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Finding Fault

A sharp ground discontinuity found at the University College hospital project in central London underlines that the London basin is more faulted than once thought. GE 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.

The building is separate to the main hospital at the end of Tottenham Court Road, being located one block eastwards on University Street next to the Rayne Institute building.

Project engineers from consultant Arup first became aware that something was up when a desk study found piling records for this building.

These revealed descriptions of thick bands of sand and of multi-coloured clay at a level where nearby boreholes had encountered grey London Clay.

“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.”

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.

“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”

“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 should make provisions to fully case the piles in the vicinity of the sandy band to prevent collapse of the bore.

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.

The retaining wall piles were constructed successfully and during excavation of the basement 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.

TCR_2

Meanwhile, 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.

Forum member Jackie Skipper, a senior geologist at the Geotechnical Consulting Group, is excited by the findings: “We are finding more and more evidence of faulting in the London Basin, and this is a particularly useful visual example in an area which is normally considered to consist of 20m thick London Clay.

“This site also confirms that a model of ‘normal’ faulting in London, where one side of the fault moves vertically relative to another but there is no horizontal offset, is inadequate to explain these features. Together with work on Crossrail and Thames Tideway projects, this points to a much more complex structure.”

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