Soil Springs for Sloped Ground Movement in Saturated Soil

Abstract Pipelines buried in sloping ground have not previously incorporated adequately defined soil-springs to represent the reality of forces applied on sloping ground. The soil-spring representation, with its three mutually orthogonal spring pairs, has been developed and defined first by ASCE (1984), and subsequently built on by others. These guidelines were based on physical modelling experiments conducted in the 1970’s and 80’s, and were composed of straight sections of rigid pipe. Those experiments saw the pipe translated relative to the soil in single directions under monotonic movement. The soils studied were largely laboratory (clean sand) and manufactured clays. These experiments define the soil springs for horizontal ground only, and do not necessarily represent the interaction occurring on sloping ground. Physical modelling in a geotechnical centrifuge has been undertaken to study the direct soil-pipe interaction in landslides. Centrifuge modelling recreates scale models in an accelerated gravity field such that the forces, stresses, and physical dimensions are multiplied representing the full-scale equivalent environment. A model pipe representing an NPS 18, ∼X52 line pipe has been designed to allow axial and bending response equivalent to the real-world pipe. Landslides were initiated by elevated pore water pressure, representing above mean seasonal rainfall in upland areas. Pipe strain was monitored using fiber optics and subsurface imaging was used to capture the strain demand (soil displacement) provided by the landslide. Pipe strain was decomposed into bending moment, as well as axial and orthogonal structural forces representing the soil loading on the pipe. Results of the pipe-soil experiments give new insight into the strain transfer between the sloping ground and the pipe. Results show that the peak bending moments shift downslope as the landslide movement progresses. A comparison of American Petroleum Institute (API), DNV, and other industry guidance for springs is made to the pipe-slide interaction results from the physical experiments.