30
If the used lower part of the helmet is the square shaped
”box” used with precast concrete piles, it is recommended
that a steel adapter be always used on the pile head in
piling class PTL3. The top of the adapter should be as
close in size to the ”box” of the helmet as possible, and
the bottom should ”sit” quite snugly inside or outside
the pile pipe. The use of an adapter is recommended in
piling class PTL2. In PTL2, when installing RR220 piles
with a ”box” measuring 250x250, RR270 piles with a
”box” measuring 300x300, or RR320 piles with a ”box”
measuring 350x350, sufficiently centric blows can also be
achieved without an adapter.
When installing a pile with an external splice sleeve at
the top, it's recommended to use an adapter piece that
passes the stroke to the pile pipe trough the sleeve. By
decision of installation crew, the embedding of pile in
soft ground layers may also be done by striking directly
to sleeve if it does not cause damages to the sleeve or
its weld. In dense ground layers and during the end-of-
driving, an adapter piece must be used to pass the stroke
to the pile pipe through the sleeve.
In Finnish soil conditions large diameter end-bearing
piles can usually be installed to a sufficient penetration
depth with relatively small hammers. Thick friction soil and
moraine layers require a sufficiently high impact energy
for the pile to penetrate effectively. In preliminary analyses
it can be estimated that when using the hammers and
pile dimensions presented in Table 26, the installation
equipment has sufficient impact energy and the hammer
is able to mobilise sufficient static geotechnical resistance
in PTL2 and PTL3 during loading tests. Actual mobilised
resistances depend largely on pile size, pile length and soil
conditions. Sufficient static resistance can be achieved
more easily with short piles securely bearing on bedrock
than with long piles bearing on moraine layers.
It is often practical to use a smaller hammer / lower
impact energy to drive large diameter piles and perform
the actual dynamic load test using a pile driving
equipment of higher impact energy.
Table 26. Combination of hammer weight [
t
] – max. pile
size that is normally able to mobilise sufficient static
resistance in PTL2/PTL3.
Hammer [t]
max. RR pile
5
RR400…RR500
7
RR500…RR700
9
RR700…RR800
7.3.1.3 Hydraulic rams
Hydraulic rams are rapid percussion pile drivers
extremely well suited for installing RR micropiles. They
are also suitable for the installation of larger piles, if full
geotechnical compressive resistance is not required of the
piles. The advantages of hydraulic rams in the installation
of RR piles include high blow rate and impact force, which
makes for quick pile installation; piles can in most cases be
installed very straight, and the installation equipment is
light and mountable on many kinds of basic machines.
The suitability of different hammer types for different pile
sizes and lengths has been determined by simulations
based on the stress wave theory. Pile sizes and lengths
suitable for different rams are presented in the end-of-
driving instructions based on the above analysis.
Hydraulic rams are effective in installing micropiles, and
experience has shown that in certain conditions piles driven by
hydraulic rams penetrate deeper than dynamic penetration
test. Even if simulation does not indicate sufficient mobilised
static geotechnical resistance due to the small settlement
caused by a single blow, the pile toe often bears reliably on
bedrock or very dense basal moraine. In conditions clearly
suitable for end-bearing piles, it is therefore often possible to
achieve sufficient reliability by a hammer/pile combination
where the calculated mobilised static geotechnical resistance
does not meet the end-of-driving criteria. In such a case, the
geotechnical resistance of piles must be ensured by dynamic
load tests (PDA measurements) or analyses based on pile
driving formulas. A pile driving equipment of higher impact
energy, such as a drop hammer or a hydraulic hammer, must
be used in the load tests.
To centre the blow and protect the pile head, a steel
adapter must be used between the pile and the tool of the
hydraulic ram. (Figure 12)
Ensuring that the impact is aligned with the pile in piling
class PTL3 requires use of piling equipment where the
hydraulic ram moves along a piling mast. This driving
mode is also recommended for other piling classes.
Figure 12. Example of an adapter between the striker of
the hydraulic ram and the pile during impact.
7.3.1.4 Pneumatic hammers
Pneumatic hammers have a slightly lower blow rate than
hydraulic rams. The suitability of different hammer types
for different pile sizes and lengths has been determined by