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 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)

 


62 ton micropiles in Metanol Kaveh project, Dayer, Boushehr, 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


Installation of micropiles near rail track operation

Construction classification
The method of grouting is typically the most sensitive construction process influencing grout/ground bond capacity. Grout/ground bond capacity varies directly with the grouting method. In the view of construction methods, the micropile classification is based primarily on the method of placement and pressure under which grouting is performed during construction. The use of drill casing and reinforcement define subclassifications. This classification is shown schematically in figure below in 4 categories.


Wall Stabilization with micropile

Ischebeck Titan micropiles
Ischebeck Titan micropiles consist of a continuously threaded, hollow stem steel reinforcement tendon combined with an OPC grout body of a minimum 25 N/mm2 strength. The profiled surface of the grout body transfers tension and/or compression forces into the ground.


Wall Stabilization with micropile

The threaded hollow stem acts as the drill rod, the reinforced bar and the injection tube. The hollow bar has more favorable values in static conditions compared to a solid bar of the same cross-sectional area, with regard to bending, shear resistance and bond friction. The continuous thread allows you to cut and couple any length required on site. Various drill bits are available to suit different ground conditions which may be experienced on site.
The grout body interacts with the surrounding ground producing friction at the ground/grout interface, and offers corrosion protection. Centralizing spacers are placed before every coupling nut to ensure a regular grout cover of a minimum 20 mm. Tension and compression loads are transferred from the tendon into the grout body and the ground.
Ischebeck Titan micropiles generate excellent shear bond with the ground and allow the safe working load deformation at the pile head to be calculated to less than 5 mm. The sketch shows an excavated grout body. Here the threaded tendon, 20 mm grout cover, the filter cake and the improved ground conditions can be seen. A longitudinal section of a grout body shows regular, fine cracks within the hardened cement paste, the cracks are directed away from the ribs of the tendon. Only conical cracks with a max. length of 10 mm; not reaching the surface of the grout body.


Micropile classification system based on type of grouting

 


Ischebeck Titan micropile
 


Typical cross section of Ischebeck Titan micropile

 



Construction sequence of Ischebeck Titan micropile
 

 


 

 

 

 

Advantages over conventional piles
• Works in compression and tension
• Does not require temporary casing
• Improved mechanical ground/grout interaction reduces overall depth
• Dramatically increased production rates
• Lightweight rotary percussive drilling equipment
• Installed in confined spaces
• Permits top down jet grouting in saturated clays and silts complete with re-bar
• Perfect as pali radice for invisible structural repairs
• Bayonet fixing eliminates pile trimming and facilitates remote de-coupling
• Self drilling micropiles provide a range of working loads from 110 kN to 3,660 kN
• Minimal noise
• Low vibration
• Minimal spoil.

Constructuion and material
Ischebeck Titan 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. Ischebeck Titan micropiles require smaller holes and work with small rigs which means less drill spoil and lower mobilization costs.
The grout body takes care of the radial friction in the soil, the stiffness against buckling and the corrosion protection. By using spacers (centralizers) at each coupler, a minimum grout cover of 20mm is achieved. The soil mechanics of the Titan micropiles, in comparison to standard reinforced concrete piles, show very little settlement due to their very good soil friction (typical settlement less than 5mm under design load). Due to 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.
The hollow steel section is far superior to a solid rod of the same cross section and steel quality with respect to bending, shear and surface friction.