How Does Structural Design of Micropiles Perform?

Micropiles can be designed as rock socketed piles in rock formation and friction piles in weathered rocks or soils to carry either compression load or tension load. All micropiles are designed to transfer load through the shaft friction over a length of pile shaft to the founding medium. End bearing at the pile tip is generally negligible for the reasons of small base bearing areas, in which the axial load cannot be effectively transferred to the base. If to be considered, the point capacity commonly does not exceed 15 to 20 % of the side  resistance. This  design  philosophy  also  inherently demands  a founding medium with sufficient thickness to carry the imposed load from the micropile. If there is a cavity below the pile toe or the pile is socketed into a boulder, there will be some transfer of the load to the surrounding sound material by arching effect or to spread the load to the underneath soils. 


If a large pile group is involved in these founding conditions, care need to be taken to avoid punching shear failure of the rock slab or bearing failure of soils underneath the boulder causing excessive settlement under the entire pile group. The ultimate load which can be supported by a single micropile is defined by the lowest of the following:

· structural shaft resistance

 

· buckling load

· failure of the grout/ground bond.

The allowable load used is the ultimate load divided by a factor of safety. However, a lower load may be specified due to limitations of stresses and/or settlements that can be accepted by the structure being underpinned.