Observational method guidance (P3295)

The design of geotechnical structures such as deep excavation and tunnels is based on the strength and deformation parameters of the surrounding soil, and the planned sequence of construction activities. Uncertainties can exist around the soil parameters as well as the construction method (e.g. material properties and measurements) actually implemented. To allow for some of these uncertainties, conservative designs are often proposed that tend to be material intensive. Through monitoring, design predictions can be compared with measured performance. Trigger values are often provided to pre-define actions in case of exceedance. Generally no allowance is made in the design to make it more efficient if measured performance is better than predicted.

The Observational Method (OM) is a framework that allows adjustment of the geotechnical design during construction based on monitoring data. Observed movements of the structure and surrounding soil can be used to supplement predictions using real time back analysis to update soil parameters and construction method. As such, using the OM can contribute to minimalising the use time and material whilst ensuring safe construction, enhancing the sustainability of the overall project.

Since the publication of CIRIA guide R185 in 1999, design standards have been updated. Eurocode 7 explicitly permits the application of the OM. The increase of computing powers opens the possibility for real time back analysis. Through optimisation of material use, applying the OM can contribute to Net Zero goals and it can be expected that the OM will be applied more often. As such, guidance is required to establish for each project whether using the observational method is feasible and to describe the responsibilities of the different project contributors to ultimately save costs, material, and time.

CIRIA proposes to update R185 and develop a new leading good practice guidance on the application of the OM in geotechnical projects.

Download a copy of the full proposal.

Project status
This project is currently seeking technical and financial support.

Further information
For further information or to get involved with this project please contact Loretta von der Tann at CIRIA.