Using the Total Apparent Spacing Approach to Optimize Foundation Drain Orientations

ABSTRACT Design of foundation drain holes in bedrock where fractures control the permeability may be optimized based on the discontinuity pattern, specifically the spacing and orientation of the predominant discontinuity sets. When suitable representative outcrops are available, either naturally or by means of excavation, joint spacing and orientation can be measured directly by traditional methods using a measuring tape and compass or pocket transit or newer methods using computer software to measure virtual joint surfaces within a digital terrain model developed from digital photogrammetry or light detection and ranging (LiDAR) surveys. When suitable outcrops are not available, joint spacing and orientation can be measured using oriented core drilling techniques or downhole imaging tools (e.g., optical and acoustic sondes) that provide digital images of the borehole wall. With an understanding of the bedrock jointing pattern obtained from one or more of the above methods, the total apparent spacing (TAS) approach can be used to optimize the orientation of drain holes. The TAS approach employs the vector dot product formula to minimize the total apparent spacing between discontinuities and maximize the number of discontinuities intercepted by the drain hole. The TAS approach was used to optimize the orientation of foundation drain holes during rehabilitation of the Gilboa Dam in Schoharie County, NY. The TAS approach has other applications, e.g., optimizing the orientation of exploratory core holes to collect discontinuity data or the orientation of grout holes for foundation grouting.