1 Soil-buried pipeline interaction for vertical downwards relative offset George P. Kouretzis 1 , Kristian Krabbenhøft, Daichao Sheng and Scott W. Sloan ARC Centre of Excellence for Geotechnical Science and Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, NSW 2308, Australia A new perspective is presented on the interaction effects for the vertical downwards offset of a pipeline relative to its surrounding soil. Instead of estimating the interaction force via shallow footing bearing capacity theory, as per common pipeline design practice, we assume that vertical movement of the pipeline in uniform soil is governed by mechanisms similar to the lateral loading of a circular pile up to its limit load. The validity of this assumption is investigated numerically with the finite element limit analysis method, and design expressions are derived for the maximum interaction force on pipelines embedded in cohesive and granular soils. For the common case of buried pipelines built in sand-backfilled trenches, the same numerical method is employed to determine the necessary trench dimensions so as to avoid interaction with the possibly much stiffer native soil, that will result in a significant increase of the force applied on the pipeline during ground movement. The described approach can be employed in project-specific analyses to optimize trench dimensions, and thus avoid unnecessary excavation costs or mitigation measures. Keywords: buried pipelines; vertical offset; trench; design; limit analysis 1. Introduction Permanent ground deformations of a magnitude sufficient to jeopardize the integrity of buried pipelines may take place in areas where their route crosses soil deposits that undergo significant volume changes due to environmental effects such as wetting-drying and freezing-thawing cycles (Rajeev & Kodikara 2011, Oswell 2011); where they occur in irrigated lands where aquifer overpumping may result in non- uniform consolidation (Wols and van Thienen, 2014); where they are subject to surface settlement associated with mine subsidence or near-surface tunnelling works (Wang et al. 2011); where they span normal or oblique seismic fault crossings (Rojhani et al. 2012); where they are embedded in potentially unstable slopes (Cocchetti et al. 2009); or where they undergo liquefaction-induced lateral spreading (O’Rourke and Liu, 2012). Regardless of whether the ground surface deformation patterns are widespread or localized, the design of buried pipelines subjected to permanent ground movement is commonly based on numerical or analytical beam- nonlinear spring models (e.g. ASCE-ALA, 2005; Karamitros et al., 2007, Karamitros et al. 2011). Input parameters of such models include the force-displacement curve of the elastoplastic soil springs which represent the surrounding native soil, or the backfill for cases of pipelines constructed in trenches. Numerous studies in the literature have focused on the development of force-displacement curves for horizontal offset (Trautmann and O’Rourke 1985, Olson 2009, Jung et al. 2013a, Yimsiri et al. 2004, di Prisco and Galli 2006, Kouretzis et al. 2013, Robert and Soga 2010, Turner 2004, Paulin et al. 1998) and vertical upwards offset (Trautmann et. al, 1985, Yimsiri et al. 2004, Jung et al. 2013b). Much less attention has been paid to 1 Corresponding author - email: Georgios.Kouretzis@newcastle.edu.au tel. +61 2 4921 6449, postal address: EA Building, University of Newcastle, Callaghan NSW 2308, Australia Page 1 of 13 Can. Geotech. J. Downloaded from www.nrcresearchpress.com by University of Newcastle on 05/05/14 For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record.