In highly seismic areas and in ground conditions prone to soil liquefaction, piled foundations are often the first choice of the design team, although not necessarily the most cost-effective solution. Once a commitment is made to a piled foundation concept, it can be difficult for the owner, architect, developer or designer to explore other options, such as ground improvement.
Geotechnical experts should be part of the design process as early as possible, so that different foundation concepts can be fully explored as part of the geotechnical and structural design options. This early involvement allows the cost, benefits, and risks of the different foundation options to be evaluated to help identify the most suitable solution.
Consider ground improvement together with structural engineering measures
It is often beneficial to understand possible ground improvement options. While in some cases ground improvement might be slightly more expensive than foundation piles, it can create opportunities for savings in the structural design.
In poor soil conditions, ground improvement can significantly increase the strength and stiffness of the soil, which then creates a stronger foundation for the building. Stiffer foundation soils can increase the robustness and resilience of the structure and reduce the extra strength that would otherwise be needed. Ground improvement could also potentially reduce the cost for concrete and reinforcement steel. The potential benefits from ground improvement should be assessed on a project-specific basis.
While not every project may benefit from ground improvement, certain poor ground conditions can be greatly improved from the use of ground improvement techniques and technologies. Rammed aggregate piers (RAP), for example, introduce stiff aggregate columns into the existing soil matrix and displace and densify the surrounding soil during their installation. This is a proven, versatile, and cost-effective solution for projects located in high seismic areas with potentially liquefiable soils. Other technologies such as deep soil mixing, dynamic compaction or the installation of stone columns can also provide suitable alternatives.
Developers, structural engineers, and geotechnical engineers all need to collaborate at project initiation to identify, clarify and communicate the required performance criteria – in other words, what the foundation system really needs to achieve. Ground improvement and structural engineering of the super-structure work together to minimise the potential for long-term static and liquefaction-induced ground settlement. A respectful and effective working relationship between structural and geotechnical engineers benefits the project by creating value for the owner, as both disciplines meet the requirements for the design and construction of a safe and efficient foundation system.
Find the best fit for the site and project
In earthquake-prone areas, there’s no unique or prescribed way to design a structure that will always stand firm on shaky ground. Different approaches will have different implications for safety, performance, schedule and cost. Project success will depend on finding the most efficient balance for the unique project circumstances. Effective communication between the structural engineer and geotechnical engineer is how to achieve this optimum balance – where the ground improvement performance and the structural engineering requirements work together to achieve project success.
The best solution is likely to emerge when the specific site conditions and performance requirements are well understood, a range of specialist views is encouraged and heard, and minds remain open to the many possible alternatives. When projects start from the ground up, and different geotechnical ground improvement options are considered in conjunction with structural engineering needs, it’s more likely that the resulting structures will be safe and resilient during future earthquakes.