The design of underground structures, especially those in direct contact with the ground, requires a process called ‘soil-structure interaction’. Why and what is the interaction? How does it take place? This post kicks off a series of blog posts on this topic.
What is the interaction?
The exact meaning of ‘soil-structure interaction’ depends on the context it is applied to, but generally speaking, for underground construction, the ground and the structure behave in a composite manner – the structure holds up the soil whilst the soil can also hold up itself to certain extent. The soil puts pressure on the structure and vice versa. The soil moves under pressure, but it moves together with the structure. The design has to verify against both structural limits and geotechnical limits.
From a design management perspective, soil-structure interactions typically involves two collaborative technical teams – geotechnical and structural. The teams can either stay separate throughout the process, managed by a design manager, or combined into a single mixed team, particularly in the case of multi-disciplinary consultancies. After an initial briefing and exchange of design strategy, each team proceed to do their own modelling and obtain results out of their own model. Results are then exchanged and discussed, with both teams incorporating changes and conducting another iteration of analysis. Results are then again exchanged and discussed and then changes done. So on and so forth. The ultimate end (at least in theory) is the convergence of all key results from the two teams. Exactly what results are to be converged and to what degree of accuracy they need to be converged is somewhat arbitrary and pragmatic, depending on many considerations such as the level of risk, available expertise, programme and budget.
Usually each team has a discipline lead managing a team of engineers and/or specialist modellers. Ofcourse under the context of soil-structure interaction, any person that is able to understand the technical aspects from both sides has a strong competitive edge in technical leadership. With the technology available nowadays, computer analysis models are almost exclusively used, even for preliminary simplified design. Each model is briefly outlined separately below:
The geotechnical model
The geotechnical model typically has the ground properties explicitly modelled and simplified structure, based on one or perhaps a few pre-defined critical design scenarios. If the ground is soil then ground water is also modelled. It also typically has construction sequence explicitly modelled to understand the sequence and timing for each stage to take place and their effects on the ground behaviour and groundwater flow. The model usually has non-linear materials and thus based on a single load combination without load factors. The geotechnical model has mainly the following purposes:
· It verifies whether the ground fails when subject to the loadings from the structure.
· It estimates the ground movement which leads to the prediction of surface settlement and impact on existing structures.
· Feeds ‘soil springs’ into the structural model
The structural model
The structural model has all structural elements explicitly modelled, with the ground simplified into ‘springs’. It can have all possible ULS and SLS load combinations and load factors explicitly modelled.
The structural model has mainly the following purposes:
· Take into account of all possible load combinations so that enveloped values can be used for structural design.
· Generates construction sequence and loadings
· Generates geometry and material properties, a simplified version of which will be fed into the geotechnical model
The interface
The direct interface between the two teams are mainly the pressure and movement at the ground-structure interface. The movement needs to converge between the two models for a given (critical) load case. However, more importantly, the two teams need to ensure they are using consistent assumptions and methodologies. This calls for sound design brief and discussion upfront before the work starts, as well as a systematic continual process to maintain and update the initial agreement. This puts the skill of design management to test.
To deal with a large amount of uncertainties, each team needs to conduct sensitivity tests on key design parameters to make sure the design is robust enough to survive the worst credible scenarios. I will make another blog post specifically on sensitivity tests.
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