Micropiles are typically recommended by the Geotechnical Engineer and designed
by the Structural Engineer as they complete the foundation layout and pile cap designs.
This leads to conservatism on both ends as neither the Geotechnical Engineer nor the
Structural Engineer has a full understanding of both the ground conditions and building
needs. This conservatism gets amplified when the site has difficult ground conditions, as
a perception of greater risk drives more caution in the design expressed both as a higher
design factor of safety and more redundancy through an increase in the number of piles.
The current practice for micropile foundations in karstic terrain is to install long,
relatively lightly loaded micropiles extending through encountered and possible voids and
fractured rock. Where voids and fractures are encountered, grout loss often occurs,
which is resolved by the specialty contractor pumping dramatic volumes of high mobility
micropile grout in an attempt to fill the hole. Additionally, fractures and soil seams
throughout the rock strata are often disregarded and the bond zone started below the
fractures and soil seams to get “good rock.” These practices can lead to excessively long
piles, an excessive number of piles, high quantities of cement use, larger than necessary
pile caps, increased potential for pile conflicts, and unnecessary delays and cost to the
owner.
It is time for these designs to be completed by the Foundation Engineer who
understands the ground conditions, how micropiles are constructed, how they perform,
and the demands of the structure. Only when the Foundation Engineer understands the
entire project and has influence during construction, including quality control and
treatment of anomalies encountered, are the risks of construction in difficult ground
mitigated. This paper will demonstrate the value of such an approach and describe how
real savings can be obtained.
Subjects: risk mitigation, micropiles, karst,
Author: Allen Cadden, PE, DGE, Philip Shull, PE