Doug Stewart Member Name
Principal Geotechnical Engineer
No one wants an expensive surprise on a construction project. While it’s well known that ground movements can occur during construction, it’s another matter to accurately predict when the earth will move and by how much.
Despite proper planning and design, significant movement of the ground and/or building components can occur during or after construction. Moving from design to reality creates the potential for variability. The impacts of these unexpected movements may include serviceability issues, structural overloading, increased construction costs, delays, and damage to adjacent infrastructure and properties. So, if this is the reality of construction and ground movement, what can you do to keep your project on track and avoid damaging consequences?
1 – Plan for the predictable – but prepare for the unpredictable
The first step is to consider what can reasonably be predicted to happen, and plan accordingly. During the development of the project design, geotechnical engineers will typically predict the scale and rate of displacement of certain project elements, such as shallow and deep footings, retaining walls, earth fills, and so on. In many cases, it will be possible to use or adapt precedents from previous projects or apply well-known design methods. In more complex situations, advanced modelling techniques may need to be used.
Whether due to lack of sufficient investigation data, poor application of the Observational Method, error, inexperience, optimism, lack of precedent, or inadequate consideration of construction site processes, instances can occur in which some types and causes of ground movement are not adequately or directly addressed during design.
Unexpected movements can occur during or after construction. Some examples include excessive deflection of retaining walls and piles, movement caused by installation or removal of piles or retaining walls, placement of fill on soft ground, or inflow of soil and water into excavations.
Protection against the consequences of such unexpected movements requires careful thought in design and construction planning. Investing in upfront geotechnical investigation, design and assessment of construction impacts can have a significant benefit, particularly when dealing with difficult ground conditions or construction near other infrastructure. If these consequences are left to be dealt with during construction, it may be too late to avoid the problem, making it very difficult – and often expensive – to resolve.
2 – Get experienced support
When it comes to understanding and managing the impacts of ground movement, the level of awareness varies significantly within the construction industry.
There’s really no substitute for experience. Although many engineers will understand the basic principles, the full complexity of potential ground movement scenarios is not covered in university courses or textbooks. While methods of movement assessment exist, there are certain circumstances in which no clear method of prediction is feasible or available.
In this case, the most reliable insights will come from engineers who’ve seen on-the-ground examples of problems and risks, situations to avoid, as well as solutions and best practices. Securing the support of experienced consultants and specialist contractors who understand how various elements of the project interact can help stakeholders avoid unexpected delays and costs during construction.
3 – Assess and communicate the risks
All construction works involve varying degrees of risk. Identifying and communicating these risks with all involved parties is critical to limit impacts on cost and schedule during construction.
The designer’s risk assessment is a very important part of the process. For this to be effective, the designer must understand the site processes and equipment that are likely to be used during construction. Once the various parties have identified the range of issues that could arise on site at different stages of the project, they can determine which may need management or design modification.
For example, the significant movements that can occur during installation of continuous flight auger (CFA) piles, driven piles or extraction of sheet piles are sometimes not considered in design or construction planning. These may be left to the foundation contractor to manage, but, in some cases, this can lead to problems since the construction method has already been locked in and there is limited opportunity to make changes at this late stage without costly and problematic implications.
Effective communication of risk among all the involved parties is sometimes difficult to achieve. This may be due to the parties having poor working relationships or working across countries or cultures with different design and construction standards and processes. The solution is for the party in charge of running the project to facilitate inclusive and regular conversation about the project’s risks and opportunities.
When risk is well communicated, all parties can manage expectations and be clear about their responsibilities. This is particularly important when a change of construction sequence, methodology or design is needed.
4 – See the big picture and consider the neighbours
To fully assess and communicate risk, you need to look beyond the limits of a particular work package and beyond a site’s boundaries.
Where a package of work has been parcelled out to a subcontractor/subconsultant, perhaps with a limited associated fee, that work may be undertaken in relative isolation from the other components of the project. The danger, then, is that the subcontractor does not extend their view beyond the immediate limits of the task, nor contemplate or communicate the broader implications of the package of work. It is up to the lead contractor/manager to overcome this compartmentalisation, confirming that the interactions are managed, and integrating all the discrete risk factors into a comprehensive picture.
Another aspect of seeing the bigger picture is being able to anticipate potential effects on adjacent structures. It may be difficult to identify the contribution of the current works to any damage or deterioration of that structure if the adjacent structure has not been adequately inspected and understood before construction begins.
5 – Keep watch and act quickly
It is possible to contain the negative impacts of ground movement risks. We have experience on projects where settlement has exceeded original expectations, yet has been effectively managed through pausing construction, analysing the circumstances, implementing a series of small modifications, and then maintaining close monitoring and review to achieve a successful outcome.
Careful, diligent monitoring of anticipated risks during the construction period, effective management and control of the work site, and the rapid application of relevant modifications can help to keep things on track even when surprises do occur.