Home - Istasazeh services - Ground improvement - What is Micropile Underpinning or Root Pile Construction?

Piles with a diameter of less than 30 cm, which are accompanied by steel reinforcement and grout injection, are referred to as micropile underpinning also known as minipiles, pin piles, needle piles, and root pile construction. There are two main approaches to ground improvement:

• Use of bearing members
• Improving soil strength characteristics by various materials

Steel bar has the first advantage and grout has the second advantage.

There are three main stages in executing micropile underpinning or root pile construction:

• Tube driving or drilling and tubing
• Grout
• Reinforcement

For the first time micropile underpinning or root pile construction was used to repair existing foundations and buildings but with the development of drilling and grout equipment, it became more widely used.

History of micropile method

In the early 1950s, when the need to repair damaged structures of World War II was felt in Europe, the famous Italian contractor Fondedile, together with Dr. Fernando Lizzi, invented the micropile. Due to the:

• Speed of execution
• Ability to execute in damaged buildings
• Proper performance

micropile method spread rapidly in Italy.

The Fondedile expanded the method in 1962 with the restoration of several historic buildings in England and in 1965 with the improvement of transport projects in Germany. In 1973, with the modification of structures in New York and Boston, micropile method was introduced to North America.

After the spread of micropile, different countries developed regulations for the design and execution of micropiles. The most famous of these is the Micropile Design and Construction Code, regulated by the Federal Highway Administration (FHWA). The French National Forever Project also conducted a comprehensive study on the behavior of micropiles.

Micropile Design & Construction

Micropile Design & Construction depend on the purpose of its use. If the purpose of micropile is to improve the soil, three issues should be considered in the design of piles:

1. Structural Design
2. Geotechnical Design
3. Cone Shear Control

Execution of micropile to improve the soil under the existing structure

In structural design, the bearing capacity of micropile structural members is calculated including

• Steel pipe
• reinforcement bar
• Cement slurry

This capacity must be with a suitable reliability, higher than the load applied to the micropile.

In geotechnical design, the frictional resistance of the micropile (root pile) wall with the surrounding soil is calculated. This frictional resistance should be with an appropiate safety factor, higher than the applied load so that it is not possible to separate the micropile from the soil before the yield of the reinforcements.

Finally, due to the high load and small diameter of the micropiles (root pile) to prevent cone shear, a suitable sheet of micropile cap is designed.

Execution of micropile method

Two methods can be used to execute micropile underpinning:

1) Use of self-drilling micropile – self-grout

2) Conventional method: This method consists of the 4 following stages:

• Drilling (if required): In general, micropile is placed by driving or drilling:
• Placement by driving due to the friction between the pipes and the soil wall around them and it effect on increasing the grout pressure of cement slurry, significantly improves the bearing performance of micropiles.
• Placement through drilling causes corrosion in different layers of soil and in practice, a larger diameter should be drilled than the outer diameter of the pipes. For this reason, drilling operations are used only in situations where pipe driving is not possible (mainly at great depths and stiff soils). Drilling operations are performed by different methods such as rotary drilling or D.T.H.
• Pipe driving: After drilling, micropile pipes are driven by pipe hammer machines in designated places. The first piece is bayonet and the next pieces are completely connected by bushing and welding, respectively. The driving operation will continue as long as it is possible to drive the pipes, and if the pipe does not sink more than 10 cm for 30 consecutive blows of the pipe hammer, the driving operation of the micropile pipe will be stopped.

In this case, until the depth of design is realized (reached), drilling is done and then the tubes pipes related to micropile are installed inside the borehole. Micropile pipes are usually in two-meter sections with an outer diameter of 76 mm and an inner diameter of 68 mm. These pipes are connected to each other by bushing and welding. Each micropile has 80 holes with a diameter of 8 mm per meter. The internal environment of the tubes should be electrified so as not to have negative effects on the grout of cement slurry.

• Grout: Grout machines consist of three parts including primary mixer, secondary mixer and grout pump. Mixing in the primary mixer is of the rapid water circulation system type and the secondary mixer is of the vane type. The grout slurry is made in the primary mixer by first pouring the desired amount of water into the mixer and then adding cement to the required ratio of water to cement. The minimum mixing time of cement slurry is 30 seconds. The ratio of water to cement is selected according to soil characteristics between 0.5 to 1.5. To maintain the prepared slurry in the primary mixer, the slurry is poured into the secondary mixer and then injected by a pump. Tubes called packers are used to inject the slurry into the micropile tubes. During grout, the packer adheres to the wall of the pipe and prevents the slurry from leakage.

For example, to close the packer for 8 to 10 m: The first stage is to close the packer at a depth of 6 m, and after the grout operation at this depth, the packer is closed at a depth of 4 m, and after the grout at this stage, the pack is closed at a depth of 2 m, and finally, after grout at this stage, the packer of head well is closed and the grout operation is completed. If slurry leakage is observed from the surface or lateral micropiles, the grout operation is stopped.

