Paul,

I forgot to mention an additional reference that presents a comprehensive assessment and discussion regarding past collective research on soil restraint of buried pipelines is presented in the ASCE publication:

"Guidelines for the Seismic Design of Oil and Gas Pipeline Systems", Committee on Gas and Liquid Fuel Pipelines, ASCE, 1984.

The previously mentiond "Guidelines for the Design of Buried Steel Pipe", July 2001, summarizes much of this information with some further refinements, but the original 1984 publication provides more historical background and further insight.