Laboratory Determination of Soil Stiffness Data for Buried Plastic Pipes

Abstract

The structural behavior of a buried flexible pipe is influenced by the applied surface loading, the pipe ring stiffness, and the stiffness of the surrounding soil. The pipe stiffness is governed by national standards and can be determined by testing. The more dominant surround stiffness depends on the soil type and its placement and is consequently greatly influenced by site installation processes. There is a relative lack of soil stiffness data, and much of the available data relates to more common installation conditions in the United States (i.e., installation beneath embankments). Described in this paper is the assessment of stiffnesses of surround types used in the United Kingdom by laboratory testing of plastic pipes in a large test tank to simulate burial in a trench. Both gravel and sand surrounds were used, and either thorough or nominal compaction was applied. Static surface stresses were then applied to the buried pipes and diametral strains recorded, which are used for back-analysis of Spangler’s modulus of soil reaction (E’). The analysis yielded significantly higher values of E’ than have previously been assumed. For a lightly compacted sand, E’ was found to be 16 MPa. However, when the same sand was thoroughly compacted, E’ was found to be 99 Mpa. For a rounded, 10-mm gravel with only nominal (i.e., light) compaction, E’ was found to be 29 MPa. Where an existing pipe is subjected to an additional surface loading, the corresponding recommend E’ values for additional diametral strain prediction are 20, 100 to 150, and 40 MPa, respectively. The results obtained are discussed in relation to currently used values of E’ and site practice.