In April 2021, the Geotechnical Working Group issued an article titled Ground movements on Underpinning – A problem with data?, with the objective of launching a discussion to try and better understand the magnitude of ground movements that can arise due to underpinning and the construction of small-scale basements, and how ground movement data recorded as part of the construction process can be used to establish a robust framework for the analysis and prediction of potential movements on new projects.
Since publication of the article, an Underpinning Focus Group has been formed by interested members of the Geotechnical Working Group. The Focus Group has been working closely with the University of Portsmouth to facilitate an MSc project, using data from a series of basement excavations within central London, which was completed towards the end of 2021 and identified some issues likely to be common to many projects:
- Often buildings involve a mixture of underpinning and piling, and this modifies the movement in different ways.
- Some buildings have inherent problems before construction starts and may already have ground movement problems before a new basement is excavated.
- There is normally no published ‘post construction report’ to say what went according to plan and what changes were made during the construction, unless an article is published, making it difficult to assess what learnings were made.
In general, the study found that there was a good level of agreement between predicted and actual movements and was able to make the following recommendations:
- Close coordination between the geotechnical / ground investigation consultants, structural engineers, movement monitoring survey companies and the construction companies is important.
- Identify and confirm the geotechnical properties, depth and influence of superficial deposits, such as river terrace deposits, brickearth and the weathered London Clay within the area of the proposed development to enable the development of a ground model that accurately reflects the ground conditions that can then be used in any subsequent design of the proposed basement structure and analysis of potential ground movements.
- Accurately assess the state of buildings and infrastructure in the vicinity of the excavation site prior to construction.
- Ensure the ground and building monitoring survey targets are in-place before any construction, ground loading or demolition work commences, as it is important to establish a baseline of background noise, vibration and any other ground disturbance.
- Make provision for monitoring sensors to remain in place once construction is complete to provide data on the long-term response of the ground and surrounding structures to basement construction.
- Although it is generally considered that the deepened foundations provided by basements should make the overground structures more stable, the subsurface geology, excavation methods, construction techniques, and the quality of workmanship, can all have different impacts on the severity and extent of ground movements affecting adjacent buildings and structures.
Following completion of this initial pilot study, three more MSc projects are presently in hand and due for completion at the end of this academic year, with the students working on the modelling aspects of the problem using real data from basement excavations in London, which, it is hoped, will lead to further detailed analysis and ultimately the establishment of a research group to continue the work that has been started.
In the long-term, the Underpinning Focus Group is looking to facilitate the collation of case study data and are presently working on ideas for a suitable on-line system to allow members and other interested parties to submit data and share their experiences, which, in time, can hopefully be used to generate guidance through suitably reviewed research of the case studies. In the meantime, if any readers of this or the previous article have data or other personal experiences that they think might be of use or would like to become actively involved and contribute to these discussions, then please contact the Geotechnical Working Group (firstname.lastname@example.org), who will be more than happy to hear from you.