Micropile Design Spreadsheet – In this case we will use the formulation of the “Guide for the design and execution of micropiles to the ground in road works” of the Ministry of Public Works, based on the friction values of the unit of measurement obtained by empirical correlations.
First of all, calculate the unit friction per micropile shaft against a compression stress, using the empirical correlations, which is obtained by the following expression:
Form fc, d = Unit friction per shaft versus stress a compression.
Rf, lim = Unit friction obtainable from Figure 1 and 2.
Fr = Minimization coefficient, which takes into account the duration of the structural function of the micropiles. For works where the duration is more than 6 months, a value of Fr = 1.65 is adopted.
Figure 1: Unit limit resistance per stem for sands and gravels
Figure 2: Limit unit strength per shank for clays and silts
IRS: Repeated Injection Selected – IR: Repetitive Injection. – IU: single global injection.
For the part of the shaft that is less than 5 meters deep (z <5 m), it is possible to use as a limit unified friction value, that corresponding to an injection of type IU.
If the micropile has to be subjected to the tensile stresses, the unitary friction per calculation shaft for the tensile stresses can be obtained from the following formula:
Formulatraccionr fc, d = Unit friction per shaft versus tensile stress
Η = coefficient that takes into account the existence of alternate loads on the micropile.
Micropiles alternately subjected to tensile and compressive load, η = 0.60
Micropills with tensile loads only, η = 0.75.
Once the unitary friction has been obtained, the friction per shaft of the micropile is calculated in the following expression:
Formula fuste micropilotesr fc, d = Unit friction per shaft versus compressive stress.
ALi = Micropile lateral area in the area included in the i-th section of the discretized zone
N = Number of tranches comprising discretization.
In case the micropile is expected to be embedded in the levels that present very firm to hard consistencies, with simple compression values of more than 100 KPa, for the calculation of the bearing capacity against the sinking of the micropile also consider the value of Resistance by tip, which is given by the expression:
FormulapuntaR p, d = Tip percussion.
R fc, d = Blade friction against stresses a compression.
Finally, from the values of friction per tip and per stem, the calculation resistance is obtained against the sinking, through the following expression:
Chargeable from the value of the friction by a tensile shaft, the resistance of calculation against the start, through the following expression:
Starting tc, d = Blade friction against tensile stress.
We: Own weight of the micropile.
Fwe: reduction coefficient, which proves justification adopts a value of 1.2.
A nominal micropile diameter of 250 mm, an injection type IU and the following soil profile:
It is obtained that the resistance per stem for a micropile of 9.0 m is 42.84Tn (14.99 Tn + 27.85 Tn) and the resistance per tip of 6.42 Tn (0.15 * 42.84 Tn), from which it follows that the calculation resistance for a Micropile of 9 m against sinking, with a diameter of 250 mm and an injection type IU is 49.27 Tn.
In case of the calculation resistance against the start of this same micropile, it will be 37.68Tn (49.27Tn * 0.75 +0.88 Tn / 1.2). A density of 2.5 Tn / m3 has been considered for the calculation of the own weight of the micropile.