Non-Conventional Practice: Recent Case Histories of the Use of Vertical Buttress Piles in Deep Excavation Support-Paper

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The paper presents twin vertical buttress piles (TBPs) as an alternative to conventional props and ground anchors for supporting deep urban excavations. Installed perpendicular to a contiguous bored-pile retaining wall and connected by reinforced-concrete capping beams, the paired piles use a push-pull action to restrain wall movement. TBPs can be placed inside the excavation for temporary use, then cut down after the permanent slabs are built, or behind the wall as a permanent, unobtrusive solution.<br /><br />Design combines conventional limit-state analysis with finite-element modelling to assess soil–structure interaction, including load transfer among the wall, capping beams and buttress piles. The paper describes three London-area residential basement projects with excavations approximately 2.3–6 metres deep. The TBPs were temporary and inside the excavation at two sites, and permanent and behind the wall at the third.<br /><br />Monitoring showed the systems were effective at limiting wall movement. Recorded lateral deflections were 2 mm at the first site, 4 mm at the second and up to 9 mm in the deepest zone at the third. These movements were generally below design predictions, and no serviceability problems were reported for neighbouring buildings. At the second site, using TBPs instead of proposed temporary bracing saved about 4.5 weeks.<br /><br />The authors conclude that TBPs can provide stable excavation support while preserving working space and reducing the risk of encroaching on neighbouring properties. Because the piles can be installed using the same equipment as the retaining wall, they can also simplify construction sequencing and improve project schedules and costs.

Keywords

twin vertical buttress piles

deep urban excavations

contiguous bored-pile retaining wall

push-pull action

reinforced-concrete capping beams

soil-structure interaction

finite-element modelling

London basement projects

lateral wall deflection

temporary excavation support