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Finding fault

Ground discontinuity at a University College hospital project in London show that the London basin is more faulted than thought. NCE reports.

Work on the foundations for a new cancer care unit at University College London Hospital has thrown up some new evidence that the ground in the London area can be more complex and discontinuous than usually thought, with complex faulting.

“When we looked at the piling records we realised that the geology was not entirely straightforward,” says Alice Berry, Arup’s geotechnical engineer for the project. “They appeared to show a sharp change in the level of the boundary between the London Clay and the Upper Mottled Beds of the underlying Lambeth Group. We realised we needed to investigate further.”

Site investigation

Arup tailored the site investigation to get as close as possible to the neighbouring Rayne Institute building. A cone penetration test from inside the basement of the existing building found a 2m thick layer of sand, while the external boreholes showed variation in the level of the top of the Lambeth Group and the core exhibited polished discontinuities of varying orientations.

“It was fascinating to see such changes in stratigraphy finally revealed on site”

Alice Berry, Arup

“Although we had some interesting results from the site investigation, it was not possible to fully understand what was going on, so we had to accommodate some uncertainty in our design,” says Berry. “We made an initial interpretation of the ground conditions and concluded that the geological anomaly did not extend across the whole site, but that the sandy bands and the piling records of the Rayne Institute could not be ignored.”

Arup used two different design stratigraphies: one “standard” and one modelling the effect of the sandy band and allowing for the higher Upper Mottled Beds. It also specified that the piling contractor make provisions to fully case the piles in the vicinity of the sandy band to prevent collapse of the bores.

Understanding anomaly

To allow a better understanding of the nature and extent of the anomaly and its impact on the construction process, Arup agreed with the piling contractor, Cementation Skanska, that the first piles should be constructed where the anomaly was expected to be present. These piles encountered a thick layer of sandy material, and very little London Clay above the top of the Lambeth Group.

In contrast, piles constructed only a few metres further away found more than 15m of London Clay. Their design was sufficiently robust in light of the as-found ground conditions, but those a little further away were reassessed by the contractor to see which of the remaining piles would need to be fully cased.

Exposed anomaly

The retaining wall piles were constructed successfully and during basement excavation it was finally possible to see the anomaly exposed. A clear inclined contact between the top of the London Clay and the Upper Mottled Beds was visible, stretching across the basement floor and up the side of the retaining wall. A sandy channel present in the Upper Mottled Beds dipped steeply from south to north close to the Rayne Institute. These observations indicated a faulted contact at this end of the site.

“Having identified and interpreted a possible anomaly, and allowed for it in the design and construction, it was fascinating to see such rapid changes in stratigraphy finally revealed on site,” says Berry.

“It goes to show how important a desk study is: if we had just assumed standard London stratigraphy and not investigated the anomaly further then we could have had a nasty surprise during the construction.”

Arup and Skanska have shared their findings with the London Basin Forum research group, to assist with its work on faulting in the London Basin.

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