## Secant Piling

#### Principles Of Construction

• Often known as “Benoto” piling, which is a type of trade name.  Usually replaced by diaphragm walling methods.
• Essentially typical bored piles, or down-the hold type bored piles, are constructed with a spacing slightly smaller than the chosen diameter of the piles.  Once a suitable length of piling has been completed, the “holes” are filled with a similar bored pile which intersects and “bites” into the two previously constructed piles.
• Usuall these piles are strengthened with a steel “H” pile, rather than the typical reinforcement cages used in standard bored piling.

#### As-Built Examples

• Large diameter circular secant pile cofferdam.
• Structure consisting of 1,200 mm diameter secant piles to a depth of 20 which then stepped into 900 mm contiguous piles for a further 20 m depth.  Each pile was probed to 10 m depth in advance.  A total of 255 number piles were constructed.
• The “cofferdam” had a 1.4 m deep concrete base slab with a concrete lining to the walls.
• An intermediate concrete roof slab was placed to form an underground box structure.
• Overall construction time  12 weeks (Heathrow Express).