Geotechnical and geophysical investigation for trenchless pipelines

Abstract

Trenchless pipeline installation involves horizontal drilling beneath obstacles that preclude trenched construction (e.g. creeks, rivers, major transport corridors and environmentally sensitive areas). Traditional geotechnical assessments for trenchless routes typically involve intrusive investigations at discrete locations. Hence, in environments with potentially variable ground conditions (e.g. river alluvium, or disturbed ground associated with railway or road embankments), considerable geotechnical risks inherently remain where trenchless installations are progressed essentially "blind" between discrete investigation locations. The consequences of not adequately managing geotechnical risks could include bore misalignment or steering issues, encountering drilling obstructions, bore refusal and overburden blow-out or frac-out. The authors' approach to assessments for trenchless alignments commences with identifying and assessing groundrelated risks based on a preliminary understanding of the site geological environment and/or geomorphological features to developing an investigation scope. The scope of site investigations is governed by technical requirements appropriate for the given project stage/budget (e.g. feasibility assessment, concept or detailed design) and investigation requirements set by authorities as a condition of approval (e.g. for railway under-track crossings). However, it is the authors' experience that often, traditional geotechnical investigation methods need to be augmented with either geophysical methods and/or other specialised investigation techniques at an early stage, in order to enable effective identification and management of geotechnical risks as a trenchless project progresses. This paper discusses various geotechnical and geophysical investigation methods that have been used to assist in managing geotechnical risks for trenchless pipelines. A number of recent project examples are presented to demonstrate the application of the methods and their effectiveness in reducing geotechnical risks.