Elephant and Castle Town Centre Development - Engineering within the Harwich Formation-Paper

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The Elephant and Castle redevelopment required excavation of six deep reinforced-concrete transfer-beam trenches above the Bakerloo Line tunnels. The excavation extended through London Clay, with the highly permeable, water-bearing Harwich Formation about 5 m below the trench bases. Although conventional Eurocode 7 uplift checks indicated adequate stability, analysis showed that uplift of the London Clay could still cause damaging movement of the tunnels.<br /><br />Groundwater measurements indicated pressure of approximately 154 kPa in the Harwich Formation. A three-dimensional Plaxis soil–structure interaction model, incorporating the tunnels, retaining walls and staged excavation sequence, was used to assess ground movements. The analysis found that a factor of safety of 2 against uplift was needed to keep tunnel movements within acceptable limits. This required reducing groundwater pressure to about 100 kPa.<br /><br />The construction team installed six wells and continuously pumped the formation, supported by backup pumps and a generator, regular system checks, enhanced tunnel monitoring, emergency procedures and measures to enable rapid excavation and backfilling if necessary. The transfer beams were successfully constructed, and monitoring confirmed that tunnel movements were within predicted limits. The case demonstrates that meeting standard uplift requirements alone may not protect nearby tunnels from movement-induced damage; project-specific ground-movement assessment and additional groundwater control can be essential.

Keywords

Elephant and Castle redevelopment

Bakerloo Line tunnels

transfer-beam trenches

London Clay excavation

Harwich Formation groundwater

uplift stability

Plaxis soil-structure interaction

tunnel movement monitoring

groundwater control

Eurocode 7