An Analysis of Soil–Pipe Interaction in Sand by Photoelastic Approach and an Analytical Approximation

In-situ stress condition is an important aspect of buried unpressurized pipelines. Empirical approaches used for preliminary design are usually based on observations, which may be associated with some errors related to the measurement method. The photoelastic approach represents an alternative, nonintrusive measurement technique to model the plane stress-strain behavior of buried pipes under different surcharge and groundwater conditions. Based on this approach, two model tests M1 and M2 were conducted to demonstrate the effects of soil arching on a pipe model buried in river sand and manufactured sand, which have similar granulometry but different angularity. In the dry state, the higher arching effect between the round particles of the river sand leads to a lower settlement and a smaller change in the stress and diameter of the pipe in M1 compared to M2. The finite element analysis confirms the experimental results quite well, while Iowa formula is sufficient to represent the diameter changes. However, after a loading and unloading cycle, M1 shows larger settlements at saturation due to the loss of arching effect in contrast to M2. The redistribution of the round particles and the re-arching in a denser state lead to a huge stress relief in the pipe of M1, while the interlocked angular particles prevent the diameter changes, so that the stresses observed in the pipe are mainly maintained in M2 at unloading and saturation. Finally, a practical analytical model to determine the pipe deformation for the dry condition problem is proposed.