Definition of micropile
Micropiles are divided into two general groups: “prefabricated micropiles” and “in situ micropiles” (Fleming, 1985). Prefabricated micropiles are elements that are pushed into the ground by impact or vibration, so during installation, they move the surrounding soil and push it forward. In-site micropiles are placed in pre-drilled boreholes and wells, or built and executed on site. So they are placed in the excavated ground.
Micropiles is often a component of in-site and drilling piles with a small diameter (usually less than 300 mm) that are often reinforced with reinforcement. Micropiles is installed by drilling a borehole or well, then placing the reinforcements in the borehole, and finally injecting the borehole.
Micropiles are able to withstand axial and lateral loads and can replace conventional piles or be used in conjunction with a pile system, depending on the design method. Micropiles are performed in ways that cause minimal damage to the soil, structure, and surroundings. They can be installed in difficult access environments and in all types of soil and ground conditions. Micropiles can be installed at any angle to the horizon using the same equipment used in injection and anchorage projects.
Because the implementation process creates little vibration and noise and can be installed in low-ceilinged spaces, it is often used to strengthen the foundations of existing structures. In cases where the underground space is limited, special drilling equipment is required.
Most of the forces applied to in-situ piles are mainly supported by reinforced concrete. In this regard, to increase their structural bearing capacity, the cross section and contact surface of the pile can be increased. In contrast, the structural bearing capacity of a micropile relies on the capacity of its durable steel elements. These steel elements occupy about half of the borehole volume. The use of special drilling and injection methods in the implementation of micropiles increases the load capacity in the soil-slurry complex.
The slurry transmits the forces in the bonding area (soil-slurry complex) to the ground in a manner similar to ground bracing, through friction and with the help of reinforcements. Due to the small diameter of the micropile, the bearing capacity of the tip is generally neglected.
The strength of the slurry-soil bonding area depends on the type of soil and the method of injection (for example, compression or gravity injection). The drilling method is also effective.
Installation steps of micropile using pipe jacketing
1) Start drilling or install temporary jacket 2) Continue drilling to the final depth 3) Extract the drillbit and rod 4) Installation of reinforcement or other reinforcement systems and Tremie grout injection 5) Temporary jack extraction and injection of secondary grout under possible pressure 6 ) Completing the piling (the jack may remain in the compressible layer)
Micropiles appeared in Italy in the early 1950s to strengthen historic foundations and monuments that had been damaged and destroyed over time, especially during World War II. At that time, a reliable system was needed to maintain the loads and weights of the structures with the least displacement, applicable in limited access spaces with the least damage to existing structures. An Italian contractor named Fondedile proposed the “palo radice” or “root piles” method to strengthen foundations.
Palo radice is a small diameter candle that is drilled and executed in site and injected after placing a light reinforcement.
Fig. 2 shows the classic arrangement of palo radices in the foundation
Although steel was scarce in post-war Europe, labor was cheap and plentiful, and most of them had high technical capabilities. Such conditions provided the basis for the development of low-reinforcement root piles, which were mainly designed and implemented by contractors.
Loading tests on these root piles recorded load capacities in excess of 400 kN.
Full-scale experiments were performed at relatively low cost. During these experiments, no rupture was recorded in the soil-slurry bonding area.
The use of root piles was developed in Italy during the 1950s. Mr. fondedile proposed this technique to strengthen the foundations of several historic structures in the United Kingdom in 1962, and in 1965 it was used in the foundation of German road and transportation projects. At the same time, for specific linguistic reasons, the term “micropile” replaced “root pile”.
Initially, the main application of micropiles was to strengthen foundations in urban environments. In 1957, engineering demands led to the emergence of the “reticoli di pali radice” system. The system consisted of several vertical and inclined micropiles interlocked in a three-dimensional network.
Micropiles were used for stabilization of gables, strengthening of coastal walls, protection of buried structures and other cases of maintenance and strengthening of soils and structures and strengthening of land. The micropile system was developed in Switzerland and Germany, and the Far East soon became the main market for the technology. The abundance of relatively cheap workers, the shortage of steel, and the urgent need to repair and rehabilitate urban environments all increased the use of micropiles in Europe. Conversely, the slow growth of micropile use in North America was a reflection of the abundance of cheap steel, relatively high labor costs, and the implementation of large projects in suburban areas. Such conditions led to the development of the technique of pile displacement, which were implemented using prescription specifications and had a lower technology than micropiles. Today, the costs of implementation around the world are almost the same, and demand for micropiles continues to grow, especially among geotechnical contractors who have the ability to build and design.