|Large diameter bored pile
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.
bored piles are particularly effective in the following applications:
- Very high
vertical loads: depending on the diameters, vertical loads per pile
vary from 2.000 to 15.000 kN.
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
- The necessity,
due to surrounding conditions, to avoid vibrations.
obstacles: non-reinforced concrete foundations can be destroyed by chiseling.
pile depth: the techniques used can reach a depth of 60 meter and more.
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.
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.
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.
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.
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.
above the ground water level
below the ground water level
below the ground water level.
with a plunging tube.
extraction of the temporary casing.