Deformation Analysis of a Pipeline River Crossing

In Manitoba, the Main (100) Line operated by TransCanada PipeLines Ltd. (TCPL) comprises six natural gas pipelines ranging from 864 mm to 1219 mm in diameter. These pipelines cross the La Salle River at Main Line Valve (MLV) 39+6.06, approximately 3 km south of St. Norbert, Manitoba. A rupture in Line 100-2 occurred at this crossing site on April 15, 1996. Investigations of the site following the line break concluded that ground movement associated with the east approach slope was likely a contributing factor in the failure of Line 100-2. It was postulated that the observed slope instability was attributable, at least in part, to a combination of the low shear strength of Lake Agassiz clay and artesian pore pressures associated with the underlying regional Upper Carbonate Aquifer (UCA), controlled in part by groundwater pumping within the City of Winnipeg. Based on the findings from the initial slope stability analyses, a more detailed deformation analysis was proposed to examine the influence of hydrogeological conditions on deformation and slope stability at the La Salle site. The numerical analyses were conducted using FLAC^2D, a two-dimensional explicit finite difference code for engineering mechanics computation. The code simulates the pre- and post-peak behaviour of geological and other materials based on specified material properties, constitutive models, and failure criteria. The deformation analysis has identified the key factors controlling stability of the east approach slope at the La Salle site, and has provided insight into probable failure mechanisms. High pressures in the UCA increase the risk of sudden large-scale slope failure at the slope toe at critical times during the year, e.g., after rapid drawdown following the spring flood, and following the drop in river level in late fall to winter levels. The analysis indicated that the risk of slope instability at this site can be reduced by lowering the aquifer pressure locally in the vicinity of the east approach slope at critical times in the year. The results of the study have demonstrated the usefulness of employing deformation analysis as a means of understanding the key factors controlling slope stability at a particular site. The approach is also compatible with the development of a comprehensive soil/pipe interaction methodology that integrates the results of ongoing in situ monitoring into pipe stress analyses.