Finding ways to visualise and map what lies underground can be challenging, but BIM represents a leap forward in what ground engineers can do. Autodesk’s Civil 3D and geotechnical analysis packages such as Plaxis make it possible to create and analyse 3D models of the ground in a more detailed, dynamic, integrated and accessible way.
The big issues
BIM’s impact is evident regarding one of ground engineering’s biggest challenges: modelling hidden features between exploratory holes. BIM does not change the complexity of identifying hidden features, but it does make them significantly easier to represent in a model. Previous software connected data points crudely and phantom boreholes were commonly created. Civil 3D allows customisation of geometry between data, permitting the insertion of geological features such as faults and pockets.
Ground modelling always requires interpolation because ground investigations only ever sample a fraction of a site. A ground investigation report (GIR) and geotechnical design report (GDR) are needed to explain and qualify the data and its interpretation, but it may not be possible for individuals to consult these alongside a model.
BIM can help solve this problem by enabling annotation of data points in the model with confidence levels or notes from the GIR and GDR, uniting these separate sets of information to give the user a fuller overview. Making confidence levels more prominent could also help to dispel the misconception that a 3D model is more accurate than a 2D visualisation. BIM doesn’t change the accuracy: the risk is still there, and its interpretation by ground engineers would be the same without BIM. But it gives them more tools to assist understanding and communicate information.
Efficiency from the outset
Ground modelling is a process of gradual refinement. It starts with a desk study to compile data from maps and topographical surveys, and extrapolate below-ground surfaces. These records are increasingly available in BIM-compatible formats from organisations such as the British Geological Survey and the Landmark Information Group, which provide geo-referenced digital 2D maps that integrate with Civil 3D. Data can also come from more specialised sources, such the Coal Authority with regard to the possible impacts of coal seams and surface mine workings.
With an initial model in place, BIM becomes instrumental in designing ground investigations. Engineers can use the model to identify the best locations for exploratory holes. With these positioned in the model, it’s easy to take off quantities and calculate ground investigation cost. Setting-out data can then be fed directly into surveying equipment or extracted as 2D drawings.
Interdisciplinary design benefits
Integrating project design models with an accurate BIM ground model enables outline designs to be positioned on the site, making it easier to analyse what’s going on. Ground investigation data can be transferred to a BIM model in real time. Inputting geometry from the model directly into analytical packages enables further insights to be gained.
A ground model can also be used to take off quantities for geotechnical structures and earthworks. And refining a BIM model with ground conditions revealed during construction and with as-built details of geotechnical structures benefits project operation, maintenance and decommissioning.
Mott MacDonald is working with data management firm Keynetix to improve the geotechnical data managing and modelling capabilities of its HoleBASE SI extension for Civil 3D, and one of our senior ground engineers sits on the Association of Geotechnical & Geoenvironmental Specialists (AGS) Data Management Working Group committee. But this level of geotechnical BIM activity needs to be reflected throughout the wider construction industry.
Despite the broad benefits offered by advanced ground models, they are not commonly requested. But we predict that the benefits of geotechnical BIM for quantifying, communicating and managing ground risk will become better recognised. This may require more resource on the modelling, but the whole process will become more efficient thereafter, offering the potential for reductions in project cost.