1. Axial resistance of micropiles : from French to Eurocode design
Roger Frank
CERMES (ENPC-LCPC), Paris, France

2. Eurocode 7-1 : check of axial capacity of pile foundations
ULS compressive or tensile resistance failure of a single pile
ULS of the pile foundation as a whole
ULS of collapse or severe damage to a supported structure by excessive displacement or differential displacements of the pile foundation
SLS in supported structure by displacements of the piles

3. Axial resistance of piles/micropiles
The design shall be based on :
- the results of static load tests
- empirical or analytical calculation methods whose   validity has been demonstrated by static load tests
- the results of dynamic load tests whose validity has   been demonstrated by static load tests
- the observed performance of a comparable pile   foundation (provided that this approach is supported   by the results of site investigation and ground testing).

4. Basic relations for axial resistance
The basic relations of Eurocode 7-1 are :
Rk = Rm/x (1)
Rd = Rk/gt (or Rd = Rbk/gb + Rsk/gs) (2)
Or Rd = Rk/gst
On the other hand : Fd = gFF (3)

5. The basic condition for all ULS is :
Fd(ULS)  Rd (4)
Eqs (1) to (4) lead to :
Fd  Rm/ gF.gt.x = Rm/FS (5)

6. Eurocode 7- 1 : Characteristic resistance from static load tests
Rk = Min { Rm,mean/1 ; Rm,min / 2 }
where Rm : measured resistance(s)
with x = x1 on the mean, or
        x = x2 on minimum value
function of the number of pile test(s) (n)

7. EC 7-1: Recommended values for the x - factors for Rk from n – static load tests French practice (Fascicule 62-V) : x ≤1.2
x for n = 1 2 3 4
 5 x1
1.40
1.30
1.20
1.10
1.00 x2
1.05

8. Factors on actions (EC 7-1 and F 62-V)
ULS, permanent and transient situations :
F = 1.35 to 1.5
ULS, accidental situations :
F = 1.0
SLS : F = 1.0

9. Factors for micropile side resistance : Eurocode 7-1 and Fascicule 62-V
ULS persistent and transient situations
s = 1.1 and st= (approach 2 of EC 7-1)
gs = gst = 1.4 (F 62-V)
ULS accidental situations
s = st = (EC 7-1)
gs = gst = 1.2 (F 62-V)

10. Factors for micropile side resistance : Eurocode 7-1 and Fascicule 62-V
SLS characteristic combinations
check the displacements (EC 7-1)
gs = gst = 1.1 on Qcreep (F 62-V)
SLS quasi-permanent combinations
gs = gst = 1.4 on Qcreep (F 62-V)

11. Conclusion
The main challenge is the prediction of displacements, whether for ULS or for SLS …
Which model(s) to use : t-z approach or others ?
How to collect the data ?
Which are the tolerance criteria ?

12. Thank you for your attention !