18
5.5.5 Resistances determined by pile driving
formulas
Pile driving formulas can be used in piling class PTL1 or PTL2,
for example, in situations where, according to the end-
of-driving table, the used pile driver is not able to mobilise
sufficient ultimate geotechnical resistance and geotechnical
resistance is ensured by a separate test loading hammer
without a dynamic load test. The pile driving formulas are used
according to PO-2016, Ch. 1, Sec. 4.5.2.5.
5.5.6 Resistances determined on the basis of ground
test results
Geotechnical resistance is determined on the basis of ground
test results according to PO-2016, Ch. 1, Sec. 4.5.2.3. It is
recommended that the so-called alternative method is
used in design, where a model factor of
≥
1.6 is used for
end-bearing and friction piles and
≥
1.95 for cohesion piles in
long-term loading and
≥
1.40 in short-term loading.
As concerns steel piles, the capacity of both smooth and
grouted friction piles can be determined on the basis of
ground investigation results, but it is recommended that
the capacity is also determined by static or dynamic load
tests. This method is highly suitable for calculating the
geotechnical compressive strength of foundation piles of
lightweight noise barriers.
The point and shaft resistance of piles can be estimated
either on the basis of the angle of friction or cohesion of soil or
directly based on sounding resistance according to PO-2016.
5.5.6.1 Special features of the geotechnical resistance of
open ended steel pipe piles
In preliminary analyses of open ended steel piles with point
reinforcement ring (a steel collar over the shaft), external
shaft resistance can be estimated to decrease by 50 %
in a dense coarse-grained soil layer or moraine layer,
and 25 % in a loose layer compared to the table values
presented in PO-2016 or static capacity formulas. Point
resistance increases with increasing pile-point area.
If no plugging occurs in the pile, internal shaft resistance
can be assumed to be half of external shaft resistance in
preliminary analyses. However, the capacity consisting of
internal shaft resistance and point resistance of the area
of the steel cross-section of the pile must not exceed the
capacity of a plugged pile of corresponding size due to
point resistance.
5.5.6.2 Geotechnical resistance of grouted CSG-RR piles
bearing on a soil layer
The dimensioning geotechnical diameter of shaft grouted
piles (
d
d
) may be larger than the diameter of the collar
(
d
0
) used with the pile. The increase in diameter is caused
by the pressurising effect of grout with this installation
method, which makes the grout both displace and mix
with the soil layers surrounding the pile.
The dimensioning geotechnical diameter can be
determined, for example, by measurements on a test pile
or by using information on shaft grouted micropiles in
corresponding soil conditions. The magnitude of
the dimensioning diameter can be evaluated using
Formula 2.
d
d
= a ·d
0
(2)
where
d
d
is the dimensioning geotechnical diameter;
a
is the coefficient that depends on soil type, grout
pressure, etc. and
d
0
is the diameter of the collar used with the pile in
question
The coefficient can vary with different soil types as follows:
clay
a
= 1.0
silt
a
= 1.0 to 1.1
sand, gravel
a
= 1.1 to 1.2
moraine
a
= 1.0 to 1.2
With shaft grouted CSG-RR piles the shaft resistance
factors of Table 16 can be used as shaft resistance
factor
K
s
tan
φ
a
, and the dimensioning of point resistance
is the same as with non-grouted piles. Values based on
sounding resistance may also be used to evaluate shaft
and point resistance, see PO-2016, Ch. 1, Tables 4.6 and
4.8.
5.5.6.3 Geotechnical resistance of RD piles drilled into
bedrock
The point resistance of a pile bearing on solid Finnish
bedrock is usually not a dimensioning factor, but
resistance is determined on the basis of the structural
resistance of the pile.
The tip of the pile is assumed to bear on bedrock when
both drilling observations and geotechnical investigations
confirm it.
Table 16. Shaft resistance factor K
s
tan
φ
a
for grouted piles in coarse-grained soil types.
Internal angle of friction of soil [
o
]
28
30
32
34
36
38
40
42
K
s
tan
φ
a
1.2
1.3
1.5
1.7
2.1
2.5
2.9
3.4