Is this project a good fit for micropiles? This is a question that I’m asked quite often. Micropiles are an excellent deep foundation option to solve two common problems – difficult ground conditions or limited access.
Let’s start with a brief description. A micropile is a drilled and grouted replacement pile less than 12 inches in diameter. Capacities typically range from 50 kips to 500 kips with higher loads sometimes achieved with larger micropiles (>12”). To install a pile, contractors typically advance casing through the overburden material to the top of rock then drill a rock socket. Steel reinforcing is lowered into the casing and rock socket then grout is tremie pumped from the pile tip to the top of the pile displacing any ground water. The grout bonds the reinforcing steel to the rock socket. The load is predominantly carried by steel reinforcing – a pipe, threaded rod(s), or both. The grout also contributes.
Difficult Ground Conditions
When deep foundations are required, and other pile solutions are not feasible due to difficult ground conditions, then micropiles are a great option. Micropiles are commonly installed in situations where driving piles or installing auger cast piles are prohibitive due to obstructions such as construction debris or boulders. Advanced overburden drilling technologies developed by the mining industry allow for installation of casing through concrete or boulders. A recent micropile project we completed required installation of micropiles by drilling through 40’ to 50’ of debris laden fill to reach original ground.
Another common use of micropiles due to difficult ground conditions is limestone karst. Karst is water soluble rock and is encountered in our region in the mountains of Virginia, West Virginia and eastern Tennessee. In geologic time, ground water has dissolved rock leaving behind pockets of soft soil and voids. The ground layers are often tilted or folded creating a situation where a pile will go through soil, then rock, then a void, and then rock again. Due to the overburden drilling systems, micropiles can be advanced through these varied conditions to competent rock. Often in karst, depth to rock varies wildly. Since micropiles are installed in sections, the pile length can be increased as required.
Micropiles are also a great solution when deep foundations are required but access is limited. When a vertical expansion is needed for an existing structure, micropiles can be installed in low headroom conditions (typically 8’ to 9’) to reinforce existing footings or to add new columns for the vertical expansion. The small rigs operate with umbilical cords so that the diesel fumes are on the outside of the building. Other limited access conditions may include drilling below bridges, between structures or in courtyards. We have installed micropiles for a hospital vertical expansion while patients were treated on the other side of the wall.