Managing uncertainty in ground conditions in design on a megaproject where ground investigation is highly variable

Ground conditions present one of the major areas of uncertainty and risk for High Speed Two (HS2) Phase One and have a significant impact on project affordability.

The ability to effectively and appropriately manage ground risk in design in a consistent manner is complicated by the varying degrees of completeness of ground investigation which may vary from asset to asset and in some areas may be limited. Designers must make best use of the data so to reflect the most likely prevailing ground conditions however, due consideration to potential variations considering gaps in data also needs to be given.

This paper documents the risk-based approach taken in scheme design (including coordinated design, spatial arrangements, approval in principal documents and detail to assist in development of a target price) where local and regional scale reviews of data were undertaken for different ground conditions encountered, and a qualitative ground investigation classification scheme created (GI Class 1-5).

This classification scheme enabled a common understanding of the gaps in information across the contract from asset to asset while also serving as a reference to define ground models and parameters in a consistent way making use of regional data sets with appropriate factors to reflect the level and extent of GI available where limitations existed. This enabled geotechnical designers across the globe to reduce the potential for overly cautious costly designs and optimise the design while giving due consideration to geotechnical risk.

Examples of the approach are presented for different classes showing the evolution of the design and risk profile as new data became available.