TECHNICAL | TUNNELLING IMPACTS
Thames Water Utilities Limited (TWUL), the largest
water and wastewater services company in UK, offers ‘Guidance on piling, heavy loads, excavations, tunnelling and dewatering’ on their website (https://www.
thameswater.co.uk/developers). The assessment criteria provided in that document are summarised in Tables 1 and 2 and they assist developers in preparation of impact assessment reports. The criteria are for guidance only, and it is based on early work by Attewell et al. (1986) and years of experience by TWUL. It is intended to represent a level of risk of damage which may be reasonably regarded as negligible for a pipe in average to good condition. It is a fact however that any pipe is potentially vulnerable to any increase in strain and joint rotation. The developers’ designers may adopt alternative criteria values, provided they can justify that the risk of damage remains negligible. Attewell et al. (1986) and Bracegirdle et al. (1996) also
offer useful guidance on the assessment of strain in cast iron pipelines.
7 THE STAGED ASSESSMENT PROCESS A more risk based, three-stage approach for pipeline assessment has been proposed by New (2019). The aim is to demonstrate the risk balance between the likelihood (based on evaluation of ground movement and ‘impact strain’ presented in Section 6) and consequences of an adverse event, and the ability to recover (i.e. emergency preparedness and resilience) during and after the works. Details for this staged assessment approach will not
be repeated here. However, a comprehensive summary of this staged assessment process is presented on Figure 18 and the key considerations are listed below: Stage 1 (Assessment scope and information retrieval) a. Identification of all utility assets which may be affected by the proposed works: i
Calculation of likely green field ground movements based on conservative assumptions.
ii Assets falling outside the 1mm settlement contour are likely to be excluded from further analysis.
iii Contours indicating minimum radius of curvature (R) or maximum curvature (1/R) can assist in identifying those assets most likely to need mitigation at this earliest stage.
b. Gathering of information needed for subsequent stages of the risk assessment (see ‘Conditions (properties) of assets’ and ‘Consequence of damage’ in Section 3 for further details).
Stage 2 (More detailed analysis) a. Longitudinal and transverse analyses (based on moderately conservative assumptions and calculation methodologies as presented in Sections 5 and 6) for those assets not eliminated by the Stage 1 preliminary assessment.
b. Produce charts showing (axial, lateral, vertical and flexural) displacements, curvature, (axial, flexural, combined tensile and compressive) strains, joint
26 | February 2022
4 2 0
-2 -4 -6 -8
-10 -12 -14 -16 -18 -20
-16 -14 -12 -10 -8 -6 -4 -2 0 210 12 14 16 4 6 8
Position change in lateral (y) direction (mm) 10° Interval
Original Prediction
4 2 0
-2 -4 -6 -8
-10 -12 -14 -16 -18 -20
-16 -14 -12 -10 -8 -6 -4 -2 0 210 12 14 16 4 6 8
Position change in lateral (y) direction (mm) 10° Interval
Original Prediction
Brick Sewer (2m ID, 0.6m thick) Depth to axis (8m)
z =90°
T+
z+
N+ y+
+ =180° =0° y
=270°
Position change in vertical (z) direction (mm)
Position change in vertical (z) direction (mm)
1m
1.6m
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