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Large diameter bored pile

Large diameter bored piles were developed principally to cope with higher building loads and the economic advantage of reducing pile cap construction. The piles are effective in a wide range of soils but are particularly economical when bearing on a firm founding strata.

Large diameter bored piles are particularly effective in the following applications:

Large diameter bore
  • Very high vertical loads: depending on the diameters, vertical loads per pile vary from 2.000 to 15.000 kN.
  • Important horizontal loads or bending moments at the pile top: the pile diameter allows very dense reinforcement cages to be used (up to 2% of the concrete section).
  • The necessity, due to surrounding conditions, to avoid vibrations.
  • Underground obstacles: non-reinforced concrete foundations can be destroyed by chiseling.
  • Large pile depth: the techniques used can reach a depth of 60 meter and more.

Often they are designed as end bearing piles, which do not depend upon skin friction for their load bearing capacity, although friction can be beneficial in appropriate ground conditions.

Large diameter boring rig

Typical diameters range from 0.75 m to 1.80 m, or more. Bored piles can be constructed either straight-shafted or underreamed. In cohesive soils, an enlarged bases of up to three shaft diameters can provide an economic solution for heavy loads, as it substantially increases the base bearing capacity. On firm soil, or competent bearing strata, the strength of the founding material can be utilized. The working load may be controlled by the structural capacity of the pile shaft (concrete). Reinforced shafts also offer considerable resistance to both lateral forces and bending moments.

To cope with difficult ground conditions, special boring techniques have been developed, using chiesels, grabs, core barrels, oscillator and vibrators. The use of bentonite suspension (instead of a temporary casing) in unstable ground is an effective alternative. In non-cohesive material, temporary or permanent casings are often used.

Construction of pile

Execution of a large diameter bored pile with a grab

The ground is removed by an excavating tool, such as a bucket, auger or core barrel, mounted on a crane equipped with an appropriate rotary table. The boring is possible in a variety of soils, from soft ground to loose rock.

While boring, a recoverable steel casing is pushed into the ground. In order to avoid any penetration of soil or of water into the casing, a soil plug should be maintained inside the casing during boring.

Large diameter bore with grab

In the case of high ground water table, the hydrostatic excess pressure can be maintained inside the casing in order to avoid decompression or erosion of the soil surrounding the pile.

When a resistant layer is reached, the pile base is cleaned and the reinforcement cage is installed in the pile, using the drilling unit. The pile is concreted using a tremmie pipe to avoid concrete segregation. The use of a tremmie pipe, or bell, is possible even under water.

Execution sequence

Execution sequence of a large diameter augered pile
  1. Drilling above the ground water level
  2. Drilling below the ground water level
  3. Drilling below the ground water level.
  4. Concreting with a plunging tube.
  5. Final extraction of the temporary casing.

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