TECHNICAL | TUNNELLING IMPACTS


c. Material i


Iron (grey or ductile); clayware; brick; concrete (unreinforced or reinforced); thermoplastic (polyethylene (PE) or polyvinyl chloride (PVC)); glass reinforced plastic (GRP)


d. Size i


Outside and inside diameters; pipe section length


e. Jointing i


Lead run; Cementitious; Flanged; Welded; Flexible; Age


f. Present conditions i


Review of repair and maintenance (R&M) history and night line flow records held by the utility will give a better indication of the condition of the pipe network than the occasional trial pit investigation.


ii


Internal survey of sewers allows identification of existing defects which may require urgent attention before commencement of the proposed works.


g. Exclusion zone i


ii


This is the minimum vertical and horizontal clearances required between the extrados of the asset and the outer face of the proposed works.


This is to allow sufficient space for any repair/ maintenance/upgrade works required for the assets.


h. Ground conditions i


Problematic soil (e.g. alluvial deposits) iii Bedding (e.g. poor workmanship)


ii


i. Pre-existing strain (if known) i


Strain arising from ground movements or other loadings caused by historic works (e.g. cross trenching, tunnelling, basements)


2 Calculations (e.g. ground movements, changes in loading) a. Tunnels, shafts, basements, boxes and foundations (including any demolition, piling, dewatering works required and any use of heavy construction plant in the vicinity of the assets) and associated vibration issues


b. Geometries and construction sequences/ methods (including both temporary and permanent works)


c. Ground movement and utility damage assessments (e.g. strain, joint rotation and pullout)


3 Consequence of damage a. Strategic, trunk or distribution main? i


Consequences of pipe failure are not necessarily related to pipe size. For instance, a water supply to a hospital or fire mains may be in a relatively small diameter pipe (distribution main) but nevertheless safety-critical in importance.


ii


Failure of a pipe within a resilient network (i.e. there are sufficient alternative supply routes) will have less impact on the supply to customers.


14 | February 2022


Geo-environment (e.g. chemical attack on pipe material)


b. Failure to meet statutory service obligations? i


There can be a substantial penalty to the utility if it cannot restore its service to customers within a pre-defined period.


c. Loss of whole-life asset value (e.g. damage, loss of capacity or downgrade)?


d. Damage to third-party assets and health and safety issues (e.g. possible flood zones, explosion)?


e. Financial and reputational impact to the utility’s business? The general public usually considers the utility is at fault even though the cause of pipe failure may be associated with other third-party activities.


4 Control (and preparedness) a. Mitigation i


ii Replacement, diversion or strengthening?


Temporary isolation? This will allow the asset to be inspected and/or tested upon completion of the proposed works before it can be safely reopened for use.


iii Pre- and post-condition survey (including additional inspection to be carried out at critical phases of the works, where necessary).


iv Locate and exercise of valves controlling the flows to minimise the extent of flooding/ damage associated with pipe failure.


v Potential risk to workers (e.g. substantial periods of man entry to install structural lining inside a sewer; collapse of brickwork during removal of temporary support)?


vi Potential risk/disruption to public (e.g. traffic diversion and intrusive traffic management, traffic delays)?


vii Potential risk to projects (e.g. availability of materials, pipes and plants; speed of construction; unforeseen additional costs and time to implement mitigation measures)?


viii Feasibility of developing a strategy for doing the minimum to preserve serviceability of the assets to reduce potential serious risks associated with mitigation works?


ix


‘Do-nothing’ approach? This relies on a robust emergency preparedness plan and an undertaking from the developer that, on completion of the works and stabilisation of the ground movements, any damage will be either repaired or the asset replaced.


b. Monitoring i


ii iii


Validation of design assumptions relied upon by the assessments


Type (e.g. ground/pipeline movements, leakage, vibration)


Location of monitoring points


iv Frequency of data collection v Data interpretation method vi Trigger levels vii Action plan (e.g. termination and/or modification of the construction / mitigation processes; change in monitoring and data review frequency).


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