Tunnelling in sand and its effect on pipelines and piles

Alec Marshall, Cambridge University, Geotechnical Engineering Group


Beneath the surface of the world’s larger cities is an intricate and congested system of tunnels, pipelines, and other buried infrastructure. New tunnel construction occurs increasingly close to existing buried structures. Tunnel designers must account for likely effects that construction activities may have on these structures.

Three areas of research are dealt with in the dissertation: (i)tunnelling in sandy ground, (ii) tunnelling beneath pipelines, and (iii) tunnelling beneath jacked piles. A series of centrifuge tests was carried out for each topic. The first series of tests on tunnels in sand served as a baseline for comparison for the subsequent tests involving pipes and piles. Tests were carried out in a symmetric condition behind the face of a transparent Perspex wall such that high-quality soil and structure deformation data could be obtained using an image-based measurement system.

The tunnelling tests illustrate that the Gaussian curve typically used to model settlement trough data is not sufficient for tunnels in sands. The use of a modified Gaussian curve is shown to provide a better fit. Trends in shape function parameters are provided which illustrate the change in settlement trough shape with depth and volume loss. The complexity of the problem due to the contractive/dilative nature of the sand is highlighted. Insights into soil behaviour are provided using contours of displacement and associated values of strain and dilation angle.

Results from the pipeline tests illustrate the importance of determining a reasonable estimate of the relative stiffness of the pipeline compared to the soil. Mechanisms of soil and pipeline displacements are provided including data that illustrates the formation of a gap beneath the tested pipes during tunnel volume loss. Comparison of pipeline bending behaviour is also made to predictions made using elastic continuum solutions.

The pile tests provide visual evidence of the mechanisms of soil and pile displacement when jacked piles are undermined by tunnel construction in sand. The development of highly dilatant zones of soil in the area of the pile bases and between the piles and the tunnel are illustrated. The potential for sudden and dramatic pile displacement is demonstrated.

Keywords: tunnel, pipe, pile, sand, centrifuge, soil-structure interaction.