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drill bit (ring bit) must be able to withstand the same loads
during use as the pile. The manufacturer of the casing
shoe and drill bit is liable for their strength according to
Ch. 2, Sec. 3.8.4.3 of PO-2016.
During drilling, the pressure under the pilot bit due to
the feed force is smaller than the flushing pressure,
which means that the flushing holes of the bit remain
open for the duration of the drilling. Should the flushing
holes get clogged, their opening can be attempted by
increasing flushing pressure to the maximum allowed and
by altering rotation speed and feed force. Unclogging the
holes may also be attempted by using air as the flushing
agent instead of a liquid. If the unclogging of the flushing
holes is unsuccessful, the pilot bit is extracted from the
casing, the holes are cleared, and drilling is continued
subsequently.
When an RD pile encounters a large stone, a boulder or
bedrock, the feed force is kept low and rotation speed
is increased. That reduces the risk of excessive lateral
displacement, inclination and curvature of the pile.
If there is the risk of an RD pile encountering wood
material in the soil during installation, it is recommended
that special bits are used to speed up drilling and ensure
successful penetration. Penetration is generally achieved
using conventional drill bits with larger RD piles, but
drilling speed decreases, and in the case of smaller RD
piles, the risk of failing to drill through wood material is
greater. Being able to drill through pieces of metal in the
soil without significant risk of breaking the equipment is
uncertain.
If an RD pile breaks or the drill bit or casing shoe gets
damaged during drilling, so that driving is no longer
possible, an attempt is made to extract the entire pile.
If that is not possible, the pile is generally rejected. In
foundation underpinning it is generally practical to make
a separate assessment of the geotechnical or structural
resistance of a damaged RD pile. The assessment can
then be used as a basis for determining the capacity of
the damaged RD pile to serve as a foundation pile.
The impacts of the installation on surrounding soil are
monitored during drilling of RD piles, and any detrimental
impacts are taken into consideration in subsequent
installation.
Drilling through coarse-grained soil layers may cause
loosening of compact soil layers or compaction of loose soil
layers. The soil supporting the pile loosens if the volume of
the removed soil material exceeds that of the RD pile.
Drilling in fine-grained soil layers may cause soil disturbance
and an increase in pore water pressure. That results in lower
soil layer strengths. Strength returns quite slowly and in the
case of overconsolidated layers only partially.
Disturbance and an increase in pore water pressure can
be prevented, for instance, by:
•
selecting a drilling method suited to the ground
conditions
•
limiting used flushing pressure
•
sequencing pile drilling or extending the duration of pi-
ling work.
Air, water, polymers or cement grout can be used as
flushing agent in drilling. The volume of the soil material
flushed out of the ground with the flushing agent should
be slightly smaller, never larger, than the volume of the RD
pile, while the amount of exiting water should equal the
amount of water used in flushing.
The excess water and/or soil material rising up with the
flushing agent may:
•
disturb the soil layers around the pile
•
result in a loss of strength in soil layers below the
foundations of adjacent structures or ones to be
underpinned
•
damage unhardened mantles of near-by freshly
installed grouted piles or other unhardened concretings
in the ground.
The risk of soil material and/or water rising increases:
•
in loose, even grained soil layers
•
in soft, fine-grained soil layers
•
in different soil layers
•
when using DTH hammer-based drilling equipment and
direct flushing beneath the water table.
If the air used as flushing agent does not exit the soil
around the RD pile, drilling is suspended.
An RD pile is drilled to the design depth in bedrock. At the
rock drilling phase, attention is paid to the colour of the
drilling mud, the penetration rate and upflow of flushing
water. It is possible to determine the type of the bedrock
based on them.
When RD piles are extending to bedrock, it must be ensured
that RD pile serving as a bearing structure bears reliably on
rock after drilling has reached the target depth in rock. This
has to be paid special attention with the eccentric drilling
method and when using concentric reamer wings.
Control blows are imparted on the heads of RD piles bearing
on rock always after extraction of the drill rods and the pilot
bit. The RD pile pipe may lift off the surface of the bedrock
when the pilot bit and drill rods are extracted from the RD
pile. When using the eccentric drilling method and a drill bit
based on a single reamer bit, or concentric drilling method
with "wing bits", the RD pile always remains the height of the
reamer portion of the pilot bit away from the bedrock.
Control blows can be imparted, for instance, by a hammer
drill on the upper end of the RD pile.