Hollow Bar Friction Anchoring Systems

Micropiles

Micropiles consist of a continuously threaded, hollow reinforcing tendon as the load carrying steel member together with a grout body of furnace (Portland) cement with a minimum of 2000psi strength which allows transfer of tension and compression forces mainly from the friction of the threaded tendon via the grout body into the soil.

The grout body takes care of the radial friction in the soil, the stiffness against buckling and the corrosion protection. A minimum grout cover of 1" is achieved. The soil mechanics of the Micropiles, in comparison to standard reinforced concrete piles (DIN 4014), show very little settlement due to its very good soil friction (typical settlement less than 3/16" under design load). Due to the excellent mechanical bond between the grout body and the soil, the movement required to activate the friction is in the range of just a few millimeters.

Micropiles are installed using a standard, rotary percussive drill rig in a one visit drilling operation. The installation procedure uses a grout flush to stabilize the annulus, consequently eliminating the use of casings. This type of installation is free from vibration and has a low noise level. Micropiles require smaller holes and work with small rigs which means less drill spoil and lower mobilization costs. The four in one, threaded hollow tendon is a lost drill rod, reinforcing member and injection tube at the same time, therefore the following installation steps become redundant:

• Removal of drill rod
• Removal of casing
• Insertion of an anchor rod
• Removal of drill spoil

The use of Micropiles in comparison to the traditional methods of installation - clearly demonstrates an increased production rate of at least double. An added benefit of the simultaneous drilling and grouting process of the Micropiles uses grout (w/c = 0,7 to 0,5) as the flushing and stabilizing fluid. Due to the unusually strong mechanical connection at the grout/ground interface and low settlement characteristics, Injection piles are very suitable for dynamic load cases, occurring in foundations for wind energy towers, noise barriers, cantilevered motorway signs, quay walls, transmission towers and seismic retrofit work.

Installing micropiles - Heartland Community College, Bloomington, IL: