TECHNICAL | TUNNELLING IMPACTS
J
Initial spacing between mid-points of adjacent pipe J
Initial Position
Right, figure 14: Joint pullout
J + ΔJ Joint pullout ΔJ
Final Position
displacement can cause leakage. Assuming the
pipeline is rigid and the joint is free to accommodate axial movement, the joint pullout will not be more than the differential ground movement between mid-points of adjacent pipes (see Attewell et al. (1986)). For an optimally aligned spigot and socket
connection, a pullout limit of 3mm can be considered for cast iron pipes as a joint leakage criterion for assessment purposes. For non-flexible joints (e.g. bolted flanges), the
pipelines should be considered as continuous linear structures with a check that the connections are not overstressed.
6.7 Transverse (crushing) strain for pipe The problem with large diameter pipe subject to load from concentrated sources from above is crushing. This can result in longitudinal cracking (see Figure 15) which
is usually developed at the quarter points of the pipe wall. The maximum transverse bending strain (εt
= 3 σv r2 (1 - K0) /2 E t 2
where σv
) at the
quarter points of the pipe wall is given by New (2019) as: εt
is the vertical stress at the pipe axis level. This can be estimated by Boussinesq-based analyses.
K0 is the ratio of horizontal to vertical ground stress.
r is the mean radius of the pipe. E is the Young’s modulus of the pipe (typically 80GPa for cast iron pipe).
t is the pipe wall thickness.
6.8 Transverse strain for masonry sewer For assessment purposes, it is conservative to assume that masonry has negligible tensile strength and each part of the outer circumference of the masonry sewer moves with the ground (full bond). The transverse sewer
Longitudinal crack
Longitudinal crack
Above, figure 15: Longitudinal cracks observed along vitrified clay pipe and cast iron pipe due to crushing failure Cast Iron Pipe
Vitrified Clay Pipe 24 | February 2022
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