Use of Porous Concrete in Secant Bored Pile Walls-Paper

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The study investigates using porous concrete in primary piles of secant bored pile walls to create drainage paths that reduce groundwater pressure on retaining walls. The piles must remain permeable while retaining enough strength to transfer horizontal earth loads to reinforced secondary piles and be cut during their installation.<br /><br />Laboratory tests identified a suitable mix using 8/16 mm aggregate, CEM III B 42.5 cement, a water-cement ratio of 0.38, 2.5% stabilizer, and 1% superplasticizer by cement mass. Depending on cement content and aggregate grading, the concrete achieved compressive strengths of approximately 3–8 MPa and permeability well above the required 1 × 10⁻⁴ m/s. Cores could be obtained after seven days of curing, indicating that the material should be sufficiently workable for subsequent secondary-pile construction.<br /><br />The main challenge is placing this unsaturated, stiff concrete underwater without washing cement paste away from the aggregate. Conventional tremie placement and free falling caused segregation. Small-scale tests showed better results with a “cone repose” method: concrete is poured over a previously placed cone while the pipe is continuously withdrawn. A ball valve is recommended to separate the initial concrete from water. The effectiveness of this method also depends strongly on the specific admixture products.<br /><br />The authors conclude that ordinary fresh-concrete tests, such as slump measurements, do not adequately assess suitability for this application. Placement tests and improved quality-control methods are needed before full-scale use. A prototype-scale trial pile was planned in Germany.

Keywords

porous concrete

secant bored pile walls

drainage paths

groundwater pressure

retaining walls

permeability

compressive strength

tremie placement

cone repose method

underwater concrete placement