Hydrodrill

Micro-Piles

By micro-piles we mean drilled piles with a diameter of less than 300 mm, its shaft is composed of a mortar or cement mix cast on-site and reinforcement formed generally by a metal tube or angular section rod.
They can be subjected to compression, traction and shearing stresses, they can be vertical, horizontal or inclined.

Classification

Depending on the method of injection there are:
- gravity micro-piles: mortar cast at atmospheric pressure.
- low pressure micro-piles: mortar cast at low pressure.
- high pressure micro-piles: positioning of a metal tube with a valve and injection of the cement mix to form a sheath (primary injection) followed by an injection of cement mix through the tube with the valve (secondary injection with controlled pressure and section).

Construction materials and elements

a. Metal reinforcement
The reinforcement can be made with:
- tubular reinforcement rods in Fe 430 or 510 steel  with diameters variable from 48 mm up to 250 mm with thickness variable from 4 mm to 20 mm. Depending on the type of reinforcement either a single rod can be used or sections jointed together with threaded sleeve couplings or welded or by male-female threaded joints formed within the walls of the tube. In the case of tubes with a valve the valves are generally formed by 3 mm thick rubber sleeves with metal rings welded to the tube.
- metal angular section rods: are generally formed by single elements or ones joined together by suitable welding.

b. Mortar and cement mixes
In the case of cast cement mortar (gravity or low pressure micro-piles) the characteristics are:

Product

Data

Cement type 325-425

Depending on the characteristics of the ground

Water to cement ratio A/C

0,5-0,6

Minimum resistance

R’ck = 25 MPa

Aggregate

Washed sand

Normally the following composition is used in the case of injection of cement mixes for all types of micro-piles:

Product

Data

Cement type 325-425

Depending on the characteristics of the ground

Water to cement ratio A/C

< 0,50

Minimum resistance

R’ck = 25 MPa


Means of execution

-
drilling: using a rotary percussion head or rotary head with variable diameters from 80 mm to 300 mm either dry, with water circulation, stabilizing mud or cement, with or without a temporary lining according to the stability of the bore hole walls. Tools used are continuous flight augers, jackhammers or cutters with triple blades, triple cones or with bits in widia. The bore hole is purged by using compressed air or the same drilling fluids or by a mechanical conveyor in the case of a continuous flight auger.
-
positioning reinforcement: with the above characteristics.
-
casting: using the above methods according to gravity, low pressure and high pressure micro-piles. In the case of high pressure micro-piles firstly the sheath is formed by injecting the cement mix at low pressure into the circular space between the reinforcing tube and the walls of the bore hole (primary injection); then the secondary injection is made through the valves after 12 to 24 hours with appropriate expansion packing introduced into the tubular reinforcement starting from the deepest valve.

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