Proceedings - 13th International Workshop on Micropiles - 2017, Vancouver, BC, Canada, (DFI)
RISK MITIGATION FOR MICROPILE FOUNDATIONS IN KARSTIC TERRAIN
Allen Cadden, PE, DGE, Philip Shull, PE
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.
|article #2625; publication #1033 (ISM- 2017)|