Micropiles can be installed at various angles from vertical and are capable of resisting both axial and lateral loads. The unique structural makeup of micropiles has an effect on the way in which they develop their load resistance and how they behave in response to these loads at various inclinations.
Micropiles develop their axial capacity primarily through the bond between grout and soil or rock in the bonded zone of the pile. Because of this, micropiles provide both tension and compression resistance thus making them useful in a variety of applications, particularly where resistance to uplift is needed in addition to resisting compression or gravity loads.
With the majority of micropile axial load being resisted by steel casing or internal reinforcing core, micropiles exhibit relatively high axial stiffness and are capable of resisting large axial loads. Micropiles that are fully bonded exhibit even higher apparent stiffness than those that are partially unbonded for the same steel area.
Because of the installation methods (down-hole hammer and rotary-percussive drilling) and equipment used, micropiles can be installed in soil and rock conditions where the use of other conventional deep foundation systems are not a reasonable alternative, such as in Karst topography or where modest subsurface obstructions or boulders are present. Micropiles can be installed through modest obstructions and boulders that would be problematic for installation of helical piles, driven piles, drilled shafts or augercast piles. They can also be drilled into pinnacled rock where achieving acceptable anchorage or bearing for other deep foundation types might be questionable.
Micropiles can be easily installed in caving soils and below the water table by either using casing or hollow bar micropile installation methods. Caving soils and elevated water can be problematic for deep foundation systems that rely on open hole drilling such as drilled shafts.
Compact and low headroom drilling equipment is available such as to make installation of micropiles possible in low-headroom or limited space applications where the installation of other types of conventional deep foundation systems is not possible. Examples of this are applications where supplemental foundation support is required in basements or areas with overhead restrictions or otherwise small or space restricted areas where relatively large equipment cannot fit.
The drilling and installation equipment used for micropiles does not produce an amount of vibration that would be harmful to structures as opposed to driven piles which can produce magnitudes of vibration that have the potential for causing settlement of adjacent structures resulting in structural damage. For this reason micropiles are well suited for use in close proximity to existing structures.
Micropiles are ideal for retrofit applications in that they can easily be installed through a core drilled hole in an existing foundation or footing and anchored to the existing foundation for load transfer.