16
8) Structural design of a pile foundation
• Pile foundations are always designed together with
the structures to be supported on it, which allows
selecting the most suitable shape, dimensions
and stiffness for the entire structure. Things to
be considered in the structural design of the pile
foundation include
- joints between piles and superstructure
- determination of positional and verticality
tolerances on the basis of pile type, soil conditions
and superstructure
- elevation of pile group foundation
- centre-to-centre pile spacing
- pile inclinations
- distances of piles to nearby structures
- distance from side of the foundation to side of
the pile
- other structural aspects to be considered
9) Foundation engineering print-outs
• Building specification (work schedule)
- soil conditions
- geotechnical works
° Work stages prior to piling and measures
affecting the work phases, piling platforms,
excavations, etc.
° Site-specific instructions related to piling, such
as instructions for installation, piling sequence,
quality control measurements, special measures
- Foundation structures
• Pile foundation drawings
• Geotechnical and structural dimensioning calculations
- normally dimensioning calculations for the
structure of an axially loaded pile can be made
and printed out easily using the pile dimensioning
program for RR and RD piles
• As-built drawing
5.3 Actions and design situations
The design actions of loads must correspond to those
specified in standard EN 1991 and the national annex to
it. They are included in publication RIL 201-1-2008 with
their application instructions. Actions caused by subsoil
displacement, such as downdrag (negative skin friction),
are treated in dimensioning as permanent actions on the
pile (for more details, see 5.9).
5.4 Geotechnical investigations
Geotechnical investigations for the design of steel piles
are generally regulated by the Finnish Building Code
and Eurocodes EN 1997-1 and EN 1997-2. PO-2016
presents the general requirements for geotechnical
investigations in building construction, requirements and
recommendations for different geotechnical classes and
foundation underpinning sites, as well as requirements
and instructions for the presentation of geotechnical
information. Valid guidelines of the Finnish Transport
Agency are observed in infrastructure construction.
Geotechnical investigations must be complemented with
sufficiently extensive investigations of nearby structures
(ducts, pipes, cables, underground structures, etc.), their
location and condition, foundation methods, as well as
sensitivity to displacement and vibration.
Geotechnical information and its assessment are
presented in the ground investigation report according to
PO-2016, Ch 1, Sec. 3.3. The ground investigation report
should indicate the following points most essential for the
design and dimensioning of steel piles:
• the characteristic value of the undrained shear
strength of soil used in dimensioning – by soil
layers, if necessary, and/or with the site divided
into different zones if the site is large and/or the
undrained shear strength of soil varies considerably
• corrosion allowance used in dimensioning
• estimated pile length and basis of estimate
• estimate of subsoil properties affecting piling, such
as the quality, stoniness and thickness of fills, dense/
stony intermediate layers, density and stoniness of
moraine, inclination of bedrock surface
• shoe type of driven pile used at site
• estimate of the occurrence of downdrag (negative
skin friction) and determination of the design value
of downdrag for different pile dimensions and
different areas of the site, if necessary
Depending on the site, the following may also be needed:
• axial spring constant of pile for service state
displacement analysis (see Sec. 5.8), is in principle
always required in PTL3 but also in PTL2 in the case
of long piles
• geotechnical parameters of soil layers to determine
extreme values of modulus of subgrade reaction
and lateral resistance, when piles are subject to a
horizontal load and/or moment.
5.5 Dimensioning methods and analyses of
geotechnical resistance
5.5.1 Selection of geotechnical dimensioning
method for steel piles
The geotechnical compressive strength of steel piles can
be determined according to PO-2016 in several ways,
whose applicability is shown in Table 15.
5.5.2 Stiffness of a piled structure
The stiffness of a piled structure is taken into account
in building construction projects according to the
instructions of PO-2016 and in civil engineering projects
according to those of NCCI7. The correlation coefficients
presented in these instructions and the design values
based on them assume that the structures are not so-
called rigid structures.