7 Design related considerations
7.1.1 The basic standards for the design of displacement piles are:
- basis of design and actions on structures: ENV 1991-1;
- structural design of concrete piles: ENV 1992-3;
- structural design of steel piles: ENV 1993-5;
- structural design of composite piles: ENV 1994-1-1;
- structural design of timber piles: ENV 1995-1-1;
- bearing resistance: ENV 1997-1.
7.1.2 This present Standard gives design rules related to execution which are not covered by the above standards and which can influence the design or the detailing of displacement piles.
7.2.1 The design of displacement piles shall establish the type and size of pile and that its installation is appropriate for the particular ground conditions and environmental constrains.
NOTE This can often be established on the basis of previous comparable experience.
7.2.2 If no comparable experience exists regarding the driveability, one or more driving tests should be carried out at chosen locations before the main work commences.
NOTE A driving test offers the possibility of investigating driving procedures, equipment, techniques to assist driving and to assess the effect of pile construction on soil behaviour and the environment. In addition driving tests can be used to establish installation criteria and give an indication of the pile length and bearing capacity.
Driveability analysis can help to define suitable driving procedures, driving equipment and driving stresses.
7.2.3 When the driveability of the piles is investigated, account shall be taken of any requirements in the design or specification regarding techniques to assist driving (e.g. jetting, pre-boring, chiselling, blasting).
7.3 Geometrical construction deviations
7.3.1 Unless specified otherwise, piles shall be installed within the following geometrical deviations:
- plan location of vertical and raking piles (measured at the working levels):
- on land: e ≤ 0,1m
- over water: in accordance with the specifications,
- inclination of vertical piles:
- i ≤ imax = 0,04 (0,04 m/m)
- inclination of raking piles:
- i ≤ imax = 0,04 (0,04 m/m)
i is the tangent to the angle between the designed and as built centre line of the pile.
7.3.2 Geometrical construction deviations shall be taken into account in the design. If the specified deviations are exceeded, the extent of a possible overloading of any structural part shall be investigated and suitable remedial measures taken if necessary.
7.3.3 Where geometrical deviations other than those stated above are required or allowed they shall be agreed before the commencement of the work.
NOTE This might be the case in regard to constructional demands (small diameter piles, walls), ground conditions, available piling equipment or low cut-off level.
7.3.4 For the recording of construction deviations the centre of a cast in place pile is considered as the centre of gravity of the longitudinal reinforcement or, for unreinforced piles the centre of the largest circle which can be drawn within the section of the pile head.
7.3.5 An acceptable limit of vertical and lateral pile displacement should be established.
7.4 Sequence of installation
7.4.1 The sequence of driving piles shall be planned.
7.4.2 This sequence should take account of:
- the detrimental effects of any lateral or vertical movement of a pile or a group of piles is minimised;
- the bearing resistance of previously installed piles is not detrimentally reduced related to the design load;
- the soil surrounding the pile is not compacted to the extent that other piles cannot be installed properly;
- the vibrations caused by driving cast in place piles do not affect adjacent fresh concreted piles or concrete which has taken initial set.
7.4.3 For piles which have displaced detrimentally, the overall pile performance shall be reassessed.
7.4.4 Local or comparable experience can influence the proposed sequence of installation.