Micropile Basics: An Overview and History
December 1, 2010
The use of micropiles has grown significantly since their conception in the 1950s, and in particular, since the mid-1980s. Micropiles have been used mainly as elements for foundation support to resist static and seismic loading conditions, and as in-situ reinforcements for slope and excavation stability. Many of these applications are for transportation structures.
Piles are divided into two general types – displacement piles and replacement piles. Displacement piles are members that are driven or vibrated into the ground, thereby displacing the surrounding soil laterally during installation. Replacement piles are placed or constructed within a previously drilled borehole, thus replacing the excavated ground. A micropile is a small-diameter (typically less than 12 inches), drilled and grouted replacement pile that typically is reinforced. A micropile is constructed by drilling a borehole, placing reinforcement, and grouting the hole. Micropiles can withstand axial and/or lateral loads, and may be considered a substitute for conventional piles or as one component in a composite soil/pile mass, depending upon the design concept employed.
Micropiles are installed by methods that cause minimal disturbance to adjacent structures, soil and the environment. They can be installed in access-restrictive environments and in all soil types and ground conditions. Micropiles can be installed at any angle below the horizontal using the same type of equipment used for ground anchor and grouting projects. Since the installation procedure causes minimal vibration and noise, and can be used in conditions of low headroom, micropiles often are used to underpin existing structures. Specialized drilling equipment often is required to install the micropiles from within existing basement facilities.
Most of the applied load on conventional cast-in-place replacement piles is structurally resisted by the reinforced concrete; increased structural capacity is achieved by increased cross-sectional and surface areas. Micropile structural capacities, by comparison, rely on high-capacity steel elements to resist most or all of the applied load. These steel elements have been reported to occupy as much as one-half of the hole volume. The special drilling and grouting methods used in micropile installation allow for high grout/ground bond values along the grout and ground interface. The grout transfers the load through friction from the reinforcement to the ground in the micropile bond zone in a manner similar to that of ground anchors. Due to the small pile diameter, any end-bearing contribution in micropiles generally is neglected. The grout/ground bond strength achieved is influenced primarily by the ground type and grouting method used, i.e., pressure grouting or gravity feed. The role of the drilling method also is influential, although less well quantified.
Micropiles were conceived in Italy in the early 1950s, in response to the demand for innovative techniques for underpinning historic buildings and monuments that had sustained damage with time, and especially during World War II. A reliable underpinning system was required to support structural loads with minimal movement, and for installation in access-restrictive environments with minimal disturbance to the existing structure. An Italian specialty contractor called Fondedile, for whom Fernando Lizzi was the technical director, developed the palo radice, or root pile, for underpinning applications. The palo rudice is a small-diameter, drilled, cast-in-place, lightly reinforced, grouted pile.
Although steel was in short supply in postwar-Europe, labor was inexpensive, abundant and often of high mechanical ability. Such conditions encouraged the development of these lightly reinforced, cast-in-place root pile elements, largely designed and installed by specialty contractors on a design-build basis.
Direct full-scale load tests were performed at relatively little cost, fostering the acquisition and publication of a wealth of testing information. No grout/ground bond failures were recorded during these early tests.
The use of root piles grew in Italy throughout the 1950s. Fondedile introduced the technology in the United Kingdom in 1962 for the underpinning of several historic structures, and by 1965, it was being used in Germany on underground urban transportation systems. For proprietary reasons, the term “micropile” replaced “root pile” at that time. Initially, the majority of micropile applications were structural underpinning in urban environments. Starting in 1957, additional engineering demands resulted in the introduction of systems of reticulated root piles. Such systems comprise multiple vertical and inclined micropiles interlocked in a three-dimensional network, creating a laterally confined soil/pile composite structure. Reticulated micropile networks were applied for slope stabilization, reinforcement of dockside walls, protection of buried structures, and other soil and structure support and ground reinforcement applications.
Other proprietary micropiles were developed in Switzerland and Germany, and the technologies quickly were exported overseas by branches or licensees of the originating contractors. The Far East soon became a major market. Fondedile introduced the use of micropiles in North America in 1973 through a number of underpinning applications in the New York and Boston areas. The micropile technology did not grow rapidly in the United States, however, until the mid-1980s, after which time an abundance of successful published case histories, consistent influence by specialty contractors, and the growing needs of consultants and owners working in old urban environments overcame the skepticism and concerns of the traditional East Coast piling market. The abundance of relatively cheap labor, the shortage of steel, and the need for reconstruction programs in urban environments promoted the growth and use of micropiles in Europe. Conversely, the slower and later growth of micropile usage in North America is reflective of the abundance of cheap steel, relatively high labor costs, and the need for capital works projects typically outside of the cities. These circumstances fostered the growth of the comparatively low-technology, driven-pile techniques governed by prescriptive specifications. Today, construction costs and technical demands are similar throughout the world, and so continue to foster the growth of micropile demand, largely through geotechnical contractors with design-build capabilities.
This article is provided through the courtesy of the U.S. Department of Transportation’s Federal Highway Administration (FHWA). It is excerpted from the document “Micropile Design and Construction Guidelines.” Preparation of the manual was funded through a cooperative effort between FHWA, DBM Contrac-tors Inc., Dywidag Systems International Inc., Hayward Baker Inc., Condon-Johnson and Associates Inc., Malcolm Drilling Co., Nicholson Construction Co., and Schnabel Foundation Co.