Article Geotechnical

Serviceability Limit State and Sustainable Shallow Foundations Design

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With the Net Zero target initiatives set, shaping the future of the construction industry around a greener approach must be one of the main focuses of the sector. Although these fixed dates appear to be long ahead, data shows that significant changes must be made immediately and in great scales, to be able to cut the carbon emissions to the required levels. The consequences of failing to do so will have a significant impact on the future of our lives. The concern is that even with the recognized importance of this matter, the regulations for UK construction industry yet fail to formally incorporate and enforce the requirements for build to a net-zero emission performance at the current time.

Nevertheless, the industry is going through some changes to improve, predominantly within the larger firms that have the resources and the funds available to invest in greener solutions, although this is considered to be nowhere near sufficient.

Studies show that these efforts are vastly shifted towards targeting reduction of emissions by focusing on the operational carbon emissions and less by means of targeting embodied carbon emissions or reducing the net carbon emissions by offsetting practices. However, research indicates that in actuality, to achieve the Net Zero targets by 2050, a fabric first approach, focusing on the embodied carbon in combination with operational carbon footprint reduction is extremely vital (1). This approach would be targeting the whole life carbon emission footprint of a project.

As engineers we have a significant impact on projects’ embodied carbon emissions, through our design of every aspect and step. In the absence of enforced guidlines, it is our responsibility to target embodied carbon emission reduction by adopting a more efficient design and/or selecting the right materials.

This article aims to challenge our approach towards serviceability limit state verification of shallow foundations, in order to achieve a more sustainable design. This topic is considered to be of significant importance, because often the shallow foundation design is governed by the serviceability limit states, particularly where cohesive deposits are present within the influence zone of the foundation. An overly conservative design can lead to requirement for larger and therefore less environmentally friendly foundations.

Design of foundations generally involves both the geotechnical and structural engineers; and given the set industry Net Zero targets, they must fully appreciate that whilst some initiatives (such as Whole Life Carbon Approved Document Z (6), UK Architects Declare Climate and Biodiversity Emergency (2) or the RIBA’s climate change plan (3)) are merging to allow a more structured and regulatory approach, they must take immediate actions through design to implement the Net Zero Performance criteria.

We recognize that the design of a foundation must satisfy the ultimate limit state and the serviceability limit states.

The serviceability limit states of the foundations, assessed in form of settlement, as described within the current Eurocodes (5) are considered to be those concerning comfort of the users during use and the aesthetic of the structure, and it depends on both the total and differential settlements. Our approach to validating the serviceability limit state generally comprises assessment of the anticipated settlement of the ground due to the applied load against a total settlement limiting value and assessment of the differential settlements across structure foundations against a differential settlement limiting value, such that the serviceability limit state criteria described above is utilised.  On this basis, we consider that a greener approach can be achieved through two possible ways: first challenging the industry accepted serviceability limit state limiting values and second, the exploring available methodologies through which the settlement is estimated.  These are to, typically, be considered by the structural engineers and the geotechnical engineers respectively.

Nonetheless it must not be forgotten that a foundation is the point of interaction between the ground and structure and therefore a lack of communication between the two disciplines can often result in an overly conservative design.

The serviceability limit state limiting values, must be chosen based on a series of factors, as described within the Eurocodes (5), such as: the rate of ground movements, the type of structure, foundations and construction material, ground conditions, the mode of deformation, the structure function and the need to ensure that there are no problems with the services entering the structure. Therefore, with reliance on the Eurocode guidance, it is evident that the limiting values must be decided upon by consideration of the factors mentioned and based on the design requirement throughout the lifetime of the structure.

However, whilst the codes provide us with clear indication of what must be considered, their guidance provided on the actual limiting values are generic and limited. This should be considered as an opportunity for critical thinking to evaluate design and therefore to provide values reasonable for the requirement of each specific project. In addition, other guidance (such as Structural Engineer’s Pocket Book Eurocodes(7), allowing determination of a value based on deflection criteria of a structure in accordance with the Eurocodes) through consideration of specific criteria as listed by Eurocode, can be utilised  to assist with the limiting value determination. A review of the Eurocodes Next Generation, it is clear that guidance as such are to be provided more clearly with a better and more clear guidance on the limiting values, on the basis of a project risk/sensitivity assessment, which is what should in reality be undertaken by the engineers continuously. Though, even with the new guidance from the Eurocode Next Generation, a thorough evaluation of the requirements of design, must still be considered.

The question raised, for the time being, is that why guidance as such is not used often enough to give limiting values specific to the design of every structure, as opposed to using the set of generic values, considering that the effects of this value on the sustainable design of the foundation can be significant, particularly on larger scale projects with numerous foundations involved.  Therefore, it is important that prior to choosing a limiting value, that consideration is given to factors such as: importance of aesthetics, lifetime of the structure or use frequency. Of course, this requires a thorough understanding of ground movement and its effects on structure. In terms of challenging the design of serviceability simit states, from a structural engineer’s side, it can be challenged that by a better understanding of differential and total settlement concept, more thoughtful and possibly less overly conservative values can be used. This applies to even where the typical values are used for the purposes of design.

In addition to challenging the limiting values, the methods of estimation of settlement must be considered by the geotechnical engineers. A thorough understanding of soil mechanics and methodologies as provided within the current Eurocodes should allow for a better design, where a less conservative approach could be taken.

We understand that risk with variable ground conditions is always present and therefore we always emphasise that settlement values are only an estimation of how the ground is likely to behave and move. Therefore, we often incorporate some level of conservatism in design. However, what should not be forgotten is that serviceability limit states verification methodologies such as numerical methods, prescriptive methods, verification through in-situ testing or observational methods are at our disposal to use in order to challenge and potentially reduce the level of conservatism through gathering of better and more representative data, in order to make our design better and more efficient.

Even with the Eurocode Next Generation coming into play with better indication of our approach to serviceability limit state and therefore resulting in a more efficient design, it is still vital that serviceability limit state limiting values are decided based on the design criteria of the structure throughout its design life, using the assessment criteria provided in the current codes. In addition, it is undeniable that further consideration given to methods of assessment of the ground and estimation of settlement can result in a better approach. The latter is most important in dealing with challenges such as variability of ground conditions which often are the reason for which a conservative approach is utilised.

  1. Built Environ (2021), How can UK Housing Projects be Brought in Line With Net-Zero Carbon Emission Targets? L Jankovic et al.
  2. Architects Declare (2019). UK Architects Declare Climate and Biodiversity Emergency [WWW Document]. UK Archit. Declare Clim. Biodivers. Emerg. URL Available at: 5.6.21).
  3. RIBA (2019). RIBA 2030 Climate Challenge [WWW Document]. 2030 Clim. Chall. URL Available at: 5.6.21).
  4. EAC (2021). Energy Efficiency of Existing Homes (No. Fourth). Environment Audit Committee.
  5. BRITISH STANDARDS INSTITUTION. (1995). Eurocode 7. Part 1, General rules (together with United Kingdom national application document). Part 1, General rules (together with United Kingdom national application document). London, British Standards Institution.
  6. N/A (2021), Whole Life Carbon Approved Document Z, N.A
  7. CRC Press (2015), Structural Engineer’s Pocket Book Eurocodes, third Edition, C. Fiona

Article provided by Ronak Amirhosseiny, Senior Geotechnical Engineer at Coffey Geotechnics Ltd