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Certain details of micropile design remain unresolved between various practitioners and such disagreements have limited the acceptability of micropiles in certain quarters. Detailed micropile design is an extremely lengthy and often complex subject, especially when micropile groups are concerned or seismic issues must be addressed. The paper provides a simplified step-by-step design approach discussing initial evaluation of feasibility, review of data, loading combinations and general considerations. Thereafter the final design steps are discussed. These include geotechnical strength limit states, other structural considerations, service limit states, corrosion protection, and some seismic considerations.

Extreme variations in ground conditions which categorize karstic limestone terrain inevitably create challenges during design and installation of deep foundation systems. For example, the elevation of rock head may vary greatly over short distances, while for substantial depths below rock head, one may anticipate the presence of major solution features. Such features may be entirely open, or may be partially or completely filled with products of the limestone degradation. High capacity micropiles of the type described by FHWA (1997) have, for some years now, proved to be a technically and economically viable deep foundation system in such terrain. Frequently, they have been the only plausible solution. This adaptability is a result of the large variety of small diameter borehole drilling techniques which can be exploited to ensure penetration in such conditions, and also the fact that micropiles transfer load by skin friction. The design implication is that if certain thickness of "good" rock is encountered, then a certain pile capacity can be guaranteed. There is therefore dramatically reduced risk of piles "punching through" into a solution feature as would occur in the case of an end-bearing pile. During the last several years, the authors have been involved in the design, construction and performance testing of high capacity micropiles in karst. This paper provides an overview of these experiences and provides recommendations as to most appropriate practice. Reference is made to several recent case histories.

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