TUNNELLING IMPACTS | TECHNICAL
Flexural displacement profile
D
Left, figure 12: Assessment of joint rotation
where S is the initial distance between P1 and P2, and ΔS is the change in the axial distance between P1 and P2. Axial compression and tension are represented by negative and positive ΔS values respectively (see Figure 11). The bending strain (εb
) and axial strain (εa
combined together to calculate the total ‘impact strain’ for a pipeline when undertaking assessments. This can sometimes result in very significant impact strain even after the axial strain reduction factor proposed by Attewell et al. (1986) has been applied. Following review of field experience and various
calculation methodologies, New (2019) concludes that axial strain may be regarded as negligible for various types of pipes at relatively shallow depths (say 2m) for the Stage 1 and Stage 2 assessment processes. However, assessors may still wish to include axial strain when assessing pipes of critical importance. It is important to note that the potential axial compressive and tensile strains cannot be relied on to reduce flexural tensile and compressive strains respectively (i.e. the beneficial axial strains are ignored) for assessment purposes.
) are usually
6.5 Joint rotation Assessment of joint rotation (ꞵ) is typically undertaken by tracking two pipes along the predicted flexural displacement profile with the left and right end of the two pipes and the joint following the flexural displacement profile (see Figure 12) until the point of maximum rotation has been established. The magnitude of joint rotation is dependent on the pipe length L which is usually assumed to be 12ft (3.66m) if as-built records are not available. Joint rotation is an important cause of failure in larger
cast iron pipe (with diameter in excess of about 300mm or 12in) because of its higher bending stiffness. This allows the generation of very large bursting forces as the spigot rotates within the socket. Figure 13 shows an image of a reconstructed cast iron pipe after suffering from socket bursting caused by joint rotation.
6.6 Joint pullout The axial ground strain may induce relative movement between pipe lengths resulting in joint pullout (see Figure 14) which should be assessed because excessive joint
Left, figure 13:
Zone of fracture associated with socket bursting
Reconstructed cast iron pipe after suffering from socket bursting caused by joint rotation
February 2022 | 23
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