The first use of micropiles dates back to the early 1950’s in Italy, where new methods of underpinning for existing structures were needed to restore structures and monuments damaged during World War II (Lizzi, 1982). Dr. Fernando Lizzi is commonly recognized as the inventor of micropiles in the form of the root pile or palo-radice.
Dr. Lizzi was a civil engineer and Technical Director with the Italian specialty foundation contractor Fondedile and obtained the first patents for root piles in Italy in 1952. This early form of micropile technology was used extensively in Europe for the restoration of various structures and monuments.
Fondedile introduced micropiles into North America in 1973 by performing a number of projects, mainly in the Northeastern United States. By the mid 1970’s a number of US specialty foundation contractors previously engaged in drilled and grouted anchor work had developed their own variants of the technology. There was slow growth of the technology in the time period between the mid 1970’s and the mid 1980’s with Fondedile closing their North American venture for economic reasons. (Bruce and Juran, 1997)
There has been a rapid growth in the specification and use of micropiles in the United States since the mid 1980’s to early 1990’s partly as a result of FHWA research efforts, trade association promotion efforts and the development of various publications offering standardized design and specification guidelines.
In the early 1990’s, the Intermodal Surface Transportation Efficiency Act (ISTEA) provided massive funding for the rehabilitation of highway infrastructure in the United States. As part of this effort, the FHWA undertook a number of research and development projects associated with specialty geotechnical construction to encourage innovation in geotechnical applications and produced several design manuals including the first on micropiles. This was the beginning of the surge in micropile use in the United States.
In 2006 and 2007 respectively, the International Building Code and the AASHTO LRFD Bridge Design Specifications incorporated design code sections for micropiles thus making way for further expansion of applications in both building and highway construction. Micropiles currently are widely specified and used in all construction sectors worldwide.
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.
A new video has been uploaded to my YouTube Micropile page: Limited Access Hollow Bar Micropile Installation. Hollow bars can be the ideal foundation solution in this application because they do not require as much hardware and equipment as conventional cased micropiles. Hollow bar segments can easily be carried by hand whereas casing and downhole hammer equipment often will require a forklift or other mechanical assistance to move. WARNING: The US Surgeon General has determined that smoking is hazardous to your health. You can view my videos at http://www.youtube.com/user/JonathanBennettPE. Let me know if there are particular things you want to see.
Details are being finalized for the 2011 DFI / ADSC Micropile Seminar to be held in Little Rock, AR. Complete information and registration details will be available soon from DFI and / or ADSC.
Date: Tuesday and Wednesday, May 3-4, 2011
Location: Peabody Hotel, Little Rock, AR, USA
DFI / ADSC Micropile Seminar
A day and a half event to discuss the history of micropiles, design and construction, industry challenges, load testing, material and equipment, quality control, and selected case histories. This seminar will include a presentation on LRFD for micropiles in response to increased interest in the subject. The event will feature presentations from the leading North American and worldwide experts on micropile design and construction.
More information will be posted as it becomes available. Make plans to attend this year’s event.
Exhibitor spaces and sponsorship opportunities are available.
Hollow bar micropiles exhibit outstanding bond transfer values and have the ability to reinforce soft/loose soils thus improving the settlement behavior of foundations utilizing them. The behavior of hollow bar (fully bonded) micropiles in soil will be examined and a case history discussed where over 1000 hollow bar micropiles were used to reinforce soils beneath heavily loaded equipment pads at an aircraft manufacturing facility in soft coastal plain soils. A model of bonded anchors / micropiles in soil is developed for characterizing and describing micropile behavior. An extensive load testing program was implemented to optimize micropile lengths and finite element modeling was utilized to quantify the amount of improvement achieved by the micropiles.
Design-build contractor GeoStructures completed the foundation reinforcement of Pennsylvania’s historic Kinzua Bridge. Tension micropiles were required in order to add supplemental lateral load resistance for the remaining sections of the former railroad bridge that was partially destroyed by a tornado in 2003. The micropiles, which anchor the existing tower piers into rock, were installed through the existing sandstone tower pier foundations. GeoStructures installed a total of 48 primary tension micropiles with load capacities ranging from 54 to 84 kips to provide additional foundation uplift restance and 48 secondary anchors to reinforce the existing tower piers.
A case history on the project was presented at the International Society for Micropiles – International Workshop on Micropiles in Washington, DC. The case history presentation can be viewed HERE.
A new video has been uploaded to my YouTube Micropile page. Micropile Drilling Process in Karst is a six-minute clip showing the drilling process used for rock socketed micropiles in Karst geology. The video shows casing advancement through overburden, drilling of the rock socket and removal of the drill string and hammer. This drilling was performed using an excavator-mounted drill rig, a down-hole-hammer overburden drilling system and seven inch casing. You can view my videos at http://www.youtube.com/user/JonathanBennettPE. Let me know if there are particular things you want to see.