The 48-inch diameter pipeline crosses three mountain ranges as well as forests, rivers, and plains. More than half the line is elevated in sections ranging from about 30 miles in length to the a few hundred feet. The remainder is buried underground.
The decision to elevate or bury the pipe depended primarily on soil conditions and the possible effects of the pipeline heat on the soil. Normal burial was used in stable soils and rocks, where thawing would not cause loss of soil support for the pipeline. Additionally, special burial techniques were used in some short sections for animal and highway crossings.
In places where melting permafrost might create soil stability conditions, the pipeline was insulated, jacketed, and installed above ground. Thawing around the aboveground supports in the most heat-sensitive areas was and is prevented by thermal devices that carry heat up through the pipes to radiators on top of the supports. from: HISTORY