Micropile Design Spreadsheet – In this case we are going to use the formulation of the “Guide for the design and execution of micropiles to the terrain in road works” of the Ministry of Public Works, based on the values of friction limit unit obtained by empirical correlations.

In the first place, we calibrate the unitary friction by micropile bole against compressive stresses, using the empirical correlations, which is obtained by means of the following expression: r fc, d = Unit friction per shaft versus compressive stress.
rf, lim = Unitary friction limit that can be obtained from Figure 1 and 2.

Fr = Coefficient of reduction, which takes into account the duration of the structural function of the micropiles. For works where the duration is longer than 6 months, a value of Fr = 1.65 is adopted.

IRS: Repetitive injection selected – IR: Repetitive injection. – IU: Global single injection.

For the part of the shaft that is less than 5 m deep (z <5 m), it should be used as limit unit friction value, the one corresponding to an injection of type IU.

In the case that the micropile must be subjected to tensile stresses, the unit friction per calculation shaft for tensile stresses can be obtained from the following formula: r fc, d = Unit friction per shaft versus tensile stress
η = coefficient that takes into account the existence of alternating loads on the micropile.

Micropiles subjected alternately to tension and compression load, η = 0.60
Micropiles subjected only to tensile loads, η = 0.75.
Once the unitary friction is obtained, the friction by shaft of the micropile is calculated by means of the following expression: r fc, d = Unit friction per shaft versus compressive stress.
ALi = Lateral area of the micropile in the zone included in the i-th section of the discretized zone
n = Number of sections comprising the discretization.

In the event that the micropile is expected to be embedded in levels that have very hard to hard consistencies, with simple compression values above 100 KPa, the value of the micropile may also be considered for the calculation of the carrying capacity. endurance, which is given by the expression: R p, d = Tip friction.
R fc, d = Rod friction against compressive stresses.

Finally, from the values of friction per point and per shaft, the calculation resistance against the subsidence is obtained, by means of the following expression: And from the value of friction by shaft to traction, the resistance of calculation against the start, by the following expression: R tc, d = Rod friction against traction efforts.
We: Own weight of the micropile.
Fwe: coefficient of reduction, which except justification adopts a value of 1.2.

Practical example
Considering a nominal micropile diameter of 250 mm, one type of IU injection and the following soil profile: It is obtained that the stem resistance for a 9.0 m micropile is 42.84Tn (14.99 Tn + 27.85 Tn) and the tip resistance of 6.42 Tn (0.15 * 42.84 Tn), from which it is deduced that the calculation resistance for a 9 m micropile against sinking, with a diameter of 250 mm and one type of IU injection is 49.27 Tn.

In the 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 calculating the micropile’s own weight.

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