When BIM meets GIM!

We all know what a BIM is (don’t we?!) An architectural BIM for construction would include design parameters, objects, object properties, products, quantities, activities, risks, modifications, verifications and asset management within a three-dimensional, digital format. These types of BIMs are a relative ‘known’ within the industry and there is an amount of certainty and confidence surrounding such models.

Let’s move onto the ‘unknown’. The nature and properties of the sub-surface are not so clear cut. However, understanding the geotechnical nature of a construction site for any development is an essential part of BIM. Meet GIM – Ground Information Model. The data from such a model contributes to design, construction and maintenance, and to the future site development. The parameters of a site-specific GIM are usually far less well understood, with values and properties that can be extremely variable. This data can be defined from observations of the sub-surface (from the surface), from excavations, boreholes, data extrapolated from previous adjacent sites (may be many years old and not reliable). All of this data is interpreted by geotechnical specialists and presented as site investigation reports for construction projects.

The process of producing a GIM begins with a desk study, where existing resources such as topographic and geological maps, site photos, field reports and borehole data are reviewed. This provides the framework for an initial model and becomes the next step of planning a site investigation strategy that aims to confirm and develop what the desk study has indicated. The model is then modified and validated by being tested and interrogated. The information may well lack metadata and appear very unreliable. Some of the information may be temporal – such as ground water seasonal changes, or even daily or hourly changes of levels – however represented in the GIM as a single aggregated value. A GIM is dynamic and will change depending on the quality and quantity of the available data and information, and this will have an effect on the resulting interpretations.

Risks are associated with using these predictions and interpretations. The ‘knowns’ and ‘unknowns’ of a site’s ground conditions are important from a project management and cost point of view. If they are recognised and understood their effects can be mitigated and managed. The point has often been rightly made that it is not the ‘known-unknowns’ but rather the ‘unknown-unknowns which cause concern!

Thank you Andrew Thompson.