Abstract: Geotechnical engineering investigation adopts results obtained from the small-diameter (≤100mm) exploratory sampling and in-situ testing to guide the design and construction of the production cast-in-situ piles. In a project practice in the coastal deeply deposited karst terrain site, even the “one exploratory borehole for one pile” method was adopted, the deviation rate with the large-diameter production pile drilling (≥800mm) construction still exceeds 20%, which made the effect from the exploratory pilot borehole reduced significantly. This paper uses the analogy method to explain the drilling effect under the complicated karst characteristics. The graphical analysis is made to illustrate the interaction between the rock mass characteristics and the production drilling. Three types of trajectory models for the drilling in the rock mass are proposed; namely, no-deviation; single deviation and multiple deviations. Factors that might induce these deviations are also discussed in the paper. The objective of this paper is to explore the essential aspects of the drilling, to enrich the basic theory of geotechnical exploration, to improve the understanding of the geotechnical exploration work by all involved parties, and to make recommendations of the technical management in Karst terrain for all involved parties.

Key words: One Exploratory Borehole for One Pile, Exploratory Drilling, Karst Morphological, Structural Plane, Deflection