Drilled & Grouted Hollow Bar Micropiles

Drilled micropiles are constructed using bespoke hollow-threaded steel bars that are simultaneously drilled and grouted. These piles are generally between 150 mm and 300 mm in diameter and installed using powerful minipiling rigs weighing between 2.5 and 10 tonnes. These piles can be installed in restricted-access areas and very-low-headroom environments, depending on the rig and loading requirements.

They are installed using rotary percussive drilling techniques. The bore’s stability is maintained by the grout flush, which is pumped through the centre of the steel bar as the pile is progressed. The steel bar is advanced into the ground using sacrificial drill bits selected to suit the ground conditions. The steel bar forms the structural element of the pile; the grout acts as the load transfer mechanism from the steel bar to the ground to achieve the geotechnical capacity. These micropiles can also be enhanced at the head with circular hollow sections to provide modest shear capacity. This technique requires comprehensive ground investigation information to design the pile for the prescribed loads.

Drilled and grouted hollow bar micropiles

Maintained load tests are carried out in conjunction with this technique. Both compression and tension tests can be undertaken on preliminary and/or working piles for design verification purposes. Preliminary testing of this pile type is relatively cost-effective and can generate significant cost and programme benefits to the main pile installation work.

This technique is suitable for use in all ground conditions and is not susceptible to issues relating to groundwater. It is particularly effective in granular soils where the geotechnical capacity regularly exceeds those calculated and achieved by more conventional methods. The technique is regularly used for underpinning unstable/distressed buildings and on overhead transmission line projects to enhance or found the tower structures. This installation process can also be used in permeation grouting where voids require to be infilled or weak soils require to be strengthened.