Micropiles are a type of deep foundation that are particularly useful in situations where there is very limited space or headroom available for the construction of conventional pile rigs. They are also known by a variety of other names, including minipiles, pin piles, needle piles, and root piles.
These piles are relatively small in diameter, typically ranging from 150 to 300 mm. They are constructed using a rotary drilling machine and are cast in place, which means that they are formed directly at the site where they will be used. Once the hole for the pile has been drilled, it is filled with grout that is rich in fine sand. This grout is injected into the hole under pressure, which ensures that it completely fills the space and provides a solid foundation for the pile.
Method Statement of Micropile Installation are:
The process of drilling a borehole for foundation purposes involves the use of a rotary drilling equipment and bentonite slurry. The diameter of the borehole should be between 150 to 300 mm. The use of bentonite slurry helps to stabilize the sides of the borehole during the drilling process.
To ensure the stability and durability of the foundation, it is necessary to install a liner that extends to the hard rock or strata. MS (Mild Steel) or HDP (High-Density Polyethylene) pipe liners are commonly used for this purpose. However, MS liners are preferred due to their reliability.
To reinforce the pile, a tor steel reinforcement cage made up of a bundle of large diameter rebars is provided. This reinforcement cage helps to increase the pile’s capacity by improving its resistance to external forces.
After installation, the pile is grouted with a rich grout made up of cement, fine sand, and water with a cement-to-water ratio of 0.5. The grout must have a compressive strength of at least 480 kg/cm2 after 28 days. An alternative method involves filling the borehole with coarse aggregate up to a specified height of approximately 1m and subsequently filling it with cement grout.
Finally, the piles are socketed adequately into the hard strata or rock to develop the necessary capacity mainly through friction in the socket. This process helps to ensure that the foundation is secure and can withstand external forces.
Fig: Micropiles
The safe load capacities for vertical loads usually range from 25 to 35 metric tons. On the other hand, for lateral or shear loads, the safe capacities generally fall within the range of 2 to 4 metric tons.
Sequence of Piling for Micropiles:
When installing piles, it is important to follow a specific sequence that ensures the load carrying capacity of previously installed piles is not diminished. This sequence should be based on the working drawings provided. It is common practice to install piles in a group from the center towards the periphery, or from one side to another to prevent soil from flowing out during the operation.
By following the proper sequence, the integrity of the pile group is maintained, and the load carrying capacity is optimized. This is particularly important in situations where the pile group will be supporting a significant load. Failure to follow the correct sequence could result in reduced load capacity, compromising the safety and stability of the structure being supported.
Therefore, it is imperative to pay close attention to the sequence of piling during installation. Any deviation from the prescribed sequence could have detrimental consequences, potentially leading to costly and time-consuming repairs. Proper planning and execution are crucial to ensure the successful installation of piles and the overall safety and stability of the structure being constructed.