Micropiles are deep foundation used when very limited space or headroom is available for execution with conventional pile rigs. These piles are also called as minipiles, pin piles, needle piles and root piles.
Micropiles are small diameter bored cast-insitu piles of diameter 150 to 300 mm constructed by rotary drilling machine and grouted using rich grout with fine sand by injection process.
Method Statement of Micropile Installation are:
- A borehole of required diameter 150 to 300 mm is drilled using bentonite slurry by a rotary drilling equipment to stabilize the sides of the bore hole.
- A liner of full depth upto hard rock / strata is necessary. M.S. or HDP pipe liners are being used MS liners are however preferable.
- Tor steel reinforcement cage of a bundle of large diameter rebars is provided as pile reinforcement.
- Pile is grouted with rich grout of cement and fine sand with water cement ratio of 0.5 and this grout shall have compressive strength of minimum 480 kg/cm2 on 28 days. Alternatively any other suitable method including filling the bore with coarse aggregate upto specified height of about 1m and filling it subsequently with cement grout can also be proposed.
- The piles are socketed adequately into the hard strata / rock necessary to develop the capacity which comes mainly in friction in the socket.
Fig: MicropilesThe safe vertical load capacities are generally from 25 to 35 MT and lateral (shear) capacities are generally from 2 to 4 MT.