TUNNELLING IMPACTS | TECHNICAL
Missing brick Missing mortar
Left, figure 3: Loss of mortar and missing brick around the arch of a masonry sewer
c. Network resilience i
Alternative supply routes within the network?
d. Emergency preparedness/contingency planning i
Review of impact models (possible flood zone and other consequence).
ii Emergency preparedness plan (EPP) should include an incident recovery plan that will allow rapid repair of a pipe failure. R&M contractors should be made aware of the works and have emergency spares and equipment immediately available for repairs.
iii Local operations staff notified of the works.
5 Consent a. Has the appropriate level of analysis and checking been carried out?
b. Is the risk ALARP (As Low As Reasonably Practicable)?
c. Response from utility: ‘Approval in Principle’; ‘No further comments’, or ‘LONO’ (Letter of No Objection)
6 Conciliation a. Further negotiations with Developer b. Court injunction (the last resort).
4 PIPE FAILURE MECHANISMS Pipes do not fail randomly and there is always an explanation for pipe failure. Generally, the following three groups of causes may be attributed to pipe failures: 1 Third-party intervention a. road works; ground movements; compaction; vibration; abnormal surface or other loads.
2 Weakened pipe a. Corrosion; casting defects (e.g. incorrect section, wormholes, porosity, joint failure).
3 Operational overload (which is unlikely given a pipe in serviceable condition) a. Over pressurisation; surge
New (2019) illustrates typical failure mechanisms for pipes made from brittle materials (e.g. cast iron, vitrified clay, unreinforced concrete) and they can be grouped into the following categories: ● Bearing fracture due to concentration of reaction or load (which may occur in any size of pipe).
● Longitudinal cracking (splitting) due to overload (with larger pipes tending to be more vulnerable).
● Circumferential crack due to flexural bending (which is most common in metal pipes with diameters of 300mm or less; or unreinforced concrete/vitrified clay pipe with aspect ratio (pipe length/pipe diameter) greater than two.
● Burst socket due to excessive joint rotation and/or shearing (which may occur in all pipe sizes).
● Spiral failure due to a combination of longitudinal flexural and transverse loading (which is commonly found in pipes between 300 – 600mm diameter).
Experience has shown that the locations of pipe failures rarely coincide with those pipe sections predicted to have the most likely chance to fail. This is because pipe failure depends on various factors which, when present in an unfavourable combination, are sufficient to cause the failure. Therefore, a probabilistic (rather than a strictly deterministic) approach is more appropriate for pipeline assessment.
5 MASONRY Many early trunk sewers were constructed in brickwork and took various geometrical forms (e.g.
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