22
In uncertain cases it is advisable to overdimension
excessively or use corrosion protection.
The usualness of conditions is established on the basis
of geotechnical investigations and historical data on the
area when there is no reason to assume the soil to be
contaminated. Special investigations can be carried out
in uncertain situations to ascertain that the separately
determined limit values for aggressive conditions are
not exceeded. The instructions of the publication ”Sillan
geotekniset suunnitteluperusteet” (Geotechnical
design criteria for bridges) are followed in corrosion
dimensioning and corrosion investigations in bridge
projects commissioned by the Finnish Transport Agency.
Use of filter fabric or plastic sheeting or pipes is
recommended around unprotected steel piles to prevent
direct contact between granules and steel in lightweight
aggregate fills.
The corrosion of piles installed in water can be estimated
on the basis of the values presented in Table 21. Higher
corrosion values must be used in unusual conditions
including sites where pile surfaces are subject to
mechanical stress caused, for example, by propeller
slipstreams of ships. The recommendations of the Table
were originally presented in standard EN 1993-5.
When cement-based injection grout, grouting mortar or
concrete is used as corrosion protection of steel piles,
corrosion dimensioning and determination of the correct
protective coating thickness can be based on Tables 4.25
ja 4.26 of PO-2016, Ch. 1.
External coatings can be used as corrosion protection on
steel piles, although overdimensioning of wall thickness
is generally the recommended method. Possible external
coatings are protective paint or 3-layer HDPE coating.
The selected coating must be able to withstand stresses
during installation. Protective painting is highly suitable
for piles that are installed in water where the purpose
of the protective painting is to prolong service life, for
instance, in the most aggressive splash zone areas. The
service life of protective painting can be assessed in the
case of the protective paint systems specified in standard
EN 12944-5. There is always the risk of damage to the
coating of piles driven into soil unless special measures
are taken. In the case of drilled piles, when there are no
stones or boulders in the soil, it may be possible to use
the strong 3-layer HDPE coatings specified in DIN 30670
without the coatings being damaged. Even then, the use
of a coating should be considered case by case and the
durability of the coating ensured, for example, by test
installations.
In the case of electric protection methods, the electric
current required by the system and the replaceability/
service life of galvanic anodes must be taken into
account.
5.8 Vertical displacements of pile foundation
If necessary, vertical displacements of pile foundations
are analysed according to PO-2016, Sec. 4.5.4. An
analysis of settlements is generally not needed with steel
piles bearing on dense moraine or bedrock, if ultimate
limit state analyses of the piles have been conducted.
However, for example, at foundation underpinning sites,
especially with long piles, the elastic compression of piles
may become a dimensioning factor, which is why the piles
are often prestressed.
The assessment of vertical displacements must cover
both the settlement of individual piles and settlement
due to group effect as well as structural rotations
and changes in inclination possibly caused by uneven
settlement.
In the case of tension piles, upward displacements should
be assessed on the basis of the same principles. If very
strict criteria have been set for the service limit state, a
separate analysis of upward displacements is needed.
5.9 Considering downdrag (negative skin friction) in
dimensioning
Negative skin friction is taken into account according to
PO-2016, Ch. 4.2.2.2 as a downward acting load.
In building construction projects, the design value
of downdrag load is obtained by multiplying the
characteristic value of negative skin friction by a factor of
1.2. When the design value of short-term loads exceeds
the design load of downdrag, downdrag load is not taken
into account in ultimate limit state analysis.
In infrastructure projects, the characteristic value is
multiplied by the partial safety factors of dead load (Load
Combination 6.10a). Downdrag load does not affect the
pile simultaneously with compressive stresses from traffic
loads.
Small diameter steel pipe piles have dimensioning
advantages over conventional pile types in terms of
negative skin friction, since the skin area is small and
the downdrag load on the piles is often smaller than
short-term loads. On the other hand, due to their low soil
displacement, driven small diameter piles, even large RD
piles when installed carefully, do not cause soil heave
and subsequent settlement in normally consolidated
clay. Thus, installation of piles does not cause significant
displacement of soil or lead to a situation where negative
skin friction should be taken into account according to
PO-2016.