Horizontal directional drilling (HDD) in urban environments creates some risk with respect to heave and subsidence induced damage to nearby utilities, pavements, and structures. Risk management practices and field quality controls are reviewed and risk factors for heave and subsidence are analyzed based upon governing geotechnical conditions. Allowable drilling pressures and ground improvement protections are discussed as primary mitigation tools. An application of geomechanical modeling is introduced, using cavity expansion theory to provide quantitative assessment of drilling mud limit pressure and minimum cover requirements. Analytical results are compared with other related published numerical analyses and methods. Findings are discussed in relation to five major HDD installations in the Honolulu vicinity. Modeling is also shown to help evaluate jet grout ground improvement for subsidence protection.
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