Grout specifications

• Grout pressure: Grout pressure in different stages of grout, at different depths, under the influence of soil type and geotechnical conditions can be variable. The maximum grout pressure is limited to 10 atmospheres.
• Amount of cement used: Depending on the geotechnical conditions and the bearing capacity of the micropiles design, the estimated amount of cement can be equivalent to each micropile length up to 100 kg. However, due to the fact that the grout should be continued up to a pressure of 10 atmospheres, so the amount of cement may be higher than the initial estimate.
• Water to cement ratio: The water to cement ratio in the grout slurry used is between 0.5 to 1.5 in normal conditions and the water to cement ratio of grout cement in other conditions can vary from 0.67 to 1. It is noteworthy that the slurry ratio in each section is determined at the suggestion of the contractor and the approval of the monitoring body.
• Type of used cement: The type of used cement in the cement grout mortar is Portland cement type one, two or five, which is determined according to the chemical conditions of the desired location.
• used water: The used water in the preparation of grout slurry, must be clean and smooth and have all the necessary conditions for used water in the manufacture of concrete. The used water should be free of any harmful substances to the cement slurry such as acids, alkalis, sugars, salts and organicmatter. The used water to make the grout slurry must contain suspended solids less than 0.2%, soluble substances less than 3.5%, chlorine less than 1%, sulfate less than 0.3% and alkalis less than 0.06%. In general, drinking water is suitable for making slurry.
• Reinforcement bar: In case of micropile bearing, it is necessary to place the reinforcement bar inside the micropile pipe and install the micropile flange. Obviously, the reinforcement bar must be installed inside the borehole before the cement traps. The flange, which is used to create a complete connection between the micropile and the foundation concrete, as well as to prevent the cone shear of the flange inside the concrete, must be welded to the pile reinforcement bar of the micropile.

Applications of micropile method in ground improvement

Extensive research on micropile (mini pile) has been conducted in specialized forums around the world and is still increasing day by day.

Investigating various aspects of micropile is done, including behavior:

• Static
• Dynamic
• Earthquake

In addition, in general, the application of micropiles in geotechnical engineering consists of two parts:

• Use in the bottom of foundations
• In situ soil improvement

Some applications of micropiles or root pile constructions are:

• Use as a bearing member under foundations
• Use as a bearing member to repair the foundations of old buildings
• Use as a bearing member under other superstructures (backpacks, oil and gas reservoirs, etc.)
• Use with the aim of improving the soil and increasing its resistance and behavioral characteristics
• Use for ground improvement and counteracting the liquefaction

Classification of micropile or root pile construction

The classification of micropiles is based on the following two criteria:

1) Design method:

The design of a single micropile or group of micropiles is very different from the design of a network of closely spaced reticulated micropiles. This has led to the definition of two categories of design for micropiles. In the first category, micropiles are loaded directly and the reinforcements bear most of the applied load. In the second category, micropiles are placed in the soil as a mesh and create a reinforced soil composite system that can withstand the applied loads. This hybrid system is the same mesh of a network of closely spaced reticulated micropiles.

In the first category, micropiles are used as an alternative to traditional piles to transfer the load of the structure to the underlying resistant layers. These micropiles are designed to function individually, even if constructed in groups.

In the second category, micropiles have a lighter reinforcement because the reinforcements, like the first category, are not directly loaded and it is the mass of soil and piles that bears the applied loads.

In addition to these two categories, another type of micropile (root pile) is used in Iran, which is known as consolidation micropile. The consolidation micropile is similar in behavior philosophy to reticulated micropile, except that it does not use reinforcing bars.

Cement slurry grout is the main part of consolidation micropile, which increases the resistance and behavioral characteristics of the soil.

2) construction method:

Grout method is the most important factor affecting the strength of the trapped part (bond) between soil and slurry in such a way that the resistance of the trapped part changes directly by changing the grout method. The classification criterion is the grout method and its pressure, and the use of casing and reinforcing bars determines the sub-sections of this classification. In this classification, micropiles are divided into 4 types:

Type A: In this method, the slurry is injected under its own weight and without pressure at the site.

Type B: In this method, the cement slurry is injected into the borehole under pressure after the steel casing is removed from the borehole. The grout pressure usually varies between 0.5 and 1 MPa and should be such as to prevent hydraulic failure.

Type C: This method shows a two-stage process in which the cement slurry is first placed under its own weight inside the borehole (as in type A) and then before the initial slurry hardens (after about 15 to 25 m), the same type of slurry is injected through reticulated grout pipes without the use of a packer, at the point of contact of the slurry with the ground and at a pressure of at least 1 MPa. This type of micropile is used only in France.

Type D: This method is similar to type C in that the cement slurry is first placed under its own weight such as type A and C or under pressure, such as type B in the borehole. After the initial slurry hardens, the next slurry is injected through mesh grout pipes at a pressure of 2 to 8 MPa. In this method, a packer is used so that special surfaces can be modified several times if necessary.

Quality control of micropile method

Depending on the purpose of executing the micropile, different quality control tests can be defined for this method, some of which are:

• Micropile compression loading test
• Micropile tensile loading test
• Micropile lateral loading test

Advantages of micropile method

• High execution speed
• Need for less equipment compared to precast or on-site pile construction equipment
• No need for special expertise to use the equipment
• Possibility of use in projects with limited space (for example, in the basement of old structures or under the decks of bridges, and so on)
• Ability to increase the bearing capacity of existing foundations due to high flexibility
• Possibility of use in urban environments due to low noise pollution
• Easy quality control tests

Animation of micropile method in soil improvement

Related contents:

Federal Highway Administration (FHWA)

Keller , Micropiles