Design of Self-Drilling Micropiles for Permanent Applications-Paper

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The paper examines the structural design and long-term corrosion protection of permanent self-drilling micropiles, focusing on their use in urban infrastructure and an uplift-reinforcement project for a parking deck. Because such projects may have service lives exceeding 50 years and are difficult to repair, protecting the steel reinforcement against corrosion is a central design requirement.<br /><br />Self-drilling micropiles use hollow, continuously threaded steel bars that are drilled and grouted simultaneously. This installation method is suitable for confined urban sites and creates a rough grout–ground interface that supports load transfer. Common protection strategies include coatings, plastic encapsulation, sacrificial steel allowance, and grout cover.<br /><br />The paper compares seven reinforcement options for a parking structure subject to groundwater uplift. Using AASHTO and IBC design provisions, it calculates a factored tensile demand of 389.5 kN per micropile and assesses the structural resistance of different systems after accounting for corrosion. All seven options meet the calculated demand.<br /><br />Most hollow-bar systems rely on corrosion allowance, often combined with galvanizing, because adding a separate plastic sheath is impractical during self-drilling installation. The Ischebeck TITAN system instead relies on grout encapsulation. Its thread geometry is intended to limit cracking in the grout near the steel; tests and technical approval support a maximum allowable crack width of 0.1 mm, which the authors present as sufficient for durable corrosion protection without relying on loss of steel section.<br /><br />The paper concludes that self-drilling micropiles offer an efficient, low-disturbance solution for restricted urban sites, but their permanent use requires corrosion protection to be explicitly incorporated into design. The appropriate approach depends on the system, ground conditions, loading, and required service life.

Keywords

self-drilling micropiles

corrosion protection

permanent reinforcement

urban infrastructure

uplift reinforcement

parking deck

hollow steel bars

grout encapsulation

galvanizing

AASHTO design