Minipile

Micropiles (also known as Minipiles) are used mainly as a piling foundation system in many types of structures from buildings to bridges, especially where headroom is a problem.
Micropiles were conceived in Italy in the early 1950s, in response to the demand for innovative techniques for underpinning historic buildings and monuments that had sustained damage with time, and especially during World War II. The use of micropiles has grown significantly since their conception in the 1950s, and in particular since the mid-1980s. Micropiles have been used mainly as foundation support elements to resist static and seismic loads, and to a lesser extent, as in-situ reinforcements to provide stabilization of slopes and excavations.
A micropile is a small-diameter (typically less than 300 mm (12 in.)), drilled and grouted non-displacement pile that is typically reinforced. A micropile is constructed by drilling a borehole, placing steel reinforcement, and grouting the hole as illustrated in figure below.


Micropile construction sequence

Micropiles can withstand relatively significant axial loads and moderate lateral loads, and may be considered a substitute for conventional driven piles or drilled shafts or as one component in a composite soil/pile mass, depending upon the design concept employed. Micropiles are installed by methods that cause minimal disturbance to adjacent structures, soil, and the environment. They can be installed where access is restrictive and in all soil types and ground conditions. Micropiles can be installed at any angle below the horizontal using the same type of equipment used for the installation of ground anchors and for grouting projects.
Most of the applied load on conventional cast-in-place drilled or non-displacement piles is structurally resisted by the reinforced concrete; increased structural capacity is achieved by increased cross-sectional and surface areas. Micropile structural capacities, by comparison, rely on high-capacity steel elements to resist most or the entire applied load. These steel elements may occupy as much as one-half of the drillhole cross section. The special drilling and grouting methods used in micropile installation allow for high grout/ground bond values along the grout/ground interface. The grout transfers the load through friction from the reinforcement to the ground in the micropile bond zone in a manner similar to that of ground anchors. Due to the small pile diameter, any end-bearing contribution in micropiles is generally neglected. The grout/ground bond strength achieved is influenced primarily by the ground type and grouting method used, i.e., pressure grouting or gravity feed. The role of the drilling method is also influential, although less well quantified.

Micropile Applications
1-Structural support
Micropile applications for structural support include foundations for new structures, underpinning of existing structures, scour protection, and seismic retrofitting of existing structures.


Micropiles for foundation support applications


Micropiles for tunnel support


Micropiles for foundation support applications      


Micropiles for bridge piers support


 Underpinning of existing structures with miropile


 Underpinning of existing structures with miropile


Underpinning of existing structures with miropile


Underpinning of existing structures with micropile, Akhtar basin project, Asalooyeh, Iran (Omran Ista)


Underpinning of existing structures with micropile, Akhtar basin project, Asalooyeh, Iran (Omran Ista)


Underpinning of existing structures with micropile, Akhtar basin project, Asalooyeh, Iran (Omran Ista)


Micropiles for foundations of new structures, Damon Darya project, Kish Island, Iran (Omran Ista)

2-In-situ reinforcement

Micropiles are used in two different ways to stabilize slopes. Lizzi (1982) suggests that
micropiles be used as reticulated network systems, which creates a stable, reinforced-soil, “gravity-retaining wall”. In this type of systems, the reinforced soil gravity mass supplies the essential resisting force, and the micropiles, encompassed by the soil, supply additional resistance to the tensile and shear forces acting on the “wall”. Alternately, Pearlman and Wolosick (1992) and Palmerton (1984) suggest that groups of individual inclined micropiles could be used to stabilize the slope because they serve to connect the moving zone (above the failure surface) to the stable zone (below the failure surface). These micropiles provide reinforcement to resist the shearing forces that develop along the failure surface. Typical configurations of inclined micropile groups for slope stabilization and earth retention are shown in figure below.


62 ton micropiles in Metanol Kaveh project, Dayer, Boushehr, Iran (Omran Ista)


Underpinning of existing structures with micropile, Fajr 2 petrochemical project, Imam Khomeini port, Iran (Omran Ista)


Typical configurations for inclined micropile walls