TECHNICAL | TUNNELLING IMPACTS


Table 1: Assessment criteria for existing TWUL pipeline and sewer assets Pipe type


Brick Sewer (red/yellow/blue brick) Cast Iron (lead-yarn joints)


Ductile Iron (lead-yarn gasket joints) Ductile Iron (rubber gasket joints) Steel


Vitrified clay Concrete (unreinforced)


Diameter (mm) N/A


N/A N/A N/A N/A <125 >125 <225


225 – 750 >750


Allowable increase in strain (µε) Tension 500 100 500 500 450 80 80 20 40 60


Table 2: Maximum rotation for existing TWUL vitrified clay and concrete pipes Diameter (mm) <375


375 – 750 750 – 1400 >1400


Compression


25% of the allowable stress 1200 700 700 450 400 400 400 400 400


Rotation (degree) N/A


0.1 0.5 2.0 1.5 0.5


See Table 2 0.5


See Table 2


Rotation (degree) 2.0 1.0 0.5 0.3


rotation and pullout along the particular section of the pipe under consideration.


c. Identify those assets likely to be particularly vulnerable based on R&M history, night line flow history and condition survey.


d. Identify those assets which are of critical importance and with high consequence of failure.


e. Those assets which fail to meet the utility assessment criteria and/or with high criticality/ vulnerability are further considered in the Stage 3 risk analyses.


Stage 3 (Evaluation of risk) a. Risk/remedy based approach i


ii


Acceptable level of risk? ALARP concept (i.e. the risk shall be ‘As Low As Reasonably Practicable’)


Importance of collaboration between developer and utility to develop a ‘reasonable and optimised’ solution which is acceptable to both parties


iii Establish/justify the need for intrusive diversionary or mitigation works (see ‘Control (and Preparedness)’ in Section 3 for further details regarding inherent risks associated with these works and alternative options).


iv Consideration of consequential losses (in terms of ‘health and safety’, failure to meet statutory requirements, and ‘commercial’).


v Risk of incorrect analysis or human errors during the formal checking and approval of reports.


b. Need for soil-structure analysis? i


Use of numerical models can be helpful for certain applications but it is generally considered to be unnecessary for pipeline assessment, given the uncertainties about the ground and pipe conditions and difficulties in establishing an appropriate set of input parameters.


28 | February 2022


ii Assessors should justify the necessity of undertaking numerical analysis (finite element or similar) and the results should be corroborated by closed form (analytical) solutions together with case history data where possible. Further guidance on the management of advanced numerical modelling in geotechnical engineering can be found in CIRIA C791 (2020).


Mitigations a. There are various engineering solutions to mitigate risk to pipelines. However, there will be risks associated with the implementation of the mitigation measures. Further details are summarised under ‘Control (and Preparedness)’ in Section 3.


b. Pre- and post-work inspections of the assets are recommended. This can inform the assessors regarding the current condition of the asset so that an appropriate set of criteria will be adopted in the impact assessment. Also, any damages/defects identified after the completion of the proposed works can then be repaired in order to restore the whole-life asset value.


c. It is important to have an Emergency Preparedness Plan (EPP) in place throughout various phases of the construction works. An incident recovery plan should be included that will allow rapid repair of a pipe failure and have provision of equipment and spares required to restore serviceability to customers promptly while repair works are being undertaken (e.g. overpumping equipment covering a sewer failure scenario).


d. It is important for developers to provide a RAMS (Risk Assessment and Method Statement) package for early review by the utility to ensure the works will be carried out within the constraints defined by the utility and be ALARP.


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