7 Considerations related to design
7.1.1 The basic standards for the design of micropiles shall be:
- for basis of design and actions on structures: EN 1991–1;
- for basis of design and actions on constructions: EN 1991–1;
- for structural design of concrete piles: EN 1992–3;
- for structural design of steel elements: EN 1993;
- for structural design of composite elements: EN 1994–1–1;
- for design of prestressed elements: prEN 10138–4;
- for bearing resistance: EN 1997–1.
7.1.2 The present document gives design rules related to execution which are not covered by the above standards and which can influence the design or details of the micropiles.
7.1.3 The design of micropiles shall establish the type and size of micropiles.
NOTE Micropiles can only be designed efficiently on the basis of sound knowledge of the construction project, of the structural requirements of the micropile system, of the geotechnical properties of the ground and of previous experience with micropiles in similar ground conditions.
7.1.4 If no comparable experience exists regarding the execution, one or more preliminary or trial micropiles shall be installed at chosen locations before the main works commence.
NOTE The installation of a preliminary micropile offers the possibility of investigating execution procedures and equipment, and to assess the effect of the micropile installation on soil behaviour and the environment.
7.1.5 The design shall take also into account the specific execution restrictions.
7.2 Geometrical construction tolerances
7.2.1 Geometrical construction tolerances shall be considered for the design and execution.
NOTE Guidance on geometrical construction tolerances are given in Annex B.
7.2.2 If the specified geometrical construction tolerances are exceeded the extent of possible overloading of any structural part shall be taken into consideration and suitable measures shall be taken as necessary.
7.3.1 The limit values (such as displacement limit, vibration limits, ….) acceptable for adjacent structures and/or services should be defined in the project specifications.
7.3.2 When micropiles are to be socketed into a bearing stratum or into rock, the minimum depth and diameter of penetration shall be given in the project specifications.
7.3.3 Wherever the starting point of the borehole is located below the water table or artesian head, special measures shall be taken into account in order to avoid piping through the boreholes or loosening of the ground.
7.3.4 Where ground conditions differ from those stipulated in the design, appropriate action shall be taken to modify the design in order to provide the required bearing resistance of the micropile or of the foundation.
7.3.5 If micropiles encounter an impenetrable obstruction prior to reaching their designed founding depth, the design shall be reviewed in the light of any available knowledge about the obstruction and in view of consequences on the bearing behaviour of the foundation.
7.3.6 When a permanent casing, lining or other method is used to protect for the fresh grout, mortar or concrete against rapid groundwater flow that might have a washing–out effect, this should be taken into account.
7.3.7 The installation of a permanent casing or lining shall be taken into account for the design of the load transfer length in the soil.
7.3.8 The design for impact driving of prefabricated elements shall be done in accordance with 7.7 of EN 12699:2000.
7.3.9 For driven steel micropiles the transferred energy of the driving system should be chosen so that the maximum calculated stress during driving should not exceed 0,9 × the characteristic yield strength of the material.
7.3.10 Where stresses are actually monitored during impact driving, these may be 20 % higher than the value stated in 7.3.9.
7.4.1 The reinforcement cage for cast in place micropiles shall be designed not only to have adequate strength in the final micropile, but also adequate strength and stiffness during its handling and during the construction of the micropile. It shall also allow the fresh grout, mortar or concrete to flow easily around each of its components.
7.4.2 Unless otherwise specified, cast in place concrete micropiles shall be reinforced over their full length.
7.4.3 A cast in place micropile may be designed as a partially unreinforced element if:
- the design actions and/or actions caused by the construction and/or actions resulting from the ground, produce only a compressive stress in the micropile and;
- the micropile is not located in a seismic area.
7.4.4 Starter bars or dowel bars for connection to a superstructure shall be in accordance with EN 1992–1–1.
7.4.5 When steel reinforcement is considered as the load bearing element of a micropile, the design shall be in accordance with EN 1993–1–1.
7.4.6 When steel reinforcement and mortar or concrete are considered as bearing members of the micropile, the design shall be in accordance with EN 1992–1–1 or EN 1994–1–1.
7.4.7 The design value of bond strength between grout, mortar or concrete and the load bearing elements of steel or cast iron in accordance with ENV 1994–1 (bars, tubes or sections) shall have been agreed before the commencement of the works.
7.4.8 The grout, mortar or concrete cover, shall be specified according to 7.6 in the project specifications.