Geotechnical categories in lightweight structure design — what you need to know

A correct assessment of ground conditions and the assignment of an appropriate geotechnical category are the foundation of safety and durability for every structure. This applies equally to large steel halls, temporary canopies and lightweight tensile membrane structures. In this article we walk through geotechnical categories comprehensively — what they mean, how they are used in practice, and how they affect the design and investment process.

Legal basis and classification of geotechnical categories

Geotechnical classification under European and Polish practice rests on three pillars: national construction regulations, Eurocode 7, and industry standards. Together, these documents create a coherent system that allows the precise definition of requirements for each type of structure depending on the ground conditions.

The classification is based on the analysis of two key parameters: ground conditions and structure type. Ground conditions are considered simple when the layers are homogeneous with known geology, the groundwater level is below the foundation level, and the site shows no unfavourable geological phenomena. Complex conditions are characterised by heterogeneous layers, variable groundwater levels or the presence of organic soils. Complicated ground conditions cover landslide-prone areas, karst terrain, mining-impact zones or expansive soils.

The consequences of misclassification can be serious. If you ignore the presence of local lenses of organic soil and assign a lower geotechnical category, you may face uneven settlement in the future, leading to damage to the envelope and the supporting structure. The cost of repairs in such cases is often many times the cost of the geotechnical investigation that would have prevented the problem.

The three geotechnical categories explained

The regulations define three geotechnical categories that determine the scope of the necessary investigation and analysis. Understanding their specifics will help you optimise the design process.

Geotechnical category 1 (GC1)

Geotechnical category 1 covers small structures with a simple form, founded in simple ground conditions. In practice this covers temporary stage canopies up to 3 metres high, advertising tents on hard standing, or small tent halls without permanent installations.

For structures classified as GC1, a geotechnical opinion is sufficient — it includes a ground reconnaissance based on macroscopic assessment and an evaluation of the suitability of the soil as a building base. You can prepare such an opinion based on archive materials, a site visit and simple field investigations.

Geotechnical category 2 (GC2)

Geotechnical category 2 covers structures with average foundation conditions or structures in simple ground conditions but with a complex form. In practice this is where you find most steel halls with girders, biogas tanks, pneumatic structures and typical tensile membrane structures.

For GC2 structures you need to prepare geotechnical documentation that includes a description of the ground and water conditions, definition of the geotechnical parameters of the building base, and a forecast of changes in the groundwater state. Your investigation should include borings, soundings and laboratory testing of the soils.

Geotechnical category 3 (GC3)

Geotechnical category 3 applies to structures founded in complicated ground conditions or to structures with a form that is sensitive to uneven settlement. This is where you classify large-span ETFE structures on post-industrial sites, tensile membrane structures with unusual geometry or steel halls on landslide-prone terrain.

For GC3 structures you need full geological-engineering documentation as well as a geotechnical design. Your documentation should contain detailed geological investigations, slope stability analyses, forecasts of the project’s impact on the geological environment, and geotechnical monitoring during and after construction.

Geotechnical categories in design practice

Correctly defining the geotechnical category has a direct impact on the design process, the choice of structural solutions and the cost of delivering your investment. Putting this knowledge into practice will let you optimise both safety and project economics.

For ETFE structures, even in apparently simple ground conditions, you should adopt at least category GC2. This is due to the specific way these structures behave under wind loading: uneven foundation settlement can lead to significant changes in the stress distribution within the envelope. Additional geotechnical analyses represent only a small percentage of total project cost, but they significantly raise the safety level of your structure.

For air-supported structures it is worth applying the “120% rule” — even if the formal criteria classify your structure under GC1, doing the investigation as for GC2 lets you define the soil parameters more precisely. This is particularly important when designing the anchoring of these structures, where uniform envelope tension directly affects the geometry and functionality of the building.

Tensile membrane structures present a particular geotechnical challenge because of their sensitivity to ground deformation. Uneven foundation settlement can lead to significant changes in envelope geometry, and as a consequence to stress concentrations and premature material wear. That is why even for small membrane structures you should carry out at least basic geotechnical investigation, including identification of the ground layers down to the active depth of the foundations.

Practical recommendations for investors and designers

Effective management of the geotechnical aspects of lightweight-structure projects requires a systematic approach and awareness of potential risks. The following recommendations will help you integrate geotechnical analyses into the investment process effectively:

At the planning stage:

• Start with a preliminary geotechnical reconnaissance already at the location-selection stage
• Use available archive materials, geological maps and information about neighbouring structures
• Tailor the scope of the geotechnical investigation to the specifics of the planned structure
• Remember the seasonal variability of ground and water conditions

During delivery:

• Include in the project schedule the time needed to carry out the geotechnical investigation
• For temporary structures, consider a modular foundation system
• Consult the investigation programme with the structural designer
• Consider geotechnical monitoring for structures sensitive to changes in ground conditions

If you need support with geotechnical analyses for your tensile membrane, steel or pneumatic structures, get in touch with us. At Abastran we combine specialised geotechnical knowledge with many years of experience in designing and delivering lightweight structures, which lets us offer comprehensive and optimised solutions.