domingo, 11 de outubro de 2009

SECANT PILLE WALLS


Figure 3.15: Drilling CFA Piles through Guide for Secant Wall


Figure 3.14: Schematic Plan View of a Secant Pile Wall


Figure 3.13: Secant CFA Pile Wall for a Light Rail System in Germany

CFA piles can be used to construct secant or tangent pile walls in a manner very similar to that of drilled shaft walls, which can be designed as cantilever or anchored walls. The most significant distinction relating to CFA piles, as opposed to other types of vertical elements, is the construction method for the vertical element. The CFA piles are intended to provide a reinforced vertical wall member having a similar function as that of a drilled shaft, slurry wall section, or sheet pile. In almost all such cases, the contractor provides designs of CFA piles for retaining structures as a design-build option. Figure 3.13 illustrates a typical CFA secant pile wall.

The major differences for CFA pile wall systems as compared to other pile types are discussed below.

  • Diameters of CFA piles are generally limited to about 1 m (40 in.).
  • Maximum depths for CFA piles are generally no more than 10 to 18 m (33 to 60 ft) with wall heights generally around 12 m (40 ft) or less. This limitation is not only related to machine capability, but also due to the fact that the reinforcement must be placed into the fluid concrete and verticality of the piles can be difficult to maintain for long piles.
  • Control of verticality is critical to keep the piles aligned, especially if a watertight structure is needed; some of the hydraulic rigs equipped with inclinometers are well-suited for this construction.
  • For secant pile systems, the sequencing of pile installation and set time and initial strength characteristics of the concrete is critical so that the excavation equipment can cut into previously drilled piles. Some contractors install primary piles using a weaker concrete mix and utilize a stronger mix for the secondary piles, which provide the main structural strength of the wall (see Figures 3.14 and 3.15). Large, high torque-capacity rigs are necessary, especially when cutting into existing concrete piles.
  • Placement of reinforcement can be more difficult in a CFA pile than in a conventional drilled shaft or slurry wall, in which the reinforcement is placed ahead of the concrete.

  • http://www.fhwa.dot.gov/engineering/geotech/pubs/gec8/03.cfm

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