Micropiles are designed by two standards, the Federal Highway Administration (FHWA) and the 2015 International Building Code (IBC). Of course, the FHWA standard applies to highway projects and the IBC standard applies to building construction. The main difference in the two standards are the allowable stresses. FHWA allows for slightly higher stresses to be imposed on the steel and grout. The following equations shown the difference.
FHWA NHI-05-039 December 2005
0.47*Asteel*Fy-steel + 0.40*Agrout*fc-grout = Compression Capacity
0.40*Asteel*Fy-steel + 0.30*Agrout*fc-grout = Compression Capacity
Note that Fy-steel is limited to a maximum of 87 ksi. This is to limit the strain to that which can be tolerated by the grout.
Within the cased micropile zone, both the steel casing and the inner reinforcing steel are combined for the total area of steel. Be mindful that the steel casing and the reinforcing steel may have different yield strengths. The lower strength value must be used to determine the steel contribution. Again, note that the maximum allowable steel strength that can be used is 87 ksi.
Asteel = Acasing + Areinforcing
Within the uncased micropile zone, only the steel reinforcing and the grout are available to carry the load.
Asteel = Areinforcing
The geotechnical capacity of the micropile is typically determined by the bond to rock within the rock socket. While end bearing can be significant in some cases, micropiles are typically designed using only the bond to rock given the small diameter of the piles.
While the best estimate of the bond capacity in rock is based on geotechnical data and local experience, allowable bond stresses within the rock socket vary from 3 ksf (20 psi) for fine grained partially weathered rock such as a silt stone to 20 ksf (138 psi) or higher for granite. Estimation of the bond value can be made by looking at the % sample recovery and rock quality designation (RQD). With that said, nothing trumps past local test data.
Micropiles can also be supported along the continuous length of the pile. The most common approach for this method is a hollow bar micropile. Hollow bar micropiles are installed by advancing a hollow threaded rod with a sacrificial bit into the ground. The pile is grouted as it is drilled into the ground. Once the hollow bar has reached its design depth, the pile installation is complete. While the material cost is higher than some micropile systems, the hollow bar avoids the need for casing and is therefore especially helpful in coastal soil profile where rock may be incredibly deep.