Micropile is a small pile with diameter ranging from 3-1/2" (+/- 90mm) to 9-7/8" (+/- 250mm). Micropiles are generally used for underpinning and retrofitting structures.
Micropile can be made by boring or drilling holes in soil or rock, then micropile execution continue by inserting a reinforcing element adequate to the hole size. The micropiles are then grouted by backfilling the hole from the bottom, using cement mortar (grout). This type of micropile has a small bearing capacity, in accordance to micropile diameter, micropile length and the type of soil to which the micropile has to transfer the loads applied to the micropile itself.
Different types of micropiles are the "High Capacity Pressure Injected Steel Micropiles" The High Capacity Steel Micropile is made using High Tensile Tubular Steel. The steel micropile is provided with "manchettes". Manchettes, are rubber sleeves installed at regular interval in the active length of high pressure injected tubular steel micropiles and are acting as check valves, allowing cement to flow outwards into the surrounding soil and preventing backflow. The High Capacity Micropile is drilled, then fixed into the ground by annular grout. Eventually the steel Micropile is subjected to multi-stage high pressure grouting through the manchettes. Micropiles, due to the very small cross section area are generally skin-friction piles. The Pressure Grouting done through the manchettes provided within the micropiles active length creates series of bulges between the annular grouting of the Micropile and the soil formation surrounding the micropile. The cement bulges protruding from the micropiles shaft are solidly conneced to the annular grouting of the micropiles. The irregular shapes of the micropiles' bulges have a sizable area which in turn translates to a sizeable increase of the micropile lateral area, creating an increase of the micropiles' shaft interface with the medium [soil] surroundind the Micropile. The end result of the process is giving this type of micropile a much higher bearing capacity as compared to non pressure grouted micropiles .
The use of high pressure injected micropiles is advantageous when using micropiles in doing underpinning of buildings, where the micropiles have to be installed in areas with limited vertical clearance (for example building basements), or underpinning by means of micropiles bridge structures with little vertical clearance. Micropiles are generally executed by use of small size equipment, therefore the execution of micropiles can be carried out in very small areas, and small access. High Capacity Micropiles are done using segmental Tubular Steel Micropile reinforcement and injected cement grout. Both materials employed to install micropiles can easily be carried into small spaces, though small access and yet the Pressure Injected Steel Micropile achieves high bearing capacities. In addition, a micropile can be installed through pre-existing foundations, then the micropile head will be properly fixed to the existing structure. If properly designed and expertly installed, this kind of micropile application can be executed very neatly, and provide a convenient mean for repairing insufficient or defective foundations and allow an efficient structural retrofit.
Micropiles used for underpinning can be precompressed prior to being connected to the pre-existing structures, in this way further settlements are generally avoided,since the piles are made active from inception. Micropile field of application is very wide. Pressure Injected Micropile is an extremely reliable technology. Micropiles are generally designed with high safety factor and are usually tested by static load test using reaction micropiles to provide the necessary loads.
Pressure injected tubular steel micropiles are a versatile technology, because of the small dimension of the equipment needed for micropile execution, and because the material needed to form the micropile can be split into small loads and can be carried even by hand, practically anywhere, narrow basements [for underpinning] , inside industrial buildings [as foundations for additional heavy machinery] small tunnels [for repair and reinforcement], mountain trails, rice fields, mountainous forested areas [for foundations of electric power transmission lines and telecom transmission towers], steep slopes [for foundations of pipelines and slope stabilization or slope protection], the list of applications of micropiles is quite large.
A sub-category of micropiles and with diameter ranging from 1" to 3" are the grouted bars [drilled and grouted], needle-piles and soil-nails [displacement elements], installed with close spacing, are employed mostly for slope stabilization, or slope protection.
Conventional foundation piles, especially large-diameter piles cannot be drilled in any soil. Sometimes the presence of boulders, old foundations or other obstruction in the subsoil makes the drilling of such piles difficult or impossible. But the penetration of, for example, large boulders present in the subsoil to make way for large-diameter concrete piles seems illogical, due to waste of time and energy involved. It would usually be much more convenient to substitute for the large- diameter piles an adequate number of micropiles, which on account of their more flexible technology, can be more easily drilled through any obstruction.