Secant Piling Shoring System-With Circular Configuration

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The paper describes a circular concrete secant-pile shoring system built for a Terminal Lift Pumping Station at the Al Madina Al Shamaliya Sewage Treatment Plant in Bahrain. The station required an excavation approximately 20 m deep within a 33 m-diagonal footprint. A 34.5 m-diameter wall was formed from alternating 900 mm hard and soft piles spaced at 675 mm centres, extending 3 m below excavation level.<br /><br />Ground conditions comprised sand to about 10.5 m depth, underlain by weathered calcisiltite/carbonate mudstone. The circular arrangement enabled external earth and water pressures to be resisted mainly through hoop compression, avoiding cross-bracing and tie-backs. Design calculations showed that the pile overlap’s compression capacity alone was insufficient for the full design pressure, so two reinforced-concrete ring beams were added. The hard piles between the beams were designed to resist the remaining pressure through bending.<br /><br />Because reliable overlap was essential for both structural performance and water tightness, a stringent verticality tolerance of 1:150 was adopted, supported by a guide wall. Pile position, verticality, concrete integrity and strength, and pile overlap were monitored during construction. Temporary casing was used through the loose upper soils.<br /><br />The completed system provided a leak-resistant excavation despite substantial seepage from the surrounding sea. Its circular configuration eliminated cross-bracing, shortened construction, and allowed full-depth hydrotesting of the pumping station. The case demonstrates how careful design, tight piling tolerances and construction quality control can make circular secant piling an effective solution for deep excavations.

Keywords

circular secant-pile shoring

Bahrain

deep excavation

terminal lift pumping station

ring beams

hoop compression

water tightness

secant piles

verticality tolerance

construction quality control