This series of papers summarizes the research project initiated in 1993 by the US Federal Highway Administration to review the state of practice, case studies and design methods of micropile group systems. Following a brief description of the recently adopted classification system for micropile design previously outlined by Bruce et al. (1997) (part 1), this paper (part 2) presents a summary of recommendations for the design of single micropiles and groups and networks of micropiles for selected engineering applications, including direct structural support and in situ soil reinforcement. Preliminary estimates of the ultimate axial and lateral capacity of micropiles, as outlined by different authors, are presented first. Design guidelines which have been developed generally through observations on full-scale testing and field experiences are discussed with regard to cohesionless soils, cohesive soils and rocks. Group and network effects are investigated and preliminary conclusions are presented along with proposed design guidelines for micropile groups. Existing analytical approaches are evaluated through comparisons with experimental data obtained by different investigators on the engineering behaviour of micropile groups and reticulated micropile networks under different loading conditions.

A major study of micropiles has recently been funded by the Federal Highway Authority (FHWA) in the United States. The subject is limited to drilled and grouted, cast-in-place, reinforced piles of nominal diameter less than 300 mm. Such piles are used for direct structural underpinning, and, when installed closely spaced in groups or networks, are used for in-situ soil reinforcement. They have become increasingly popular throughout the world since their inception in Italy in 1952, and are widely used in association with urban and industrial development and redevelopment projects. In the series of five papers, the major findings of the FHWA study are presented. Part 1 in the current paper, deals with the background to the FHWA study, characteristics and definitions, classification, historical development and types of application. Part 2 summarizes design, with distinctions drawn between single piles, and groups and networks of piles. Part 3 describes construction techniques, with reference to various national practices. Part 4 discusses testing procedures, while Part 5 provides summaries of well documented case histories which illustrate the application of the principles introduced in the first four parts. As an integral feature of Parts 2, 3 and 4, the gaps in current knowledge are highlighted, as a possible guide to future research.

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Recent advances in tactile sensor technology have produced very thin sensors allowing the simultaneous measurement of normal pressure at multiple points. Such sensors were used under the base of a laboratory model of a rigid strip footing (76.2mm x 609.6mm). The developed contact stresses are peaked along the centerline and lower along the edges. This distribution, visually observed for the first time, matches well the anticipated theoretical distribution and is quite different than the even distribution routinely assumed in design. The insight obtained by the observed contact stress distribution can assist in a better geotechnical and structural economical design of shallow foundations. BACKGROUND A successful and economic design of a shallow foundation requires the knowledge of the maximum bearing pressure of the soil under the footing. Design methods are based on Bearing Capacity theories assuming that the contact pressures beneath a footing are uniform. In the case of eccentric loading conditions, the design methods are simplified and a linear approximation of the actual stress distribution is used. In reality, however, the distribution of contact stresses is neither constant nor linear.

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