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  • Writer's pictureSi Shen

Soil structure interaction (3) – Eurocode 7 design approach

Updated: May 4, 2021

Failure modes in underground structures

For underground structures, as long as a structure is present to support the ground, the design is always done for two parts - (1) structure and (2) ground. For the most commonly seen type of underground structures, such as building foundations or retaining walls, there are failure modes associated with both the structure and the ground, and the failure modes in the two categories can be completely independent. Take an embedded retaining wall as an example:

  • The wall could fall over on its face whilst the structure is completely intact (overturning failure in the ground)

  • It could also snap/bend in the middle whilst the lower part remains firmly buried in the ground (structural failure)

However, there are also plenty of situations where only one of the two categories is checked.

  • Where the ground is self-supporting (e.g. slope stability, unsupported tunnel excavation face), the check is done for the ground only.

  • Where there is no failure mode for the ground (e.g. the ground around a completed tunnel in its final and fully supported form is self-equalising), the check is done for the structure only.

Design partial factors

Eurocode 7 is the design code for underground structures, and any structures where soil-structure interaction is relevant. See my previous post on what ‘soil-structure interaction’ means. The ‘prescriptive’ design by calculations, which is the most common design procedure as we are taught in universities, requires us to place ‘partial factors’ on top of ‘characteristic’ values. See my previous post for what ‘characteristic values’ and ‘partial factors’ mean.

In Eurocode 7, partial factors could be applied to three components:

  • Actions. Applying partial factors to the actions is mostly the same as to the effect of actions

  • Materials. In geotechnical engineering, this means ground parameters

  • Resistance (of the ground or structure)

A fundamental approach for Eurocode 7 calculations is to make sure that the factored effects of actions is no greater than the factored resistance.

Eurocode 7 provides three different ‘design approaches’ available for the choice by the engineer. For each design approach, the way that partial factors are applied is different. Largely speaking (with exceptions for some structures), the following is how partial factors are applied:

  • Design approach 1: partial factors are applied to actions and materials, and not to resistance (unity factor).

    • Combination 1. This is mostly a structural check. Partial factors are only applied to actions. Partial factors for actions are the same as those for structural design (e.g. Eurocode 2 if it is concrete)

    • Combination 2. This is mostly a geotechnical check. Partial factors are generally not applied to actions (with the exception of live load), but applied to material properties of the ground. Piles and ground anchors have their exceptional rules - a partial factor is applied to the resistance as well

  • Design approach 2: partial factors are applied to actions and resistance, but not to materials

  • Design approach 3: partial factors are applied to actions and to material, but not to resistance

For United Kingdom, the default choice is ‘Design Approach 1’. In a future post, I will discuss about the relative ease of applying these design approaches in finite element models.

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