TECHNICAL | TUNNELLING IMPACTS


Original brickworks around sewer crown


Right, figure 6:


Collapse of masonry sewer caused by a storm event


Collapsed sewer section


adopt numerical models (finite element or similar)


which assume masonry as linear elastic material with ‘unrestricted’ tensile strength and modulus of elasticity equal in both tension and compression. It is important to note that a numerical model must not assume tensile properties that do not exist or cannot be justified.


Masonry sewers were generally designed for


sustaining gravity flows without surcharge. However, the continuous expansion of population in cities has led to a significant increase in volumes of sewage passing into the network which can generate surcharge pressures. This can result in unacceptable hoop tension in the


Δσvo


CPRin-situ σvo / P > 1.33 = existing soil overburden pressure, Δσvo (σvo


CPRexcavation - Δσvo


) / P < 1.33


Collapse


Above, figure 7: Illustration of failure mechanism associated with CPR dropping below 1.33 as a result of excavation (Note: σvo


= change in soil overburden pressure due to excavation, h = head of surcharge pressure, γw 18 | February 2022 = unit weight of water & P = surcharge pressure)


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