Pipeline Route Assessment at River Crossings and in Steep Terrain: Geophysical, Hydrologic and Geotechnical Characterization

Abstract

Pipeline river crossings and sections of pipeline routes where steep terrain requires directionally drilled borings have the highest chance of being successfully designed and constructed if subsurface geological conditions are understood. In this paper we present results of geophysical surveys conducted to characterize the subsurface at two pipeline river crossings and at a site where steep topography would likely require directional boring below the face of a steep hillside. The objective is to help assess and minimize the risk in engineering design in difficult terrain by analyzing subsurface geology from geophysical data and vertical geotechnical borings, and evaluating the dynamic behavior of the river itself through hydrologic analysis. Risk factors can be assigned on the basis of lithology and environmental considerations relating to the level of potential impact in different parts of the crossing. The laterally heterogeneous nature of river channels, consisting of stacked paleochannels and floodplains could require a significant number of vertical geotechnical borings for adequate characterization of the entire crossing. We find that a combination of electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and ground penetrating radar (GPR) data can efficiently provide us with an understanding of electrical and mechanical properties from which lateral variations and depth extent of lithology along the proposed boring can be inferred. Confirmatory vertical geotechnical borings allow us to verify our interpretation at two locations. Geophysical data are used to laterally extrapolate the lithologic interpretation and define, in conjunction with surface water hydrologic considerations, the minimum depth of directionally drilled borings and optimum locations of ingress/egress points. The investment in a geological assessment study to understand subsurface conditions prior to beginning horizontal boring operations is essential to mitigate risk and ultimately may save money. In the case of steep terrain, geophysical data can provide valuable information on the vertical and lateral variations in subsurface properties in areas where it would be impossible to safely drill vertical borings. Our last case history is an example of the geological information that can be efficiently inferred from geophysical surveys conducted in steep terrain.