Application of 3D finite-element analysis to predict ground displacement from horizontal directional drilling in highly compressible organic clays

Department of Infrastructure, Veda Associates Ltd, London, United Kingdom

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Abstract

Horizontal Directional Drilling (HDD) is a proven trenchless methodology for utilities installation below critical infrastructure that limits predicted ground displacement to 5mm. In very soft, highly compressible organic clays, the option to HDD is discounted as small ground displacements are considered unachievable. Very soft, highly compressible organic clays are problematic due to high water content, low permeability and high compressibility, which cause excess pore pressures and time-dependent consolidation during and after drilling. The resultant effects are settlement and distortion; risks increase if annular pressures approach hydraulic-fracture thresholds or if ground loss occurs. This paper presents an HDD case study to install two high-voltage circuits below a National Railway in Tilbury, where the drill alignment below track is within a very soft, slightly sandy, very silty, organic clay. This study applies Finite-element software PLAXIS 3D to predict; ground displacement, long-term consolidation, and assess bore stability. The benefit of the software is the HDD bore geometry, staged construction (i.e., pilot bore, reaming), and idealised bore pressures can be modelled. PLAXIS has developed Soft Soil model specifically designed for highly compressible clays, and organic soils, similar to the Alluvium deposits recorded either side of the railway. The Soft Soil parameters calibrated from triaxial, 1D oedometer, and in-situ testing are screened to identify the high-influence parameters.