TECHNICAL | TUNNELLING IMPACTS
the Gaussian models. Recent experience in London
has confirmed that a volume loss of 1% for tunnelling in stiff cohesive materials (e.g. London Clay) has usually not been exceeded by using Earth Pressure Balance Machines (EPBMs) and this is generally considered as a ‘moderately conservative’ assumption for assessment purposes: this relates to a single tunnel driven in ground undisturbed by other tunnels. Historically however, it is clear from the literature that for twin tunnels the driving of a second tunnel parallel to the first yields larger volume losses. This is confirmed by recent measurements reported by
Wan et al. (2017) during the construction of twin tunnels (7.1m bore diameter, axis depth of 34.5m and 16.3m separation) below Hyde Park in London for Crossrail (now the Elizabeth Line). The surface volume loss during the excavations for the first tunnel being about 0.8% but for the second tunnel increased to about 1.4%. This increase is attributed to a softening of the clay caused by the first tunnel excavations. This would indicate that the impact assessment for the second of twin tunnels a volume loss of greater than 1% should be considered by designers. It is important to note that the predicted ground
movements based on the Gaussian models cited above are neither conservative nor un-conservative. It is the assessors’ responsibility to establish and justify the parameter values adopted in their analyses. Given the uncertainties in the input parameters, more complex models are unlikely to be beneficial/necessary to Stages 1 and 2 of the assessment processes (see Section 7).
6.2.2 Shafts, basements/boxes, dewatering Tunnel construction often require construction of shafts which are used for tunnel boring machine (TBM) launching, access, and maintenance purposes and the shaft construction itself can lead to significant ground movements. Based on the original predictive equations proposed by New & Bowers (1994) and an extensive database of measurements taken in London, New (2017) has proposed a generic equation for predicting settlement at a distance from the shaft wall: Sd
= αH (1- d/nH)2
where Sd
is the settlement at a distance d from the shaft wall.
n is a simple multiple of the shaft depth H to a distance d from the shaft wall where settlement becomes zero. (Note: n = 1 for the original New & Bowers (1994) equation).
α is an empirical constant and αH is the settlement at the shaft wall.
The n and α values are dependent on shaft diameter, ground conditions and construction method. The assessors will have to establish and justify their parameter choices based on appropriate empirical data (e.g. case histories). CIRIA C760 (2017) provides good guidance on ground
movements associated with construction of basements and boxes.
20 | February 2022
Dewatering (either route wide, local, short-term or
long-term) is commonly required to facilitate excavation for shafts, basements and boxes, and this can give rise to significant ground movements. Local dewatering (especially in the presence of alluvial deposits) can result in damaging localised differential ground movements. Designers for the dewatering system should work with the assessors to minimise the risk of damage to the pipelines and other utility assets.
6.2.3 Foundation & heavy/abnormal loading The application of foundation loadings will result in ground movements and potential overloading of underlying utilities. The construction and use of piled foundations in close proximity to a pipeline may result in damage which can be caused by: ● Excessive pile loading onto the pipeline (e.g. shaft load shedding, end-bearing pressure)
● Uncontrollable ground loss during pile boring within difficult ground conditions (e.g. water bearing gravels)
● Excessive vibration during pile construction (e.g. driven piles, encountering physical obstructions)
The use of temporary casings, sleeving, non-vibratory/ non-percussive piling methods (e.g. silent piling) and redesign of the foundation layout, backed-up by the calculation of associated impacts, are the common ways to mitigate the risks mentioned above. Construction works normally involve the use of
sizeable plant (e.g. cranes, piling rigs, load loaders, self-propelled modular transporters (SPMTs)) which can impose damaging loadings onto pipelines. The ALARP risk approach is to ensure these heavy/abnormal loads are positioned outside the zone of influence of the pipeline, which is an area commonly defined by drawing 45 degree lines upward and away from the pipeline. This can also be achieved by using bridging structures to carry the load away from the pipeline. If it is not possible to position the loads outside the
zone of influence, both longitudinal and transverse analyses for the pipeline will be required. Simple analytical solutions (e.g. Boussinesq approach) are generally adopted for the assessment. It is unlikely to be beneficial to undertake soil-structure interaction analysis given the uncertainties regarding the ground and pipe conditions.
6.2.4 Demolition Analysis should be undertaken to assess the potential ground movements and associated impacts on the pipeline due to demolition works. Careful control of demolition operation is important to minimise the level of vibration and impact loads transmitted to the pipelines.
6.2.5 Vibration Construction works may generate vibrations (dynamic ground strains) that can be damaging to local structures, including utility apparatus. However, there are now non-vibratory methods available that can
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