TUNNELLING IMPACTS | TECHNICAL
masonry which can lead to sewer collapse (see Figure 6). Stability of masonry sewers relies on sufficient
confinement during any surcharge events. The consideration of the Confinement/Pressure Ratio (CPR) gives an indication of the vulnerability of masonry sewers when subject to surcharge pressure. CPR is defined as the ratio of soil overburden pressure at the sewer axis level (σvo
) to the water pressure within the
sewer (P). For assessment purposes, the surcharge pressure within a sewer is commonly taken as the hydrostatic pressure to the levels of the controlling nearby manholes or other sources of pressure relief. If a masonry sewer is in good condition and other
significant destabilising loadings are absent, a CPR in excess of 1.33 is generally considered to be acceptable. This criterion is also applicable to sewers constructed using materials vulnerable to surcharge pressures such as unreinforced concrete (see Figure 7). CPR check is also a useful tool for assessing
potential vulnerability of unbolted water transmission tunnels. CPRs in excess of 1.33 under surge pressure and 1.5 under operating pressure have been found to be acceptable. Reduction in soil overburden pressure is usually
caused by excavations for various structures (e.g. stations, basements) and can produce potentially damaging unload of confining compressive hoop loads in both sewers and water tunnels. Therefore, this issue should be considered at the earliest planning stage for infrastructure and buildings in order to avoid a design which is unacceptable to the utilities. Many masonry sewers are commonly subject to
frequent cycles of large loadings from heavy traffic and vibration. A fatigue-type racking motion within the masonry, caused by rotation of principal stresses and such loadings from a rolling axle, can result in sewer failure.
6 CONCEPT OF ‘IMPACT STRAIN’ For assessment purposes, the parameter ‘strain’ rather than ‘stress’ is adopted as the damage criteria. This is because it is displacements both in the ground and of the pipeline which are actually measured and used to monitor and control the works. The conversion to stress can be done but this introduces an unnecessary uncertainty in the choice of appropriate moduli. It is self-evident that a pipe will fail when the
movement/load imposed upon it exceeds its strain limit. In order to assess the impacts on the pipe as a result of the proposed construction works, it is important to understand the existing state of strain within the pipe which is unknowable in many circumstances. This leads to the introduction of the concept of ‘impact strain’ for assessment purposes. ‘Impact strain’ is defined as the strain arising from
ground movements or other loadings caused by the proposed construction works. It does not include any strains arising from any other causes (including pre- existing ground movement, ground load, operating pressures or normal vehicular loading). This ‘impact
strain’ may be that predicted during the assessment process or back-calculated from field measurements undertaken during the works. The calculated impact strain should be compared
with the assessment criteria advised by the relevant utility. This will provide an indication of the risk of damage to the pipe associated with the proposed construction works.
6.1 KEY ASSESSMENT ASSUMPTIONS For Stages 1 and 2 of the assessment process (see Section 7) it is conservatively assumed that: 1 ‘Green field’ ground movement is adopted. 2 Longitudinal flexural (bending) of the pipe follows this ‘green field’ ground movement (i.e. stiffness of the pipe is ignored).
3 The joints are stiff so that the pipe is assumed to be a continuous linear structure. This allows the calculation of maximum longitudinal flexural (bending) stain.
4 The joints are fully flexible so that the limiting condition is in joint rotation.
5 In the absence of site specific information, a 3.66m (12ft) long pipe is used for the calculation of joint rotation for UK metal pipes. It is also useful to review dimensions provided in the Standards and/or manufacturer specifications for other pipe materials.
6 Flexural strain is calculated at the pipe extrados. a Metal/plastic pipes – the neutral axis is at the geometric centre (i.e. lever arm equal to external pipe radius).
b Materials of low/negligible tensile strength (e.g. masonry, vitrified clay, unreinforced concrete) – the neutral axis is at the pipe extrados (i.e. lever arm equal to external pipe diameter).
7 Total ground movements are simply obtained by superposition, as appropriate.
6.2 SOURCES OF GROUND DISTURBANCES 6.2.1 Tunnels Gaussian models (e.g. O’Reilly & New (1982) reprinted in 2015; New & O’Reilly (1991); Leca & New (2007)) have been commonly used to calculate green field tunnelling induced movements and associated slope, curvature and strain because they are straightforward to apply and have stood the test of time in their application. The authors are not aware of any body of data that would contradict their general application for initial assessment of near surface pipelines with a depth to axis of less than say, 3m. The ‘ribbon sink’ model by New & Bowers (1994) is
useful for calculating ground movement in stiff cohesive materials at depth (especially for estimating movements in the vicinity of the tunnel under construction). Three dimensional movements from complex tunnel geometries can be calculated using this model. Volume loss (Vl) and trough width parameter (K)
are the two key input parameters for the calculation of tunnelling-induced ground movements based on
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